signals travel down the spinal cord to the muscles

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The Anatomy of the Spinal Cord

It forms the nervous system connection between the brain and the body

Associated Conditions

Rehabilitation.

The spinal cord is part of the central nervous system. This long structure runs down the center of your back, and it mediates messages between the brain and the peripheral nerves. The spinal cord is primarily composed of nerves, which are organized in systematic pathways, also described as tracts. 

The spine (backbone) encloses and protects the spinal cord. Damage to the spinal cord can occur as a result of problems such as traumatic injuries, infections, and disease. Treatment for conditions that affect the spinal cord often includes rehabilitation and may also involve medication and/or surgery. 

The spinal cord is adjacent to and below the medulla, which is the lowest part of the brain. The top region of the spinal cord extends down from the medulla all the way to the lower back.

Throughout the spinal cord, there is a consistent arrangement of nerves. The sensory nerve pathways are located toward the posterior (back) of the spinal cord, while the motor nerve pathways run along the lateral (sides) and anterior (front) regions of the spinal cord.

Cerebrospinal fluid (CSF) , with nutrients and immune cells, flows around the spinal cord. Meninges, which are layers of protective connective tissue, surround the spinal cord and CSF.

Meninges are composed of three thin layers—the innermost pia mater, the middle arachnoid mater, and the outermost dura mater. All of these structures—the spinal cord, CSF, and meninges—are enclosed in the backbone, which is also referred to as the spine and the vertebral column. 

Structure and Location

From top to bottom, the regions of the spinal cord include the cervical, thoracic, lumbar, and sacral levels. Each of these levels corresponds to spinal nerves that emerge from the spinal cord toward structures of the body, such as the arms, legs, and trunk. 

The deep, central area of the spinal cord is referred to as gray matter, and the portion that is located nearer to the outer edge of the spinal cord is referred to as white matter.

A coating called myelin (a type of fat) insulates all nerves. The white matter tends to have more myelination than the grey matter, giving it a whiter appearance when viewed with a microscope. 

The grey matter of the spinal cord is shaped somewhat like an open-winged butterfly lying across the center of the spinal cord. This butterfly-shaped gray matter contains nerve roots. The white matter is composed of several tracts (pathways) that travel up and down the spinal cord. 

Regions of the spinal cord include the following.

Anterior Horn

This region is the frontal portion of the gray matter of the spinal cord, and it is composed of nerves that send motor signals to the spinal nerves. 

Lateral and Anterior Tracts

These white matter pathways carry motor signals down the spinal cord in the corticospinal tract. This tract travels all the way down the spinal cord at the front and sides of the white matter regions.

Motor control of the voluntary muscles (muscles you choose to move) travels through the spinal cord in the corticospinal tract. Motor signals are initiated in the motor strip, a region of the cerebral cortex of the brain.

These motor signals travel down the internal capsule, and then cross over to the other side of the body in the brain stem. From there, these messages are sent to the anterior horn and the lateral and anterior tracts of the spinal cord. The motor message exits the spinal cord through the ventral root (the front portion) of the spinal nerves. 

Dorsal Horn

This area is the posterior region of the grey matter. The spinal nerves deliver sensory messages such as light touch, position sense, and vibration to the dorsal horns. 

Posterior Tracts

Also described as the spinothalamic tract, this is a long, white matter pathway that extends all the way up the spine to the brain. The spinal cord mediates sensation coming from the skin, bones, and internal organs.

Your skin detects these sensations and sends messages from peripheral sensory nerves (embedded in the skin) to the spinal nerves, then to the dorsal horn and up through the spinothalamic tracts, crossing over to the other side of the spinal cord before reaching the brain.

Eventually, these messages reach the brain stem, then the thalamus, and then the sensory strip, which is directly behind the motor strip in the cerebral cortex of the brain.

Lateral Horn

The lateral horns of the spinal cord are located at the two sides of the gray matter. This area is composed of nerves that mediate autonomic functions of the body. The autonomic nervous system regulates involuntary functions (actions you don't purposely control), such as digestion and breathing. 

The primary role of the spinal cord is to relay sensory, motor, and autonomic messages between the brain and the rest of the body.   Myelinated nerves along the pathways of the spinal cord send electrical signals to each other to facilitate these actions. 

The motor messages sent through the corticospinal tract eventually reach the corresponding muscle as the spinal nerve branches into smaller peripheral motor nerves that extend to the target muscle. As a result of this nerve stimulation, you can voluntarily move your arms, legs, neck, back, and abdominal muscles. 

Your spinal cord sends messages from your peripheral sensory nerves to your brain, allowing you to detect sensations that include light touch, vibration, pain, temperature, and the position of your body.

The spinal cord sends messages to regulate the internal organs of the body. This includes control of smooth muscles, such as the muscles that move your lungs, stomach, intestines, bladder, and uterus. 

There are a number of medical problems that can affect the spinal cord. Disease of the spinal cord is often described as myelopathy . These conditions cause impairment of motor, sensory, and/or autonomic function.

Myelopathy also often causes spasticity , which is stiffness of the affected arm and/or leg. The symptoms of any spinal cord problem typically correspond to the section of the spinal cord that is impaired. Sometimes, spinal cord injuries also affect functions that are controlled by areas below the level of spinal cord damage due to disruption of the spinal cord tracts.

Diagnosis of a spinal cord condition can include tests such as a physical examination, spinal imaging, nerve conduction studies (NCV), and/or electromyography (EMG).

Conditions that affect the spine include:

Multiple sclerosis (MS)

This is a demyelinating condition that may affect the brain and/or spine. Multiple sclerosis lesions in the spine can cause weakness, sensory loss, tingling, and pain, and they may affect bowel and bladder function.  

Spinal Cord Compression

When the spinal cord is placed under physical pressure , weakness, sensory loss, and autonomic deficits can occur. Severe degenerative disease of the bone or cartilage of the spine can cause these structures to fall out of place—potentially resulting in physical impingement on the spinal cord. Metastatic (spreading throughout the body) cancer can cause spinal cord compression as well.

An injury can cause the spine to move out of place and can even cause a spine fracture (break), which can injure the spinal cord . Injuries may also cause spinal cord compression due to bleeding, and an injury can directly damage the spinal cord.

Amyotrophic Lateral Sclerosis (ALS)

Sometimes called Lou Gehrig's disease , this is a rare condition characterized by the gradual degeneration of the motor neurons located in the spinal cord. ALS causes progressive weakness and, eventually, a complete loss of muscle control. As a result, most individuals affected by ALS need a high level of supportive care.

Currently, there is no cure for ALS. However, medications such as Radicava (edaravone), Rilutek (riluzole), Relyvrio (sodium phenylbutyrate/ taurursodiol), and Qalsody (tofersen) can help relieve the symptoms and improve the quality of life for people with this condition.

An infection or inflammation of the meninges, often described as spinal meningitis, can cause symptoms such as a headache, stiff neck, fever, nausea, and vomiting. Episodes of bacterial meningitis require antibiotics. Other types of meningitis may require anti-inflammatory therapy or other treatments that target the cause. 

Vitamin B12 Deficiency

A deficit in this vitamin can cause many medical issues , including anemia, nerve damage, and subacute combined degeneration of the spinal cord, which is a very rare demyelinating condition that can cause weakness, sensory loss, and stiffness.  

Spinal cord cancer is not common, but tumors can develop in any region of the spinal cord. Late-stage cancer often metastasizes to the spine and/or spinal cord, causing spinal cord compression.  Meningeal carcinomatosis is the spread of cancer cells throughout the meninges and CSF.

Spinal Cord Infarct

If the blood supply to the spinal cord is interrupted, an area of the spine might not receive an adequate supply of blood. This can lead to severe damage, with resulting loss of spinal cord function. 

Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy is a hereditary condition that can cause substantial muscle weakness. SMA is characterized by degeneration of the motor neurons in the spinal cord. There are a few treatments used for SMA, including Spinraza (nusinersen) and Zolgensma (onasemnogene abeparvovec).

This contagious viral infection is usually preventable with a vaccine. In some instances, the infection involves one or more regions of the spinal cord, causing muscle paralysis of the areas that are controlled by the affected spinal cord regions.

Spinal cord diseases and injuries typically require medical and/or surgical interventions. Treatment may include steroids to reduce inflammation or antibiotics to target bacterial infections. Certain neurological conditions, such as MS, ALS, and SMA, also can improve with prescription medications indicated for the specific conditions. 

If you have spinal cord compression, you may need surgery to reduce pressure on your spinal cord due to cancer or a bone impingement. Cancer may require treatment with chemotherapy and radiation therapy. 

Therapies also usually include physical therapy and rehabilitation exercises. Some people may need to use a cane, walker, or wheelchair while recovering from a condition involving the spinal cord. 

Cho TA. Spinal cord functional anatomy . Continuum (Minneap Minn) . 2015;21(1 Spinal Cord Disorders):13-35. doi:10.1212/01.CON.0000461082.25876.4a

National Institute of Neurological Disorders and Stroke. Transverse myelitis fact sheet .

Ziu E, Mesfin FB. Cancer, spinal metastasis . StatPearls.

ALS Association. FDA-Approved Drugs for Treating ALS .

National Institute of Neurological Disorders and Stroke. Meningitis and encephalitis .

MedlinePlus. Vitamin B12 .

National Institute for Neurological Disorders and Stroke. Spinal muscular atrophy fact sheet .

Menant JC, Gandevia SC. Poliomyelitis . Handb Clin Neurol . 2018;159:337-344. doi:10.1016/B978-0-444-63916-5.00021-5

By Heidi Moawad, MD Dr. Moawad is a neurologist and expert in brain health. She regularly writes and edits health content for medical books and publications.

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Neuroanatomy, motor neuron.

Lindsay C. Zayia ; Prasanna Tadi .

Affiliations

Last Update: July 24, 2023 .

• Introduction

While the term “motor neuron” evokes the idea that there is only one type of neuron that conducts movement, this is far from the truth. In fact, within the classification of a “motor neuron,” there lies both upper and lower motor neurons, which are entirely different in terms of their origins, synapse points, pathways, neurotransmitters, and lesion characteristics. Overall, motor neurons (or motoneurons) comprise various tightly controlled, complex circuits throughout the body that allows for both voluntary and involuntary movements through the innervation of effector muscles and glands. The upper and lower motor neurons form a two-neuron circuit. The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body. Understanding the difference between upper and lower motor neurons, as well as the pathway that they take, is crucial to being able to not only diagnose these neuronal injuries but also localize the lesions efficiently.

• Structure and Function

The upper and lower motor neurons together comprise a two-neuron pathway that is responsible for movement. Upper and lower motor neurons utilize different neurotransmitters to relay their signals. Upper motor neurons use glutamate, while lower motor neurons use acetylcholine. [1]

To perform a movement, a signal must begin in the primary motor cortex of the brain, which is in the precentral gyrus. In the primary motor cortex are the cell bodies of the upper motor neurons, referred to as Betz cells. [2] Specifically, these cells are located in layer 5 of the motor cortex and have long apical dendrites that branch up into layer 1. [3] The upper motor neuron is responsible for integrating all of the excitatory and inhibitory signals from the cortex and translating it into a signal that will initiate or inhibit voluntary movement. Thalamocortical neurons and callosal projection neurons regulate upper motor neurons. While the mechanism of regulation by these entities is not completely understood, it is thought that the majority of the excitatory input to these neurons comes from neurons located in layers 2, 3, and 5 of the motor cortex. The axons of the upper motor neuron travel down through the posterior limb of the internal capsule. From there, they continue through the cerebral peduncles in the midbrain, longitudinal pontine fibers, and eventually the medullary pyramids. It is at this location that the majority (approximately 90%) of the fibers will decussate and continue down the spinal cord on the contralateral side of the body as the lateral corticospinal tract. The lateral corticospinal tract is the largest descending pathway and is located in the lateral funiculus. This tract will synapse directly onto the lower motor neuron in the anterior horn of the spinal cord. The pyramidal tract fibers that did not decussate at the medulla comprise the anterior corticospinal tract, which is much smaller than the lateral corticospinal tract. This tract is located near the anterior median fissure and is responsible for axial and proximal limb movement and control, which helps with posture. Although it does not decussate at in the medulla, this tract does decussate at the spinal level being innervated. [4] [5] [6]

The lower motor neuron is responsible for transmitting the signal from the upper motor neuron to the effector muscle to perform a movement. There are three broad types of lower motor neurons: somatic motor neurons, special visceral efferent (branchial) motor neurons, and general visceral motor neurons. [1]

Somatic motor neurons are in the brainstem and further divide into three categories: alpha, beta, and gamma. Alpha motor neurons innervate extrafusal muscle fibers and are the primary means of skeletal muscle contraction. The large alpha motor neuron cell body can be either in the brainstem or spinal cord. In the spinal cord, the cell bodies are found in the anterior horn and thus are called anterior horn cells. From the anterior horn cell, a single axon goes on to innervate many muscle fibers within a single muscle. The properties of these muscle fibers are nearly identical, allowing for controlled, synchronous movement of the motor unit upon depolarization of the lower motor neuron. Beta motor neurons are poorly characterized, but it has been established that they innervate both extrafusal and intrafusal fibers. Gamma motor neurons innervate muscle spindles and dictate their sensitivity. These neurons primarily respond to stretch of the muscle spindle. Despite being named a “motor neuron,” these neurons do not directly cause any motor function. It is thought that they get activated along with alpha motor neurons and fine-tune the muscle contraction (alpha-gamma coactivation). A disruption in either alpha or gamma motor neurons will result in a disruption of muscle tone. [7] [1]

Lower motor neurons also play a role in the somatic reflex arc. When muscle spindles detect a sudden stretch, a signal travels down the afferent nerve fibers. These nerve fibers synapse either directly onto the alpha motor neuron (monosynaptic reflex arc), or onto interneurons, which then synapse onto the alpha motor neuron (polysynaptic reflex arc). The lower motor neuron innervates the effector muscle, allowing for a quick muscle response. A reflex arc allows for interpretation of and reaction on the stimulus immediately through the spinal cord, bypassing the brain, resulting in a faster effector response. [1] [8]

