The Depolarizing Nature of the Trigger 27 



that most graded electrical events — receptor and synaptic potentials 

 — arise in regions of membrane whose conductance characteristics 

 are not affected by the passage of electric currents, although they 

 may be profoundly altered by the application of other types of 

 stimulus energy (see, however, the discussion of electroreceptors 

 in Chapter 4), A corollary to this doctrine of 'electrical inexcit- 

 ability'*"'*^ is that neither synaptic nor receptor types of electro- 

 genesis are actively propagated to adjacent regions of the mem- 

 brane, and local currents, flowing as a result of an applied stimulus, 

 spread only decrementally along the cell. These currents thus 

 resemble the electrotonic spread of artificially generated anodal, 

 or sub-threshold cathodal, electric shocks, in that there is no 

 active propagation of the electrical disturbance to distant parts of 

 the cell. Potentials of this type may also resemble in their 

 behavior the local response which can be recorded from electric- 

 ally excitable membrane. And, although Hodgkin showed that 

 such local activity can propagate to varying extents along an axon, 

 these distances are exceedingly short unless a full action potential 

 is triggered. The local response however shares with the action 

 potential the property of refractoriness, i.e. a period of absolute, 

 and then relative, inexcitability following previous activity. Thus, 

 after one local response has decayed to zero, a finite interval must 

 elapse before a second response can be evoked in that region of 

 the nerve membrane. No such restriction applies to the receptor 

 potential of any sensory cell so far examined, whether neural or 

 non-neural. Sub-maximal receptor potentials will summate at all 

 intervals of time, as was first demonstrated with the Pacinian 

 corpuscle — a particularly accessible type of mammalian mechano- 

 sensory neuron.^- ^^ The response characteristics of these receptors 

 are dominated by the mechanical properties of the capsule which 

 surrounds the nerve-ending (fig. 23). As a rule, the intact 

 receptor adapts very rapidly, even to prolonged stimuli, and the 

 receptor potentials (occurring in response to mechanical stimula- 

 tion) decay within a few milliseconds. These brief responses can 

 be continuously graded in amplitude by variations in stimulus 

 intensity, and electrical summation occurs at brief intervals of 

 stimulation, indicating absence of any appreciable period of 

 absolute refractoriness (fig. 9). 



By the early 1950s, therefore, several functional distinctions 



