Principles of Stimulus Coding 7 



than to sodium, and when this permeability differential is con- 

 sidered with the metabolically maintained separation of sodium 

 and potassium ions on different sides of the membrane, the result 

 is a concentration of potassium forty times greater inside the 

 membrane than in the external solution, and an internal sodium 

 concentration only one-tenth that of its external concentration. 

 Now, since the membrane has at all times a finite permeability to 

 potassium ions, this species tends to diffuse outward across the 

 cell membrane. The associated large organic anions are unable to 

 follow, and the result is an electrostatic charge which makes the 

 inside of the membrane negative to the external environment of 

 the cell. This potential gradient constitutes the so-called resting 

 potential of the cell membrane; its magnitude can be theoretically 

 calculated from the following equation of Nernst: 



E=^RTlF\n{KJK;) 



where E is the potential difference in millivolts, T the absolute 

 temperature, R the gas constant, F the Faraday constant, and 

 K^ and K^ are the concentration of potassium ions on the outside 

 and the inside of the membrane respectively. In most nerve cells 

 this relationship only approximates the electrical and ionic con- 

 centration conditions which exist across the membrane. The 

 reasons for this departure from predicted theory are several and 

 not entirely understood. One fairly obvious possibility is that 

 the discrepancy results from the finite permeability of the mem- 

 brane to sodium ions, so that this ion species continuously diffuses 

 inward. The resulting potential would thus tend to reduce 

 substantially that created by the outward diffusion of potassium. 

 In addition, the permeability characteristics of the membrane are 

 not constant and, as will be discussed below, are rather dependent 

 upon the voltage gradient across the neuronal membrane. None 

 the less, most nerve cells in the resting state exhibit a steady 

 potential difference of about 75 millivolts across the membrane, 

 the inside being negative to the outside. 



It will now be useful to examine in greater detail the sequential 

 permeability changes which occur during the action potential. 

 As mentioned above, the earliest event in the sequence is a 

 marked increase in membrane permeability toward sodium ions. 

 Now, an electrical, as well as a concentration, gradient occurs 



