Which of the following is a true statement about the peripheral nervous system?

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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 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

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)

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. The impulses from the axon cross a synapse (the junction between two nerve cells) to the dendrite of another cell.

Each large axon is surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one.

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 nerve signals (electrical impulses) to be conducted along the nerve fiber with speed and accuracy. When the myelin sheath is damaged (called demyelination), nerves do not conduct electrical impulses normally.

Peripheral nerve disorders can affect

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.

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

Certain disorders cause progressive deterioration of the nerve cells in the spinal cord and brain that control muscle movement (motor neuron diseases Amyotrophic Lateral Sclerosis (ALS) and Other Motor Neuron Diseases (MNDs) Motor neuron diseases are characterized by progressive deterioration of the nerve cells that initiate muscle movement. As a result, the muscles stimulated by these nerves deteriorate, become... read more ) as well as in the peripheral nerves. Motor neuron diseases can resemble peripheral nerve disorders, which affect nerve cells outside the brain and spinal cord rather than those in the spinal cord or brain. Motor neuron diseases may be caused by viruses (such as the polio Polio Polio is a highly contagious, sometimes fatal enterovirus infection that affects nerves and can cause permanent muscle weakness, paralysis, and other symptoms. Polio is caused by a virus and... read more virus), be inherited, or have no clear known cause (such as amyotrophic lateral sclerosis Amyotrophic Lateral Sclerosis (ALS) and Other Motor Neuron Diseases (MNDs) Motor neuron diseases are characterized by progressive deterioration of the nerve cells that initiate muscle movement. As a result, the muscles stimulated by these nerves deteriorate, become... read more ).

Novichok was developed in Russia and has been used in assassination attempts. Curare has been used to help relax muscles during surgery and to paralyze and kill when placed on the tip of poison darts.

Disorders that affect muscle rather than nerves (as peripheral nerve disorders do) also cause muscle weakness. Muscle disorders may be categorized as

• Hereditary, such as Duchenne muscular dystrophy, Becker muscular dystrophy Duchenne Muscular Dystrophy and Becker Muscular Dystrophy 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 , familial periodic paralysis Familial Periodic Paralysis Familial periodic paralysis is a rare inherited disorder that causes sudden attacks of weakness and paralysis. There are four different forms, which involve abnormalities in how electrolytes... read more , limb-girdle muscular dystrophy Limb-Girdle Dystrophy Limb-girdle dystrophy encompasses a group of muscular dystrophies that can be inherited in various ways. The muscles of the shoulder or pelvis are affected. Muscular dystrophies are a group... read more , myotonia congenita Myotonia Congenita Myotonia congenita is an inherited disorder that causes muscle stiffness and muscle enlargement. It affects what most people think of as muscle, the type that can be contracted to move the various... read more (Thomsen disease), and myotonic dystrophy Myotonic Dystrophy Myotonic dystrophy is a rare muscular dystrophy. This disorder affects the ability to relax the muscles at will. Myotonia refers to delayed relaxation after muscle contraction, which can cause... read more (Steinert disease)

Doctors do tests to determine whether the cause of weakness is a muscle, a neuromuscular junction, or a nerve disorder.

• 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.

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

Tests may include the following:

• 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)

• 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

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