nerve cell

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Elongated cell that transmits information rapidly between different parts of the body, the basic functional unit of the nervous system. Each nerve cell has a cell body, containing the nucleus, from which trail processes called dendrites, responsible for receiving incoming signals. The unit of information is the nerve impulse, a travelling wave of chemical and electrical changes involving the membrane of the nerve cell. The cell's longest process, the axon, carries impulses away from the cell body. The brain contains many nerve cells.

A reflex arc is a simple example of how nerve cells help to control and coordinate processes in the body. It involves the use of three types of nerve cell. These are the sensory nerve cell, the intermediate (relay) nerve cell and the motor nerve cell. Where each nerve cell connects with the next there is a tiny gap called a synapse. Impulses can cross this gap.

The impulse involves the passage of sodium and potassium ions across the nerve-cell membrane. Sequential changes in the permeability of the membrane to positive sodium (Na⁺) ions and potassium (K⁺) ions produce electrical signals called action potentials. Impulses are received by the cell body and passed, as a pulse of electric charge, along the axon. The axon terminates at the synapse, a specialized area closely linked to the next cell (which may be another nerve cell or a specialized effector cell such as a muscle). On reaching the synapse, the impulse releases a chemical neurotransmitter, which diffuses across to the neighbouring cell and there stimulates another impulse or the action of the effector cell.

Nerve impulses travel quickly – in humans, they may reach speeds of 160 m/525 ft per second, although many are much slower.