The name is absent

40

We have extended previous work by using this model to create a linear approximation of the
photoreceptor electrical response at different input magnitudes, and use this model predict
the change in frequency response of the cell with stimulus intensity.

In the circuit, the capacitor C represents the membrane capacitance, R_m the membrane
resistance, and the inductor L and series resistance R↑ the contribution of 7⅛. The circuit
works as follows: injected current charges the membrane capacitance with a time constant
dependent on R_m and C. After a delay, the inductor L representing the contribution of
HCN channels begins to turn on and shunt current through its branch of the circuit with a
time constant dependent on L and R_i. This causes a sag in the voltage response, shown in
figures 3.6 A and B.

To determine the values for the equivalent circuit, hyperpolarizing current pulses were
delivered to rods and cones, and the voltage responses were recorded (figure 3.6 A and B).
From these responses the parameters for each component of the circuit were estimated,
and the corresponding frequency response predicted. Derivation of the model temporal re-
sponse is given in Appendix A.l. The membrane capacitance and whole cell resistance were
determined by fitting the first hyperpolarizing pulses that failed to activate R- Parameters
for L and Ri at each input level were determined using a least-squares fit for the equivalent
circuit voltage response. This model shows that as the hyperpolarizing input magnitude in-
creases, the inductance L and series resistance R_i in the equivalent circuit decrease in both
rods and cones (figure 3.6 A3,4 and B3,4). This causes a corresponding increase in the res-
onant frequency and damping factor (figure 3.6 Al,2 and Bl,2). Derivation of the model

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