A nerve is an enclosed, cable-like bundle of peripheral axons (the long, slender projections of neurons). A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system. In the central nervous system, the analogous structures are known as tracts.
Neurons are sometimes called nerve cells, though this term is technically inaccurate since many neurons do not form nerves, and nerves also include non-neuronal Schwann cells that coat the axons in myelin.
Nerves can also be categorized into two groups based on where they connect to the central nervous system:
Each nerve is covered externally by a dense sheath of connective tissue, the epineurium. Underlying this is a layer of flat cells, the perineurium, which forms a complete sleeve around a bundle of axons. Perineurial septae extend into the nerve and subdivide it into several bundles of fibers. Surrounding each such fibre is the endoneurium. 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 fibers, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibers. Nerves are bundled along with blood vessels, since the neurons of a nerve have fairly high energy requirements.
Within the endoneurium, the individual nerve fibers are surrounded by a low protein liquid called endoneurial fluid. The endoneurium has properties analogous to the blood-brain barrier, in that it prevents certain molecules from crossing from the blood into the endoneurial fluid. In this respect, endoneurial fluid is similar to cerebro-spinal fluid in the central nervous system. During the development of nerve edema from nerve irritation or injury, the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized using Magnetic resonance neurography, and thus MR neurography can identify nerve irritation and/or injury.
Nerves can be categorized into two groups based on function: