Fig. 5.

The effect of external cesium on the responses of an octopus cell to steps of injected current. A, External cesium (15 mm) decreased the current required for action potential initiation, reduced the delayed depolarizing sag in responses to hyperpolarizing current steps, and increased the input resistance of the cell, particularly in the hyperpolarizing voltage range. Resting potentials (in mV): Control, −61;15 mm Cs⁺, −66;Wash, −63. Current pulses: from 1.4 to −1.4 nA in −0.2 nA steps. B, The voltage–current relation derived from the traces in A. C, The membrane time constant of the cell in normal saline and 15 mmcesium. Hyperpolarizing responses of comparable magnitude in the two conditions were fit by single exponential functions (darker traces). The time constant increased by over an order of magnitude in the presence of external cesium. D, External cesium increased both the size and width of the action potential modestly and reversibly decreased the current necessary for their initiation from 2 to 1.6 nA.