EXCITATION BY HYPERPOL ARIZING POTENTIALS 



327 



10 mV 



Fig. 1. Depolarizing electrogenesis of crayfish mechanoreceptor^ sense organ 

 and the effects it evokes in the electrically excitable portion of the cell. {Top) — A 

 weak stretch stimulus ( [ ) caused a depolarization of about 7 mV across the 

 membrane of the cell body. This was maintained until the stretch was released 

 (|). (.Middle) — Records at lower amplification. A weak stimulus produced a 

 low-frequency discharge of spikes. Increased stretch (second arrow) caused a 

 higher frequency discharge which continued with some slowing as long as the 

 stimulus was applied. The spikes generated during the depolarization develop a 

 hyperpolarizing undershoot which is absent when the response is evoked by a 

 single electrical stimulus. {Bottom) — Three increasingly larger stimulations are 

 shown in a-c. The spikes produced at a high frequency by the strongest 

 stimulus (c) were diminished in amplitude and at the end were no longer 

 evoked, while the receptor continued to respond with its sustained depolarization. 

 D-F. The return of responsiveness of the electrically excitable membrane after 

 its inactivation. Note that the average level of the depolarization produced by 

 the mechanoreceptor dendrites is graded with the degree of the stimulus. (From 

 Eyzaguirre and Kuffler, 1955.) 



SPIKE GENERATING SENSORY CELLS 



The two different electrogenic components that are found in the primary 

 sensory neurons and in the final path neuron of secondary receptors behave 

 quite differently to apphed currents. The long-lasting depolarizing generator 

 potential decreases with depolarizing current and increases when hyper- 

 polarizing currents are applied (Fig. 2). The centripetal messages of spikes, 

 however, increase in number and frequency with depolarizing currents and 

 decrease with hyperpolarizing. These differences are, of course, explicable as 

 manifestations of two fundamentally different activities: one, in an electrically 



