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Overview of the Peripheral Nervous System
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 page 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
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: Insulating a Nerve Fiber)
Peripheral nerve disorders can affect one nerve (mononeuropathy—see page Mononeuropathy), two or more peripheral nerves in separate areas of the body (multiple mononeuropathy—see page Multiple Mononeuropathy), or many nerves throughout the body but usually in about the same areas on both sides of the body (polyneuropathy—see page Polyneuropathy).
Dysfunction of peripheral nerves may also result from damage to
A spinal nerve root (the part of the spinal nerve connected to the spinal cord—see page Overview of Spinal Cord Disorders)
A plexus (a network of nerve fibers, where fibers are sorted and recombined to serve a particular area of the body—see page Plexus Disorders)
The neuromuscular junction (where the nerve and muscle connect—see page Overview of Neuromuscular Junction Disorders)
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
To diagnose a peripheral nerve disorder, doctors ask people to describe their symptoms, including
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 page 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 page 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 page Hereditary Neuropathies).
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 page Advance Directives).
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