Merck Manual

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Overview of the Peripheral Nervous System


Michael Rubin

, MDCM, New York Presbyterian Hospital-Cornell Medical Center

Last full review/revision Oct 2019| Content last modified Oct 2019
Click here for the Professional Version
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The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord.

Thus, the peripheral nervous system includes

  • The nerves that connect the head, face, eyes, nose, muscles, and ears to the brain (cranial nerves)

  • The nerves that connect the spinal cord to the rest of the body, including the 31 pairs of spinal nerves

  • More than 100 billion nerve cells that run throughout the body

Using the Brain to Move a Muscle

Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate in the brain, as when a person consciously decides to move a muscle—for example, to pick up a book.

Or the impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to sense pain, as when they step on a sharp rock, or to sense temperature, as when they pick up a very hot cup of coffee. This information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways:

  • The sensory nerve pathway to the brain

  • The motor nerve pathway to the muscle

Using the Brain to Move a Muscle
  • If sensory receptors in the skin detect pain or a change in temperature, they transmit an impulse (signal), which ultimately reaches the brain.

  • The impulse travels along a sensory nerve to the spinal cord.

  • The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord.

  • The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord.

  • The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information, located deep in the brain.

  • The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors).

  • The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse.

  • The nerve carrying the impulse crosses to the opposite side at the base of the brain.

  • The impulse is sent down the spinal cord.

  • The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord.

  • The impulse travels out of the spinal cord along the length of the motor nerve.

  • At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where the impulse stimulates the muscle to move.

Dysfunction of peripheral nerves may result from damage to any part of the nerve:

  • Axon (the part that sends messages)

  • Body of the nerve cell

  • Myelin sheath (the membranes that surround the axon and that function much like insulation around electrical wires, enabling nerve impulses to travel quickly)

Damage to the myelin sheath is called demyelination, as occurs in multiple sclerosis.

Typical Structure of a Nerve Cell

A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. Each large axon is wrapped in layers of a fat called myelin.

Typical Structure of a Nerve Cell

Insulating a Nerve Fiber

Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation around an electrical wire, the myelin sheath enables electrical impulses to be conducted along the nerve fiber rapidly.

When the myelin sheath is damaged, nerves do not conduct electrical impulses normally.

Insulating a Nerve Fiber

Peripheral nerve disorders can affect

  • One nerve (mononeuropathy)

  • Two or more peripheral nerves in separate areas of the body (multiple mononeuropathy)

  • Many nerves throughout the body but usually in about the same areas on both sides of the body (polyneuropathy)

  • A spinal nerve root (the part of the spinal nerve connected to the spinal cord)

  • A plexus (a network of nerve fibers, where fibers are sorted and recombined to serve a particular area of the body)

  • The neuromuscular junction (where the nerve and muscle connect)

If motor nerves (which control muscle movement) are damaged, muscles may weaken or become paralyzed. If sensory nerves (which carry sensory information—about such things as pain, temperature, and vibration) are damaged, abnormal sensations may be felt or sensation may be lost.

Motor nerve (neuron) disorders involve deterioration of the nerves that control muscle movement. These disorders are rare, but they are progressive and often fatal.


Peripheral nerve disorders can be hereditary or acquired (caused by exposure to toxins, injury, infections, or metabolic or inflammatory disorders).


Some Causes of Peripheral Nerve Disorders



Motor neuron diseases


Acquired diseases that begin suddenly

Polio, infections due to coxsackievirus or other enteroviruses (rare), and West Nile virus infection

Acquired diseases that are chronic

Nerve root disorders


Neurofibromas (soft, fleshy growths of nerve tissue)


A herniated disk, infections, injuries, metastatic cancer, osteoarthritis, and rheumatoid arthritis

Plexus disorders


Acute brachial neuritis, autoimmune disorders, damage during delivery in newborns, diabetes mellitus, a hematoma (a pocket of blood), severe injuries (such as those due to high-speed motor vehicle accidents), metastatic cancer, neurofibromatosis (rare), and nerve tumors

