Fig. 3. Simulated muscle fibres are able to support spikes in response to rapid depolarizations (A; black trace), which is enhanced when successive stimuli are separated by a hyperpolarizing interval (A; red trace). (B) The rate of recovery of the graded action potentials in response to the second depolarization is faster when the cell is hyperpolarized by injecting 0.7 nA of current during the interstimulus interval (red traces) than when no current is injected (black traces). (C) A recording of the value of the sodium inactivation variable, h, between two depolarizing stimuli separated by an interval of 1 ms shows that h recovers more effectively when 2 nA of hyperpolarizing current is injected during the interstimulus interval (red trace) compared with `control' (black trace). (D) If, however, the value of h between the paired stimuli against an interpulse potential of –70 mV is adjusted to the value that is recorded at the same time but against an interpulse potential of –90 mV (red trace), the recovery of the second action potential matches that obtained against a resting potential of –90 mV.