signals travel down the spinal cord to the muscles

• Type 2 Diabetes
• Heart Disease
• Digestive Health
• Multiple Sclerosis
• COVID-19 Vaccines
• Occupational Therapy
• Healthy Aging
• Health Insurance
• Public Health
• Patient Rights
• Caregivers & Loved Ones
• End of Life Concerns
• Health News
• Thyroid Test Analyzer
• Doctor Discussion Guides
• Hemoglobin A1c Test Analyzer
• Lipid Test Analyzer
• Complete Blood Count (CBC) Analyzer
• What to Buy
• Editorial Process
• Meet Our Medical Expert Board

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), and Relyvrio (sodium phenylbutyrate/ taurursodiol) 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 Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.  

By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts.

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

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.

Share This Book

How the Spinal Cord Works

What is the central nervous system.

The brain is the center of our thoughts, the interpreter of our external environment, and the origin of control over body movement. Like a central computer, it interprets information from our eyes (sight), ears (sound), nose (smell), tongue (taste), and skin (touch), as well as from internal organs such as the stomach.

The spinal cord is the highway for communication between the body and the brain. When the spinal cord is injured, the exchange of information between the brain and other parts of the body is disrupted.

How does the central nervous system differ from other systems of the body?

Most systems and organs of the body control just one function, but the central nervous system does many jobs at the same time. It controls all voluntary movement, such as speech and walking, and involuntary movements, such as blinking and breathing. It is also the core of our thoughts, perceptions, and emotions.

How does the central nervous system protect itself from injury?

The central nervous system is better protected than any other system or organ in the body. Its main line of defense is the bones of the skull and spinal column, which create a hard physical barrier to injury. A fluid-filled space below the bones, called the syrnix, provides shock absorbance.

Unfortunately, this protection can be a double-edged sword. When an injury to the central nervous system occurs, the soft tissue of the brain and spinal cord swells, causing pressure because of the confined space. The swelling makes the injury worse unless it is rapidly relieved. Fractured bones can lead to further damage and the possibility of infection.

Why can’t the central nervous system repair itself after injury?

Many organs and tissues in the body can recover after injury without intervention. Unfortunately, some cells of the central nervous system are so specialized that they cannot divide and create new cells. As a result, recovery from a brain or spinal cord injury is much more difficult.

The complexity of the central nervous system makes the formation of the right connections between brain and spinal cord cells very difficult. It is a huge challenge for scientists to recreate the central nervous system that existed before the injury.

Cells of the central nervous system

Neurons connect with one another to send and receive messages in the brain and spinal cord. Many neurons working together are responsible for every decision made, every emotion or sensation felt, and every action taken.

The complexity of the central nervous system is amazing: there are approximately 100 billion neurons in the brain and spinal cord combined. As many as 10,000 different subtypes of neurons have been identified, each specialized to send and receive certain types of information. Each neuron is made up of a cell body, which houses the nucleus. Axons and dendrites form extensions from the cell body.

Astrocytes , a kind of glial cell, are the primary support cells of the brain and spinal cord. They make and secrete proteins called neurotrophic factors. They also break down and remove proteins or chemicals that might be harmful to neurons (for example, glutamate, a neurotransmitter that in excess causes cells to become overexcited and die by a process called excitotoxicity).

Astrocytes aren’t always beneficial: after injury, they divide to make new cells that surround the injury site, forming a glial scar that is a barrier to regenerating axons.

Microglia are immune cells for the brain. After injury, they migrate to the site of injury to help clear away dead and dying cells. They can also produce small molecules called cytokines that trigger cells of the immune system to respond to the injury site. This clean-up process is likely to play an important role in recovery of function following a spinal injury.

Synapses and neurotransmission

Messages are passed from neuron to neuron through synapses, small gaps between the cells, with the help of chemicals called neurotransmitters. To transmit an action potential message across a synapse, neurotransmitter molecules are released from one neuron (the “pre-synaptic” neuron) across the gap to the next neuron (the “post-synaptic” neuron). The process continues until the message reaches its destination.

There are millions and millions of connections between neurons within the spinal cord alone. These connections are made during development, using positive (neurotrophic factors) and negative (inhibitory proteins) signals to fine-tune them. Amazingly, a single axon can form synapses with as many as 1,000 other neurons.

What causes paralysis?

There is a logical and physical topographical organization to the anatomy of the central nervous system, which is an elaborate web of closely connected neural pathways. This ordered relationship means that different segmental levels of the cord control different things, and injury to a particular part of the cord will have an impact on neighboring parts of the body.

Paralysis occurs when communication between the brain and spinal cord fails. This can result from injury to neurons in the brain (a stroke), or in the spinal cord. Trauma to the spinal cord affects only the areas below the level of injury. However, poliomyelitis (a viral infection) or Lou Gehrig’s disease (amyotrophic lateral sclerosis, or ALS) can affect neurons in the entire spinal cord.