Branchial motor neurons innervate the muscles of the head and neck that derive from the branchial arches. They are in the brainstem. The branchial motor neurons and sensory neurons together form the nuclei of cranial nerves V, VII, IX, X, and XI. [1]

Visceral motor neurons contribute to both the sympathetic and parasympathetic functions of the autonomic nervous system. In the sympathetic nervous system, central motor neurons are present in the spinal cord from T1-L2. They appear in the intermediolateral (IML) nucleus. Motor neurons from this nucleus have three different pathways. The first two pathways are to the prevertebral and paravertebral ganglia, from which peripheral neurons go on to innervate the heart, colon, intestines, kidneys, and lungs. The third possible pathway in this system is to the catecholamine-producing chromaffin cells of the adrenal medulla. By targeting these three pathways, the visceral motor neurons in the sympathetic division contribute to the “fight-or-flight” response. On the other hand, in the parasympathetic system, the visceral motor neurons help give rise to cranial nerves III, VII, IX, and X. Besides in the brainstem, these visceral motor neurons contribute to the parasympathetic system in the spinal cord at levels S2-S4. Similarly to the sympathetic division, these motor neurons directly innervate ganglia in the heart, pancreas, lungs, and kidneys. Thus, in both divisions of the autonomic system, these lower motor neurons take on a different role than somatic motor neurons in that they do not directly innervate an effector muscle, and instead innervate ganglia. [1]

• Blood Supply and Lymphatics

The primary motor cortex is supplied primarily by the middle cerebral artery (MCA), along with the anterior cerebral artery (ACA). The MCA supplies the area of the primary motor cortex that is responsible for the upper limbs and face, while the ACA supplies blood to the area that controls the lower limbs. As previously discussed, the upper motor neurons continue down as the pyramidal tract, which receives vascular supply from the lenticulostriate arteries. Once this tract reaches the brainstem, the paramedian branches of the basilar artery become the primary source of blood. At the caudal medulla level, the anterior spinal artery supplies most of the blood. This artery continues to provide blood to the lateral and anterior corticospinal tracts and anterior horn cells in the spinal cord. [9]

• Surgical Considerations

Because there are numerous causes of upper and lower motor neuron dysfunction and injury, surgical consideration requires individualization for each patient. The overall goal of any form of treatment, surgical or not, should be focused on reducing pain and preserving or enhancing day-to-day functionality. [10]

Surgical intervention may also help prevent upper extremity deformity due to contractures or spasticity that may present with an upper motor neuron injury. Examples of surgical procedures that are options for a patient with severe upper motor neuron disease include tendon lengthening, muscle origin release, myotomy, tenotomy, neurectomy, arthrodesis, and joint osteocapsular release. Surgical interventions are chosen based on an individual patient’s level of functioning. [10]

• Clinical Significance

Upper and lower motor neuron lesions cause very different clinical findings. An upper motor neuron lesion is a lesion anywhere from the cortex to the corticospinal tract. This lesion causes hyperreflexia, spasticity, and a positive Babinski reflex, presenting as an upward response of the big toe when the plantar surface of the foot is stroked, with other toes fanning out. On the other hand, lower motor neuron lesions are lesions anywhere from the anterior horn of the spinal cord, peripheral nerve, neuromuscular junction, or muscle. This type of lesion causes hyporeflexia, flaccid paralysis, and atrophy.

Knowledge of the anatomy of the motor neurons is critical to the ability to localize the lesion when faced with a patient who presents with a weakness that is likely due to a motor neuron injury. Focusing mainly on the lateral corticospinal tract, it is essential to keep in mind that this neuronal pathway decussates at the level of the pyramids in the medulla. This crossing means that an upper motor neuron lesion above the medulla will cause symptoms on the contralateral side of the body. However, a lesion to the lateral corticospinal tract after it decussates will present on the ipsilateral side of the body.

Upper motor neuron syndrome occurs when there is injury anywhere to the descending tract before the anterior horn of the spinal cord (cortex, internal capsule, pyramidal tract, lateral corticospinal tract). Examples of pathology that cause upper motor neuron symptoms are strokes, traumatic brain injury, spinal cord injury, amyotrophic lateral sclerosis (ALS), primary lateral sclerosis (PLS), multiple sclerosis (MS), or anoxic brain injury. There are both positive and negative features of the upper motor neuron syndrome. Positive features include hyperreflexia (abnormally brisk reflexes), spasticity (a brisk stretch of muscles causes a sudden increase in tone followed by decreased muscle resistance), and a positive Babinski reflex. Negative features include impaired motor control, easy fatiguability, weakness, and loss of dexterity. [11] [5] [10]

Lower motor neuron syndrome occurs when there is an injury to the anterior horn cells or the peripheral nerve. Diseases of the neuromuscular junction or muscle itself may mimic a lower motor neuron lesion and are important to consider in the differential diagnosis. Similarly to an upper motor neuron lesion, the patient with a lower motor neuron lesion will present with weakness; however, distinct lower motor neuron lesion findings will include hyporeflexia, flaccid paralysis, fasciculations, and atrophy.  

There are many forms of motor neuron disease, the most common of which is amyotrophic lateral sclerosis (ALS). This disease is unique in that it presents with both upper and motor neuron signs. The patient will typically present with weakness, along with spastic paralysis and hyperreflexia in the lower limbs and flaccid paralysis and hyporeflexia in the upper limbs. The patient may also present with fasciculations in both the tongue and extremities. Of note, there is no sensory loss. ALS is a progressive neurogenerative disease, and eventually, the patient will have serious dysarthria, dysphagia, extreme weakness, and dyspnea. The estimated median survival is 2 to 4 years, with the most common cause of death being respiratory failure. [12]

One group of genetic disorders that causes lower motor neuron disease is spinal muscular atrophy (SMA). There are many different forms of SMA, but all of them are characterized by degeneration of the motor nuclei in the brainstem, in addition to the anterior horn cells found in the spinal cord. One specific type of SMA is spinobulbar muscular atrophy (Kennedy disease). This x-linked disease usually presents in adulthood (age 30 to 50). First presenting signs typically include tremor, lower extremity weakness, and orolingual fasciculations. This pathology is a progressive disease that is later characterized by the above symptoms in addition to atrophy of limb, bulbar, and facial muscles. [13]

Poliomyelitis is another lower motor neuron disease. This disease results from poliovirus and results in the destruction of the anterior horn cells. Subsequently, the affected patient will experience weakness and lower motor neuron symptoms, including flaccid paralysis in the lower limbs. Usually, this presents asymmetrically. The patient may also provide a history of muscle aches or muscle spasm that occurred in the recent past. Unfortunately, this weakness and paralysis may extend up to involve the respiratory muscles. Many patients will recover some strength, but may later decompensate into “postpolio syndrome,” which is characterized by the onset of additional weakness, pain, and/or atrophy. Among other viral causes of anterior horn cell destruction are coxsackievirus, West Nile virus, and echovirus. [13]

While most cranial nerves are innervated by upper motor neurons bilaterally, cranial nerves VII and XII are the exceptions, as they receive only unilateral input from the contralateral side of the brain. Specifically, damage to the corticobulbar tract and/or facial nerve causes a unique presentation depending on whether the damage occurred in the upper vs. lower motor neuron. The forehead region is dually innervated by corticobulbar tracts from each side of the brain, while the rest of the face below the forehead is innervated primarily by the lower motor neuron of CN VII. An upper motor neuron lesion of the facial nerve can occur anywhere in the corticobulbar tract rostral to the facial motor nucleus on the pons. If an upper motor neuron lesion occurs, the forehead will be spared due to its dual innervation. However, a lower motor neuron lesion of CN VII results in flaccid paralysis of the entire ipsilateral side of the face. [14]

Overall, clinicians should consider motor neuron disease whenever a patient presents with weakness and any of the previously described motor neuron lesion signs without significant sensory loss. Referral to a neurologist for subsequent testing is warranted in these cases.

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Figure showing Labeled Motor Neuron. Contributed by Katherine Humphries

Disclosure: Lindsay Zayia declares no relevant financial relationships with ineligible companies.

Disclosure: Prasanna Tadi declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Zayia LC, Tadi P. Neuroanatomy, Motor Neuron. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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14.3 The Brain and Spinal Cord

Learning objectives.

By the end of this section, you will be able to:

• Name the major regions of the adult brain
• Describe the connections between the cerebrum and brain stem through the diencephalon, and from those regions into the spinal cord
• Recognize the complex connections within the subcortical structures of the basal nuclei
• Explain the arrangement of gray and white matter in the spinal cord

The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.

The Cerebrum

The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum ( Figure 14.3.1 ). The wrinkled portion is the cerebral cortex , and the rest of the structure is beneath that outer covering. There is a large separation between the two sides of the cerebrum called the longitudinal fissure . It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere . Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.

Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The complexity of the cerebrum is different across vertebrate species. The cerebrum of the most primitive vertebrates is not much more than the connection for the sense of smell. In mammals, the cerebrum comprises the outer gray matter that is the cortex (from the Latin word meaning “bark of a tree”) and several deep nuclei that belong to three important functional groups. The basal nuclei are responsible for cognitive processing, the most important function being that associated with planning movements. The basal forebrain contains nuclei that are important in learning and memory. The limbic cortex is the region of the cerebral cortex that is part of the limbic system , a collection of structures involved in emotion, memory, and behavior.

Cerebral Cortex

The cerebrum is covered by a continuous layer of gray matter that wraps around either side of the forebrain—the cerebral cortex. This thin, extensive region of wrinkled gray matter is responsible for the higher functions of the nervous system. A gyrus (plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex.

The head is limited by the size of the birth canal, and the brain must fit inside the cranial cavity of the skull. Extensive folding in the cerebral cortex enables more gray matter to fit into this limited space. If the gray matter of the cortex were peeled off of the cerebrum and laid out flat, its surface area would be roughly equal to one square meter.

The folding of the cortex maximizes the amount of gray matter in the cranial cavity. During embryonic development, as the telencephalon expands within the skull, the brain goes through a regular course of growth that results in everyone’s brain having a similar pattern of folds. The surface of the brain can be mapped on the basis of the locations of large gyri and sulci. Using these landmarks, the cortex can be separated into four major regions, or lobes ( Figure 14.3.2 ). The lateral sulcus that separates the temporal lobe from the other regions is one such landmark. Superior to the lateral sulcus are the parietal lobe and frontal lobe , which are separated from each other by the central sulcus . The posterior region of the cortex is the occipital lobe , which has no obvious anatomical border between it and the parietal or temporal lobes on the lateral surface of the brain. From the medial surface, an obvious landmark separating the parietal and occipital lobes is called the parieto-occipital sulcus . The fact that there is no obvious anatomical border between these lobes is consistent with the functions of these regions being interrelated.

Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy—the cytoarchitecture—of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann’s areas , which is still used today to describe the anatomical distinctions within the cortex ( Figure 14.3.3 ). The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.

The temporal lobe is associated with primary auditory sensation, known as Brodmann’s areas 41 and 42 in the superior temporal lobe. Because regions of the temporal lobe are part of the limbic system, memory is an important function associated with that lobe. Memory is essentially a sensory function; memories are recalled sensations such as the smell of Mom’s baking or the sound of a barking dog. Even memories of movement are really the memory of sensory feedback from those movements, such as stretching muscles or the movement of the skin around a joint. Structures in the temporal lobe are responsible for establishing long-term memory, but the ultimate location of those memories is usually in the region in which the sensory perception was processed.

The main sensation associated with the parietal lobe is somatosensation , meaning the general sensations associated with the body. Posterior to the central sulcus is the postcentral gyrus , the primary somatosensory cortex, which is identified as Brodmann’s areas 1, 2, and 3. All of the tactile senses are processed in this area, including touch, pressure, tickle, pain, itch, and vibration, as well as more general senses of the body such as proprioception and kinesthesia , which are the senses of body position and movement, respectively.

Anterior to the central sulcus is the frontal lobe, which is primarily associated with motor functions. The precentral gyrus is the primary motor cortex. Cells from this region of the cerebral cortex are the upper motor neurons that instruct cells in the spinal cord and brain stem (lower motor neurons) to move skeletal muscles. Anterior to this region are a few areas that are associated with planned movements. The premotor area is responsible for storing learned movement algorithms which are instructions for complex movements. Different algorithms activate the upper motor neurons in the correct sequence when a complex motor activity is performed. The frontal eye fields are important in eliciting scanning eye movements and in attending to visual stimuli. Broca’s area is responsible for the production of language, or controlling movements responsible for speech; in the vast majority of people, it is located only on the left side. Anterior to these regions is the prefrontal lobe , which serves cognitive functions that can be the basis of personality, short-term memory, and consciousness. The prefrontal lobotomy is an outdated mode of treatment for personality disorders (psychiatric conditions) that profoundly affected the personality of the patient.

Area 17, as Brodmann described it, is also known as the primary visual cortex. Adjacent to that are areas 18 and 19, which constitute subsequent regions of visual processing. Area 22 is the primary auditory cortex, and it is followed by area 23, which further processes auditory information. Area 4 is the primary motor cortex in the precentral gyrus, whereas area 6 is the premotor cortex. These areas suggest some specialization within the cortex for functional processing, both in sensory and motor regions. The fact that Brodmann’s areas correlate so closely to functional localization in the cerebral cortex demonstrates the strong link between structure and function in these regions.

Areas 1, 2, 3, 4, 17, and 22 are each described as primary cortical areas. The adjoining regions are each referred to as association areas. Primary areas are where sensory information is initially received from the thalamus for conscious perception, or—in the case of the primary motor cortex—where descending commands are sent down to the brain stem or spinal cord to execute movements ( Figure 14.3.4 ).

