FIG. 2.

Influence of I _h on membrane potential responses to current injections. A Hyperpolarizing current steps produced a time-dependent, depolarizing sag, returning the membrane potential towards its zero current potential of −57 mV. The amplitude of the sag varied with current amplitude, which was −50 pA (red trace) or −100 pA (black trace). A depolarizing overshoot occurred at the end of the current step. B In a different cell an action potential occurred at the termination of the −75 pA current step (zero current potential = −67 mV). C Reversal potential of I _h was estimated from extrapolation of current responses to 40 ms voltage ramps from −139 to −109 mV after conditioning voltages to −119, −129 and −139 mV for 1 s. Voltage commands are shown above the current responses. Current extrapolations intersected where conductance = 0, i.e., the reversal potential which was −35.4 mV for the cell shown (dashed line). D Hyperpolarization-activated current increased when extracellular K⁺ was raised from 5.8 mM (control, black trace) to 30 mM (red trace). Currents shown are in response to a voltage step from −79 to −159 mV.