The information pathways

Specialized neurons carry messages from the skin, muscles, joints, and internal organs to the spinal cord about pain, temperature, touch, vibration, and proprioception. These messages are then relayed to the brain along one of two pathways: the spinothalmic tract and the lemniscal pathway. These pathways are in different locations in the spinal cord, so an injury might not affect them in the same way or to the same degree.

Each segment of the spinal cord receives sensory input from a particular region of the body. Scientists have mapped these areas and determined the “receptive” fields for each level of the spinal cord. Neighboring fields overlap each other, so the lines on the diagram are approximate.

Voluntary and involuntary movement

Over one million axons travel through the spinal cord, including the longest axons in the central nervous system.

Neurons in the motor cortex, the region of the brain that controls voluntary movement, send their axons through the corticospinal tract to connect with motor neurons in the spinal cord. The spinal motor neurons project out of the cord to the correct muscles via the ventral root. These connections control conscious movements, such as writing and running.

Information also flows in the opposite direction resulting in involuntary movement. Sensory neurons provide feedback to the brain via the dorsal root. Some of this sensory information is conveyed directly to lower motor neurons before it reaches the brain, resulting in involuntary, or reflex movements. The remaining sensory information travels back to the cortex.

How the spinal cord and muscles work together

The spinal cord is divided into five sections: the cervical, thoracic, lumbar, sacral, and coccygeal regions. The level of injury determines the extent of paralysis and/or loss of sensation. No two injuries are alike.

This diagram illustrates the connections between the major skeletal muscle groups and each level of the spinal cord. A similar organization exists for the spinal control of the internal organs.

How the spinal cord and internal organs work together

In addition to the control of voluntary movement, the central nervous system contains the sympathetic and parasympathetic pathways that control the “fight or flight” response to danger and regulation of bodily functions. These include hormone release, movement of food through the stomach and intestines, and the sensations from and muscular control to all internal organs.

This diagram illustrates these pathways and the level of the spinal cord projecting to each organ.

What happens following a spinal cord injury?

A common set of biological events take place following spinal cord injury:

• Cells from the immune system migrate to the injury site, causing additional damage to some neurons and death to others that survived the initial trauma.
• The death of oligodendrocytes causes axons to lose their myelination, which greatly impairs the conduction of action potential, messages, or renders the remaining connections useless. The neuronal information highway is further disrupted because many axons are severed, cutting off the lines of communication between the brain and muscles and between the body’s sensory systems and the brain.
• Within several weeks of the initial injury, the area of tissue damage has been cleared away by microglia, and a fluid-filled cavity surrounded by a glial scar is left behind. Molecules that inhibit regrowth of severed axons are now expressed at this site. The cavitation is called a syrinx, which acts as a barrier to the reconnection of the two sides of the damaged spinal cord.

Although spinal cord injury causes complex damage, a surprising amount of the basic circuitry to control movement and process information can remain intact. This is because the spinal cord is arranged in layers of circuitry. Many of the connections and neuronal cell bodies forming this circuitry above and below the site of injury survive the trauma. An important question to research scientists is, how much do these surviving neurons “know?” Can they regenerate and make new, correct connections?

Intervention strategies

Research points to a multiplicity of possible interventions to promote recovery from a spinal injury. Some would be delivered immediately following the injury; others are less time-specific and involve rebuilding and reconnecting the injured cord. Clearly, both approaches are important: limiting degeneration will enhance the probability of greater recovery, while stimulating regeneration will build upon the remaining system to restore lost connectivity and perhaps to prevent further degeneration.

The following are some of the intervention strategies supported by funding from the Christopher & Dana Reeve Foundation. This is not a comprehensive list of all possible interventions.

Treatments immediately following an accident:

• Limiting initial degeneration Recent research has shown that there are at least three different mechanisms of cell death at play in neuronal and oligodendrocyte loss after injury: necrosis, excitotoxicity, and apoptosis.
• Treating inflammation Soon after injury, the spinal cord swells and proteins from the immune system invade the injured zone. This swelling and inflammation may foster secondary damage to the cord after the initial injury. So it is important to treat the inflammatory response as quickly as possible. Labs pursuing this approach include the Schwab Lab .

Longer-term treatments:

• Stimulating axonal growth Nerve fertilizers called neurotrophins can promote cell survival by blocking apoptosis and stimulate axonal growth. Each neurotrophin has a very specific target cell function. Some selectively prevent oligodendrocyte cell death, others promote axon regrowth or neuron survival, and still others serve multiple functions. Labs pursuing this approach include the Black Lab and the Parada Lab.
• Promoting new growth through substrate or guidance molecules Substrate and guidance molecules may improve targeting once axons have been encouraged to regenerate past the lesion site. These proteins act as roadmaps, steering axons to their correct targets. This is a critical function because even if axons do survive, they must reconnect with the correct targets. Labs pursuing this approach include the Black Lab, the Mendell Lab, and the Parada Lab.
• Blocking molecules that inhibit regeneration There are molecules within the brain and spinal cord that prevent neurons from dividing and axons from growing. Overcoming inhibition can stimulate axonal regrowth and regeneration and is likely to be an important component of regenerative therapies. The Schwab Lab is pursuing this approach.
• Supplying new cells to replace lost ones Stem cells, which are isolated from the CNS and can divide to form new cells, may replace lost neurons and gila. These stem cells must be harvested, treated to encourage growth, and then injected into the injured cord. Labs pursuing such an approach include the Bunge Lab and the Gage Lab.
• Building bridges to span the lesion cavity Bridges may be needed to reconnect the severed sections of the injured spinal cord. Scientists must determine how best to build these bridges and what molecules to use to encourage new growth and enhance survival of new connections. The Bunge Lab is pursuing this approach.

