Nerve

A nerve is a bundle of axons in the peripheral nervous system, along with supporting connective tissue and blood vessels, that transmits electrical impulses between the central nervous system and peripheral targets. A typical nerve contains axons of many neurons surrounded by three connective tissue layers: the endoneurium around each axon, the perineurium around fascicles, and the epineurium encasing the whole nerve. Axons may be myelinated by Schwann cells, which enables faster conduction through saltatory conduction at nodes of Ranvier. The vasa nervorum supply the nerve with blood.

Nerves are classified by the fibers they carry: sensory (afferent), motor (efferent), or mixed. Spinal nerves and cranial nerves are examples of peripheral nerves. In the central nervous system, signals are transmitted by tracts rather than nerves, so the term nerve is generally reserved for peripheral pathways. Conduction velocity depends on fiber diameter and myelination; larger, myelinated fibers conduct more rapidly than small or unmyelinated ones.

Nerve injury and disease affect function in the target region. In the peripheral nervous system, Wallerian degeneration occurs distal to the injury, and axons can regenerate to some extent under the guidance of Schwann cells, though regrowth is slow and may be imperfect. Diagnosis often involves nerve conduction studies and electromyography. Common clinical issues include compression neuropathies (for example, carpal tunnel syndrome) and traumatic nerve injuries, which can cause motor and sensory deficits in the supplied areas.