(Fig. 1b). This phenomenon is called anodal break excitation or rebound spiking.

The value of threshold depends on the duration of the stimulus (Fig. 1c); brief stimuli are required to be larger to evoke an action potential. Threshold also depends on more subtle features of the stimulus, such as its speed of onset. For a short time after an action potential has occurred, it is impossible to evoke a second one (Fig. 1d). This period is referred to as the absolute refractory period (ARP). After the ARP comes the relative refractory period (RRP), in which an action potential can be evoked, but only by a larger stimulus than was required to evoke the first action potential. Stimulation by an ongoing suprathreshold stimulus leads to repetitive firing at a rate that is constant once any transients have settled out (Fig. 2a). The rate of repetitive firing increases with increasing depolarization (Fig. 2b), eventually approaching the limit imposed by the ARP.

Once initiated, the action potential propagates down the axon at an approximately constant velocity. The leading edge of the action potential depolarizes adjacent unexcited portions of the axon, eventually bringing them to threshold. In the wake of the action potential, the membrane is refractory, preventing reexcitation of previously active portions of the cell. In unmyelinated axons, the action potential travels smoothly, with constant shape and at constant velocity. In myelinated axons, conduction is saltatory: The action potential "jumps" nearly instantaneously from one node of Ranvier to the next, greatly increasing the speed of propagation.

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