Fig. 11. Contribution of Na+ channel inactivation to PAD-mediated presynaptic inhibition. A–D, The parameters of Na+ channel inactivation (h) were set to the following: Aαh= 0.08 mV−1 msec−1; Bαh = −45 mV; Cαh = 1 mV; and Aβh = 0.4 mV−1msec−1; Bβh = −41 mV; Cβh = 2 mV (same as in all preceding simulations; Erest of −70 mV). A control orthodromic spike is generated at t = 5 msec, and a PAD with an amplitude of 22 mV and a duration of 200 msec (ECl of −40 mV; GCl = 300 nS) is generated at location 400 μm at t = 45 msec. V, Membrane potential; m, Na+ channel activation variable; h, Na+ channel inactivation variable; and n, K+ channel activation variable. A, the PAD alone did not induce any inactivation of Na+channels. B, During the occurrence of PAD, the amplitude of orthodromic spikes was reduced because of shunting mechanisms. The inhibition of orthodromic spikes is identical whatever the delay between the onset of the PAD and that of the orthodromic spike (fourtraces are superimposed). C, During the PAD, the amount of inhibition is identical for all orthodromic spikes during a 60 Hz spike train. D, A large-amplitude PAD (ECl of −35 mV; GCl of 300 nS) generated a train of antidromic spikes without adaptation. E, F, The inactivation threshold was lowered using the following parameters of Na+ channel (h): Aαh = 0.08 mV−1msec−1; Bαh = −50 mV; Cαh = 1 mV; and Aβh = 0.4 mV−1msec−1; Bβh = −36 mV; Cβh = 2 mV. E, The resting membrane potential was set to Erest of −70 mV and during a 22 mV PAD (ECl of −40 mV; GCl of 300 nS) a very small Na+ channel inactivation occurred. F, When an orthodromic spike occurred during the PAD, Na+ channel recovery from inactivation became slower. Note that there is almost no reduction of spike amplitude when the delay between the onset of the PAD and that of the orthodromic spike increased. G, During a train of orthodromic spikes, the inactivation of Na+ channel accumulated and resulted in a decrease in the spike amplitude. H, With such parameters, only one antidromic spike could be generated by large PADs (ECl of −35 mV; GCl of 300 nS) because of the fast adaptation that results from Na+ channel inactivation. The asteriskindicates a second transient that is not propagated.