Functions of the Cerebral Cortex

The cerebrum is the seat of many of the higher mental functions, such as memory and learning, language, and conscious perception, which are the subjects of subtests of the mental status exam. The cerebral cortex is the thin layer of gray matter on the outside of the cerebrum. It is approximately a millimeter thick in most regions and highly folded to fit within the limited space of the cranial vault. These higher functions are distributed across various regions of the cortex, and specific locations can be said to be responsible for particular functions. There is a limited set of regions, for example, that are involved in language function, and they can be subdivided on the basis of the particular part of language function that each governs.

A number of other regions, which extend beyond these primary or association areas of the cortex, are referred to as integrative areas. These areas are found in the spaces between the domains for particular sensory or motor functions, and they integrate multisensory information, or process sensory or motor information in more complex ways. Consider, for example, the posterior parietal cortex that lies between the somatosensory cortex and visual cortex regions. This has been ascribed to the coordination of visual and motor functions, such as reaching to pick up a glass. The somatosensory function that would be part of this is the proprioceptive feedback from moving the arm and hand. The weight of the glass, based on what it contains, will influence how those movements are executed.

Cognitive Abilities

Assessment of cerebral functions is directed at cognitive abilities. The abilities assessed through the mental status exam can be separated into four groups: orientation and memory, language and speech, sensorium, and judgment and abstract reasoning.

Orientation and Memory

Orientation is the patient’s awareness of his or her immediate circumstances. It is awareness of time, not in terms of the clock, but of the date and what is occurring around the patient. It is awareness of place, such that a patient should know where he or she is and why. It is also awareness of who the patient is—recognizing personal identity and being able to relate that to the examiner. The initial tests of orientation are based on the questions, “Do you know what the date is?” or “Do you know where you are?” or “What is your name?” Further understanding of a patient’s awareness of orientation can come from questions that address remote memory, such as “Who is the President of the United States?”, or asking what happened on a specific date.

There are also specific tasks to address memory. One is the three-word recall test. The patient is given three words to recall, such as book, clock, and shovel. After a short interval, during which other parts of the interview continue, the patient is asked to recall the three words. Other tasks that assess memory—aside from those related to orientation—have the patient recite the months of the year in reverse order to avoid the overlearned sequence and focus on the memory of the months in an order, or to spell common words backwards, or to recite a list of numbers back.

Memory is largely a function of the temporal lobe, along with structures beneath the cerebral cortex such as the hippocampus and the amygdala. The storage of memory requires these structures of the medial temporal lobe. A famous case of a man who had both medial temporal lobes removed to treat intractable epilepsy provided insight into the relationship between the structures of the brain and the function of memory.

Henry Molaison, who was referred to as patient HM when he was alive, had epilepsy localized to both of his medial temporal lobes. In 1953, a bilateral lobectomy was performed that alleviated the epilepsy but resulted in the inability for HM to form new memories—a condition called anterograde amnesia . HM was able to recall most events from before his surgery, although there was a partial loss of earlier memories, which is referred to as retrograde amnesia . HM became the subject of extensive studies into how memory works. What he was unable to do was form new memories of what happened to him, what are now called episodic memory . Episodic memory is autobiographical in nature, such as remembering riding a bicycle as a child around the neighborhood, as opposed to the procedural memory of how to ride a bike. HM also retained his short-term memory , such as what is tested by the three-word task described above. After a brief period, those memories would dissipate or decay and not be stored in the long-term because the medial temporal lobe structures were removed.

The difference in short-term, procedural, and episodic memory, as evidenced by patient HM, suggests that there are different parts of the brain responsible for those functions. The long-term storage of episodic memory requires the hippocampus and related medial temporal structures, and the location of those memories is in the multimodal integration areas of the cerebral cortex. However, short-term memory—also called working or active memory—is localized to the prefrontal lobe. Because patient HM had only lost his medial temporal lobe—and lost very little of his previous memories, and did not lose the ability to form new short-term memories—it was concluded that the function of the hippocampus, and adjacent structures in the medial temporal lobe, is to move (or consolidate) short-term memories (in the pre-frontal lobe) to long-term memory (in the temporal lobe).

The prefrontal cortex can also be tested for the ability to organize information. In one subtest of the mental status exam called set generation, the patient is asked to generate a list of words that all start with the same letter, but not to include proper nouns or names. The expectation is that a person can generate such a list of at least 10 words within 1 minute. Many people can likely do this much more quickly, but the standard separates the accepted normal from those with compromised prefrontal cortices.

External Website

Read this article to learn about a young man who texts his fiancée in a panic as he finds that he is having trouble remembering things. At the hospital, a neurologist administers the mental status exam, which is mostly normal except for the three-word recall test. The young man could not recall them even 30 seconds after hearing them and repeating them back to the doctor. An undiscovered mass in the mediastinum region was found to be Hodgkin’s lymphoma, a type of cancer that affects the immune system and likely caused antibodies to attack the nervous system. The patient eventually regained his ability to remember, though the events in the hospital were always elusive. Considering that the effects on memory were temporary, but resulted in the loss of the specific events of the hospital stay, what regions of the brain were likely to have been affected by the antibodies and what type of memory does that represent?

Language and Speech

Language is, arguably, a very human aspect of neurological function. There are certainly strides being made in understanding communication in other species, but much of what makes the human experience seemingly unique is its basis in language. Any understanding of our species is necessarily reflective, as suggested by the question “What am I?” And the fundamental answer to this question is suggested by the famous quote by René Descartes: “Cogito Ergo Sum” (translated from Latin as “I think, therefore I am”). Formulating an understanding of yourself is largely describing who you are to yourself. It is a confusing topic to delve into, but language is certainly at the core of what it means to be self-aware.

The neurological exam has two specific subtests that address language. One measures the ability of the patient to understand language by asking them to follow a set of instructions to perform an action, such as “touch your right finger to your left elbow and then to your right knee.” Another subtest assesses the fluency and coherency of language by having the patient generate descriptions of objects or scenes depicted in drawings, and by reciting sentences or explaining a written passage. Language, however, is important in so many ways in the neurological exam. The patient needs to know what to do, whether it is as simple as explaining how the knee-jerk reflex is going to be performed, or asking a question such as “What is your name?” Often, language deficits can be determined without specific subtests; if a person cannot reply to a question properly, there may be a problem with the reception of language.

An important example of multimodal integrative areas is associated with language function ( Figure 14.3.5 ). Adjacent to the auditory association cortex, at the end of the lateral sulcus just anterior to the visual cortex, is Wernicke’s area . In the lateral aspect of the frontal lobe, just anterior to the region of the motor cortex associated with the head and neck, is Broca’s area. Both regions were originally described on the basis of losses of speech and language, which is called aphasia . The aphasia associated with Broca’s area is known as an expressive aphasia , which means that speech production is compromised. This type of aphasia is often described as non-fluency because the ability to say some words leads to broken or halting speech. Grammar can also appear to be lost. The aphasia associated with Wernicke’s area is known as a receptive aphasia , which is not a loss of speech production, but a loss of understanding of content. Patients, after recovering from acute forms of this aphasia, report not being able to understand what is said to them or what they are saying themselves, but they often cannot keep from talking.

The two regions are connected by white matter tracts that run between the posterior temporal lobe and the lateral aspect of the frontal lobe. Conduction aphasia associated with damage to this connection refers to the problem of connecting the understanding of language to the production of speech. This is a very rare condition, but is likely to present as an inability to faithfully repeat spoken language.

Those parts of the brain involved in the reception and interpretation of sensory stimuli are referred to collectively as the sensorium. The cerebral cortex has several regions that are necessary for sensory perception. From the primary cortical areas of the somatosensory, visual, auditory, and gustatory senses to the association areas that process information in these modalities, the cerebral cortex is the seat of conscious sensory perception. In contrast, sensory information can also be processed by deeper brain regions, which we may vaguely describe as subconscious—for instance, we are not constantly aware of the proprioceptive information that the cerebellum uses to maintain balance. Several of the subtests can reveal activity associated with these sensory modalities, such as being able to hear a question or see a picture. Two subtests assess specific functions of these cortical areas.

The first is praxis , a practical exercise in which the patient performs a task completely on the basis of verbal description without any demonstration from the examiner. For example, the patient can be told to take their left hand and place it palm down on their left thigh, then flip it over so the palm is facing up, and then repeat this four times. The examiner describes the activity without any movements on their part to suggest how the movements are to be performed. The patient needs to understand the instructions, transform them into movements, and use sensory feedback, both visual and proprioceptive, to perform the movements correctly.

The second subtest for sensory perception is gnosis , which involves two tasks. The first task, known as stereognosis , involves the naming of objects strictly on the basis of the somatosensory information that comes from manipulating them. The patient keeps their eyes closed and is given a common object, such as a coin, that they have to identify. The patient should be able to indicate the particular type of coin, such as a dime versus a penny, or a nickel versus a quarter, on the basis of the sensory cues involved. For example, the size, thickness, or weight of the coin may be an indication, or to differentiate the pairs of coins suggested here, the smooth or corrugated edge of the coin will correspond to the particular denomination. The second task, graphesthesia , is to recognize numbers or letters written on the palm of the hand with a dull pointer, such as a pen cap.

Praxis and gnosis are related to the conscious perception and cortical processing of sensory information. Being able to transform verbal commands into a sequence of motor responses, or to manipulate and recognize a common object and associate it with a name for that object. Both subtests have language components because language function is integral to these functions. The relationship between the words that describe actions, or the nouns that represent objects, and the cerebral location of these concepts is suggested to be localized to particular cortical areas. Certain aphasias can be characterized by a deficit of verbs or nouns, known as V impairment or N impairment, or may be classified as V–N dissociation. Patients have difficulty using one type of word over the other. To describe what is happening in a photograph as part of the expressive language subtest, a patient will use active- or image-based language. The lack of one or the other of these components of language can relate to the ability to use verbs or nouns. Damage to the region at which the frontal and temporal lobes meet, including the region known as the insula, is associated with V impairment; damage to the middle and inferior temporal lobe is associated with N impairment.

Judgment and Abstract Reasoning

Planning and producing responses requires an ability to make sense of the world around us. Making judgments and reasoning in the abstract are necessary to produce movements as part of larger responses. For example, when your alarm goes off, do you hit the snooze button or jump out of bed? Is 10 extra minutes in bed worth the extra rush to get ready for your day? Will hitting the snooze button multiple times lead to feeling more rested or result in a panic as you run late? How you mentally process these questions can affect your whole day.

The prefrontal cortex is responsible for the functions responsible for planning and making decisions. In the mental status exam, the subtest that assesses judgment and reasoning is directed at three aspects of frontal lobe function. First, the examiner asks questions about problem solving, such as “If you see a house on fire, what would you do?” The patient is also asked to interpret common proverbs, such as “Don’t look a gift horse in the mouth.” Additionally, pairs of words are compared for similarities, such as apple and orange, or lamp and cabinet.

The prefrontal cortex is composed of the regions of the frontal lobe that are not directly related to specific motor functions. The most posterior region of the frontal lobe, the precentral gyrus, is the primary motor cortex. Anterior to that are the premotor cortex, Broca’s area, and the frontal eye fields, which are all related to planning certain types of movements. Anterior to what could be described as motor association areas are the regions of the prefrontal cortex. They are the regions in which judgment, abstract reasoning, and working memory are localized. The antecedents to planning certain movements are judging whether those movements should be made, as in the example of deciding whether to hit the snooze button.

To an extent, the prefrontal cortex may be related to personality. The neurological exam does not necessarily assess personality, but it can be within the realm of neurology or psychiatry. A clinical situation that suggests this link between the prefrontal cortex and personality comes from the story of Phineas Gage, the railroad worker from the mid-1800s who had a metal spike impale his prefrontal cortex. There are suggestions that the steel rod led to changes in his personality. A man who was a quiet, dependable railroad worker became a raucous, irritable drunkard. Later anecdotal evidence from his life suggests that he was able to support himself, although he had to relocate and take on a different career as a stagecoach driver.

A psychiatric practice to deal with various disorders was the prefrontal lobotomy. This procedure was common in the 1940s and early 1950s, until antipsychotic drugs became available. The connections between the prefrontal cortex and other regions of the brain were severed. The disorders associated with this procedure included some aspects of what are now referred to as personality disorders, but also included mood disorders and psychoses. Depictions of lobotomies in popular media suggest a link between cutting the white matter of the prefrontal cortex and changes in a patient’s mood and personality, though this correlation is not well understood.

Everyday Connections –  Left Brain, Right Brain

Popular media often refer to right-brained and left-brained people, as if the brain were two independent halves that work differently for different people. This is a popular misinterpretation of an important neurological phenomenon. As an extreme measure to deal with a debilitating condition, the corpus callosum may be sectioned to overcome intractable epilepsy. When the connections between the two cerebral hemispheres are cut, interesting effects can be observed.

If a person with an intact corpus callosum is asked to put their hands in their pockets and describe what is there on the basis of what their hands feel, they might say that they have keys in their right pocket and loose change in the left. They may even be able to count the coins in their pocket and say if they can afford to buy a candy bar from the vending machine. If a person with a sectioned corpus callosum is given the same instructions, they will do something quite peculiar. They will only put their right hand in their pocket and say they have keys there. They will not even move their left hand, much less report that there is loose change in the left pocket.

The reason for this is that the language functions of the cerebral cortex are localized to the left hemisphere in 95 percent of the population. Additionally, the left hemisphere is connected to the right side of the body through the corticospinal tract and the ascending tracts of the spinal cord. Motor commands from the precentral gyrus control the opposite side of the body, whereas sensory information processed by the postcentral gyrus is received from the opposite side of the body. For a verbal command to initiate movement of the right arm and hand, the left side of the brain needs to be connected by the corpus callosum. Language is processed in the left side of the brain and directly influences the left brain and right arm motor functions, but is sent to influence the right brain and left arm motor functions through the corpus callosum. Likewise, the left-handed sensory perception of what is in the left pocket travels across the corpus callosum from the right brain, so no verbal report on those contents would be possible if the hand happened to be in the pocket.