Related Content

Causes of Paralysis

Common causes of paralysis include acute flaccid myelitis, a...

Secondary Conditions

Secondary conditions refer to a range of complications that ...

Sexual Health

Paralysis often impacts people’s sexuality, including chan...

Depression is a serious medical illness that involves the br...

Staying Active

Discover sport and recreation activities for all levels of i...

Learn more tips, tools, and advice about paralysis and how t...

Spinal Cord Glossary

Our glossary has spinal cord terms designed to help you as y...

The central nervous system controls most functions of the bo...

Winter Safety Tips

Winter can be a brutal season, especially for those living w...

COVID-19 and Spinal Cord Injury

Symptoms of COVID-19 appear 2-14 days after exposure to the ...

The National Paralysis Resource Center website is supported by the Administration for Community Living (ACL), U.S. Department of Health and Human Services (HHS) as part of a financial assistance award totaling $10,000,000 with 100 percent funding by ACL/HHS. The contents are those of the author(s) and do not necessarily represent the official views of, nor an endorsement by, ACL/HHS, or the U.S. Government.

An official website of the United States government

The .gov means it's official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you're on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings
• Browse Titles

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.

StatPearls [Internet].

Neuroanatomy, corticospinal cord tract.

Adriana L. Natali ; Vamsi Reddy ; Bruno Bordoni .

Affiliations

Last Update: August 14, 2023 .

• Introduction

The corticospinal tract, AKA, the pyramidal tract, is the major neuronal pathway providing voluntary motor function. This tract connects the cortex to the spinal cord to enable movement of the distal extremities. [1]  As the corticospinal tract travels down the brain stem, a majority of its fibers decussate to the contralateral side within the medulla then continues to travel down the spinal cord to provide innervation to the distal extremities and muscle groups. Various collateral pathways exist which do not follow this pathway, leading to variability amongst individuals. This structure continues to develop after birth, with maturation taking place during puberty, due to rising levels of androgens. Clinically, the corticospinal tract is important in ischemic infarcts, rehabilitation, and various neurodegenerative disorders.

• Structure and Function

The corticospinal tract originates primarily from the frontoparietal cortices, including the primary motor cortex, secondary motor area, and somatosensory cortex. [2]  The corticospinal tracts then come together to form bundles, which travel through the internal capsule and cerebral peduncles. The bundles then travel down to the brainstem. [3]  As the tract reaches the pons, the bundles take on a more compact structure and continue to condense as they descend. As a result, the neural structure of the corticospinal tract takes up more surface area in the upper pons than in the lower pons. [4]  As the corticospinal tract continues to travel down into the medulla, 75 to 90% of the fibers will decussate to the contralateral side via the pyramidal decussation. [5] [3] The 5 to 15% of fibers that do not decussate within the pyramidal decussation make up the anterior corticospinal tract. This tract extends into the spinal cord, but only travels down to the levels of the lower thoracic cord. Various collaterals also exist for the corticospinal tract, with the aberrant pyramidal tract being the most representative. The aberrant pyramidal tract separates from the corticospinal tract within the midbrain and pons, then descends through the medial lemniscus. [6]  This collateral pathway may provide an alternative motor pathway in the case of a cerebral infarct, which will be a topic of discussion below. [2]

After leaving the brainstem and entering the spinal cord, the fibers run down through the anterior and lateral corticospinal tract. When they get to their target level, the fibers of the anterior corticospinal tract decussate through the anterior white commissure before synapsing to a neuron in the anterior horn of the gray matter. The lateral corticospinal tract fibers have previously decussated at the level of the pyramid and synapse at a neuron on the anterior horn when they get to the appropriate level. These neurons, known as anterior horn cells, then project to the limbs and axial muscles to provide voluntary motor function. [3]  

During embryologic development, there is an overgrowth of axons distributed throughout the cortex, which incorporate into the corticospinal tract, and as development progresses, many of these axons are eliminated. Gray matter development begins a few weeks after the corticospinal tract axons reach the spinal cord. As growth continues, the corticospinal tract axons will reach the lower part of the cervical spinal cord by 24 weeks gestation. [3] [7]  After birth, the corticospinal tract continues to develop. The tract is then refined, and motor control develops. The research proposes that refinement of the corticospinal tract happens through the elimination of transient termination and growth within the gray matter of the spinal cord.; this is followed by developing control of the corticospinal tract’s role in voluntary motor function. [2]  The tract continues development through puberty, which is when the gender differences in white matter emerge. Studies have shown that androgens play a role in axonal development through the proliferation of neural cell bodies and the prevention of cell death following axonal injury. As a result, the development of white matter in males and females diverges during adolescence. [8]

