6 The Physiology of Sense Organs 



The form and duration of the individual pulses in the fre- 

 quency code have relatively little bearing upon the way the code 

 is read by the central nervous system. What is important is the 

 number of pulsed events and their frequency of occurrence, and 

 it is this information that must be transmitted in an undistorted 

 fashion. In all metazoan animals the currency unit of the code is 

 the propagated action potential, also called the nervous impulse. 

 This is the electrical trace of a brief series of events — actually 

 changes in the permeability characteristics of the nerve cell mem- 

 brane — ^which can be triggered in many regions of the cell by an 

 appropriate electrical stimulus, and which, once triggered, can 

 regenerate itself in adjacent membrane areas; thus, it can be 

 propagated to all parts of a cell. The membrane changes occur 

 in sequence, beginning with an increase in the permeability to 

 sodium ions (sodium activation) which rises to a peak value 

 within half a millisecond and is then shut off (sodium inactivation). 

 Shortly after the start of sodium activation a slowly-increasing 

 specific enhancement occurs in the membrane permeability to 

 potassium ions. As will be described below, both types of 

 permeability change are intimately involved in the production of 

 the potential changes associated with the nerve impulse. Here it 

 should be emphasized, however, that the potential changes are 

 not merely by-products of the alterations in membrane perme- 

 ability; the form and duration of the action potential is strongly 

 influenced by these electrical gradients, without which the perme- 

 ability changes could neither be triggered nor concluded. ^^ 



For both sodium and potassium ions there are concentration 

 gradients across the membrane of nerve cells, potassium being 

 more concentrated within the cell and sodium more concentrated 

 in the extracellular fluid. These gradients are maintained both 

 metabolically — by the selective active transport of sodium ions 

 outward across the cell membrane, and potassium ions inward — 

 and also by a Donnan type of electrochemical equilibrium. The 

 latter exists by virtue of the particular permeability properties of 

 the membrane which resists the outward diffusion of large organic 

 anions entrapped within the cytoplasm. The electrostatic attrac- 

 tion of these indiffusible negative groups thus contributes to the 

 maintainance of the high intracellular potassium level. At rest, 

 the nerve membrane is a good deal more permeable to potassium 



