PHOTOSENSORY TRANSDUCTION IN CILIATES .3. THE TEMPORAL RELATION BETWEEN MEMBRANE-POTENTIALS AND PHOTOMOTILE RESPONSES IN BLEPHARISMA-JAPONICUM

Blepharisma japonicum exhibits a step-up photophobic response when sub jected to an increase in light stimulus intensity. This response is ch aracterized by the stop reaction after a period of delay followed by b ackward swimming (lateral rotation). The latency of the stop response decreased and duration of the lateral rotation increased as the intens ity of light stimuli was raised. A step-increase in light intensity el icited a graded membrane depolarization (photic receptor potential), a s measured by intracellular microelectrode. When the amplitude of rece ptor potential exceeded a threshold depolarization for membrane excita tion (15-25 mV), an all-or-none action potential of 50-65 mV in amplit ude was evoked which also occurred with some latency. Light stimuli of higher intensity (suprathreshold) elicited action potential which was followed by a membrane after-depolarization. Increasing the intensity of stimuli caused generation of an action potential with shorter lag period and prolonged after-depolarization. The action spectra for the latency of stop reaction, receptor potential amplitude and cell photor esponsiveness showed maxima at 460, 530 and 580 nm. The analysis of te mporal relationships between the electrophysiological responses and th e motile events showed that latency of an action potential, induced by the receptor potential, correlates well with the latency of a cell st op response. Also the duration of membrane after-depolarization resemb led the time period of the cell's backward swimming (cell rotation). T he data obtained indicate that the primary reaction initiated by light absorption in the photoreceptor pigment (blepharismin) is converted i nto the observed electrical potential changes, which in turn results i n the photomotile response of Blepharisma cells.