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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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FIG. lo. 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 (I) caused a 

 depolarization of about 7 mv across the membrane of the cell body. This was maintained until the 

 stretch was released ( 1 ). Aliddle: 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 

 to C. The spikes produced at high frequency by the strongest stimulus (C) were diminished in ampli- 

 tude and at the end were no longer evoked, while the receptor continued to respond with its sus- 

 tained, summated depolarization. D to 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 & KufHer 

 (66).] 



b) P0STSVN.\PTIC POTENTI.ALS DURING HYPERPOL.^RIZ.^- 



TiON AND DEPOLARIZATION. P.s.p.'s Can bc produccd 

 during hyperpolarization of the cell, while spike 

 electrogenesis may be blocked (fig. 1 2). These differ- 

 ent effects may be ascribed directly to the different 

 modes of excitation of the electrogenic membrane 

 components. The effects produced by depolarization 

 are somewhat more complicated but can also be 

 accounted for on the same basis. Superposition of 

 depolarization by a brief extrinsic electrical stimulus 

 and that of a depolarizing p.s.p. enhances the excita- 

 tion of the electrically excitable membrane (4, 60, 

 79). The spike thus arises earlier on the p.s.p. since 

 the critical level of depolarization is thereby attained 

 earlier. 



Sustained depolarization, in some cells even when 

 rather small, blocks spike electrogenesis (fig. 13} 

 probably (cf. 95, 96) by the augmentation of sodium 

 inactivation and potassium conductance that it causes 

 in electrically excitable membrane (113). Electrical 

 inexcitaijility of synaptic transducer action permits 

 the continued development of p.s.p.'s after the spike 

 can no longer be produced by direct or neural stimuli 

 (figs. II and 13). Other manifestations of synaptic 

 activity can also be evoked when the spike generating 

 membrane is inactivated by ionically induced depolar- 

 ization (50). The generator potential of a sense organ 

 (fig. 10) may also continue to be produced even 

 though that sustained depolarization inactivates the 

 electrically excitable membrane and no spikes can 



