Optical studies of membrane state during action potential propagation.

One of the most striking phenomena in biology is the action potential (AP), a nonlinear pulse with threshold and amplitude saturation (all-or-none-behavior) that propagates along neurons and other cells. In the classical interpretation the AP is considered to be an electrical phenomenon - a regenerating current flowing in a "biological cable". In contrast, the thermodynamic interpretation has emphasized that conservation laws necessitate pulses and that pulses must manifest as transient changes of all observables of the system (electrical, mechanical, thermal, etc.