The Depolarizing Nature of the Trigger 21 



during anodal stimulation. It is important to note that the cathod- 

 ally-evoked activity could be controlled over a limited range of 

 amplitude and duration by variations in stimulus strength, 

 although the relationship was found to be by no means a linear one. 

 Nevertheless, the implication was quite clear that axons, previously 

 thought to be capable of responding only by strictly all-or-none 

 propagated changes in potential, were also capable of graded, 

 localized electrical activity. According to the accepted theory of 

 membrane excitation by electrical currents, the graded activity 

 first observed by Hodgkin represents the barely unsuccessful 

 attempt of the sodium activation mechanism to attain the explosive 

 self-generating conditions associated with action potentials. The 

 rate of onset of sodium activation is an exponential function of 

 membrane depolarization. This rate must be at a minimum in the 

 present case, thus allowing not only the delayed increase in 

 potassium conductance, but sodium inactivation also, to interfere 

 successfully with the activation process. With a barely perceptible 

 increase in stimulus intensity, the local response will increase at a 

 progressively greater rate, reaching a higher amplitude before it is 

 removed by the restorative processes just mentioned. At threshold, 

 an impulse will be generated at maximum latency by the response, 

 and as the stimulus strength is increased still further, the rate of 

 rise of the response also increases, with an attendant decrease in 

 latency for spike triggering. This is an important point; for, as 

 will be discussed in a later chapter, local response growth appears 

 to be an important controlling factor in discharge frequency of 

 sensory neurons. It should be noted, however, that most naturally- 

 occurring membrane changes which evoke local responses in sen- 

 sory neurons are of considerably longer duration than the transient 

 electrical shocks used in Hodgkin's experiments, and significantly 

 diff^erent strength-latency relationships are to be expected. 



The discovery of the axonal local response was a conceptual 

 breakthrough which extended the theoretical, as well as the 

 practical, implications of the neuron as a responding unit. How- 

 ever, the major problems with regard to anatomical and physio- 

 logical modification of the regions of sensory neurons concerned 

 with analog stimulus replication remained unanswered. Yet, it 

 was becoming increasingly clear that the generation of conducted 

 impulses must logically be preceded by a sustained electrical 



