THE SYNAPTIC MECHANISM FOR POSTSYNAPTIC INHIBITION 79 



Fig. 6. b and d are intracellular records of i.p.s.p. and e.p.s.p. evoked in a BST 

 motoneuron at Si segmental level by a la quadriceps and a BST volley, respec- 

 tively, as shown in the inset diagram. The upper traces of a, c are the potentials 

 electrotonically conducted from the motoneurons along their motor a.xons and 

 recorded from an isolated filament of the Si ventral root, one electrode being on 

 the filament about 1 mm from its exit from the cord, the other at least 20 mm 

 distally on the isolated filament as shown by the two arrows in the inset diagram. 

 A shows potentials produced by a la quadriceps volley, c by a la BST volley 



(Araki ef al., 1960). 



This demonstration of an i.p.s.p. electrotonically propagated to the ventral 

 root establishes that inhibitory synaptic action hyperpolarizes motoneurons 

 that have not had their membrane potential lowered by microelectrode 

 impalement. A similar conclusion may be drawn from the invariable demon- 

 stration by Araki et al. (1960) that a residuum of reflex inhibitory action 

 continues for many milliseconds after the initial brief phase, as has been 

 illustrated above (Fig. 5c). We may therefore conclude that, even before 

 impalement by microelectrodes, the membrane potential is more depolarized 

 than the equilibrium potential for the i.p.s.p., i.e. that Er is less than E-^p^^ 

 This conclusion is of importance when considering the ionic mechanism 

 responsible for the inhibitory action. 



We have seen that the effects produced in the size and direction of the 

 i.p.s.p. by varying the initial membrane potential correspond precisely to the 

 changes that would be expected if the currents generating the i.p.s.p. were 

 due to ions moving down their electrochemical gradients. These currents 



