SYNAPTIC AND EPHAPTIC TRANSMISSION 



157 



FIG. 8. The different ionic mechanisms evoked by transducer 

 actions in electrically excitable and synaptic membranes, and 

 some consequences of the different excitabilities. The depolari- 

 zation caused by an electrical stimulus is regenerative in the 

 electrically excitable membrane and produces the all-or-none 

 spike. The electrically inexcitable synaptic membrane can 

 produce either depolarizing or hyperpolarizing p.s.p.'s which 

 do not react back on the transducer actions. This insensitivity 

 to electrical effects results in responses graded in proportion to 

 the available chemical stimulus. The depolarizing p.s.p. can 

 act as a stimulus for the electrically excitable membrane, while 

 the hyperpolarizing is inhibitory to the latter. [From Grund- 

 fest (96).] 



The kinetics of this reversible desensitization have 

 been studied thus far only in frog muscle endplates 

 (fig. 11). The nature of the processes involved C127) 

 is not yet clear; but neither the loss of responsivene.ss 

 nor its recovery are controlled by the membrane po- 

 tential. 



Desensitization may be slow and unimportant 

 relative to the excitatory events that occur at synapses 

 in response to their normal neural activation. How 

 ever, it might become a disturbing factor if trans- 

 mitters are continuously released locally or svstem- 

 ically. This situation could result from the action of 

 drugs or might arise from a pathological state. Rapidly 

 developing desensitization has not yet been described, 

 but it might account for the successively decreased 

 p.s.p.'s sometimes produced by a train of stimuli. 

 This process has been termed 'defacilitation' (33, 

 186). Decrease in the generator potential of sense 

 organs acted upon by a constant stimulus, such as is 

 seen in the rapidly adapting stretch receptors of cray- 

 fish (66), might be accounted for by a desensitization 

 phenomenon. 





100 msec 



i—r 



15 msec 

 T M M 



FIG. g. Soine consequences of the differently excitable electrogenic mechanisms in neurons, a.' 

 The cat motoneuron excited antidromically at high frequencies (140, 205, 280 and 630 per sec.) 

 produces pulsatile spikes, only their after -potentials fusing. [From Brock et al. (25).] b: The p.s.p.'s 

 produced by orthodromic stimuli (205 and 280 per sec.) summate, a higher average level of the de- 

 polarization being produced by the higher frequency of stimulation. The summated response is 

 maintained as long as the afferent stimuli are delivered (lower record of each set). The amplitude 

 calibration applies to the p.s.p.'s of this .set which were taken at about lox the amplification of a. 

 [From Brock et al. (24).] c: Repetitive activity evoked in the rabbit cervical syinpathetic neuron by 

 stimulating the preganglionic supply at approximately 80, 100, 120 and 150 per sec. At the time 

 scale of the records the first p.s.p. is not shown (cf. fig. 17C). The p.s.p. evokes a large spike; but 

 even at the lowest frequency, the spikes caused by the subsequent p.s.p.'s are small, while the p.s.p.'s 

 themselves are summed and sustained. This synaptic depolarization, increasing at higher frequencies 

 of afferent drive, inactivates the spike-generating membrane. \i\.er the second depressed spike the 

 responses progressively decrease, and at the highest frequency disappear. The p.s.p.'s are generated 

 as long as there is an influx of presynaptic stimuli. [From Eccles (64).] 



