SYNAPTIC AND EPHAPTIC TRANSMISSION 



FIG. 2. Some properties of depolarizing postsynaptic poten- 

 tials. .-1, B: The intracellularly recorded e.p.p. of a mammalian 

 muscle fiber is evoked by neural stimuli during hyperpolariza- 

 tion of the muscle fiber membrane through another intracellu- 

 lar electrode. The impaled hyperpolarized fiber did not re- 

 spond with a spike or contraction, but others unaffected by the 

 polarizing current and excited by the neural \olley contracted. 

 The resulting movement pulled the microelectrode out of the 

 tested muscle fiber producing the artifact seen at the end of 

 each record. The response in B is smaller than that in A, 

 partly because it is generated at a less hyperpolaiized membrane 

 as is described in the text. However, it is also broader than the 

 response in A, indicating that the recording microelectrode was 

 probably some distance from the focus of the e.p.p. The effects 

 of recording at various distances from this focus are shown in 

 C, D and E. The amplitude of the e.p.p. falls sharply (C); the 

 rising phase is prolonged somewhat (i)) and the falling phase 

 even more (£) as the electrode is moved farther from the focus. 

 [From Boyd & Martin (23).] 



by electron microscopic studies of eel electroplaques 



(95). 



These cells possess three functionally distinct types 

 of membrane. One major surface is composed of 

 membrane that does not respond electrogenically to 

 any type of stimulation and has a very low electrical 

 resistance. The other major surface of each cell is 

 diffusely innervated and, presumably only under the 

 presynaptic terminals, there is excitable membrane 

 of the synaptic type which responds only to neural or 

 to chemical stimuli. Intermingled with this electri- 



cally inexcitable membrane component is one that is 

 electrically excitable and produces a spike. Electron 

 microscopy has as yet not been able to discern differ- 

 ences between the two different components of the 

 excitable membrane, nor between their structures and 

 those of the nonresponsive membrane (95, 143). 



Two functionally quite different junctions, in squid 

 and crayfish respectively, appear to be similar when 

 observed by electron microscopy C'75)- However, 

 that activating the giant axon of squid is electrically 

 ine.xcitable and thus conforms to the extended defini- 

 tion of synapses given above. On the other hand, the 

 junction between a medial giant fiber and the motor 

 giant axon of the crayfish (83), as will be discussed 

 below, appears to resemble the ephaptic junctions of 

 septate giant axons (125). 



The inai)ility of present day microscopic techniques 

 to differentiate the structures of membranes which 

 differ profoundly in their functional properties indi- 

 cates that the differences which determine these 

 properties must be at the molecular level. Probably, 

 as microscopic methods develop, the difficulty of 

 visualizing molecular differences will be overcome. 

 At present, however, the chief tools available for 

 analyzing these structures are electrophysiological 

 obser\ations of function and of the disturbance in 

 function produced by various experimental means, 

 including the use of chemical agents (cf. gg-ioi; 163). 



Types of Postsynaptic Potentials 



•Synaptic electrogenesis differs from that of the 

 spike by being relatively small and, when more than 

 one nerve fiber is available to excite it, is graded in 

 amplitude depending on the strength of the stimulus 

 to the nerve. Furthermore, two varieties of p.s.p.'s can 

 occur. One, like the spike, tends to decrease the resting 

 potential, hence is a depolarizing p.s.p. The other 

 tends to increase the resting potential and is therefore 

 a hyperpolarizing p.s.p. The two varieties of p.s.p.'s 

 are present in different proportions in different cells. 

 Some cells generate only depolarizing, others only 

 hyperpolarizing p.s.p.'s, while in a third group both 

 types of responses are produced usually, and perhaps 

 always, by stimulation of different neural inflows. All 

 vertebrate muscle fibers thus far known, their em- 

 bryological relatives the electroplaques of most elec- 

 tric organs and some neurons develop only a depolar- 

 izing p.s.p. Certain gland cells are at present known 

 in which a hyperpolarizing p.s.p. is the sole electro- 

 genesis (144, 146). The crayfish stretch receptor, 

 likewise, produces a hyperpolarizing p.s.p. C'So), but 



