158 



Ml 



DONALD M. MAYNARD 



B 



92 



stimulus 



30 per sec. 



a 



40per sec 



66per sec 



92 per sec 



J. 



SO /sec 



80/sec. 



seconds 



5 

 seconds 



Fig. 12. Paradoxical driving, effects of various parameters of inhibition {PainiUnis), 

 same preparation as in Fig. 11. Diagram of oscilloscope records, a. Variation of 

 latent pause with duration of inhibitory train, frequently of pulses within train, 

 and frequency of train repetition. Vertical line gives mean of about seven values; 

 horizontal line, range of values, b. Same inhibitor stimulation, slower time scale. 

 Inhibitor trains indicated by small, thick vertical lines; post-inhibitory responses, 

 where they occur, by large vertical lines. 

 MECHANISMS OF INHIBITION 



Membrane potential changes associated with inhibition have been studied 

 by Terzuolo and BiiUock (1958) and Otani and Bullock (1959). The following 

 account is derived from their work. 



Inhibitory postsynaptic potentials (i.p.s.p.) recorded from follower cells 

 in Pamdirus mterruptus may be hyperpolarizing, depolarizing, or occasionally, 

 both (Fig. 14). Hyperpolarizing potentials generally occur in units with 

 spontaneously occurring generator potentials; depolarizing potentials in pure 

 followers with no spontaneous activity. In contrast to the excitatory p.s.p. 

 initiated by intrinsic neurons, i.p.s.p.'s always facilitate, at times requiring up 

 to 1 sec for complete facilitation, and at higher frequencies summate to a 

 maintained plateau potential. Individual facilitated i.p.s.p.'s usually last 

 several milliseconds and may reach values of several millivolts. Both initial 

 depolarizing and later hyperpolarizing phases of the biphasic i.p.s.p. figured 

 (Fig. 14) facilitated, but only the hyperpolarizing phase summated. With 

 prolonged inhibition, both the individual potential and the summed plateau 

 tend to adapt, returning toward initial membrane potential values (Fig. 16). 