Watch the video titled “The Man With Two Brains” to see the neuroscientist Michael Gazzaniga introduce a patient he has worked with for years who has had his corpus callosum cut, separating his two cerebral hemispheres. A few tests are run to demonstrate how this manifests in tests of cerebral function. Unlike normal people, this patient can perform two independent tasks at the same time because the lines of communication between the right and left sides of his brain have been removed. Whereas a person with an intact corpus callosum cannot overcome the dominance of one hemisphere over the other, this patient can. If the left cerebral hemisphere is dominant in the majority of people, why would right-handedness be most common?

The Mental Status Exam

The cerebrum, particularly the cerebral cortex, is the location of important cognitive functions that are the focus of the mental status exam. The regionalization of the cortex, initially described on the basis of anatomical evidence of cytoarchitecture, reveals the distribution of functionally distinct areas. Cortical regions can be described as primary sensory or motor areas, association areas, or multimodal integration areas. The functions attributed to these regions include attention, memory, language, speech, sensation, judgment, and abstract reasoning.

The mental status exam addresses these cognitive abilities through a series of subtests designed to elicit particular behaviors ascribed to these functions. The loss of neurological function can illustrate the location of damage to the cerebrum. Memory functions are attributed to the temporal lobe, particularly the medial temporal lobe structures known as the hippocampus and amygdala, along with the adjacent cortex. Evidence of the importance of these structures comes from the side effects of a bilateral temporal lobectomy that were studied in detail in patient HM.

Losses of language and speech functions, known as aphasias, are associated with damage to the important integration areas in the left hemisphere known as Broca’s or Wernicke’s areas, as well as the connections in the white matter between them. Different types of aphasia are named for the particular structures that are damaged. Assessment of the functions of the sensorium includes praxis and gnosis. The subtests related to these functions depend on multimodal integration, as well as language-dependent processing.

The prefrontal cortex contains structures important for planning, judgment, reasoning, and working memory. Damage to these areas can result in changes to personality, mood, and behavior. The famous case of Phineas Gage suggests a role for this cortex in personality, as does the outdated practice of prefrontal lobectomy.

Subcortical structures

Beneath the cerebral cortex are sets of nuclei known as subcortical nuclei that augment cortical processes. The nuclei of the basal forebrain serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the basal forebrain. The hippocampus and amygdala are medial-lobe structures that, along with the adjacent cortex, are involved in long-term memory formation and emotional responses. The basal nuclei are a set of nuclei in the cerebrum responsible for comparing cortical processing with the general state of activity in the nervous system to influence the likelihood of movement taking place. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep the urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)

The major structures of the basal nuclei that control movement are the caudate , putamen , and globus pallidus , which are located deep in the cerebrum. The caudate is a long nucleus that follows the basic C-shape of the cerebrum from the frontal lobe, through the parietal and occipital lobes, into the temporal lobe. The putamen is mostly deep in the anterior regions of the frontal and parietal lobes. Together, the caudate and putamen are called the striatum . The globus pallidus is a layered nucleus that lies just medial to the putamen; they are called the lenticular nuclei because they look like curved pieces fitting together like lenses. The globus pallidus has two subdivisions, the external and internal segments, which are lateral and medial, respectively. These nuclei are depicted in a frontal section of the brain in Figure 14.3.6 .

The basal nuclei in the cerebrum are connected with a few more nuclei in the brain stem that together act as a functional group that forms a motor pathway. Two streams of information processing take place in the basal nuclei. All input to the basal nuclei is from the cortex into the striatum ( Figure 14.3.7 ). The direct pathway is the projection of axons from the striatum to the globus pallidus internal segment (GPi) and the substantia nigra pars reticulata (SNr). The GPi/SNr then projects to the thalamus, which projects back to the cortex. The indirect pathway is the projection of axons from the striatum to the globus pallidus external segment (GPe), then to the subthalamic nucleus (STN), and finally to GPi/SNr. The two streams both target the GPi/SNr, but one has a direct projection and the other goes through a few intervening nuclei. The direct pathway causes the disinhibition of the thalamus (inhibition of one cell on a target cell that then inhibits the first cell), whereas the indirect pathway causes, or reinforces, the normal inhibition of the thalamus. The thalamus then can either excite the cortex (as a result of the direct pathway) or fail to excite the cortex (as a result of the indirect pathway).

The switch between the two pathways is the substantia nigra pars compacta , which projects to the striatum and releases the neurotransmitter dopamine. Dopamine receptors are either excitatory (D1-type receptors) or inhibitory (D2-type receptors). The direct pathway is activated by dopamine, and the indirect pathway is inhibited by dopamine. When the substantia nigra pars compacta is firing, it signals to the basal nuclei that the body is in an active state, and movement will be more likely. When the substantia nigra pars compacta is silent, the body is in a passive state, and movement is inhibited. To illustrate this situation, while a student is sitting listening to a lecture, the substantia nigra pars compacta would be silent and the student less likely to get up and walk around. Likewise, while the professor is lecturing, and walking around at the front of the classroom, the professor’s substantia nigra pars compacta would be active, in keeping with his or her activity level.

Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the direct pathway is the shorter pathway through the system that results in increased activity in the cerebral cortex and increased motor activity. The direct pathway is described as resulting in “disinhibition” of the thalamus. What does disinhibition mean? What are the two neurons doing individually to cause this?

Watch this video to learn about the basal nuclei (also known as the basal ganglia), which have two pathways that process information within the cerebrum. As shown in this video, the indirect pathway is the longer pathway through the system that results in decreased activity in the cerebral cortex, and therefore less motor activity. The indirect pathway has an extra couple of connections in it, including disinhibition of the subthalamic nucleus. What is the end result on the thalamus, and therefore on movement initiated by the cerebral cortex?

Everyday Connections –  The Myth of Left Brain/Right Brain

There is a persistent myth that people are “right-brained” or “left-brained,” which is an oversimplification of an important concept about the cerebral hemispheres. There is some lateralization of function, in which the left side of the brain is devoted to language function and the right side is devoted to spatial and nonverbal reasoning. Whereas these functions are predominantly associated with those sides of the brain, there is no monopoly by either side on these functions. Many pervasive functions, such as language, are distributed globally around the cerebrum.

Some of the support for this misconception has come from studies of split brains. A drastic way to deal with a rare and devastating neurological condition (intractable epilepsy) is to separate the two hemispheres of the brain. After sectioning the corpus callosum, a split-brained patient will have trouble producing verbal responses on the basis of sensory information processed on the right side of the cerebrum, leading to the idea that the left side is responsible for language function.

However, there are well-documented cases of language functions lost from damage to the right side of the brain. The deficits seen in damage to the left side of the brain are classified as aphasia, a loss of speech function; damage on the right side can affect the use of language. Right-side damage can result in a loss of ability to understand figurative aspects of speech, such as jokes, irony, or metaphors. Nonverbal aspects of speech can be affected by damage to the right side, such as facial expression or body language, and right-side damage can lead to a “flat affect” in speech, or a loss of emotional expression in speech—sounding like a robot when talking. Damage to language areas on the right side causes a condition called aprosodia where the patient has difficulty understanding or expressing the figurative part of speech.

The Diencephalon

The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction , or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean).

The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus ( Figure 14.3.8 ). There are other structures, such as the epithalamus , which contains the pineal gland, or the subthalamus , which includes the subthalamic nucleus that is part of the basal nuclei.

The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention.

The cerebrum also sends information down to the thalamus, which usually communicates motor commands. This involves interactions with the cerebellum and other nuclei in the brain stem. The cerebrum interacts with the basal nuclei, which involves connections with the thalamus. The primary output of the basal nuclei is to the thalamus, which relays that output to the cerebral cortex. The cortex also sends information to the thalamus that will then influence the effects of the basal nuclei.

Hypothalamus

Inferior and slightly anterior to the thalamus is the hypothalamus , the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system.

The midbrain and the pons and medulla of the hindbrain are collectively referred to as the “brain stem” ( Figure 14.3.9 ). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems.

The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.

One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. It is separated into the tectum and tegmentum , from the Latin words for roof and floor, respectively. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal.

The tectum is composed of four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not.

The tegmentum is continuous with the gray matter of the rest of the brain stem. Throughout the midbrain, pons, and medulla, the tegmentum contains the nuclei that receive and send information through the cranial nerves, as well as regions that regulate important functions such as those of the cardiovascular and respiratory systems.

The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. The bridge-like white matter is only the anterior surface of the pons; the gray matter beneath that is a continuation of the tegmentum from the midbrain. Gray matter in the tegmentum region of the pons contains neurons receiving descending input from the forebrain that is sent to the cerebellum.

The medulla is the region known as the myelencephalon in the embryonic brain. The initial portion of the name, “myel,” refers to the significant white matter found in this region—especially on its exterior, which is continuous with the white matter of the spinal cord. The tegmentum of the midbrain and pons continues into the medulla because this gray matter is responsible for processing cranial nerve information. A diffuse region of gray matter throughout the brain stem, known as the reticular formation , is related to sleep and wakefulness, such as general brain activity and attention.

The Cerebellum

The cerebellum , as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain ( Figure 14.3.10 ). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.

Descending fibers from the cerebrum have branches that connect to neurons in the pons. Those neurons project into the cerebellum, providing a copy of motor commands sent to the spinal cord. Sensory information from the periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the medulla known as the inferior olive . Fibers from this nucleus enter the cerebellum and are compared with the descending commands from the cerebrum. If the primary motor cortex of the frontal lobe sends a command down to the spinal cord to initiate walking, a copy of that instruction is sent to the cerebellum. Sensory feedback from the muscles and joints, proprioceptive information about the movements of walking, and sensations of balance are sent to the cerebellum through the inferior olive and the cerebellum compares them. If walking is not coordinated, perhaps because the ground is uneven or a strong wind is blowing, then the cerebellum sends out a corrective command to compensate for the difference between the original cortical command and the sensory feedback. The output of the cerebellum is into the midbrain, which then sends a descending input to the spinal cord to correct the messages going to skeletal muscles.

The Spinal Cord

The description of the CNS is concentrated on the structures of the brain, but the spinal cord is another major organ of the system. Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. As the spinal cord continues to develop in the newborn, anatomical features mark its surface. The anterior midline is marked by the anterior median fissure , and the posterior midline is marked by the posterior median sulcus . Axons enter the posterior side through the dorsal (posterior) nerve root , which marks the posterolateral sulcus on either side. The axons emerging from the anterior side do so through the ventral (anterior) nerve root . Note that it is common to see the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord. You should learn to be comfortable with both.

On the whole, the posterior regions are responsible for sensory functions and the anterior regions are associated with motor functions. This comes from the initial development of the spinal cord, which is divided into the basal plate and the alar plate . The basal plate is closest to the ventral midline of the neural tube, which will become the anterior face of the spinal cord and gives rise to motor neurons. The alar plate is on the dorsal side of the neural tube and gives rise to neurons that will receive sensory input from the periphery.

The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year, but the skeleton continues to grow. The nerves that emerge from the spinal cord pass through the intervertebral formina at the respective levels. As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail and is named the cauda equina . The sacral spinal cord is at the level of the upper lumbar vertebral bones. The spinal nerves extend from their various levels to the proper level of the vertebral column.

In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure 14.3.11 , the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing. The anterior horn sends out motor signals to the skeletal muscles. The lateral horn , which is only found in the thoracic, upper lumbar, and sacral regions, is the central component of the sympathetic division of the autonomic nervous system.

Some of the largest neurons of the spinal cord are the multipolar motor neurons in the anterior horn. The fibers that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body.

White Column

Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. Between the two posterior horns of gray matter are the posterior columns . Between the two anterior horns, and bounded by the axons of motor neurons emerging from that gray matter area, are the anterior columns . The white matter on either side of the spinal cord, between the posterior horn and the axons of the anterior horn neurons, are the lateral columns . The posterior columns are composed of axons of ascending tracts. The anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts—the latter carrying motor commands down from the brain to the spinal cord to control output to the periphery.

Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible fo r?

Disorders of the…Basal Nuclei

Parkinson’s disease is a disorder of the basal nuclei, specifically of the substantia nigra, that demonstrates the effects of the direct and indirect pathways. Parkinson’s disease is the result of neurons in the substantia nigra pars compacta dying. These neurons release dopamine into the striatum. Without that modulatory influence, the basal nuclei are stuck in the indirect pathway, without the direct pathway being activated. The direct pathway is responsible for increasing cortical movement commands. The increased activity of the indirect pathway results in the hypokinetic disorder of Parkinson’s disease.

Parkinson’s disease is neurodegenerative, meaning that neurons die that cannot be replaced, so there is no cure for the disorder. Treatments for Parkinson’s disease are aimed at increasing dopamine levels in the striatum. Currently, the most common way of doing that is by providing the amino acid L-DOPA, which is a precursor to the neurotransmitter dopamine and can cross the blood-brain barrier. With levels of the precursor elevated, the remaining cells of the substantia nigra pars compacta can make more neurotransmitter and have a greater effect. Unfortunately, the patient will become less responsive to L-DOPA treatment as time progresses, and it can cause increased dopamine levels elsewhere in the brain, which are associated with psychosis or schizophrenia.

Visit this site for a thorough explanation of Parkinson’s disease.

Compared with the nearest evolutionary relative, the chimpanzee, the human has a brain that is huge. At a point in the past, a common ancestor gave rise to the two species of humans and chimpanzees. That evolutionary history is long and is still an area of intense study. But something happened to increase the size of the human brain relative to the chimpanzee. Read this article in which the author explores the current understanding of why this happened.

According to one hypothesis about the expansion of brain size, what tissue might have been sacrificed so energy was available to grow our larger brain? Based on what you know about that tissue and nervous tissue, why would there be a trade-off between them in terms of energy use?

Everyday Connection –  How Much of Your Brain Do You Use?

