Principles of Stimulus Coding 9 



of the only effective trigger for an action potential — membrane 

 depolarization. The only known way to initiate a nervous 

 impulse is to pass an outward current across an inactive region of 

 cell membrane and thereby depolarize it below its threshold for 

 excitation. As soon as this occurs, sodium activation is triggered, 

 and the explosive increase in membrane permeability to this ion 

 is the result. Now, the dense inward current which occurs during 

 the rising phase of the action potential spreads longitudinally 

 within the axon from the active region and recrosses the mem- 

 brane (probably as an outward flow of potassium ions) in adjacent 

 areas. Momentarily, the resting capacitative charge on the 

 membrane is reduced by this outward current, and this de- 

 polarization triggers the increase in sodium conductance in the 

 new region of membrane. Thus, the currents which are generated 

 by the impulse itself are responsible for its renewed appearance 

 farther along the nerve. Now, action currents probably spread in 

 both directions along the axon from their immediate point of 

 entry. In the region of the nerve through which the action 

 potential has just passed, however, a refractory state of the mem- 

 brane is encountered — perhaps due to persistent sodium inactiva- 

 tion — and the currents fail to trigger another impulse at that locus. 

 By the time the membrane has recovered sufficiently to support 

 another impulse, the first is a considerable distance away, and the 

 fraction of its current crossing the earlier locus of activity is small 

 indeed. 



Action potentials, therefore, appear to constitute just the type 

 of coupled, time-dependent mechanisms which would be free 

 from distortion, except at relatively high frequencies. The energy 

 for such mechanisms derives from the existence of the metabolic- 

 ally maintained ionic and potential gradients across the neuronal 

 membrane. The time constants of the membrane events con- 

 trolling the permeability to sodium and potassium ions determine 

 the amplitude and temporal characteristics of the action potential; 

 it is the number and frequency of these transients which con- 

 stitute the meaningful code of the transmitted signal. 



An investigation of the quantitative relationships between the 



magnitude of the applied stimulus and the frequency of the 



consequent nervous discharge could not be examined rigorously 



until means were found for recording the electrical activity of 



S.O.— B 



