40 The Physiology of Sense Organs 



anatomical arrangement of the sensory system at hand makes this 

 an obvious conclusion. 



Summary 



The events that occur in a primary sensory neuron upon exposure 

 to an adequate stimulus may be summarized as follows. The 

 absorbed stimulus energy (or some fraction of it) effects a change 

 in the resting membrane potential of the cell, a change which is 

 related to stimulus intensity. This change in potential may 

 persist throughout the period of stimulus application and can 

 have either electrical polarity. SinQC^ such changes occur jn 

 regions of the cell membrane that are electrically-inexcitable, the 

 currents at the source of this receptor potential may be maintained 

 for long periods, although they spread decrementally throughout 

 the neuron; their electrotonic derivative at any point decreases 

 with distance and is critically dependent upon membrane 

 geometry. If a region of low-threshold electrically-excitable 

 membrane is sufficiently close to the source of a depolarizing 

 receptor potential, or in any case, if the electrotonic potential is of 

 sufficient amplitude, impulses may be initiated throughout the 

 period of time during which the depolarized state exceeds the 

 threshold amplitude. The nature of the control of impulse 

 frequency by such supra-threshold depolarizations will be 

 examined in the next chapter. 