Have you ever heard the claim that humans only use 10 percent of their brains? Maybe you have seen an advertisement on a website saying that there is a secret to unlocking the full potential of your mind—as if there were 90 percent of your brain sitting idle, just waiting for you to use it. If you see an ad like that, don’t click. It isn’t true.

An easy way to see how much of the brain a person uses is to take measurements of brain activity while performing a task. An example of this kind of measurement is functional magnetic resonance imaging (fMRI), which generates a map of the most active areas and can be generated and presented in three dimensions ( Figure 14.3.12 ). This procedure is different from the standard MRI technique because it is measuring changes in the tissue in time with an experimental condition or event.

The underlying assumption is that active nervous tissue will have greater blood flow. By having the subject perform a visual task, activity all over the brain can be measured. Consider this possible experiment: the subject is told to look at a screen with a black dot in the middle (a fixation point). A photograph of a face is projected on the screen away from the center. The subject has to look at the photograph and decipher what it is. The subject has been instructed to push a button if the photograph is of someone they recognize. The photograph might be of a celebrity, so the subject would press the button, or it might be of a random person unknown to the subject, so the subject would not press the button.

In this task, visual sensory areas would be active, integrating areas would be active, motor areas responsible for moving the eyes would be active, and motor areas for pressing the button with a finger would be active. Those areas are distributed all around the brain and the fMRI images would show activity in more than just 10 percent of the brain (some evidence suggests that about 80 percent of the brain is using energy—based on blood flow to the tissue—during well-defined tasks similar to the one suggested above). This task does not even include all of the functions the brain performs. There is no language response, the body is mostly lying still in the MRI machine, and it does not consider the autonomic functions that would be ongoing in the background.

Chapter Review

Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.

Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.

The adult brain is separated into four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. The cerebrum is the largest portion and contains the cerebral cortex and subcortical nuclei. It is divided into two halves by the longitudinal fissure.

The cortex is separated into the frontal, parietal, temporal, and occipital lobes. The frontal lobe is responsible for motor functions, from planning movements through executing commands to be sent to the spinal cord and periphery. The most anterior portion of the frontal lobe is the prefrontal cortex, which is associated with aspects of personality through its influence on motor responses in decision-making.

The other lobes are responsible for sensory functions. The parietal lobe is where somatosensation is processed. The occipital lobe is where visual processing begins, although the other parts of the brain can contribute to visual function. The temporal lobe contains the cortical area for auditory processing, but also has regions crucial for memory formation.

Nuclei beneath the cerebral cortex, known as the subcortical nuclei, are responsible for augmenting cortical functions. The basal nuclei receive input from cortical areas and compare it with the general state of the individual through the activity of a dopamine-releasing nucleus. The output influences the activity of part of the thalamus that can then increase or decrease cortical activity that often results in changes to motor commands. The basal forebrain is responsible for modulating cortical activity in attention and memory. The limbic system includes deep cerebral nuclei that are responsible for emotion and memory.

The diencephalon includes the thalamus and the hypothalamus, along with some other structures. The thalamus is a relay between the cerebrum and the rest of the nervous system. The hypothalamus coordinates homeostatic functions through the autonomic and endocrine systems.

The brain stem is composed of the midbrain, pons, and medulla. It controls the head and neck region of the body through the cranial nerves. There are control centers in the brain stem that regulate the cardiovascular and respiratory systems.

The cerebellum is connected to the brain stem, primarily at the pons, where it receives a copy of the descending input from the cerebrum to the spinal cord. It can compare this with sensory feedback input through the medulla and send output through the midbrain that can correct motor commands for coordination.

Interactive Link Questions

Both cells are inhibitory. The first cell inhibits the second one. Therefore, the second cell can no longer inhibit its target. This is disinhibition of that target across two synapses.

By disinhibiting the subthalamic nucleus, the indirect pathway increases excitation of the globus pallidus internal segment. That, in turn, inhibits the thalamus, which is the opposite effect of the direct pathway that disinhibits the thalamus.

Watch this video to learn about the gray matter of the spinal cord that receives input from fibers of the dorsal (posterior) root and sends information out through the fibers of the ventral (anterior) root. As discussed in this video, these connections represent the interactions of the CNS with peripheral structures for both sensory and motor functions. The cervical and lumbar spinal cords have enlargements as a result of larger populations of neurons. What are these enlargements responsible for?

There are more motor neurons in the anterior horns that are responsible for movement in the limbs. The cervical enlargement is for the arms, and the lumbar enlargement is for the legs.

Energy is needed for the brain to develop and perform higher cognitive functions. That energy is not available for the muscle tissues to develop and function. The hypothesis suggests that humans have larger brains and less muscle mass, and chimpanzees have the smaller brains but more muscle mass.

In 2003, the Nobel Prize in Physiology or Medicine was awarded to Paul C. Lauterbur and Sir Peter Mansfield for discoveries related to magnetic resonance imaging (MRI). This is a tool to see the structures of the body (not just the nervous system) that depends on magnetic fields associated with certain atomic nuclei. The utility of this technique in the nervous system is that fat tissue and water appear as different shades between black and white. Because white matter is fatty (from myelin) and gray matter is not, they can be easily distinguished in MRI images. Visit the Nobel Prize website to play an interactive game that demonstrates the use of this technology and compares it with other types of imaging technologies. Also, the results from an MRI session are compared with images obtained from x-ray or computed tomography. How do the imaging techniques shown in this game indicate the separation of white and gray matter compared with the freshly dissected tissue shown earlier?

MRI uses the relative amount of water in tissue to distinguish different areas, so gray and white matter in the nervous system can be seen clearly in these images.

Visit this site to read about a woman that notices that her daughter is having trouble walking up the stairs. This leads to the discovery of a hereditary condition that affects the brain and spinal cord. The electromyography and MRI tests indicated deficiencies in the spinal cord and cerebellum, both of which are responsible for controlling coordinated movements. To what functional division of the nervous system would these structures belong?

They are part of the somatic nervous system, which is responsible for voluntary movements such as walking or climbing the stairs.

Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure 14.3.13 demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin . Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.

The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the CNS—for example, a frontal section of the brain or cross section of the spinal cord.

Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the CNS is referred to as a nucleus . In the PNS, a cluster of neuron cell bodies is referred to as a ganglion . Figure 14.3.14 indicates how the term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is a center of some function in the CNS. There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected together and were once called the basal ganglia before “ganglion” became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the “basal nuclei” to avoid confusion.

Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve . There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the PNS, the term is nerve, but if they are CNS, the term is tract. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons ( Figure 14.3.15 ). A similar situation outside of science can be described for some roads. Imagine a road called “Broad Street” in a town called “Anyville.” The road leaves Anyville and goes to the next town over, called “Hometown.” When the road crosses the line between the two towns and is in Hometown, its name changes to “Main Street.” That is the idea behind the naming of the retinal axons. In the PNS, they are called the optic nerve, and in the CNS, they are the optic tract. Table 14.1 helps to clarify which of these terms apply to the central or peripheral nervous systems.

Visit the Nobel Prize web site to play an interactive game that demonstrates the use of this technology and compares it with other types of imaging technologies.

In 2003, the Nobel Prize in Physiology or Medicine was awarded to Paul C. Lauterbur and Sir Peter Mansfield for discoveries related to magnetic resonance imaging (MRI). This is a tool to see the structures of the body (not just the nervous system) that depends on magnetic fields associated with certain atomic nuclei. The utility of this technique in the nervous system is that fat tissue and water appear as different shades between black and white. Because white matter is fatty (from myelin) and gray matter is not, they can be easily distinguished in MRI images.

Also, the results from an MRI session are compared with images obtained from X-ray or computed tomography. How do the imaging techniques shown in this game indicate the separation of white and gray matter compared with the freshly dissected tissue shown earlier?

Review Questions

Critical thinking questions.

1. Damage to specific regions of the cerebral cortex, such as through a stroke, can result in specific losses of function. What functions would likely be lost by a stroke in the temporal lobe?

2. Why do the anatomical inputs to the cerebellum suggest that it can compare motor commands and sensory feedback?

Answers for Critical Thinking Questions

• The temporal lobe has sensory functions associated with hearing and vision, as well as being important for memory. A stroke in the temporal lobe can result in specific sensory deficits in these systems (known as agnosias) or losses in memory.
• A copy of descending input from the cerebrum to the spinal cord, through the pons, and sensory feedback from the spinal cord and special senses like balance, through the medulla, both go to the cerebellum. It can therefore send output through the midbrain that will correct spinal cord control of skeletal muscle movements.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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40 Spinal Motor Control and Proprioception

• Glossary terms

Key Takeaways

• Test Yourself

Additional Review

The motor system refers to the nerve cells that are used to control our body. The key roles of the motor system are to plan, control, and execute voluntary (deliberate) movements, and to control involuntary (subconscious or automatic) functions, such as digesting food. The motor system is sometimes described as a top-down process: in a voluntary movement, neural activity in the frontal lobe sends commands down to motor neurons located in the brainstem or spinal cord, which in turn activate muscle groups. In reality, motor control is more of a loop, rapidly communicating between the sensory cortex and motor cortex. Sensory information about limb position, posture, and objects in contact with the skin inform the descending motor plan. Simultaneously, the motor plan provides predictions about upcoming movement.

There are multiple levels of control. Within the spinal cord, simple reflexes can function without higher input from the brain. Slightly more complex spinal control occurs when central pattern generators function during repetitive movements like walking. The motor and premotor cortices in the brain are responsible for the planning and execution of voluntary movements. And finally, the basal ganglia and cerebellum modulate the responses of the neurons in the motor cortex to help with coordination, motor learning, and balance.

This lesson explores the lowest level of control at the level of the spinal cord.

Alpha Motor Neurons

Muscle fibers are innervated by alpha motor neurons . Alpha motor neurons are also called lower motor neurons because they are not located in higher brain areas. These cells are the only cells that directly command muscle contraction. The cell bodies of the alpha motor neurons are located in the central nervous system in the ventral horn of the spinal cord. Their axons leave the spinal cord via the ventral roots and travel to the muscle via efferent peripheral spinal nerves.

Like the sensory systems, the motor system is also organized in a topographic fashion, referred to as ‘somatotopic organization’. Within the spinal cord, alpha motor neurons that innervate muscles in the arms and legs are located in the lateral portion of the ventral horn, whereas alpha motor neurons that innervate muscles in the trunk are located in the medial portion.

Location of Alpha Motor Neurons

The structure of the spinal cord is reviewed in Chapter 26 . Examining cross sections of the spinal cord at different levels reveals that there is a non-uniform distribution of lower motor neurons. This is determined by the size of the ventral horn at the different levels of the spinal cord. At the cervical enlargement the larger ventral horns are due to the presence of more lower motor neurons that function in movement of the arms. At the lumbar enlargement the larger ventral horns are due to the presence of more lower motor neurons that function in movement of the legs.

Neuromuscular Junction

The lower motor neurons communicate with muscle fibers (muscle cells) at the neuromuscular junction (NMJ). The NMJ is similar to other chemical synapses, however the postsynaptic cell is a muscle cell separated by about 30 nm. The presynaptic cell is the motor neuron and the postsynaptic site is the sarcolemma, the cell membrane of the long cylindrical muscle fibers (muscle cells). The neuromuscular junction is one of the largest synapses in the body and one of the most well-studied because of its peripheral location.

The neuromuscular junction is located midway down the length of the muscle fiber. The muscle fiber will contract in response to depolarization traveling down the sarcolemma of the muscle fiber. The synapse is located midway down the length of the muscle fiber so that the postsynaptic signal can travel in both directions down the long muscle fiber and quickly activate a contraction along the entire cell.

Acetylcholine is the neurotransmitter released at the neuromuscular junction (NMJ), and it acts upon ligand-gated, non-selective cation channels called nicotinic acetylcholine receptors that are present in postjunctional folds of the muscle fiber. Nicotinic acetylcholine receptors allow the influx of sodium ions into the muscle cell. The depolarization will cause nearby voltage-gated channels to open and fire an action potential in the muscle fiber. In a healthy system, an action potential in the motor neurons always causes an action potential in the muscle cell. The action potential leads to contraction of the muscle fiber.

To review, an action potential traveling down the motor neuron (presynaptic cell) will cause the release of acetylcholine into the synapse. Acetylcholine binds to postsynaptic nicotinic acetylcholine receptors that are located in the folded sarcolemma (increasing surface area), causing depolarization of muscle fibers and ultimately muscle contraction.

Acetylcholinesterase , an enzyme that breaks down acetylcholine and terminates its action, is present in the synaptic cleft of the neuromuscular junction. Muscle contraction must be tightly controlled. Thus, the actions of acetylcholinesterase are very important to cease muscle contraction quickly.

Clinical Application: Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness, resulting in difficulty with speech, trouble with movement and swallowing, drooping eyelids, and double vision. Each year, an estimated 20 out of a million people get diagnosed with MG.

The muscle weakness seen in MG results from immune system-mediated destruction of the nicotinic acetylcholine receptors expressed at the NMJ. Thus, when the lower motor neuron releases acetylcholine, the muscle cells are unable to detect this release, so they fail to contract appropriately.

One therapeutic strategy involves inhibition of acetylcholinesterase, the enzyme that degrades acetylcholine. This causes the synaptic acetylcholine to remain in the synapse longer, increasing the chance that receptors get activated. Alternatively, autoimmune diseases like MG can be improved with immunosuppressant therapy. With successful treatment, MG usually does not result in changes in lifespan.

Motor Units

Importantly, there are many more muscles cells than there are motor neurons. One alpha motor neuron can innervate multiple fibers within one muscle due to the branching of motor neuron axons. Each axon terminal synapses (innervates) a single muscle fiber. A motor neuron and all the fibers innervated by it are called a motor unit . The muscle fibers within one motor unit are often spread throughout the muscle to spread the contraction throughout the full muscle. Further, motor units in a muscle usually contract asynchronously to help protect the muscle from fatigue. A graded contraction of the muscle is produced by activating varying numbers of motor units.

