SYNAPTIC AND EPHAPTIC TRANSMISSION 



173 



Stimuli give rise to graded summative depolariza- 

 tions. These diffusely generated depolarizations can 

 act as stimuli for the contractile mechanism, causing 

 localized graded contractions (34, 35, 132). 



Some salivary gland cells also generate only de- 

 polarizing p.s.p.'s (fig. 20) and these are produced 

 by stimulation of either the sympathetic or para- 

 sympathetic nerves (144). Chemical stimulation by 

 epinephrine, pilocarpine or acetylcholine then all 

 cause the same type of electrogenesis, but it is not 

 known whether all the excitants activate a single 

 variety of electrogenic membrane or whether there 

 are distinct, although similarly electrogenic, cholino- 

 ceptive and adrenoceptive components. As is the 

 case with the electrically inexcitable muscle fibers, 

 the synaptic electrogenesis of gland cells is also asso- 

 ciated with and itself probably effects other cellular 

 activity, in this case secretion.'^ 



Torpedo and Raia elcctroplaqucs also generate 

 only depolarizing p.s.p.'s but not to electrical stimuli 

 (95). The cells which are derived from skeletal 

 muscles therefore are in reality constituted from end- 

 plates. A specialization of the.se and other electro- 

 plaques permits series additions of the voltages pro- 

 duced by each cell; hence the electric organs generate 

 considerable voltage. The discharges are under con- 

 trol of the nervous system and in .some forms this 

 may be useful for protection or aggression. The 

 p.s.p.'s are Ijrief in Torj>edo but long-lasting in Raia. 



Vertebrate skeletal muscle fibers of the ' twitch' 

 system and autonomic ganglia combine depolarizing 

 p.s.p.'s and spike-generating membrane (cf figs. 9, 

 27), but the autonomic neuron may also produce 

 hyperpolarizing p.s.p.'s since there are indications 

 thai inhil)ilion may occur (64, 134). In both cases 



' It was noted earlier (p. 154) that bioelectric responses of 

 transmissional and conductile processes are essentially passive 

 events resulting from the mo%ement of ions in obedience to 

 charged electrochemical equilibrium states. The change from 

 one state to another is the active phenomenon, due to specific 

 processes, transducer actions which are the responses of excit- 

 able membrane to appropriate stimuli. In gland cells, the se- 

 cretory activity of the output component (fig. 16) probably 

 occurs at membrane .structures that are intimately mingled 

 with those of the input component. .Secretory electrogenesis 

 thus is probably superimposed on the p.s.p.'s of the transducer 

 input, and this is suggested also by the independence of the 

 gland electrogenesis from electrochemical conditions (fig. 20 

 E and F). In some respects, therefore, electrogenesis of gland 

 cells may differ from that of 'pure' p.s.p.'s of neurons or end- 

 plates. The details of these diflferences cannot now be specified 

 since little is known about the nature of active transport 

 mechanisms, such as arc probably involved in secretion. 



the p.s.p.'s have much longer durations than do the 

 spikes. In muscle fibers the spike energizes the proc- 

 esses of contraction by a mechanism that is not yet 

 known (cf 121). Eel electroplaques also generate 

 both depolarizing p.s.p.'s and spikes (cf figs. 3, 6, 

 13), but the contractile machinery is missing in these 

 modified muscle fibers. Eel electroplaques, like 

 neurons, are diffusely innervated Isy many nerve 

 fibers. Since the area of their innervated surface is 

 more than 10 mm-, their study has provided some 

 data that are not readily obtained with the much 

 smaller nerve cells. The results, however, very prob- 

 ably apply to the general case of synaptic transmis- 

 sion as will be described below (cf. 95). 



Postjunctional Cells with Hyperpolarizing 

 Postsynaptic Potentials 



If cells capable of generating spikes were endowed 

 only with hyperpolarizing p.s.p.'s, transmfssional 

 excitation of the electrically excitable responses would 

 not occur, for in all cases known the spike is triggered 

 by depolarization. Thus, it may be expected that 

 cells in which solely hyperpolarizing synaptic elec- 

 trogenesis occurs would be of restricted functional 

 significance. From intracellular recordings two cases 

 are known and in neither are spikes generated. These 

 are salivary gland cells (144, 146; cf fig. 20) and L- 

 cells of the fish retina (102, 184, 191 ; cf al.so footnote 

 4, above). As noted earlier, the memijrane trans- 

 ducer actions and electrochemical effects of hyper- 

 polarization are consistent with secretory activity; 

 hence neurally evoked hyperpolarizing p.s.p.'s of 

 glands have functional validity. 



Postjunctional Cells ivith Both Types of 

 Postsynaptic Potentials 



Two varieties may be expected and both types 

 occur: /) electrically ine.xcitable cells which do not 

 generate spikes and 2) cells which produce spikes as 

 well as the p.s.p.'s. A clear case of the former is 

 found in some salivary gland cells (fig. 20) in which 

 each type of synpatic electrogenesis is probably asso- 

 ciated with a different form of secretory activity. The 

 different p.s.p.'s are specifically produced by stimu- 

 lation of the two autonomic nerve supplies. Stim- 

 ulation by cholinomimetic and adrenomimetic 

 substances evokes oppositely signed electrogenesis. 

 The R-G and Y-B cells of fish retina also produce 