• Physiologic Variants

Due to the complex nature of the corticospinal tract, many physiologic variants exist. The collateral pathway known as the aberrant pyramidal tract has been observed in some patients as traveling through the medial lemniscus from the midbrain to the pons until it reached the medulla where it rejoined the corticospinal tract. [6]  Variants have also been observed between men and women following puberty, due to surges in androgens. The neuroprotective nature of testosterone leads to physiologic differences between individuals following adolescence. [8]  Studies have shown anatomical and physiologic differences of this structure exist across individuals, and those differences are a continued topic of research.

• Clinical Significance

Knowledge of the corticospinal tract is of the utmost importance in many clinical scenarios. Preservation and recovery of the corticospinal tract are necessary for the recovery of impaired motor function following a brain injury. [2]  During the event of an acute ischemic stroke, hypo-perfused tissue may be potentially salvageable through timely reperfusion therapy. Areas where the corticospinal tract is contained within a small, dense area, such as the pons, have shown less of a correlation between motor impairment and the size of the ischemic lesion. Studies have proven that the extent of motor impairment during acute ischemic stroke depends on the extent of the corticospinal tract involved in the lesion. [1] [4] [9] [4]  Motor paralysis is a debilitating result of an ischemic infarct, for which rehabilitation has proven to be the most effective treatment modality. [10]  Patients who have the highest degree of improvement following an acute ischemic stroke had superior integrity of the corticospinal tract than those with fewer improvements during rehabilitation.

Damage to the corticospinal tract has correlations with neuromyelitis optica (NMO) and multiple sclerosis (MS). Both autoimmune diseases involve an inflammatory process that causes extensive damage to neurologic structures involved in the corticospinal tract resulting in extensive neurologic disability, including optic neuritis and transverse myelitis. [11] [12]

Compromise of the corticospinal tract during development presents may present as a tract that is completely absent, hypoplastic, or malformed. Disorders with the absence of corticospinal tracts include anencephaly, where there is a failure of the rostral neural tube to close; congenital aqueduct stenosis with a narrowing of the cerebral aqueduct; and microcephaly, which is a defect in proliferation. Underdeveloped corticospinal tracts present in lissencephaly, a defect in migration leading to absent gyration, Walker-Warburg syndrome, migration deficiencies yielding cerebro-ocular dysplasia with muscular atrophy; holoprosencephaly, and the failure of the brain hemispheres to separate. Corticospinal tract malformations usually involve diffuse brain malformation and are most often associated with an abnormal trajectory of the pathway. [3]  These pathologies present with a range of problems, including the lack of motor control due to the involvement of the corticospinal tract.

• Review Questions
• Access free multiple choice questions on this topic.
• Comment on this article.

lateral corticospinal tract. Image courtesy S Bhimji MD

Disclosure: Adriana Natali declares no relevant financial relationships with ineligible companies.

Disclosure: Vamsi Reddy declares no relevant financial relationships with ineligible companies.

Disclosure: Bruno Bordoni 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 Natali AL, Reddy V, Bordoni B. Neuroanatomy, Corticospinal Cord Tract. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.

In this Page

Bulk download.

• Bulk download StatPearls data from FTP

Related information

• PMC PubMed Central citations
• PubMed Links to PubMed

Similar articles in PubMed

• Corticospinal Tract Lesion. [StatPearls. 2023] Corticospinal Tract Lesion. Van Wittenberghe IC, Peterson DC. StatPearls. 2023 Jan
• Review Neuroplasticity of spinal cord injury and repair. [Handb Clin Neurol. 2022] Review Neuroplasticity of spinal cord injury and repair. Martin JH. Handb Clin Neurol. 2022; 184:317-330.
• Neuroanatomy, Lateral Corticospinal Tract. [StatPearls. 2023] Neuroanatomy, Lateral Corticospinal Tract. Javed K, Reddy V, Lui F. StatPearls. 2023 Jan
• The porcine corticospinal decussation: A combined neuronal tracing and tractography study. [Brain Res Bull. 2018] The porcine corticospinal decussation: A combined neuronal tracing and tractography study. Bech J, Glud AN, Sangill R, Petersen M, Frandsen J, Orlowski D, West MJ, Pedersen M, Sørensen JCH, Dyrby TB, et al. Brain Res Bull. 2018 Sep; 142:253-262. Epub 2018 Aug 4.
• Review [The pyramidal tract. Recent anatomic and physiologic findings]. [Rev Neurol (Paris). 1984] Review [The pyramidal tract. Recent anatomic and physiologic findings]. Armand J. Rev Neurol (Paris). 1984; 140(5):309-29.

