The Control of Impulse Frequency 63 



visual pigment. Indeed, it has now been shown that the rate of 

 reconstitution of visual pigment in vitro^^' '' as well as in vivo^^' •* 

 parallels very closely the recovery of visual sensitivity (fig. 28). 

 Nevertheless, the loss in visual sensitivity occasioned by exposure 

 to rather weak light cannot be entirely explained on the basis of 

 simple chemical kinetics; thus, neural adaptation cannot be ruled 

 out as a contributing factor. 



10 



09 



0-8 0-7 0-6 



Fraction of rhodopsin present 



0-5 



Fig. 28. The logarithm of visual threshold (o=the threshold for 

 the dark-adapted eye) plotted as a function of available rhodopsin 

 within the receptor cells. Data were obtained from the living human 

 eye. (From Rushton,'^ Fig. 3.) 



Summary 



The control of impulse frequency in primary sensory neurons is 

 complex. It can involve changes in the responsiveness of the 

 spike-initiating regions of the membrane to extrinsic currents, not 

 only in regard to the rate of rise of local sub-threshold responses, 



