THE SYNAPTIC MECHANISM FOR POSTSYNAPTIC INHIBITION 73 



Fig. 2. In a, the mean curve of the i.p.s.p. of Fig. 1g is plotted as a continuous 

 line, and, on the basis of the electric time constant of the membrane determined 

 from Fig. 1i-l, it is analysed to give the time course of the postsynaptic current 

 generating it, as shown by the broken line. In b, the flow of these inhibitory 

 postsynaptic currents is shown. In c there is a formal electrical diagram showing 

 capacity, resistance and battery of the membrane of a standard motoneuron as 

 "seen" by a microelectrode in the soma; on the right side there is in addition a 

 representation of the inhibitory subsynaptic areas of the membrane that are 

 activated in producing the la i.p.s.p. Maximum activation of these areas would be 

 indicated by closing the switch (Curtis and Eccies, 1959). Reproduced by per- 

 mission from the Journal of Physiology. D. Diagrammatic representation of 

 current that flows as the i.p.s.p. generated in the soma-dendritic membrane 

 spreads electrotonically to hyperpolarize the initial segment (IS) which is the 

 site of initiation of impulses discharged from the motoneuron. 



When the motoneuronal membrane is set at a sufficiently high level of 

 hyperpolarization by the application of a steady background current, there is 

 reversal of the inhibitory synaptic current, as is shown by the reversed polarity 

 of the i.p.s.p.'s in the three lower records of Fig. 3a. Plotting of the series 

 partly illustrated in Fig. 3a shows that there is an equilibrium potential of 

 about — 80mV at which the i.p.s.p. is zero, and of course the flow of in- 

 hibitory current is then also zero (Fig. 3b). This equilibrium potential for the 

 i.p.s.p. (^i.p.s.p.) was in Fig. 3b and c at about 6 mV more hyperpolarized 

 than the normal resting potential. The influence of membrane depolarization 

 in increasing the i.p.s.p. and of hyperpolarization in reversing it leads to the 

 postulate that the inhibitory current is due to the net movement of ions down 

 their electrochemical gradients, and that there is no requirement of a supply 



