Overview of the Peripheral Nervous System: Peripheral Nerve Disorders: Merck Manual Home Edition

Brain, Spinal Cord, and Nerve Disorders

Peripheral Nerve Disorders

Overview of the Peripheral Nervous System

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Overview of 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 head, face, eyes, nose, muscles, and ears to the brain (cranial nerves—see Cranial Nerve Disorders: 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

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 Multiple Sclerosis (MS) and Related Disorders:Overview of Demyelinating Disorders)

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

Dysfunction of peripheral nerves may also result from damage to

A spinal nerve root (the part of the spinal nerve next 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. Some peripheral nerve disorders are progressive and fatal.

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.