Fig. 12.
The effects of adding the hyperpolarization-activated conductance gh on subthreshold resonance properties. A: effects of adding the conductance for the h-current (gh = 0.5gL) to the responses of a passive model cell with a slow (top trace, τm = 50 ms) or fast (bottom trace, τm = 15 ms) membrane time constant. Hyperpolarizing DC current was used to set Vm to −70 mV in the models with gh. Dashed horizontal lines show the magnitude of the membrane depolarization and hyperpolarization elicited by a step stimulus of this amplitude in the purely passive model cell. Note the asymmetry in the voltage responses to step currents. Vertical arrows show the steady-state membrane voltage change in response to depolarizing (ΔV1) and hyperpolarizing (ΔV2) step current pulses (current stimulus shown schematically at the bottom). Activation of Ih by the hyperpolarizing step resulted in a depolarizing membrane voltage “sag” toward rest (the sag ratio = peak voltage/steady-state voltage, was 1.31; see companion paper) and a decrease in the amplitude of ΔV2. The rectification ratio of the step responses was 1.44. The conductance gh caused resonance of the lower voltage envelope of the large-amplitude chirp response in the fast τm but not in the slow τm model cell. B1: combined effects of adding gh (gh = 0.5gL) and gM (gM = 80gL) to the fast τm model cell. The amount of asymmetry of the step responses was decreased compared with the trace in A, whereas the chirp response now showed resonance in both the upper and lower voltage envelopes, resulting in a more symmetrical impedance profile (fres = 3.6 Hz). B2: traces showing the magnitude and time course of Ih (top) and IM (bottom) during the voltage response shown in B1. C: superimposed impedance graphs for the purely passive model cell (dashed line), the model cell with gh (black line), and the model cell with both gh and gM (gray line).