Recent Activity

• Neuroanatomy, Corticospinal Cord Tract - StatPearls Neuroanatomy, Corticospinal Cord Tract - StatPearls

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

Connect with NLM

National Library of Medicine 8600 Rockville Pike Bethesda, MD 20894

Web Policies FOIA HHS Vulnerability Disclosure

Help Accessibility Careers

Advertisement

How does the spine form?

• Share Content on Facebook
• Share Content on LinkedIn
• Share Content on Flipboard
• Share Content on Reddit
• Share Content via Email

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?
• Can people get bone marrow transplants from baboons?
• Why is sitting in a chair for long periods bad for your back?
• How Prenatal Testing Works
• How Ultrasound Works
• How Bones Work
• How FOP Works
• How Osteogenesis Imperfecta Works
• Why do a child's bones heal faster than an adult's?
• How do broken bones heal?
• How Body Farms Work

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

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

Tips and real-world stories

to live your life to the fullest with limited mobility

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. 

Keep reading

• Search Please fill out this field.
• Manage Your Subscription
• Give a Gift Subscription
• Sweepstakes

Parkinson’s Patient Able to Walk Again After Spinal Implant: ‘A Rebirth’

With the new device, 63-year-old Marc Gauthier says he frequently walks up to 3.7 miles without falling

A man in France was the first patient to receive a spinal implant created to treat advanced Parkinson’s disease — and has been walking without problems since.

According to The Guardian , the patient, 63-year-old Marc Gauthier, was diagnosed with the degenerative neurological disease — which nearly 10 million people live with — more than two decades ago.

As a result, he dealt with severe mobility problems, including balance and severe gait impairments.

“I practically could not walk anymore without falling frequently, several times a day,” he told The Guardian . “In some situations, such as entering a lift, I’d trample on the spot, as though I was frozen there, you might say.”

Since receiving the implant, which intends to restore normal signaling from the spine to the leg muscles, Gauthier has been able to walk without falling. He told the outlet that the experience was “a rebirth.”

“Right now, I’m not even afraid of the stairs anymore,” he said. “Every Sunday I go to the lake, and I walk around 6 kilometers [about 3.7 miles]. It’s incredible.”

According to the Swiss team behind the implant, who published a study in Nature Medicine , they began by creating an anatomical map of Gauthier’s spinal cord that marked the specific locations involved in signaling his legs to move.

The researchers then implanted a system of electrodes at these locations, which allowed stimulation to be delivered to Gauthier's spinal neurons. He wears a sensor on each of his legs and when he begins walking, the implant turns on automatically, delivering stimulation to the spine. 

The goal of the implant is to correct abnormal signals sent from the brain — which travel down the spine — to the patient’s legs to restore normal movement.

According to the study's findings, the device improved Gauthier’s walking and mobility impairments, and the longtime Parkinson’s patient also reported significant improvements in his quality of life.

Jocelyne Bloch, a neurosurgeon and professor at the CHUV Lausanne University Hospital in Switzerland, who co-led the study, said that this targeted stimulation has previously been used on paraplegic patients.

“It is impressive to see how by electrically stimulating the spinal cord in a targeted manner, in the same way as we have done with paraplegic patients, we can correct walking disorders caused by Parkinson’s disease,” Bloch told The Guardian .

“At no point is [the patient] controlled by the machine,” Eduardo Martin Moraud, a professor of neural engineering at the Swiss university, said.

Rather, “it’s just enhancing his capacity to walk,” Moraud added.

Though effective for Gauthier, the implant has yet to be tested in a full clinical trial, which the study said is necessary to demonstrate its clinical efficacy.

The Swiss researchers have already enrolled six more patients to assess whether the apparent benefits can be replicated, per The Guardian .

Never miss a story — sign up for  PEOPLE's free daily newsletter  to stay up-to-date on the best of what PEOPLE has to offer, from celebrity news to compelling human interest stories.

Professor Karunesh Ganguly, a neurologist at the University of California San Francisco who “specializes in neurological rehabilitation, particularly for patients with gait or walking disorders” was not involved in the study — but said that its implications are “exciting.”

“This study describes a new approach for modulating the spinal cord in order to improve gait in Parkinson’s disease [and the] treatment can also potentially address freezing of gait, which is currently hard to treat,” Ganguly told The Guardian .

“It will be exciting to see how this generalizes to a larger population of patients,” he added.

By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts.

Neurons that carry messages from the spinal cord or the brain to the muscles and glands are called ________ neurons.

Hii!! I believe the answer is motor neurons. (:

Motor neurons transmit impulses to skeletal muscle and smooth muscle from the spinal cord.

What are motor neurons?

Motor neurons are part of the central nervous system( CNS ), it connects the spinal cord to the glands, smooth muscles, and skeletal muscle and transmits signals.

These are the special type of brain cells, that come into two types upper and lower motor neurons . lower motor neuron connects by upper motor neuron which originates from the brain.

