allornone

All-or-none is a principle in physiology and related disciplines describing a response pattern in which a stimulus either triggers a response with a fixed, characteristic magnitude or fails to trigger it at all. The most familiar example is the action potential in nerve and muscle cells, where depolarization beyond a threshold triggers a spike of nearly identical amplitude; subthreshold stimuli do not evoke a spike, and stronger stimuli do not increase the spike’s size but can influence firing rate or recruitment of additional cells.

In neurons, once threshold is reached, voltage-gated conductances produce a stereotyped spike. The amplitude remains relatively constant across supra-threshold stimuli, while stimulus intensity is coded by the frequency and pattern of spikes rather than by larger spikes. Subthreshold inputs may summate temporally or spatially to reach threshold and trigger firing.

Beyond neurophysiology, the concept appears in other domains. In pharmacology, some receptor responses are described as all-or-none, in which a population of receptors activates in a switch-like manner. In electronics, digital circuits behave in an all-or-none fashion, producing a definite output only when inputs cross a threshold. In biology more broadly, certain gene regulatory and developmental processes show bistable or switch-like, all-or-none behavior that can act as a control mechanism.

Limitations exist: real systems exhibit variability in threshold, noise, and context-dependent modulation, so the all-or-none description is a useful but simplified model of complex biological responses.