Habituation in Stentor: a response-dependent process.

The contractile protozoan Stentor coeruleus habituates during repetition of the mechanical stimulus used to elicit the initial contractions. This decrement in response probability was found to be highly correlated with a reduction in receptor potential amplitude, while the amplitude of the action potentials that triggered the contractions did not change. When mechanical stimulation elicited receptor potentials and not action potentials, the receptor potential did not habituate significantly. Conversely, action potentials repetitively elicited by current pulses habituated animals to mechanical stimuli. Similarly, voltage steps used to simulate action potentials produced pronounced decrements in receptor currents recorded from voltage-clamped cells, while mechanical stimulation produced only small decrements. Thus, habituation depends primarily on action potential production, while mechanical stimulation itself makes a much smaller, but significant, contribution. The temporal relation between mechanical stimuli and action potentials, when both occur, is inconsequential in determining the rate and degree of habituation produced.