142 



PHYSIOLOGICAL TRIGGERS 



generating spikes (no). It is explicable in terms of electrogenic properties of 

 electrically inexci table, postsynaptic membrane (e.g. fig. 8). While these cells 

 have not yet been studied with intracellular electrodes, it also seems necessary 

 (lOo) to invoke an early phase of sodium conductance and overshoot in the 

 responses of Torpedo electroplaques. This could come about by different kinetics 

 of the several ion conductance changes in the electrogenic reaction of the post- 

 synaptic membrane as in the production of the spike by the electrically excit- 

 able membrane. 



The membrane component which alters to increase sodium conductance is 

 presumably absent from the transducer of the inhibitory p.s.p., the response 

 normally being in the direction of increasing the internal negativity. As de- 

 scribed above, increase in conductance of either potassium or chloride, or both 

 would cause this electrogenic activity. As with the excitatory p.s.p., the maxi- 

 mum of hyper- or repolarization (cf. 134) would depend upon the equilibrium 

 determined by electrochemical conditions. If the interior is already so negative 

 (e.g. by applied hyper-polarization), that increased chloride conductance would 

 drive this ion out, or increased potassium conductance force more of these into 

 the cell, the electrogenic response should be in the direction of decreasing the 

 membrane potential toward its equilibrium value. This has been observed in 

 invertebrate muscle fibers (74), the crayfish stretch receptor (134) and cat 

 motoneurons (fig. 8; refs. 48, 50, 64). On some occasions this depolarizing 

 response may even discharge the cell, presumably when the latter is undergoing 

 some form of anodal or postanodal excitation. 



i) Magnitude and Form of P.S.P. A specific carrier molecule or a membrane 

 pore probably changes its properties in an all-or-none fashion. Thus, the mem- 

 brane transducer molecules probably become completely sodium, potassium, 

 or chloride 'electrodes' and, were one able to probe the membrane potential of 

 these molecules in isolation, the swing of the potential would probably be maxi- 

 mal. However, under actual conditions, the effects are averaged over some area, 

 and electrotonic spread of the change in charge at the active loci decreases the 

 amplitude and distorts the form of the electrogenic response. Thus, even the 

 briefest possible electrogenic action would be observed with a rising time and a 

 decay time. Since membrane conductance is higher during the active phase 

 than at rest, the rising phase would be relatively more rapid, while the decaying 

 phase would reflect the time constant of the resting membrane. In short, the 

 response is essentially ballistic. Involvement of a larger number of action units, 

 but still for the same very brief time, would increase the 'throw' of the ballistic 

 system, i.e., the amplitude of the response, but would not alter its form. This is 

 generally found to be the case for many p.s.p. 's, and it is therefore inferred that 

 the action of the synai)tic transmitter is brief compared with the time constant 

 of the membrane. The short time of action may be caused by rapid destruction 

 of the transmitter, its inactivation, or diffusion if only a small, threshold 



