352 H. MCLENNAN 



Fig. 2. Inhibitory postsynaptic potentials recorded from a biceps-semitendinosus 

 motor neuron in response to stimulation of quadriceps nerve. A double-barrelled 

 microelectrode was employed, and a steady background current was passed 

 through one barrel to pre-set the membrane potential, while the voltage change 

 due to nerve stimulation was recorded through the other. In the absence of 

 applied current the membrane potential was —74 mV. A depolarizing current 

 increased the amplitude of the i.p.s.p., while an hyperpolarizing current caused 

 a reversal of its polarity at a membrane potential level near —82 mV. (Coombs 

 et al., 1955a). (Courtesy of Sir John Eccles and the Journal of Physiology) 



lead to a transient hyperpolarization. The level of the resting potential of the 

 cell under observation should therefore be raised. (2) An increased polari- 

 zation results in a decrease in amplitude of an evoked i.p.s.p. (see Fig. 2), 

 and this effect also should be observable upon "artificial" administration of 

 the postulated transmitter. (3) There should be an observable change in the 

 amphtude of an evoked e.p.s.p. Hyperpolarization of the cell removes its 

 resting potential farther from the equilibrium potential of the e.p.s.p. (0 mV), 

 and it might be expected from this that the amplitude of an evoked e.p.s.p. 

 would be greater (by analogy with the changes in i.p.s.p.'s shown in Fig. 2). 

 However, Curtis et al. (1959) have argued that the increased permeability to 

 small ions brought about by activation of the inhibitory subsynaptic mem- 

 brane would, by reason of the increased membrane conductance, reduce the 

 effectiveness of the current produced by the action of the excitatory trans- 

 mitter, and would thus lead to a smaller observed e.p.s.p. In summary, one 

 would expect that application of a solution containing inhibitory transmitter 



