جهد الفعل
Stimuli that have the potential to harm or kill demand rapid detection. This is fundamentally distinct from navigation or exploration, in terms of the timescales available for response. Most motile species harbour a form of phobic or emergency response distinct from their steady state locomotion. Escape reactions are not strictly oriented—but commonly involve backward movement, sometimes with a negatively geotactic component. In bacteria and archaea, action potential-like phenomena have been observed in biofilms and also single cells such as cable bacteria. The archaeon "Halobacterium salinarium" shows a photophobic response characterized by a 180° reversal of its swimming direction induced by a reversal in the direction of flagellar rotation. At least some aspects of this response are likely mediated by changes in membrane potential by bacteriorhodopsin, a light-driven proton pump. Action potential-like phenomena in prokaryotes are dissimilar from classical eukaryotic action potentials. The former are less reproducible, slower and exhibit a broader distribution in pulse amplitude and duration.