= Absolute neuronal refractory period
{title2=$t_{\mathrm{abs}}$}
During an <absolute neuronal refractory period>, the excitable region cannot initiate a second distinct regenerative <action potential> under the physiological stimulation conditions being considered. Extensive <sodium> <ion-channel inactivation> is central for a fast <sodium>-dependent <action potential>. “Cannot fire” concerns regeneration; an imposed pulse can still move <membrane potential> or leave a recording artifact. Failure of just one fixed-amplitude test pulse does not establish an <absolute neuronal refractory period>: the same failure can occur in the <relative neuronal refractory period>.
The boundary is an operational question about distinguishable <action potentials> and the range of stimulation tested. A drawn vertical line is useful in the <phenomenological paired-pulse recovery> model, where it is imposed explicitly; it should not be mistaken for a universal sharp biochemical switch.
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