Interstimulus Interval (ms)

Figure 1 Basic properties of the action potential. (a) Traces show responses of a simulated space-clamped squid axon (T = 6.3°C) to intracellularly injected current pulses of duration 0.5 msec (top trace). The simulated recording configuration is shown in the inset. Sufficiently large inputs evoke all-or-nothing action potentials (middle trace). The response is minimal to subthreshold stimuli (bottom trace). The inset shows the basic recording configuration. (b) A simulation demonstrating anode-break excitation in response to the offset of a hyperpolarizing current pulse (duration = 10 msec). (c) Current threshold (the minimal amplitude of a current step necessary to evoke an action potential) plotted vs stimulus duration. (d) Simulation results demonstrating refractoriness. Two current pulses (duration = 0.5 msec each) were delivered to the model, with interstimulus interval (ISI) varied systematically. The first pulse had magnitude twice the threshold for evoking an action potential. The y-axis shows the magnitude of the second pulse necessary to evoke a spike. For ISI <15 msec, threshold is above its normal value (dashed line). During the relative refractory period (RRP), threshold is elevated; during the absolute refractory period (ARP), it is not possible to evoke a second action potential.



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