FIGURE 5.

Membrane currents and intrinsic dynamics of the rod photoreceptor. All recordings in solution B (0.75 mM K⁺). (A) Current responses of a rod photoreceptor to voltage-clamp pulses ranging from −110 to 22 mV, in 11-mV increments. The large outward currents, presumably carried by K⁺, contain only a delayed-rectifierlike component. Slowly activated, small, but still distinguishable, currents are seen for voltages ranging from −30 mV to −100 mV. (B) Voltage responses of the rod photoreceptor to current-clamp pulses ranging from −16 to 80 pA, in 8-pA increments. The voltage responses show slow overshoots for a broad range of current steps. (C) Current-voltage relationship, obtained from the current responses at the end of each of the voltage-clamp pulses shown in A. (D) Voltage-current relationship, obtained from the voltage responses at the end of each of the current-clamp pulses shown in B. (E) Effective impulse responses obtained as described in Methods. The impulse responses have undershoots in two ranges of membrane voltage: slow undershoots (asterisks) over a broad range near the resting potential, probably due to the I_h channel; fast undershoots (arrows) with marked depolarization, probably due to the delayed-rectifier K⁺ channel. (E, inset) All the impulse responses in panel E are plotted on a semilogarithmic scale. All impulse responses are convex. Two impulse responses (arrows), much faster than the others, correspond to mean currents 75 and 100 pA, presumably due to the outward delayed-rectifier K⁺ conductance. The initial segments of impulse responses reflect series resistance of the recording pipette (indicated by brackets).