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Overview of the Peripheral Nervous System

by Michael Rubin, MDCM

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—see Overview of the 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 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 and the motor nerve pathway to the 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 sensory processing center 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, the impulse crosses from the motor nerve to the motor end plate on the muscle, where it 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—see Figure: )

Peripheral nerve disorders can affect one nerve (mononeuropathy—see Mononeuropathy), two or more peripheral nerves in separate areas of the body (multiple mononeuropathy—see Multiple Mononeuropathy), or many nerves throughout the body but usually in about the same areas on both sides of the body (polyneuropathy—see Polyneuropathy).

Dysfunction of peripheral nerves may also result from damage to

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. Some peripheral nerve disorders are progressive and 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

Type

Examples

Motor neuron diseases

Hereditary

Spinal muscular atrophy types I–IV

Acquired diseases that begin suddenly

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

Acquired diseases that are chronic

Amyotrophic lateral sclerosis (Lou Gehrig disease), primary lateral sclerosis, progressive pseudobulbar palsy, progressive muscular atrophy, paraneoplastic syndromes, postpolio syndrome, and progressive bulbar palsy

Nerve root disorders

Hereditary

Neurofibromas (soft, fleshy growths of nerve tissue)

Acquired

A herniated disk, infections, injuries, and metastatic cancer

Plexus disorders

Acquired

Acute brachial neuritis, 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

Hereditary adult-onset neuropathies, hereditary sensory and motor neuropathies (such as Charcot-Marie-Tooth disease), and hereditary sensory and autonomic neuropathies

Infectious

Hepatitis C, herpes zoster, HIV infection, Lyme disease, and syphilis

In developing countries, diphtheria, leprosy, and parasite infections

Inflammatory

Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy

Ischemic (due to blockage of the blood supply)

Vasculitis (inflammation of blood vessels)

Metabolic

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

Pressure-related (called nerve entrapment syndromes)

Carpal tunnel syndrome, cubital tunnel syndrome (ulnar nerve palsy or neuropathy), radial nerve palsy, peroneal nerve palsy, and tarsal tunnel syndrome

Toxins

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

Muscle disorders

Hereditary

Duchenne muscular dystrophy, Becker muscular dystrophy, familial periodic paralysis, limb-girdle muscular dystrophy, myotonia congenita (Thomsen disease), and myotonic dystrophy (Steinert disease)

Endocrine

Acromegaly (excessive growth due to overproduction of growth hormone), Cushing syndrome, diabetes mellitus, hyperthyroidism (an overactive thyroid gland), and hypothyroidism (an underactive thyroid gland)

Inflammatory

Infections (usually viral) and polymyositis and dermatomyositis

Metabolic

Lipid and glycogen storage diseases and hypokalemia (low potassium levels)

Diagnosis

  • 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 and 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 (see Physical Examination) can help doctors identify the cause. They evaluate sensation—whether people can feel stimuli normally or have any abnormal sensations such as tingling. They also evaluate muscle strength, reflexes, cranial nerves, and internal body processes (regulated by the autonomic nervous system).

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

Tests may include electromyography and nerve conduction studies (see Tests for Brain, Spinal Cord, and Nerve Disorders : Electromyography and Nerve Conduction Studies), imaging tests, and a biopsy of muscle and nerve. Genetic testing may be done if doctors suspect a hereditary neuropathy (see Hereditary Neuropathies).

Treatment

  • Treatment of the cause

  • 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. It 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

Because some peripheral nerve disorders reduce 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 (see Advance Directives).

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