Motor units differ in size. Small motor units are motor units that innervate fewer muscle fibers and thus control fine movements. Small motor units are located in the eyes and fingers, both of which function in fine and precise movements. Large motor units innervate many muscle fibers and are typically found in weight-bearing muscles like the thighs.

The group of motor neurons that innervate all the fibers of one muscle is called a motor pool .

Types of Motor Units

In addition to the size of the motor unit, the types of muscle fibers that are innervated by motor units can also differ. There are three different types of motor units:

• Slow motor units . Slow motor units are slow to contract and generate less force but can work for a long time. They are used in endurance exercise like jogging.
• Fast fatigue-resistant motor units . Fast fatigue-resistant motor units are quick to contract (though not as fast the fast fatigable motor units). These motor units generate more force that then slow motor units, but are much more resistant to fatigue than the fast fatigable motor units.
• Fast fatigable motor units . Fast fatigable motor units are quickest to contract and generate the most force. These motor units are more prone to fatigue due to decreased number of mitochondria within the muscles. Fast fatigable motor units generate a lot of force quickly, but also tire quickly. They are used mostly in high intensity exercise like lifting weights and sprinting.

Muscle Activation

Action potential triggered in a muscle.

When an action potential is triggered in a muscle it causes a muscle twitch (contraction) that is followed by a period of relaxation. A muscle twitch shows an increase in tension after a short delay (latent period). During the contraction period, the muscle tension increased, and then has a long relaxation period, causing the muscle tension to be increased long after the initial stimulus.

When a stimulus occurs during the long relaxation period following a muscle twitch, the newly generated muscle twitch will summate to increase the strength of the overall muscle contraction. Shortening the time between stimuli will result in unfused tetanus and if stimuli are very close together will result in fused tetanus . Therefore, a higher rate of action potentials in the alpha motor neuron will generate more muscle contraction.

Muscle Recruitment

In addition to the rate of action potentials changing the force of the muscle contraction, muscle recruitment can also increase the strength of contraction within a muscle. When generating motor activity, the smallest motor units will be activated to contract first due to their size and increased excitability to move the load. Increasingly larger motor units are recruited to lift heavier loads. Recruitment of motor units allows for us to generate appropriate muscle tension to move a given load. For example, if you need to pick up a pencil, then we do not need to use the same force as if we were trying to pick up a 20-pound weight. Only smaller motor units would be activated to pick up the pencil, and increasingly larger motor units would be recruited to lift the heavier load of the 20-pound weight.

Alpha Motor Neuron Inputs

The alpha motor neurons that directly cause muscle contraction receive inputs from three different sources.

• Sensory cells from the dorsal root ganglion that provide sensory information from the muscles through proprioception.
• Upper motor neurons from the motor cortex in the brain and brain stem that are responsible for initiating voluntary movement.
• Interneurons in the spinal cord. These represent the largest input to the alpha motor neurons and can either provide excitation or inhibition to the alpha motor neuron.

Proprioception

Raise your arms above your head. Even without seeing your arms, your nervous system has mechanisms that inform you about the location and position of your body parts, including how much your joints are bent. This sense is called proprioception and is critically important for coordinated movement and motor reflexes that contribute to those tiny, rapid adjustments that are made while maintaining balance. Proprioceptive information ascends through the spinal cord and into the brain via the dorsal column-medial lemniscus tract . Proprioception is also processed in the primary somatosensory cortex.

Proprioception refers to the “body sense” that informs us about how our bodies are positioned and moving in space. Proprioceptors are receptors that provide proprioception information.

There are two main types of proprioceptors:

• Muscle spindles measure muscle stretch (muscle length) and transmit this sensory information via 1a sensory afferent fibers. Muscle spindles are nested within and arranged parallel to the extrafusal muscle fibers.
• Golgi Tendon Organs measure muscle tension and transmit this sensory information via 1b sensory afferent fibers. Golgi tendon organs are located between the extrafusal muscle fibers and their points of attachment at the bone.

Muscle Spindles

Extrafusal and intrafusal muscle fibers.

Muscle spindles are fibrous capsules that are located within muscles. Intrafusal muscle fibers are special muscle fibers that are located within the fibrous capsule of the muscle spindle. The intrafusal muscle fibers are innervated by gamma motor neurons that will cause them to contract.

Extrafusal muscle fibers , however, make up the bulk of the muscle and are located outside of the muscle spindle. The extrafusal muscle fibers are stimulated to contract by the alpha motor neurons .

Muscle Spindles Function

Group 1a sensory afferent axons , which have a large diameter and are heavily myelinated, wrap around the intrafusal fibers contained within the muscle spindle. These sensory afferent fibers will signal when the intrafusal fibers of the muscle spindle are experiencing stretch and communicate information about muscle length.

Within the spinal cord, a single sensory 1a afferent axon synapses on every alpha motor neuron within the motor pool that innervates the muscle that contains the muscle spindle. This allows for a fast and powerful contraction of the muscle in response to a change in muscle stretch.

Gamma Motor Neuron Function

When the muscle experiences a stretch and the extrafusal fibers are stretched, the muscle spindle and the intrafusal fibers are also stretched (due to being within the muscle and surrounded by the extrafusal fibers ). When the muscle spindle stretches, the 1a sensory axon will start to fire action potentials. The sensory axon synapses with an alpha motor neuron that will then cause the extrafusal muscle fibers to contract. As the extrafusal fibers contract and the muscle shortens, the muscle spindle goes slack, and the 1a axon will no longer fire action potentials as it is no longer being stretched. The gamma motor neuron is then activated that innervates the intrafusal muscle fibers , causing the intrafusal fibers to contract, allowing the muscle spindle to sense stretch again.  Therefore, the gamma motor neuron is critical for allowing the muscle spindle to continue providing information about muscle stretch even when the muscle has experienced contraction.

Golgi Tendon Organ

The Golgi tendon organ is a proprioceptor that measures muscle tension, or the force of contraction. They also contribute to our detection of weight, as we lift something heavy for example. Golgi tendon organs are located in the tendon that connects the muscle to bone. The tendon is made up of collagen fibrils and the Group 1b sensory axons are intertwined within the collagen fibrils. When the muscle experiences an increase in tension, the collagen fibrils surrounding the 1b sensory axon physically squeeze the 1b axon, opening mechanically-gated ion channels within the 1b sensory axon.

The purpose of the Golgi tendon organ is to allow for an optimal range of tension for the muscle and to protect the muscle from injury due to being overloaded. This is accomplished through a negative feedback loop controlled by the Golgi tendon organ. When the muscle experiences an increase in muscle tension, the 1b sensory axon starts to fire action potentials. The sensory neuron synapses onto an inhibitory interneuron within the spinal cord, which when active will release GABA onto the alpha motor neuron that innervates the same muscle that experienced the increase in muscle tension to begin with. When GABA binds to the alpha motor neuron, it will decrease firing of the alpha motor neuron, leading to a decrease in contraction of the muscle. This is an example of negative feedback as the increased muscle tension ultimately leads to physiological changes that decrease muscle tension.

• Motor neuron cell bodies are located in the ventral horn of the spinal cord.
• Motor neuron axons are located in the peripheral nervous system and travel to muscles via spinal nerves.
• Acetylcholine is released at the neuromuscular junction and acts upon ionotropic nicotinic acetylcholine receptors.
• The spinal cord is topographically organized.
• Muscle twitches can summate to increase muscle tension.
• A motor unit is an alpha motor neuron and all of the motor fibers that it innervates. Motor units differ in size, recruitment, and power.
• Muscle spindles and Golgi tendon organs are proprioceptors that communicate information about the location and position of the body.

Test Yourself!

• What is the difference between a motor unit and a motor pool?

Attributions

Portions of this chapter were remixed and revised from the following sources:

• Foundations of Neuroscience by Casey Henley. The original work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
• Open Neuroscience Initiative by Austin Lim. The original work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License .

Media Attributions

• MotorControlRegions © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Alpha Motor Neuron © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Spinal Cord Map © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Untitled_Artwork 1 © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• NMJ Ion Flow © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• NeuromuscularJunction © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Myasthenia Gravis © Posey and Spiller adapted by Valerie Hedges is licensed under a Public Domain license
• Motor Unit And Pool © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Types of motor units © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Muscle_Twitch_Myogram © OpenStax adapted by Valerie Hedges is licensed under a CC BY (Attribution) license
• Twitch_vs_unfused_tetanus_vs_fused_tetanus © Daniel Walsh and Alan Sved adapted by Valerie Hedges is licensed under a CC BY-SA (Attribution ShareAlike) license
• Motor unit recruitment © Daniel Walsh and Alan Sved is licensed under a CC BY-SA (Attribution ShareAlike) license
• Motor neuron inputs © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Muscle Spindle © Casey Henley adapted by Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Gamma motor neuron function © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• Golgi tendon organ © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• golgi tendon organ signaling © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license

traveling from the CNS to the body

Toward the edge

Toward the middle

The synapse between a motor neuron and a muscle fiber

a motor neuron and all of the muscle fibers that is innervates

The group of motor neurons that innervate all the fibers of one muscle

Body sense that allows for understanding of location and position of body parts

sensory receptors that provide information about proprioception

proprioceptors that communicate muscle length (muscle stretch)

muscle fibers that are located inside of the muscle spindle capsule

muscle fibers that are outside of the muscle spindle capsule. Extrafusal fibers make up the bulk of the muscle.

Proprioceptor that communicates information about muscle length

Proprioceptor that measures muscle tension

Introduction to Neuroscience Copyright © 2022 by Valerie Hedges is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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12.3: The Function of Nervous Tissue

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Learning Objectives

• Distinguish the major functions of the nervous system: sensation, integration, and response
• List the sequence of events in a simple sensory receptor–motor response pathway

Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful. An example is summarized in Figure \(\PageIndex{1}\).

Fi gure 12.3.1: Testing the Water. (1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.

Imagine you are about to take a shower in the morning before going to school. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.

Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor . When you place your hand under the shower (Figure \(\PageIndex{2}\)), the cell membrane of the thermoreceptors changes its electrical state (voltage). The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a graded potential . If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the axon. You have learned about this type of signaling before, with respect to the interaction of nerves and muscles at the neuromuscular junction. The voltage at which such a signal is generated is called the threshold , and the resulting electrical signal is called an action potential . In this example, the action potential travels—a process known as propagation —along the axon from the axon hillock to the axon terminals and into the synaptic end bulbs. When this signal reaches the end bulbs, it causes the release of a signaling molecule called a neurotransmitter .

The neurotransmitter diffuses across the short distance of the synapse and binds to a receptor protein of the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins. If that graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. The target of this neuron is another neuron in the thalamus of the brain, the part of the CNS that acts as a relay for sensory information. At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the cerebral cortex , the outermost layer of gray matter in the brain, where conscious perception of that water temperature begins.

Within the cerebral cortex, information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren't ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): The Motor Response. On the basis of the sensory input and the integration in the CNS, a motor response is formulated and executed.

A region of the cortex is specialized for sending signals down to the spinal cord for movement. The upper motor neuron is in this region, called the precentral gyrus of the frontal cortex , which has an axon that extends all the way down the spinal cord. At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a lower motor neuron . This second motor neuron is responsible for causing muscle fibers to contract. In the manner described in the chapter on muscle tissue, an action potential travels along the motor neuron axon into the periphery. The axon terminates on muscle fibers at the neuromuscular junction. Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential. When the lower motor neuron excites the muscle fiber, it contracts. All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions.

CAREER CONNECTIONS: Neurophysiologist

Understanding how the nervous system works could be a driving force in your career. Studying neurophysiology is a very rewarding path to follow. It means that there is a lot of work to do, but the rewards are worth the effort.

The career path of a research scientist can be straightforward: college, graduate school, postdoctoral research, academic research position at a university. A Bachelor’s degree in science will get you started, and for neurophysiology that might be in biology, psychology, computer science, engineering, or neuroscience. But the real specialization comes in graduate school. There are many different programs out there to study the nervous system, not just neuroscience itself. Most graduate programs are doctoral, meaning that a Master’s degree is not part of the work. These are usually considered five-year programs, with the first two years dedicated to course work and finding a research mentor, and the last three years dedicated to finding a research topic and pursuing that with a near single-mindedness. The research will usually result in a few publications in scientific journals, which will make up the bulk of a doctoral dissertation. After graduating with a Ph.D., researchers will go on to find specialized work called a postdoctoral fellowship within established labs. In this position, a researcher starts to establish their own research career with the hopes of finding an academic position at a research university.

Other options are available if you are interested in how the nervous system works. Especially for neurophysiology, a medical degree might be more suitable so you can learn about the clinical applications of neurophysiology and possibly work with human subjects. An academic career is not a necessity. Biotechnology firms are eager to find motivated scientists ready to tackle the tough questions about how the nervous system works so that therapeutic chemicals can be tested on some of the most challenging disorders such as Alzheimer’s disease or Parkinson’s disease, or spinal cord injury.

Others with a medical degree and a specialization in neuroscience go on to work directly with patients, diagnosing and treating mental disorders. You can do this as a psychiatrist, a neuropsychologist, a neuroscience nurse, or a neurodiagnostic technician, among other possible career paths.

Chapter Review

Sensation starts with the activation of a sensory ending, such as the thermoreceptor in the skin sensing the temperature of the water. The sensory endings in the skin initiate an electrical signal that travels along the sensory axon within a nerve into the spinal cord, where it synapses with a neuron in the gray matter of the spinal cord. The temperature information represented in that electrical signal is passed to the next neuron by a chemical signal that diffuses across the small gap of the synapse and initiates a new electrical signal in the target cell. That signal travels through the sensory pathway to the brain, passing through the thalamus, where conscious perception of the water temperature is made possible by the cerebral cortex. Following integration of that information with other cognitive processes and sensory information, the brain sends a command back down to the spinal cord to initiate a motor response by controlling a skeletal muscle. The motor pathway is composed of two cells, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the cerebral cortex and synapses on a cell in the gray matter of the spinal cord. The lower motor neuron is that cell in the gray matter of the spinal cord and its axon extends into the periphery where it synapses with a skeletal muscle in a neuromuscular junction.

