SYNAPTIC AND EPHAPTIC TRANSMISSION 



153 



is more apt than 'hyperpolarizing' p.s.p., btit the 

 lattei term may be extended to denote a tendency 

 to maintain as well as to increase the resting potential. 



Interrelations of Postsynaptic Potentials and Spikes 



It has been noted above that the p.s.p. is not ac- 

 tively propagated as is the spike. Thus, the transmis- 

 sional electrogenesis of a p.s.p. is confined to the syn- 

 aptic site. While their local electrical activity can be 

 recorded in or about the cells that produce it (cf. 

 51, 70), p.s.p.'s do not, in general, evoke activit\' in 

 other cells, their effects being confined to the cell in 

 which they originate. 



To elicit 'distant' actions in the next postjtinctional 

 cell, the prejunctional cell must generate a spike. 

 Thus, transmissional activity in a synaptically linked 

 chain of units is consummated only if the p.s.p. of each 

 unit evokes a spike. When the depolarizing p.s.p. in 

 one of the linked elements is insufficient to elicit a 

 spike, the transmissional chain is broken. Likewise, if 

 at one synaptic site, inhibitory p.s.p. is sufficiently 

 large to block the spike of that cell, the chain is also 

 broken. 



Thus, spikes and p.s.p.'s are functionally interre- 

 lated. The former command the secretory activity at 

 presynaptic terminals of their cell, and the released 

 transmitter agent then evokes the p.s.p. of the next 

 cell, which may or may not itself elicit a new spike, to 

 repeat the process. It should be noted that while 

 hyperpolarizing p.s.p.'s can inhibit spike production, 

 they are themselves evoked by an excitatory activity 

 in the presynaptic cell that propagates within the 

 latter and effects the hyperpolarization of the post- 

 junctional synaptic membrane through the secretory 

 activity that it calls forth in the presynaptic terminals. 

 In other words, a p.s.p., whether excitatory or 

 inhibitory, always represents an active process, a 

 response of subsynaptic membrane to an appropriate 

 excitant. 



As was noted abo\e, and will be descriiied in more 

 detail below, the electrically inexcitable synaptic 

 electrogenic membrane has different properties from 

 those which generate the spike. The properties even of 

 simple synaptic s\stems are therefore compounded 

 from and subject to the various properties of the differ- 

 ent electrogenic components. The multiplicity of syn- 

 aptic transfers in the central nervous system makes the 

 synaptic properties a dominant factor, although those 

 of conductile electrogenesis are also important. Since 

 the amount and type of synaptic electrogenesis deter- 



mines the occurrence or absence of spikes, factors 

 which modify p.s.p.'s are therefore of sjreat significance 

 in the central nervous system. Among these are the 

 effects of pharmacological agents or synaptic drugs, 

 and their use as experimental tools has already been 

 mentioned. However, other agents and physiological 

 conditions may affect production of p.s.p.'s. For ex- 

 ample, the synaptic membrane may be altered in its 

 properties by previous activity (cf 95, 97; and below) 

 and this could affect synaptic electrogenesis. The 

 physiological properties of the presynaptic terminals 

 may also be changed by various conditions, including 

 previous activity. This change might affect the amount 

 or nature of the transmitter agent released under the 

 new circumstances and thereby aflfect transmission. 

 Thus, the magnesium ion interferes with release of 

 transmitter agents from the presynaptic terminals 

 (cf. 52). Neuromuscular transmission is then depressed 

 or blocked. The calcium ion acts reciprocally and in 

 excess antagonizes the effects of excess magnesium 

 ion. 



SPECIFIC PROPERTIES OF SYN.APTIC ELECTROGENESIS 



Evidence Against Electrical Stimulation of 

 Postsynaptic Potentials 



The existence of varieties of postjunctional cells in 

 which p.s.p.'s are generated without spikes, e.g. in 

 Torpedo and Raia electroplaques, invertebrate and 

 vertebrate muscle fibers and gland cells (cf. 95, 97), 

 provides one kind of direct evidence for electrical 

 inexcitability of synaptic membrane (figs. 4/I, 5; cf. 

 fig. 20). An electrical stimulus which does not fire the 

 presynaptic nerve fibers evokes no electrical activity 

 in these cells. Responses are only produced by afferent 

 neural activity or by chemicals which thus mimic the 

 action of the transmitter agent (fig. 5). 



Even in those cells which also generate spikes, the 

 p.s.p. is produced only by neural or chemical stimuli. 

 Direct electrical stimuli applied to the cell, or its local 

 circuit excitation by antidromic invasion, evoke only 

 spikes without p.s.p.'s (fig. 6). Finally, the occurrence 

 of spikes and of absolute refractoriness which is their 

 concomitant does not preclude the independent 

 development of p.s.p.'s. The electrogenesis of the 

 latter then can be superimposed upon that of the 

 spike, i.e. it can be evoked during the absolute refrac- 

 tory period (figs. 6, 7). Together therefore, these three 

 types of data provide direct evidence that the p.s.p.'s 

 are generated by membrane that is not itself electri- 



