A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system.

A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system.

Each axon is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.

Nerves are categorized into three groups based on the direction that signals are conducted:

Nerves are categorized into three groups based on the direction that signals are conducted:

• Afferent nerves conduct sensory information from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin. Bundles of afferent fibers are known as sensory nerves.

• Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands. Bundles of these fibres are known as efferent nerves.

• Mixed nerves contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. All spinal nerves are mixed nerves, and some of the cranial nerves are also mixed nerves.

Nerves can be categorized into two groups based on where they connect to the central nervous system:

• Spinal nerves innervate (distribute to/stimulate) much of the body, and connect through the vertebral column to the spinal cord and thus to the central nervous system. They are given letter-number designations according to the vertebra through which they connect to the spinal column.

• Cranial nerves innervate parts of the head, and connect directly to the brain (especially to the brainstem). They are typically assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included. In addition, cranial nerves have descriptive names.