November 15, 2007 by TMO
A nerve is an enclosed, cable-like bundle of axons (the long, slender projection of a neuron). Neurons are sometimes called nerve cells, though actually many neurons do not form nerves, and nerves also include the non-axon glial cells that ensheath the axons in myelin.
Nerves are part of the peripheral nervous system.
Afferent nerves convey sensory signals to the central nervous system, for example from skin or organs, while efferent nerves conduct stimulatory signals from the central nervous system to the muscles and glands. Afferent and efferent axons are often arranged together, forming mixed nerves. For example, the median nerve controls motor and sensory function in the hand.
Each peripheral nerve is covered externally by a dense sheath of connective tissue, the epineurium.
Underlying this is a layer of flat cells forming a complete sleeve, the perineurium. Perineurial septa extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurial sheath. This forms an unbroken tube which extends from the surface of the spinal cord to the level at which the axon synapses with its muscle fibres or ends in sensory receptors. The endoneurial sheath consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres. Peripheral nerves are richly supplied with blood.
Most nerves connect to the central nervous system through the spinal cord. The twelve cranial nerves, however, connect directly to parts of the brain.
Spinal nerves are given letter-number combinations according to the vertebra through which they connect to the spinal column. Cranial nerves are assigned numbers, usually expressed as Roman numerals from I to XII. In addition, most major nerves have descriptive names. Inside the central nervous system, distinguishable bundles of axons are termed tracts rather than nerves.
The signals that nerves carry, sometimes called nerve impulses, are also known as action potentials. These are rapidly (up to 120 m/s) travelling electrical waves. Motor neurons innervate or activate muscles groups.