Review Questions

Q. If a thermoreceptor is sensitive to temperature sensations, what would a chemoreceptor be sensitive to?

C. molecules

D. vibration

Q. Which of these locations is where the greatest level of integration is taking place in the example of testing the temperature of the shower?

A. skeletal muscle

B. spinal cord

C. thalamus

D. cerebral cortex

Q. How long does all the signaling through the sensory pathway, within the central nervous system, and through the motor command pathway take?

A. 1 to 2 minutes

B. 1 to 2 seconds

C. fraction of a second

D. varies with graded potential

Q. What is the target of an upper motor neuron?

A. cerebral cortex

B. lower motor neuron

C. skeletal muscle

D. thalamus

Critical Thinking Questions

Q. Sensory fibers, or pathways, are referred to as “afferent.” Motor fibers, or pathways, are referred to as “efferent.” What can you infer about the meaning of these two terms (afferent and efferent) in a structural or anatomical context?

A. Afferent means “toward,” as in sensory information traveling from the periphery into the CNS. Efferent means “away from,” as in motor commands that travel from the brain down the spinal cord and out into the periphery.

Q. If a person has a motor disorder and cannot move their arm voluntarily, but their muscles have tone, which motor neuron—upper or lower—is probably affected? Explain why.

A. The upper motor neuron would be affected because it is carrying the command from the brain down.

• Spinal Cord Anatomy

The brain and spinal cord make up the central nervous system. The spinal cord, simply put, is an extension of the brain. It is an ovoid shaped column of nerve tissue that extends from the brain down to the second lumbar vertebrae. It allows us to control our arms, legs, and our bathroom habits, among many other things. 

The spinal cord is enclosed in protective tissues called the meninges. The meninges form a protective sack around the spinal cord. Within the spinal (or  dural ) sac, the spinal cord is surrounded by a nourishing fluid called cerebrospinal fluid.  The dural sac is further protected by the bones of the spinal column.

The internal anatomy of the spinal cord is quite complex. To keep things simple, the center of the cord consists of gray mater . White mater is arranged in tracts around the  gray mater. It consists of axons that transmit impulses to and from the brain or between levels of gray mater within the spinal cord.

The spinal cord has two basic functions. The spinal cord carries sensory impulses to the brain (i.e. allows us to feel) and motor impulses (i.e. allows us to move our muscles) from the brain. The spinal cord also controls stretch reflexes and controls our bowel and bladder functions. 

The spinal cord also acts as a nerve center between the brain and the rest of our body. Thirty-one pairs of nerves exit from the spinal cord to innervate our body. 

Labeled Cross Section of Spinal Cord

Spinal Nerves

There are 31 pairs of spinal nerves that arise from the spinal cord. Each spinal nerve corresponds to the level it emerges from: there are 8 cervical, 12 thoracic (chest), 5 lumbar (lower back), and 5 sacral, and one coccygeal (tailbone) nerves. Each spinal nerve is attached to the spinal cord by two roots: a dorsal (or posterior ) sensory root and a ventral (or anterior ) motor root.

The fibers of the sensory root carry sensory impulses to the spinal cord —pain, temperature, touch and position sense ( proprioception )—from tendons, joints and body surfaces. The motor roots carry impulses from the spinal cord to the muscles. The spinal nerves exit the spinal cord and pass through the intervertebral foramen. 

Nerve Plexus

A nerve plexus is a network of multiple nerves. The spinal nerves in each part of the spine cluster together to form a plexus. Listed below are the named plexuses: 

Map of Dermatomes

A dermatome is a band or region of skin supplied by a single sensory nerve. Sensory nerves carry sensory impulses to the spinal cord. Sensory impulses include pain, temperature, touch and position sense (proprioception)—from tendons, joints and body surfaces. Every part of the body has a dermatome that is supplied by a spinal nerve. The exception to this rule is the face, which is supplied by the cranial nerves.

Key to Spinal Nerve Regions

Each pair of spinal nerves links to one of four regions of the body.

• Cervical Region (green): 8 pairs of nerves supply the skin covering the back of the head, the neck, shoulders, arms and hands.
• Thoracic Region (blue): 12 pairs of thoracic nerves supply the skin on the chest, back and under arms
• Lumbar Region (pink): 5 pairs of lumbar nerves supply the skin on the lower abdomen, thighs and fronts of the legs
• Sacral Region (yellow): Sacral nerves supply the skin on the rears of the legs, the feet and genital areas

Levels of principal dermatomes

• C5 — clavicles
• C5, C6, C7 —  lateral parts of the upper limb
• C8, T1 — medial sides of the upper limb
• C6, C7, C8 — hand
• C8 — ring and little fingers
• T4 — level of nipples
• T10 — level of umbilicus
• T12 — inguinal or groin regions
• L1 L2 L3 L4  — anterior and inner services of lower limb
• L4, L5, S1 — foot
• L4 — medial side of big toe
• S1. S2, L5  — posterior and outer surfaces of lower limbs
• S1 — lateral margin of foot and little toe
• S2, S3, S4 — perineum

Receptor Arc

In the nervous system there is a “closed loop” system of sensation (sensory), decision (brain), and reactions (motor). This process is carried out through the activity of afferent neurons (sensory), interneurons (spinal cord), and efferent (motor) neurons. This closed loop forms a receptor reflex arc. While most nerve stimuli are processed through the brain, this is receptor reflex arc is processed at the level of the spinal cord to allow for a lightning fast response. 

Receptor reflex arcs allow for automatic responses to dangerous stimuli (pain, abnormal positioning of a limb) before you would otherwise think to respond to them.  An example of this is the automatic withdrawal response that occurs when you accidentally touch something that is very hot.  The receptor arc automatically responds by withdrawing your arm, before you can even think to do so — i.e. before you can even process that you’ve touched something hot. 

More information about your nerves, spine and back

• Anatomy Terms
• Lumbar Spine Anatomy (lower back)
• Spinal Cord Injury

Dr. Andrew Chung is a Spine Surgeon at Sonoran Spine in Tempe, Arizona. He is a graduate of the Philadelphia College of Osteopathic Medicine and was formerly Spine Surgeon Clinical Fellow at Cedars-Sinai, Spine Surgery Fellow at Keck Hospital, University of Southern California and Chief Resident and an Instructor of Orthopedic Surgery in the Department of Orthopedic Surgery at the Mayo Clinic in Arizona. Dr. Chung's research .

Electromyography (EMG) and Nerve Conduction Studies

• What are EMG and nerve conduction studies? |
• Why would I need an EMG or nerve conduction study? |
• What happens during an EMG or nerve conduction study? |
• What are the problems with an EMG or nerve conduction study? |

Your brain tells your muscles what to do by sending electrical signals to them. The signals travel down your spinal cord and then through different nerves to your muscles.

What are EMG and nerve conduction studies?

EMG and nerve conduction studies are tests to see if you have muscle weakness or loss of feeling from an injury to your spinal cord, muscles, or nerves.

To do an EMG, doctors insert small needles into a muscle to record your muscle’s electrical activity when it’s resting and when it’s active

To do nerve conduction studies, doctors use skin sensors or needles to send small electric shocks through different nerves to see how well your nerves work

Why would I need an EMG or nerve conduction study?

Doctors may do an EMG, nerve conduction study, or both if you have symptoms like tingling or muscle weakness. These symptoms can be caused by many problems in different parts of your body. An EMG or nerve conduction study can help your doctor tell if your symptoms are caused by nerve or muscle problems such as:

Muscular dystrophy

Carpal tunnel syndrome

Amyotrophic lateral sclerosis (ALS)

What happens during an EMG or nerve conduction study?

During an EMG:

Doctors put needles into your muscle

The needles are connected by wires to a machine that records your muscle's electrical activity while you move it and relax it

During a nerve conduction study:

Doctors put a sticky sensor on your skin over the nerve they are testing

They stimulate another part of the nerve with a small electric shock

A machine measures how fast the electrical signal travels down the nerve

What are the problems with an EMG or nerve conduction study?

The needles and electric shocks can be uncomfortable or hurt. You may have some bruises afterward.

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How does the spine form?

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In a way, your spine is the keystone that holds your body together. It's at the center of your axial skeleton -- the flexible column that holds up the center of your body. At the top of your axial skeleton is your skull, and your coccyx, or tailbone, is at the other end. Together, all these bones shelter some of the most important organs in your body. Your brain sits in its protective skull casing, and the organs in your chest are protected by 12 pairs of ribs, all of which attach directly to your spine.

As if that wasn't enough, your spine is also what lets you move your arms and legs. Your arms attach to your spine and ribs via the collarbones and shoulder blades. Your legs attach to the spine through your hips. When you want to move your arms and legs, signals travel down your spinal cord, which is enclosed in your spine. Nerves carry the signals from the spinal cord to the muscles you want to move.

Arms and legs aren't the only parts of your body that move around, though -- your axial skeleton is also flexible. Discs provide lubrication between each of the 26 bones in your spine. As long as your discs are healthy, your vertebrae don't grind together when you bend and twist. Muscles attach to protrusions on the vertebrae called processes . When the muscles contract, they pull on these leverlike surfaces, and your vertebrae move.

Since your spine has a lot of responsibility, it's not surprising that it has to develop in exactly the right way in order for the whole system to work. So where does it come from, and what can go wrong as it grows?

Spinal Development

If you've read How Pregnancy Works , you already know about how a baby develops in a woman's body. A man's sperm joins with a woman's egg, creating a one-celled zygote . That one cell divides into two, which divide into four, over and over until there are about 100 cells. At this point, a few days after conception, the zygote becomes a blastocyst , and that's when the earliest beginnings of spinal development start to happen.

A blastocyst starts off as a collection of similar cells, but it doesn't stay that way for long. It develops three cell layers, called germ layers , in a process called gastrulation. The layers are the ectoderm, mesoderm and endoderm. Most of the body's innermost organs are formed from the endoderm, while most of the external features, like skin and hair, come from the ectoderm. The spine, part of the middle of the body, comes from the mesoderm. These layers are distinct within 12 days of the egg's fertilization.

The road from undifferentiated cells to a whole body is complex, so here's a rundown of what happens just in terms of the spine:

• The layers of the blastocyst move to where they're needed, arranging themselves to build a body.
• Cells from the mesoderm get together to form a structure called the notochord . This structure provides some support for the developing embryo.
• About 25 days after fertilization, the ectoderm above the notochord folds. The folds form a canal called the neural tube, which will become the central nervous system.
• Mesenchymal cells from the mesoderm form groups called somites on either side of the neural tube. These are like tiny building blocks that will become the vertebrae. Since an embryo develops a tail that disappears as it grows, there are more somites than vertebrae.
• When the embryo is 6 or 7 weeks old, ossification , or bone formation, begins. The somites harden, eventually becoming vertebrae. How Bones Work explains exactly what's happening during ossification. The notochord becomes part of the discs that lubricate the connections between the vertebrae.

In order for the spine to grow correctly, everything has to happen without a hitch, from migration of the blastocyst's layers to ossification. If the spinal column doesn't close correctly, the result can be one of a number of birth defects. Among the most common are neural tube defects, which include spina bifida and anencephaly, or a lack of brain development. Getting enough folate and folic acid during the earliest days of pregnancy reduces the risk of these defects.

You can learn more about the human body and related topics by following the links on the next page.

A fetus's spine looks much different from an adult's spine. While in the womb, a fetus's spine has one C-shaped curve. Once the baby is born and starts walking, the spine settles into four curves that hold the body up while distributing weight.

Lots More Information

Related howstuffworks articles.

• How can scientists use an inkjet printer to make bones?
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• Why is sitting in a chair for long periods bad for your back?
• How Prenatal Testing Works
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• How Bones Work
• How FOP Works
• How Osteogenesis Imperfecta Works
• Why do a child's bones heal faster than an adult's?
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More Great Links

• Human Embryology
• Spina Bifida Association
• Tortora, Gerald J. and Sandra Reynolds Grabowsi. "Principles of Anatomy and Physiology." Ninth edition. John Wiley & Sons, Inc. New York. 2000.
• Mayo Clinic. "Fetal Development: What happens in the first trimester?" 7/25/2007 (6/9/2009) http://www.mayoclinic.com/health/prenatal-care/PR00112
• Medline Plus. "Neural Tube Defects." (6/9/2009) http://www.nlm.nih.gov/medlineplus/neuraltubedefects.html#cat1
• Merck. "Brain and Spinal Cord Defects." Merck Manuals. (6/9/2009) http://www.merck.com/mmhe/sec23/ch265/ch265h.html
• National Institute on Alcohol Abuse and Alcoholism. "Embryonic Development of the Nervous System." 2/2005. (6/9/2009) http://www.niaaa.nih.gov/Resources/GraphicsGallery/FetalAlcoholSyndrome/Embryonic.htm
• Universities of Fribourg, Lausanne and Bern. "Human Embryology." Embryology.ch. (6/9/2009) http://www.embryology.ch/genericpages/moduleembryoen.html

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The Spinal Cord: Three Types of Signals That It Sends

• Health & Assessment

There are three types of signals that are carried from your body to your brain through your spinal cord. The three signals are:

• Sensory- signals that evoke feelings like temperature, touch, pain, and pressure
• Motor- signals that cause voluntary movements
• Reflex- signals that cause involuntary movements.

The different types of signals are sent out and received in different ways.

Spinal Cord Signals: Sensory

Sensory signals alert us to feelings both inside and outside the body. These feelings include temperature, touch, pain, and pressure. These signals tell us when we are hot or cold, injured or safe. The nerves in the body send signals along their pathway to the spinal cord, then to the brain. Once the signals reach your brain, your body registers the sensation, and you are able to feel it. Sensory signals travel so fast that it seems like you get the message instantaneously.

