SYNAPTIC AND EPHAPTIC TRANSMISSION 



169 



A. 



JV. 



A. 



B 



-V 



/v. 



-V 



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JV 



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8' 



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FIG. 18. All-oi-none and graded responsiveness in an eel 

 clectroplaque. Two traces are recorded simultaneously, re- 

 peated at the rate of 5 per sec. The upper longer trace of each 

 set is the zero base line for an internal microelectrode. It also 

 carries the monitoring signal of a stimulus applied to the cell 

 and shows that the stimulus strength remained constant in 

 each of the two series. The lower trace of each set is that of the 

 potential recorded with the microelectrode. The distances be- 

 tween the two represent the resting potential, about 70 mv. 

 The weak stimulus in A, before the cell was treated with drug, 

 at first produced only a subthreshold electrotonic depolariza- 

 tion. The seventh repetition of the stimulus is followed by a 

 spike. The shorter latency at which successive spikes then de- 

 velop indicates continued growth of excitability and its per- 

 sistence through the 200 msec, intervals between stimuli. The 

 resting potential remained unchanged. B and B' . The sequence 

 of growth in response in the cell after 84 min. exposure to 500 

 Mg per ml of physostigmine. The resting potential was not 

 affected by the drug, which eliminated synaptic excitability 

 and converted the all-or-none response of the electrically 

 excitable membrane component to graded responsiveness. The 

 testing stimulus was slightly stronger than before applying the 

 drug, and the first trace seen (upper set of S) evoked a distinct. 



synaptic processes. The one descrif)ed just above is 

 summation where each successive p.s.p. is no larger 

 (cf. fig. 27.4), and may indeed be smaller, than its 

 predecessor. The excitatory action leading to the 

 overt effect would be the increased total depolariza- 

 tion produced b\- the summed effects of the repeated 

 p.s.p.'s. The overt effect would appear as a facilita- 

 tion because of the profound functional difference 

 between the local processes at the motoneuronal or 

 neuromuscular synaptic junction and their production 

 of an explosive propagated spike which triggers the 

 contractile mechanism. 



Essentially the same overt result, but an activity 

 involving more complex synaptic processes, would 

 (jccur if the successive p.s.p.'s augmented as a result 

 of the repetitive stimulation. This synaptic facilitation 

 will be discussed further in relation to heterosynaptic 

 and homosynaptic excitatory phenomena (p. 184). It 

 would seem to involve augmented responsiveness of 

 the synaptic membrane to the transmitter agent, the 

 converse to the decreased responsiveness in desensiti- 

 zation. As noted in that connection, defacilitation 

 probably is ascribable to desensitization. Both facili- 

 tation and defacilitation, however, may be only 

 apparent effects on the synaptic membrane, their real 

 cause residing elsewhere. For example, facilitation 

 could result from successively larger quantities of 

 transmitter released from the presynaptic terminals. 

 The converse, progressive exhaustion of the trans- 

 iTiitter and decrease of the amount emitted at each 

 impulse, would lead to defacilitation. 



As is also the case with other electrical stimuli, the 

 depolarizing p.s.p. first evokes a graded local response 

 of the electrically excitable membrane (4) and the 

 two depolarizations then sum to cause the explosive 

 response of the spike (figs. 3, 7). The addition of 

 hyperpolarizing p.s.p. to the depolarizing diminishes 

 the magnitude of the latter and its excitatory effect. 

 If the depolarizing p.s.p. then falls below the critical 

 level, a spike is no longer elicited and the effect of 

 hyperpolarizing p.s.p.'s is therefore inhibitory. It 

 should be noted that inhibition may occur even 

 though considerable depolarization is still generated. 

 In other words, the countervailing inhibitory p.s.p. 



though small, graded response. During the course of repetitive 

 stimulation at 5 per sec. the response grew, at first gradually 

 and then more rapidly, indicating that the rise of excitability 

 is non-linear. The series illustrated ends before the response 

 could grow to an amplitude as large as that of the spike, but 

 in other experiments this was observed. [From .Mtamirano el 

 al. (6).l 