Peripheral nerve disorders


Hereditary neuropathies (such as Charcot-Marie-Tooth disease)


In developing countries, diphtheria, leprosy, and parasite infections


Ischemic (due to blockage of the blood supply)

Vasculitis (inflammation of blood vessels)


Amyloidosis, diabetes mellitus, vitamin B deficiencies, undernutrition due to chronic excess consumption of alcohol, and kidney failure

Pressure-related (called nerve entrapment syndromes)


Arsenic, lead, and mercury

Neuromuscular junction disorders


Botulism in infants, Eaton-Lambert syndrome, myasthenia gravis, and dysfunction caused by exposure to insecticides or nerve gas (such as sarin) or by use of certain drugs

* Amyotrophic lateral sclerosis affects nerves in the brain as well as those in the spinal cord.

Disorders that may resemble peripheral nerve disorders

Neuromuscular junction disorders are distinct from peripheral nerve disorders, although they may have similar consequences such as muscle weakness. The neuromuscular junction is where the ends of peripheral nerve fibers connect to special sites on a muscle’s membrane. The nerve fibers release a chemical messenger (neurotransmitter) that sends a nerve impulse across the neuromuscular junction and signals a muscle to contract. Neuromuscular junction disorders include infant botulism, Eaton-Lambert syndrome, myasthenia gravis, and dysfunction caused by insecticides, nerve gas (such as sarin), or certain drugs.

Disorders that affect muscle rather than nerves, like peripheral nerve disorders cause muscle weakness. Doctors do tests to determine whether the cause of weakness is a muscle or a nerve disorder. Muscle disorders may be categorized as


  • A doctor's evaluation

  • Possibly electromyography and nerve conduction studies, imaging tests, or a biopsy

  • For a suspected hereditary neuropathy, genetic testing

To diagnose a peripheral nerve disorder, doctors ask people to describe their symptoms, including

  • When the symptoms started

  • Which symptoms appeared first

  • How the symptoms have changed over time

  • Which body parts are affected

  • What relieves and what worsens the symptoms

Doctors also ask about possible causes, such as whether people have had any infections or other disorders, whether they may have been exposed to toxins, and whether any family members have had similar symptoms. This information gives doctors clues about the cause of symptoms.

A thorough physical and neurologic examination can help doctors identify the cause. They evaluate the following:

What doctors find during the examination may suggest possible causes and the tests that need to be done.

Tests may include the following:

  • Electromyography and nerve conduction studies to help doctors determine whether the problem is in the nerves or the muscles

  • Imaging tests to check for abnormalities (such as tumors) affecting the cranial nerves or the spinal cord and to rule out other causes of the symptoms

  • A biopsy of muscle and nerve to identify the type of problem (such as whether nerves are demyelinated or inflamed)

  • Genetic testing (blood tests to detect the abnormal gene) if doctors suspect a hereditary neuropathy


  • Treatment of the cause when possible

  • Symptom relief

  • Possibly physical, occupational, and speech and language therapy

If a disorder is causing symptoms, it is treated if possible. Otherwise, doctors focus on relieving symptoms.

Care provided by a team of several types of health care practitioners (a multidisciplinary team) can help people cope with progressive disability. The team may include

  • Physical therapists to help people continue to use their muscles

  • Occupational therapists to recommend assistive devices that can help people do their daily activities (such as devices to help with walking)

  • Speech and language therapists to help people communicate

  • Specialists to help with specific problems, such as difficulty swallowing or breathing

If a peripheral nerve disorder reduces lifespan, the person, family members, and caregivers must talk frankly with health care practitioners about health care decisions in case the person becomes unable to make health care decisions. The best approach is to prepare a legal document that states the person’s wishes about health care decisions (called advance directives) in case the person becomes unable to make health care decisions.

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