Photostimulation of deciliated specimens of chlorella-free P. bursaria elicited a transient depolarization of the membrane potential. The amplitude of this receptor potential became larger as light intensity was increased and the relationship showed a Michaelis equation. The action spectrum of the receptor potential showed two peaks at around 420 and 560 nm. When constant current was injected to depolarize the membrane gradually, the receptor potential showed a decrease in amplitude. The potential at which the response disappeared depended on the external concentration of Ca2+ (27mV/log [Ca2+]o), and the amplitude of the receptor potential was also dependent upon external Ca2+ concentration. Therefore, the receptor potential is primarily caused by a transient increase in the membrane conductance to Ca2+.

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