64 The Physiology of Sense Organs 



but also in the threshold for full impulse generation, ^n slowly- 

 adapting cells these changes, if they occur, stabilize with time, so 

 that a steady rate of firing can persist during most of the time that 

 the stimulus is applied to the sense organ. /The impulse frequency^ 

 V, r during this period is usually related to the amplitude of the 

 A C^^eceptor potential in a linear mannerV^It follows from this that 

 the logarithmic relationship which is often observed between the 

 , stimulus and the impulse frequency must result from a trans- 



"ij, formation at a step prior to that of generation of the action 

 potential (i.e.x^n the coupling of the stimulus energy to the 

 generator of the receptor potential). Changes in the amplitude 

 of the receptor potential can occur subsequent to the application 

 of a constant stimulus, with predictable consequences for impulse 

 frequency. In some mechanoreceptors, this degradation in 

 amplitude results from an uncoupling of the nerve-ending from 

 the stimulus energy, as a consequence of the mechanical properties 

 of the end-organ. Thus, a large component of sensory adaptation 

 can be mechanical in origin. Non-neural ad aptation c an also play 

 a significant role in goyeming the response of other types of sense 

 organs, for example in primary photoreceptor cells. The decreases 

 iiTthe amplitude of the receptor potential with time which may be 

 seen in these cells may result partially from the bleaching of 

 available visual pigment; but other, less understood, processes 

 also are implicated, and they may be no less important. 



