SYNAPTIC AND EPHAPTIC TRANSMISSION 



l8n 



FIG. 28. The time courses of facilita- 

 tion and direct inhibition (magnitude of 

 response as percent of control value) 

 tested on monosynaptic reflexes are 

 nearly symmetrical. Facilitation (upper 

 curves^, A in an extensor and B in a flexor 

 muscle. Inhibition (lower curves'), i4 in a 

 flexor and B in an extensor muscle. 

 [From Lloyd (140).] 



e 10 12Msec 2 4 



12 Msec. 



refractoriness of the presynaptic fibers, no impulses 

 arrive at the synapses. At slightly longer intervals, 

 relative refractoriness or persistent absolute refrac- 

 toriness of the previously discharged postjunctional 

 cells causes a depressed testing response, but then an 

 interval is reached when the testing response can 

 become many times higher than it would have been 

 without the preceding conditioning activity. As noted 

 above, the facilitation in the simplest cases lasts 

 about 15 msec. (fig. 28), decreasing continuously 

 from its peak value during this interval. It is likely, 

 although this has not as yet been generally estab- 

 lished, that the synaptic drive of the testing volley is 

 also increased by antecedent activity of the nerve 

 fibers. This enhancement may take place in the pre- 

 synaptic fibers themselves. For example, invasion of 

 the terminal branches by the conductile activity may 

 be partial for a single volley and larger for a subse- 

 quent. Also, the amount of transmitter released by 

 the second activity may be larger. In many junc- 

 tional systems, the prolonged stimulation of the pre- 

 synaptic nerve at relatively high frequencies for 

 some time thereafter increases the effects produced 

 by a subsequent single testing stimulus (76, 87, 124, 

 135, 139). This phenomenon, post-tetanic (cf 118) 



or postactivating (59) potentiation, may likewise 

 depend upon the mechanisms just described. In- 

 creased synaptic drive may also involve the post- 

 synaptic membrane as, for instance, by a temporary 

 change in the excitability of the membrane to the 

 transmitter agent. These residual presynaptic and 

 postsynaptic effects may alter synaptic drive in either 

 direction and act without relation to the residual 

 p.s.p. from the first volley. Thus, the homosynaptic 

 facilitation which occurs in the eel electroplaque 

 (fig. 29) lasts for about i sec, whereas the p.s.p. 

 lasts only 2 to 3 msec. (4, 5, 6). 



e) HETEROSYN.'SiPTic FACILITATION. Hetcrosynaptic 

 testing eliminates the complications introduced in 

 homosynaptic facilitation except the refractoriness 

 of the discharged postjunctional cells. The facilita- 

 tion now may start at very brief intervals between 

 the stimuli. Strictly speaking, however, hetcro- 

 synaptic testing involves spatial factors for the 

 terminations of one pathway may activate different 

 synaptic sites than do those of the other. The facilita- 

 tion is therefore likely to take place by electrotonic 

 additions of the depolarization produced by the test- 

 ing stimulus onto the residual level of general de- 