The major function of these neurons is to send commands for movement of smooth muscles to help to do functions like speaking, breathing, swallowing , move.

sensory neurons carry impulses from sensory organs to the central nervous system , which differs from the motor neuron.

Therefore motor neurons carry signals from the brain and spinal cord to the muscles

Learn more about neurons :

https://brainly.com/question/1515004

📚 Related Questions

• Should we put animals to unnatural and painful situations in the pursuit of knowledge
• Which act addresses abandoned hazardous waste sites with significant contamination?
• When the organisms of a particular population seek habitats that are unevenly spaced, the distribution of the individuals in the population is?

The answer is Clumped if that's one of the options.

Explanation:

When assessing orientation, the nurse completes the assessment by asking which questions? select all that apply. "can you tell me where you are?" "would you count from 1 to 10 backward, please?" "what is your name?" "what day of the week is it?" "what did you eat for breakfast today?" submit answer?

Questions like "can you tell me where you are?, what is your name? what day of the week is it?, and what did you eat for breakfast today?" should be included. They are all significant in determining   a patient’s  cognitive status.

Details : When assessing orientation, the nurse completes the assessment by

• What features or variables of the plant leaves might affect the rate of photosynthesis?

The features that might affect how leaves process and perform rate of photosynthesis is determined by its surface area, chlorophyll and stomata.

The surface area of the leaf influences its performance to photosynthesis through the amount of light captured by it which is needed for photosynthesis to occur. As well chlorophyll which enables the leaf to absorb light and be processed. Lastly, stomata which is responsible for carbon dioxide and gas exchange.

The outer layer of cells that will form the structures that will provide protection and nourishment for the newly formed organism is the ____.

Which form of birth control protects 100 percent against pregnancy and sexually transmitted infections?

Which of the choices below lists the successive stages in coal formation from first step to last?

The five successive stages of coal formation are as follows:

step 1: plants decay

step 2: peat is formed

step 3: peat changes to lignite (soft brown coal made from peat)

step 4: bituminous coal is formed (black and brittle and very polluting)

step 5: anthracite coal is formed (highest amount of carbon and clean burning)

Details : Which of the choices below lists the successive stages in coal formation

The myocardium is thickest in the ________ ventricle because it must work the hardest to pump blood to the entire body.

What is the difference between scientific knowledge and knowledge acquired "unconsciously"?

The activity that puts the most continuous pressure on the lumbar region of your lower back is:

The highest pressure that can be felt on the lumbar region of the spine is when you are in a seated position .There is one more condition when maximum pressure can be felt and that is leaning in forward position with a heavy weight.To avoid this one should not hold weight in this position.In this case the weight should be hold closer to the body to reduce the pressure on the disc and spine.

A sitting posture is the activity that puts the most continuous pressure on the lumbar region of your lower back.

Generally, the highest pressure that can be felt on the lumbar region of the spine is when you are in a seated position .

However, not holding weight in the sitting position helps to prevent continuous pressure on the lumbar region of your lower back.

In conclusion, the sitting posture is the activity that puts the most continuous pressure on the lumbar region of your lower back.

Read more about lumbar region

brainly.com/question/17624205

The branching neuron structures that receive neural impulses from other neurons and convey them toward the cell body are called _____.

Details : The branching neuron structures that receive neural impulses from

Using data from the table above, select the best explanation for why that cell will be able to eliminate waste most efficiently?\

The commonly used acronym for the condition of joint pain due to wearing down of the articular cartilage, sometimes resulting in bone-on-bone contact, is __________.

The __________ is the long thin structure which carries the electricity generated by neurons.

Study an ecosystem of your choice, such as a meadow, a patch of forest, a garden, or an area of wetland. determine and list five major plant species and five major animal species in your ecosystem

Details : Study an ecosystem of your choice, such as a meadow, a patch of forest,

"some __________ neurons are specialized to detect stimuli, whereas __________ neurons send signals to the effectors of the nervous system."

Some afferent neurons are specialized to detect stimuli, whereas efferent neurons send signals to the effectors of the nervous system .

What are Neurons?

Neurons may be characterized as nerve cells that significantly sends or receives signal to and from the brain . These cells typically communicated with other cells of the body with the help of a junction which is known as a Synapse .

According to the physiology of the function of neurons, sensory neurons perceive as a set of information or messages from various sense organs like eyes, ears, nose, tongue, and skin and send them directly to the brain .

While the function of motor neurons is understood by the fact that these neurons send the perceived information and messages from the brain to the entire body at the target site .

Afferent neurons carry information from sensory receptors of the skin and other organs while efferent neurons carry motor information away from the central nervous system.

Therefore, some afferent neurons are specialized to detect stimuli, whereas efferent neurons send signals to the effectors of the nervous system .

To learn more about Afferent and efferent nerves , refer to the link:

https://brainly.com/question/27994849

Structures found within a eukaryotic cell and performing specific tasks are collectively termed _________

You see a high proportion of winged individuals in a population of the bean aphid. the most likely explanation is that

The most likely explanation of as to why there is a high proportion of winged individuals in a population of bean aphid was because there is a presence of high population density or another reason is that there is already a high population if before the aphids develop or has developed.

