Fig. 6.

Sodium channel availability during spontaneous and stimulated action potential firing in the same neuron. A: voltage-clamp recordings of sodium current during spontaneous firing, with availability measured using a test step to −8 mV added at various times before, during, and after a spike as in Fig. 5. B: sodium channel availability (current during second pulse normalized to current evoked by a first pulse to −88 mV) plotted as a function of the time during the firing cycle. C: sodium current during the action potential, showing incomplete inactivation during the falling phase. D–F: same analysis but using waveform of higher firing frequency evoked by steady current injection in the same neuron. G–I: same analysis but for a firing frequency near maximal for the same neuron.