A nerve is an enclosed, cable-like bundle of axons (nerve fibers, the long and slender projections of neurons) in the peripheral nervous system. A nerve provides a common path for the electrochemical nerve impulses that are transported along each of the axons towards peripheral organs.
In the central nervous system, the analogous structures are known as tracts. Neurons are sometimes called nerve cells, though this term is possibly misleading since lots of neurons do not form nerves and nerves also consist of non-neuronal Schwann cells that envelop the axons in myelin.
Each nerve is a cordlike structure containing bundles of axons. Inside a nerve, each axon is enveloped by a layer of connective tissue referred to as the endoneurium. The axons are bundled together into groups known as fascicles, and each fascicle is covered in a coating of connective tissue called the perineurium. Finally, the whole nerve is wrapped in a layer of connective tissue called the epineurium.
Nerves are categorized into three groups based upon the direction that signals are conducted:
- Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin.
- Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands.
- Mixed nerves consist of both afferent as well as efferent axons, and thus manage both incoming sensory info and outbound muscle commands in the very same bundle.
- Nerves can be classified into two groups based on where they attach to the central nervous system:
- Spinal nerves innervate (disperse to/stimulate) much of the body, and attach via the vertebral column to the spinal cord and therefore to the central nervous system. They are given letter-number denominations according to the vertebra through which they connect to the spinal column.
- Cranial nerves innervate portions of the head, and connect straight to the brain (especially to the brainstem). They are typically designated Roman characters from 1 to 12, although cranial nerve zero is often included. In addition, cranial nerves have descriptive names.