SYNAPTIC AND EPHAPTIC TRANSMISSION 1 55 



A - r\ B 



FIG. 5. Electrogenic action of acetylcholine on the elec- 

 trically inexcitable membrane of Torpedo elcctroplaques. Intra- 

 arterial injections of 10 /ng (/), 5 /ig (//) and 2.5 /ig (/!') in the 

 presence of physostigmine produced electrical activity, the 

 larger amounts evoking the larger responses. The neurally 

 evoked discharge of Torpedo organ lasts only a few msec. (cf. 

 95). The long duration of the response produced by injections 

 of acetylcholine presumably is due to sustained depolarization 

 of the electrically inexcitable elcctroplaques by an excess of the 

 administered transmitter agent. /// indicates a control in which 

 only perfusion fluid was injected. The elcctroplaques were 

 probably depolarized in the 'resting' state, and the 'hyper- 

 polarization' seen in this record may have been caused by 

 temporary dilution of the depolarizing e.vcitant. Calibrations: 

 0.5 mv, and seconds. [From Feldberg & Fessard (74).] 



excitable membranes utilize the electrical polarization 

 or resting potential of the cell. This appears as a 

 potential difference across the cell membrane with its 

 interior negative relative to the exterior. At rest, the 

 membrane has a rather high resistance, indicating 

 that it presents a considerable barrier to the penetra- 

 tion of ions. The physiological electrogcnic response 

 of the membrane to an appropriate stimulus, its 

 transducer action (94), is the temporary alteration of 

 its permittivity to ions. The electrical change is its 

 consequence, derived from the prior, metabolically 

 energized unequal distribution of ions and the resting 

 potential. 



Whereas the spike is generated by temporally 

 sequential processes comprising first enhanced sodium 

 conductance, then enhanced potassium conductance 

 and sodium inactivation(ii3),^ the transducer actions 

 of svnaptic membrane involve different ionic events. 



- Recent data on eel elcctroplaques (3) indicate that a process 

 of potassium inactivation may be involved in spike production 

 (95). The participation of other, potential-insensitive processes 

 is discussed below in connection with graded responses of 

 electrically excitable membrane. 



FIG. 6. Some differences between electrically and neurally 

 excitable responses. A, B: Weak and strong depolarizing elec- 

 trical stimuli to the eel electroplaque excited the cell directly, 

 the latter with almost no latency. C, D: The stimuli were ap- 

 plied in the reverse direction. These are ineffective for the 

 electrically excitable membrane but stimulate the cell indirectly 

 by way of the nerve terminals supplying the synaptic mem- 

 brane. The weak indirect stimulus evoked only a p.s.p. after a 

 latency of almost 2 msec. (C). The very strong stimulus (i)) 

 shortened the latency to about 1.7 msec, and the larger p.s.p. 

 evoked a spike with very brief delay. No p.s.p.'s were produced 

 by the direct stimuli. However, the strong direct stimulus (B) 

 also excited the nerve fibers which csokcd a p.s.p. that occurred 

 with the same latency as in C and D but appearing this time on 

 the falling phase of the directly elicited spike. The p.s.p. there- 

 fore occurred while the electrically excitable membrane was 

 absolutely refractory. [From Altamirano et al. (4).] 



Depolarizing p.s.p.'s are caused by a general increa.se 

 of permittivity to all ions (71 ; cf. 52, 60) which tends 

 to abolish the resting potential. Electrogenesis of 

 hyperpolarizing p.s.p.'s probably involves increased 

 permittivity for K+ and Cl^ (60, 61; Grundfest 

 el al., in preparation). Each ion species then moves in 

 the direction of its electrochemical gradient, K+ 

 outward and Cl^ inward. Loss of positive charges and 

 gain of negative thus account for the increased 

 internal negativity. 



The immediate consequences of electrical inexcita- 

 bility of synaptic transducer actions are made appar- 

 ent by the diagram of figure 8. Depolarization is the 

 stimulus that initiates transducer action of an elec- 

 trically excitable membrane. The entry of Na""", 

 forced inward because of the high concentration of 

 this ion in the external medium, causes further de- 

 polarization. This electrogenic response to the trans- 



