Excitation and Nerve Conduction

Abstract

Living cells are generally distinguished by the maintenance of an ionic composition in their cytoplasm which differs from that in the tissue fluid bathing them. The cytoplasm is rich in K⁺ ions and low in Na⁺, while in the surrounding milieu the reverse holds, and this imbalance is held with considerable stability by homeostatic mechanisms (see Chap. 11). This differential ionic distribution creates the possibility of developing a property called excitability which is found in many different biological systems, including unicellular animals and plants and the nervous systems of higher animals. A simple form of excitability is seen when a touch to the exterior of the animal gives rise to a resulting muscular movement, and indeed until the invention of the electrometer in the nineteenth century this was the only way in which the occurrence of excitation could be detected. Subsequently the production of action potentials, impulses of electrical activity, could be seen independent of muscular contraction.