Overview of the peripheral nervous system

[Monty]

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

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.
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.