nerves

nerves The word ‘nerve’ is used in a variety of ways. In classical anatomy, nerves are tough, whitish threads and cables that are found throughout most of the tissues of our body. They form great, tree-like networks that can be traced back to the spinal cord or brain. Like the trunks, branches, and twigs of a tree, the further away a nerve is from the central nervous system, the smaller it is, in general (from more than 1 cm, down to much less than 1 mm in diameter). The role of nerves in communicating information is obvious to anyone who has cut a nerve in an accident. Sensation may be lost in the skin and other tissues to which the severed nerve connects, or there may be spontaneous pain in the affected region; and if the damaged nerve is connected to muscles, they become weak or paralysed. Perhaps the best-known nerve is the sciatic nerve, which when inflamed or under pressure causes pain, tenderness, and weakness in the back of the leg (sciatica). All such nerves are part of the peripheral nervous system.

Such a peripheral nerve contains microscopic fibres called axons. Groups of a few axons are wrapped together in a delicate fibrous sheath, and bundles of such bundles are again enclosed in a fibrous sheath to form the whole nerve.

Axons are also known as nerve fibres. They are long processes (thin extensions from the main cell body) of neurons, the cells of the nervous system, which, to complicate things further, are also known as ‘nerve cells’. Axons conduct brief electrical pulses called ‘nerve impulses’ (technically, action potentials), which originate in the sensitive nerve endings in the skin or other tissues, or at the cell body of a neuron. Impulses convey sensory messages in these afferent fibres into the central nervous system, and motor commands in efferent fibres from central neurons out to muscles and glands throughout the body.

Axons conduct impulses at speeds from about 0.5–100 m/sec. The velocity increases with the diameter of the axon (typically between 0.1 and 2 μm) and is even further speeded up if the axon is covered (as many of them are) by insulating fatty material, called a myelin sheath. In general, axons run uninterrupted along the entire length of a peripheral nerve as well as some distance inside the central nervous system. Thus they can be extraordinarily long. For instance, sensory axons extend from the skin of the big toe to the base of the brain; and motor axons run from the motor cortex of the cerebral hemispheres right down to motor neurons at the bottom of the spinal cord.

Nerve cells share many basic features with cells elsewhere in the body. They have a cell body, with a nucleus containing genetic information, in the DNA of the chromosomes; and cytoplasm containing organelles, which are involved in metabolism and the production of specific proteins necessary for the proper functioning of the neuron; all covered with a thin membrane. They differ from other cells in having two types of fibre-like processes. The cell body is surrounded by a bush of up to ten or so dendrites (Greek dendron: wood or tree), but typically has only one axon, which may have many branches. Axons are specialized not only for the conduction of impulses but also for the secretion, at their tips, of chemical messengers that affect the cells that they contact (muscle or gland cells, or the dendrites or cell bodies of other neurons).

The junction between the terminal of an axon and the membrane of another neuron is called a synapse, with a very narrow synaptic cleft between the two. The axon terminal contains many tiny droplets called synaptic vesicles, close to the zone of contact. A chemical neurotransmitter (sometimes more than one kind) is stored in these vesicles and is released into the cleft when an action potential invades the terminal. This released transmitter then binds to receptors, specialized protein molecules in the membrane of the next cell, ultimately causing electrical excitation or inhibition of that cell. There are many different transmitters, but the most widespread excitatory ones are acetylcholine, noradrenaline, and glutamate; glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters.

Finally, the term ‘nerves’ has a colloquial usage, denoting anxiety or apprehension, or as an ‘explanation’ of unusual behaviour. While this adds further to the ambiguity of this over-used word, it is at least a popular recognition of the role of the nervous system in determining such ‘mental’ functions as the emotions and character.

Laurence Garey

See also grey matter; nervous system; white matter.