Nerves

Nerves are bundles of axons in the peripheral nervous system that transmit information between the central nervous system and the body. Each nerve contains nerve fibers enclosed by connective tissue layers: endoneurium around individual fibers, perineurium around fascicles, and epineurium around the whole nerve. In the PNS, most axons are myelinated by Schwann cells, and gaps between myelin segments, known as nodes of Ranvier, enable saltatory conduction and faster signal transmission.

Nerves can be sensory (afferent), motor (efferent), or mixed. They are organized into cranial and spinal nerves, with spinal nerves forming plexuses such as the cervical, brachial, and lumbosacral plexuses, from which peripheral nerves arise to innervate regions and organs.

Functionally, nerves transmit electrical impulses that convey sensory information or initiate motor activity. Conduction velocity depends on axon diameter and myelination. Nerves require a blood supply via the vasa nervorum to maintain function and support repair after injury.

Injury can cause axonal damage or demyelination. In the peripheral nervous system, distal degeneration (Wallerian degeneration) is followed by potential regeneration guided by Schwann cells, though regrowth is slow and often incomplete. Central nervous system injuries regenerate poorly. Clinically, nerve damage may present with numbness, weakness, or neuropathic pain and may require imaging, decompression, or surgical repair.