Solid fats have a high proportion of _____. glycerol saturated fatty acids unsaturated fatty acids fish and vegetable oils

this answer is saturated fatty acids

Details : Solid fats have a high proportion of _____. glycerol saturated fatty

What characteristics of bacteria make certain species pathogenic and hard to combat?

Neurons in virtually every brain area use these neurotransmitters to communicate with other neurons. they are

Neurons found in the center of the spinal cord that receive information from the sensory neurons and send commands to the muscles through the motor neurons are called ________.

The nurse is explaining possible side effects of corticosteroids to the caregiver of a child diagnosed with rheumatic fever. the caregiver comments, “i don't understand what hirsutism means.” the nurse would be correct in explaining that hirsutism is:

Details : The nurse is explaining possible side effects of corticosteroids

The nurse is conducting a prenatal class explaining the various activities which will occur within the first 4 hours after birth. the nurse determines the session is successful when the couples correctly choose which reason for the use of an antibiotic ointment?

The nurse is assigned to watch the cardiac monitors in the constant care unit and notes four different clients displaying arrhythmias. which arrhythmia is the nurse's highest priority

What does it mean to be unicellular

The unequal distribution of charges across the cell membrane results in the ______________.

Details : The unequal distribution of charges across the cell membrane results

Suppose you have a range of cells with numbers and a few blank entries. if you sort the range in ascending order (smallest to largest), where will the blank cells be located after sorting?

What are the important elements found in the human body and part of many organic compounds?

Other Questions

• The idea that many media products are digital files of ones and zeros sold in physical containers, is known as:
• When conducting a swot analysis, budgets, ratios, and sales reports can be used to identify:?
• Which organizational pattern argues that there is a direct relationship between two things?
• Prove that if the product ab is a square matrix, then the product ba is defined.
• When we use a particular measure to assess a psychological characteristic such as prejudice, that measure becomes?
• What will it cost to tile a kitchen floor that is 12 feet wide by 20 feet long if the tile cost $8.91 per square yard?
• Which global integration and coordination forces would mnes, such as mercedes-benz, encounter?
• When alissa awakened during a period of rem sleep, she reported experiencing intense fear. a pet scan most likely would reveal that her rem sleep was associated with increased activity in her?
• Which best describes the differences between the republican and democratic party platforms?
• Which colony had its charter revoked because of mismanagement, according to king william?
• When assessing orientation, the nurse completes the assessment by asking which questions? select all that apply. "can you tell me where you are?" "would you count from 1 to 10 backward, please?" "what is your name?" "what day of the week is it?" "what did you eat for breakfast today?" submit answer?
• Which party has recently emerged as a third-party alternative to the two major political parties?
• Is y=x-9 a quadratic equation
• Which statement is true regarding DNA? DNA
• Nine coins are placed in a 3x3 matrix with some face up and some face down. you can represent the state of the coins using a 3x3 matrix with values 0 (heads) and 1 (tails). here are some examples: 0 0 0 1 0 1 1 1 0 0 1 0 0 0 1 1 0 0 0 0 0 1 0 0 0 0 1 each state can also be represented using a binary number. for example, the preceding matrices correspond to the numbers: 000010000 101001100 110100001 there are a total of 512 possibilities, so you can use decimal numbers 0, 1, 2, 3,...,511 to represent all the states of the matrix. write a program that prompts the user to enter a number between 0 and 511 and displays the corresponding matrix with the characters h and t.
• The area of greenland is 840 000 square miles. what is this number in scientific notation

• International edition
• Australia edition
• Europe edition

Parkinson’s patient able to walk 6km without problems after spinal implant

Marc, 63, had freezing of gait before becoming first with advanced Parkinson’s to be fitted with device restoring normal signalling

The first patient to receive a spinal implant to treat advanced Parkinson’s disease has described experiencing “a rebirth” after the treatment allowed him to walk again without falling over.

Marc, 63, from Bordeaux, France, was diagnosed with the degenerative disease more than 20 years ago and had developed severe mobility problems, including balance impairments and freezing of gait. After receiving the implant, which aims to restore normal signalling to the leg muscles from the spine, he has been able to walk more normally and regained his independence.

“I practically could not walk any more without falling frequently, several times a day. In some situations, such as entering a lift, I’d trample on the spot, as though I was frozen there, you might say,” he said. “Right now, I’m not even afraid of the stairs any more. Every Sunday I go to the lake, and I walk around 6 kilometres [3.7 miles]. It’s incredible.”

The implant is yet to be tested in a full clinical trial. But the Swiss team, who have a longstanding programme to develop brain-machine interfaces to overcome paralysis , hope that their technology could offer an entirely new approach to treating movement deficits in those with Parkinson’s disease.