Another thing that sensory signals do is they tell your brain what your body position is in space. That means it tells you if you are standing, sitting, leaning, etc. They also tell you where your body parts are in relation to the rest of your body. They are the reason you can reach out with your arm and move the right distance to grab something without going too far or coming up short.

Spinal Cord Signals: Motor

Motor signals cause voluntary movement. They tell your muscles to move when you want them to. The signals also tell your muscles specifically when and how to move. These signals are not like sensory signals. These signals don’t begin at the nerve endings; they begin inside the brain. From there, they travel through the spinal cord, out to the spinal nerves, and then on to the parts of the body.

These signals also occur so quickly that you feel like your body gets the message instantaneously.  In fact, you don’t even realize you are consciously choosing to send the message to your body to move. An example of this would be when you reach out to hug someone.

Spinal Cord Signals: Reflex

Reflex signals cause involuntary movements. This means that the movement was not conscious. You did not decide to make it. A muscle spasm is a good example of this type of movement. Reflex signals that cause movement do not come from your brain.

A reflex signal comes from the nerves in your body, like sensory signals, but instead of going to your brain, they stop at the spinal cord . Once they reach the spinal cord, they loop through and go straight back to the body part they came from. Reflex signals are designed to protect your body. This is why they happen so quickly. They do not have to wait to reach the brain and then wait for the brain to choose a reaction.

Reflex signals are initiated when the nerves in a muscle are irritated by being stretched or pushed on. This triggers the nerves to send a message to the spinal cord . When the signal reaches the spinal cord, it goes back through at the same level it came in, returning to the muscle that initiated the signal. Once it gets back to the starting point, the signal causes the muscle to react by squeezing or contracting.

Spinal Cord Damage

When the spinal cord is damaged, it prevents these signals from being sent correctly. That is why a spinal cord injury causes loss of movement, sensation, and reflexes. The areas affected are the parts of the body that connects to the spinal cord at the level of the injury or lower. Some people have partial injury, where they still have nerves that are working below the injury, and some people have complete injury. Sometimes people even lose voluntary motion in a body part but still retain reflex motion.

Every injury is unique. It is good to talk to your doctor about your function to determine what types of signals your body has the capability of sending and receiving. Then you can work to optimize your movement to the best of your ability.

Author:   Annie Beth Donahue is a professional writer with a health and disability focus. 

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How does the brain transfer signals to each body part to move? Yulissa, 11, Virginia

Dear Yulissa,

Your brain weighs less than 3 pounds but has the power to move your whole body. That’s because it’s part of your nervous system .

Your brain and the spinal cord that runs down your back make up your central nervous system. You also have a peripheral nervous system made up of nerve cells. These connect your brain and spinal cord to all the other parts of your body.

I talked about how your brain signals your body to move with my friend Samantha Gizerian , professor of Integrative Physiology and Neuroscience at Washington State University.

She told me that the brain processes movement in three steps.

First, your senses tell your brain what’s going on around you and what position your body is in. Next, your brain uses that sensory input to plan how to move. Then, your brain sends signals that tell your muscles to contract.

One part of the brain that’s involved in movement is the motor cortex .

“It sits right before your ears but up at the top,” Gizerian said. “It’s the part that sends the signals down to your muscles to tell them to move. But it also has a map. So, there’s a particular part of motor cortex that moves your head. There’s another part that moves your hands and your fingers. There’s another part that moves your arms, your legs, your feet.”

The map is called your homunculus . That means “little man” because the map can be drawn to look like a person with huge hands. The size of the body part in the drawing shows how much of your motor cortex is dedicated to moving that part of your body.

So, let’s say you want to write. How does the motor cortex tell your hand to move?

“The part of the motor cortex that’s responsible for the parts of the hand that will be moving fires a signal,” Gizerian said. “Those signals travel down through the brain stem and spinal cord and pass that signal on to a neuron that goes out to the muscles. Then the muscles contract.”

Neurons are nerves cells. The ones that come from your brain and spinal cord out to your muscles are called motor neurons—because they motor you around. You have about 500,000 motor neurons in your body!

Neurons use electrical and chemical signals to tell your muscles to contract or relax. That makes your muscles pull on your bones, which is how your body moves.

To make sure your movements are smooth, another part of your brain steps in. The cerebellum is the bump at the back of your brain. The signals your motor cortex sends to your muscles are copied there. It helps correct your motion and provides muscle memory.

An activity like writing is what neuroscientists call a motor program. When you run a motor program over and over, your brain gets lots of feedback from your joints, skin and muscles. It uses that feedback to make tiny corrections in your motor program. Eventually, writing is easy and beautiful.

It’s all thanks to your brain .

Dr. Universe

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Table of Contents

The spinal cord is essentially the headquarters of the peripheral nervous system (PNS). It is responsible for the transmission of information between the PNS and the central nervous system (CNS). Curiously, with the spinal cord alone, many autonomic functions and even voluntary movements can occur. Not all actions of the body necessarily need the brain to be carried out! Within the spinal cord are nerve tracts that carry sensory information to regulate motor control. These nerve tracts even play a role in consciousness and awareness. Here, we’ll take a look at the anatomy and functions that make up the purpose of the spinal cord. 

Neuronal Pathways in the Spinal Cord

There are many different collections of nerve tracts that run up and down the spinal cord, inputting information to the thalamus, cerebral cortex, and cerebellum. These tracts are responsible for the transmission of sensory information as it relates to pain, temperature, touch, pressure, proprioception, and more. Once the action potentials, or neuronal transmissions, reached their ultimate destination in the brain, the signal can then be interpreted to create and execute a motor plan.

For example, let’s look at the pathway for pain and temperature: starting at the bottom of the spinal cord (or particular vertebral level, depending on where sensory information is being transmitted from), sensory neurons transmit neuronal information into the spinal cord at the point of decussation. 

(Decussation is the crossing over of nerves from one side of the spinal cord’s midline to the other. The particular bundles of nerve fibers that do this crossing are called commissures.) The information must first have synapsed onto an interneuron in order to enter the spinal cord and decussate.

(A quick distinction: An interneuron is known as such because the whole of its anatomical structure – cell body and axon – is located within the CNS, in the spinal cord and/or brain. On the other hand, a ganglion is a collection of neuron cell bodies outside of the CNS. Their axons then travel into the CNS for transmitting neuronal signals.)

The masses of interneurons within the spinal cord are what make up the gray matter, the butterfly shape in the center of the spinal cord – remember that this gray matter is made up of cell bodies. The white matter surrounding the butterfly is a mass of myelinated nerve fibers.

After the information is passed through a synapse in either the dorsal or ventral horns (gray matter), the interneuron decussates across the spinal cord and ascends the spinal tract, to ultimately pass this information to the brain. Once the interneurons make the transition of traveling either up or down to transmit information, they then become known as intersegmental neurons. Otherwise, they are called intra segmental neurons – staying within the same singular segment. 

The bundle of nerve fibers that travel up and down the spinal cord is called the spinothalamic tract. This tract passes through the medulla oblongata on its way up to the thalamus (most sensory information passes through the thalamus before anywhere else in the brain). The tract then exits the thalamus, where it transitions into mostly white matter (myelin-sheathed axons) to ultimately synapse in the primary sensory level of the cerebral cortex. 

Anatomy of the Vertebrae and Spinal Cord

The vertebrae, along with the cerebrospinal fluid (CSF) which flows throughout the central canal along the entire length of the spinal cord, are essential to the safety structure of the spinal cord. The CSF supplies a buffer of sorts that absorbs any shock that may result from possible impact, while the vertebrae surround the spinal cord, maintaining its structure and providing a hard exterior for increased protection. 

The spine – the poster portion of the vertebral column – is made up of the posterior, or dorsal, parts of the vertebrae called spinous processes (the pokey parts of the vertebrae that create the bumps you see on your back when you bend over). The main bodies of the vertebrae are on the anterior, or ventral, side of the vertebral column. The center of the vertebral column through which the spinal cord passes is known as the vertebral foramen, or the vertebral canal. 

Surrounding the spinal cord and the brain are three meninges, or membranes, called the dura mater (the outermost layer), the arachnoid (middle layer), and the pia mater (the innermost membrane). These, in addition to the CSF, function not only for the protection of the spinal cord, but also the regulation of the neuronal environment by removing toxins, circulating nutrients, and containing blood vessels vital to the organs of the nervous system. The space between the pia mater and the arachnoid membrane is called the subarachnoid space and is also filled with CSF. 

Nerves of the Spinal Cord

The spinal nerves, the lifeline of communication for the PNS, originating from the spinal cord are – listed from the top, down, or superior to inferior – 8 pairs of cervical nerves (C1-C8), 12 pairs of thoracic spinal nerves (T1-T12), 5 pairs of lumbar spinal nerves (L1-L5), 5 pairs of sacral spinal nerves (S1-S5), and one single coccygeal nerve, all of them being “mixed nerves,” both sensory and motor. 

The nerves emerging from the spinal cord are not simply extensions of the spinal nerve, rather, a “reorganization” of the axons of those nerves that ultimately follow different routes. When axons of different spinal nerves join they form what is called a systemic nerve. Systemic nerves are formed at four places along the entirety of the vertebral column – these sites are referred to individually as a nerve plexus (“plexus” is typically used in reference to describe a network of nerve fibers that have no associated cell bodies). 

Two of these nerve plexuses are located at the cervical level, one at the lumbar level, and one at the sacral level. 

• Cervical Plexus : made up of axons from spinal nerves C1-C5, axons of this plexus innervate the posterior neck and head and also connects to the phrenic nerve (controls the diaphragm). 
• Brachial Plexus : made up of axons from spinal nerves C4-T1, this plexus innervates the arms. The radial nerve comes from this plexus and gives rise to the axillary nerves that innervate the armpit. (More on this another time.)
• Lumbar Plexus : comprised of all the lumbar spinal nerves and innervates the pelvic region and anterior leg. A major nerve from this plexus is the femoral nerve.
• Sacral Plexus : this plexus comes is composed of the lower lumbar nerves, L4 and L5, and sacral nerves, S1 to S4. The most prominent nerve to arise from this plexus is the sciatic nerve.

Inside the Spinal Cord

Let’s do one of those image interpretation tests: When you look at the center of the spinal cord, what do you see? Most people see this shape as either an “H” or a butterfly – I’m here to tell you that if you see an “H,” you need to work on your handwriting. The wings of the butterfly make up the portion of the gray matter known as the lateral gray horn. This gray matter is surrounded by white matter that doesn’t go by any particular name but is responsible for transmitting information up and down the spinal cord, to and from the brain. 

There are two branches that project from either side of the spinal cord: the branch on the anterior, or ventral, side is known as the ventral root, and the one on the dorsal, or posterior side, is called the dorsal root. Located within the dorsal root is a structure called the dorsal root ganglion. Sensory information is passed through the dorsal root ganglion after passing through the point at which the two branches meet. This joining of the dorsal and ventral branches creates a spinal nerve that can be both types of sensory nerves: general sensory and motor (because of this, they are called “mixed nerves”). 

(The dorsal root ganglion functions as the “sensory branch” of the spinal nerve and is therefore known as the “spinal ganglion” as well. Within this ganglion are the cell bodies of somatic and visceral sensory nerve fibers. The ventral root, on the other hand, functions as the motor branch of the spinal nerve, sending signals down from the brain based on information gathered from sensory neurons.)

The anatomical regions of the gray matter butterfly each are known as the following: 

• The horns extending toward the dorsal side of the butterfly are called the dorsal gray horns . This is where sensory nerve fibers synapse onto interneurons.
• There are two processes in between the two butterfly wings pointing outward called the lateral gray horns . Now, these are quite unique in that they are only located at the thoracic and lumbar levels of the spinal cord. They are also the home of autonomic neuron cell bodies. 
• Lastly, the horns that point toward the ventral side of the spinal cord are known as the ventral gray horns . This is where the cell bodies of somatic motor neurons, also known as alpha motor neurons or lower motor neurons, are located.
• In the very center of the gray butterfly is the central canal . As previously mentioned, through this canal runs CSF which distributes nutrients and removes toxins from the neuronal environment and provides a buffer for any possible traumatic impact to the spinal cord or vertebral column. CSF goes on to distribute throughout the rest of the body by exiting the central canal at the bottom of the spinal cord.
• The area just above the central canal is called the posterior gray commissure , where the nerve fibers decussate (remember, this is to cross from one side of the midline to the other). Opposite of this is the anterior gray commissure. 

Inside the spinal nerve (the point at which the dorsal and ventral roots join) are both sensory neurons, which send information to the CNS, as well as motor neurons, which send commands out to the skeletal muscles (effectors) from the brain or spinal cord, depending on the type of movement and muscle to which the command is being sent. The sensory neurons pass through the dorsal root branch, and the motor neurons pass through the ventral root branch. 

As the transitional structure between the CNS and PNS, the spinal cord is another absolutely essential part of the nervous system that has many different functions in distributing information for sensory and motor functions, including continuous autonomic processes. The extent of its reach in functionality throughout the entire human body is enormous and still being explored!

• Biga, L. M., Dawson, S., Harwell, A., Hopkins, R., Kaufmann, J., LeMaster, M., … Matern, P. (n.d.). 13.3 spinal and cranial nerves – Anatomy & physiology. Retrieved from https://open.oregonstate.education/aandp/chapter/13-3-spinal-and-cranial-nerves/
• Fink. (2013, January 17). The spinal cord & spinal tracts; part 1 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&view=detail&mid=7D51F1D2918F665B2CC97D51F1D2918F665B2CC9&FORM=VIREFink. (2013, January 17). The spinal cord & spinal tracts; part 2 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&&view=detail&mid=D0413958C915777BBCEBD0413958C915777BBCEB&&FORM=VDRVRV

Image source

http://vanat.cvm.umn.edu/neurLab2/SpinalPath.html

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