Overview

A nerve is a cordlike structure made up of many specialized nerve cells working together. Each individual cell is a specialized nerve cell, commonly called a neuron. Nerves collect and transmit electrical signals — often described as messages — that allow organs, muscles and sensory receptors to communicate with the central nervous system.

Structure and components

In the peripheral nervous system a typical nerve contains bundles of axons (the long processes of neurons) grouped into fascicles. These fascicles are wrapped by connective tissue layers: the endoneurium surrounding single axons, the perineurium around fascicles, and the epineurium around the whole nerve. Many axons are surrounded by myelin, a fatty insulating sheath formed by Schwann cells in peripheral nerves; myelin increases conduction speed. Nerves also carry small blood vessels that supply oxygen and nutrients.

Types and directions of signaling

Nerves are commonly classified by the direction and type of signals they carry. Major categories include:

  • Sensory (afferent) nerves that bring information from receptors toward the central nervous system.
  • Motor (efferent) nerves that carry commands from the CNS to muscles and glands.
  • Mixed nerves that contain both sensory and motor fibers and are common in limbs.
  • Autonomic nerves that control involuntary functions such as heart rate, digestion and blood vessel tone.

Relationship with the central and peripheral systems

The nervous system is organized into the central nervous structures and the peripheral network. The central division — the central nervous system (see the acronym) — comprises the brain and the spinal cord. Nerves that branch away from the spinal cord and reach the rest of the body form the peripheral nervous system. Together, these components constitute the overall nervous system, enabling sensation, movement, reflexes and higher functions such as thought.

Development, history, and distinctions

Historically, studies of nerves established basic principles about directionality of function (sensory versus motor) and about electrical nature of signaling. In anatomy, it is important to distinguish between a nerve — a peripheral bundle of axons — and a tract, which is a comparable bundle inside the CNS. Similarly, a single neuron is an individual cell, while a nerve is the macroscopic cable made from many axons and supportive tissues.

Clinical importance and examples

Nerve health is central to movement, sensation and autonomic control. Common clinical issues include compression injuries (such as carpal tunnel affecting the median nerve), traumatic transection, diabetic and toxic neuropathies, and inflammatory conditions. Peripheral nerves have some ability to regenerate when damaged; this capacity is limited in central pathways. Physicians evaluate nerves with physical examination (reflexes, strength, sensation) and specialized tests such as nerve conduction studies and electromyography. Notable named nerves include the sciatic nerve in the leg and the median and ulnar nerves in the arm, each with distinct functional territories and clinical consequences when injured.