“It is impressive to see how by electrically stimulating the spinal cord in a targeted manner, in the same way as we have done with paraplegic patients, we can correct walking disorders caused by Parkinson’s disease,” said Jocelyne Bloch, neurosurgeon and professor at the CHUV Lausanne University hospital, who co-led the work.

Parkinson’s disease is caused by the progressive loss of dopamine-producing neurons. For about 90% of patients with advanced illness, this leads to difficulties with walking, including balance deficits and freezing of gait. Conventional treatments, such as the drug Levodopa, can improve symptoms but are unable to completely restore normal movement. The implant aims to overcome this by directly targeting the spinal area responsible for activating leg muscles during walking.

First, the team developed a personalised anatomical map of Marc’s spinal cord that identified the precise locations that were involved in signalling to the leg to move. Electrodes were then implanted at these locations, allowing stimulation to be delivered directly into the spine.

The patient wears a movement sensor on each leg and when walking is initiated the implant automatically switches on and begins delivering pulses of stimulation to the spinal neurons. The aim is to correct abnormal signals that are sent from the brain, down the spine, to the legs in order to restore normal movement. “At no point is [the patient] controlled by the machine,” said Prof Eduardo Martin Moraud, of Lausanne University hospital. “It’s just enhancing his capacity to walk.”

The study, published in Nature Medicine , found that the implant improved walking and balance deficits and when Marc’s walking was analysed it more closely resembled that of healthy controls than that of other Parkinson’s patients. Marc also reported significant improvements in his quality of life.

The authors said a full clinical trial was needed to demonstrate clinical efficacy and have enrolled a further six patients to assess whether the apparent benefits are replicated. “At this stage it’s a proof of concept,” said Prof Grégoire Courtine, a neuroscientist at EPFL, who co-led the work. “Of course it’s not tomorrow, it will be at least five years of development and testing.”

Prof Karunesh Ganguly, a neurologist at University of California San Francisco, who was not involved in the work, said: “This study describes a new approach for modulating the spinal cord in order to improve gait in Parkinson’s disease [and the] treatment can also potentially address freezing of gait, which is currently hard to treat. It will be exciting to see how this generalises to a larger population of patients.”

• Parkinson's disease
• Neuroscience
• Medical research

Most viewed

Enter search terms to find related medical topics, multimedia and more.

Advanced Search:

• Use “ “ for exact phrases.
• For example: “pediatric abdominal pain”
• Use – to remove results with certain keywords.
• For example: abdominal pain -pediatric
• Use OR to account for alternate keywords.
• For example: teenager OR adolescent

Spinal nerves carry nerve impulses to and from the spinal cord through two nerve roots:

Motor (anterior) root: Located toward the front, this root carries impulses from the spinal cord to muscles to stimulate muscle movement.

Sensory (posterior) root: Located toward the back, this root carries sensory information about touch, position, pain, and temperature from the body to the spinal cord.

In the center of the spinal cord, a butterfly-shaped area of gray matter helps relay impulses to and from spinal nerves. The "wings" are called horns.

Motor (anterior) horns: These horns contain nerve cells that carry signals from the brain or spinal cord through the motor root to muscles.

Posterior (sensory) horns: These horns contain nerve cells that receive signals about pain, temperature, and other sensory information through the sensory root from nerve cells outside the spinal cord.

Impulses travel up (to the brain) or down (from the brain) the spinal cord through distinct pathways (tracts). Each tract carries a different type of nerve signal either going to or from the brain. The following are examples:

Lateral spinothalamic tract: Signals about pain and temperature, received by the sensory horn, travel through this tract to the brain.

Dorsal columns: Signals about the position of the arms and legs travel through the dorsal columns to the brain.

Corticospinal tracts: Signals to move a muscle travel from the brain through these tracts to the motor horn, which routes them to the muscle.

• IN THIS TOPIC

Enter search terms to find related medical topics, multimedia and more.

Advanced Search:

• Use “ “ for exact phrases.
• For example: “pediatric abdominal pain”
• Use – to remove results with certain keywords.
• For example: abdominal pain -pediatric
• Use OR to account for alternate keywords.
• For example: teenager OR adolescent

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.

• 3D Models (0)
• Calculators (0)

• Lab Test (0)

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.

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

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 Introduction to Muscular Dystrophies and Related Disorders Muscular dystrophies are a group of inherited muscle disorders in which one or more genes needed for normal muscle structure and function are defective, leading to muscle weakness of varying... read more

Amyotrophic lateral sclerosis (ALS) Amyotrophic Lateral Sclerosis (ALS) Your muscles move when your brain sends a signal to them through your nerves. Motor neurons are the nerves that send your muscles the signals to move. Signals travel from the brain through your... read more

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

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

Was This Page Helpful?

Test your knowledge

Brought to you by Merck & Co, Inc., Rahway, NJ, USA (known as MSD outside the US and Canada)—dedicated to using leading-edge science to save and improve lives around the world. Learn more about the Merck Manuals and our commitment to Global Medical Knowledge .

• Permissions
• Cookie Settings
• Terms of use
• Veterinary Edition

• IN THIS TOPIC