The name is absent

37

3.3.2 HCN contribution to photoreceptor membrane frequency response

Frequency-chirped sinusoidal current stimuli according to equation 2.1 were injected into
both rods and cones in order to examine the contribution of HCN channels to the electrical
frequency response of these cells. All other currents were blocked (see Solutions in Meth-
ods), and the voltage responses were recorded (figure 3.5 Al and Bl). The magnitude of the
Fourier transform of the stimulus and response were computed, and the impedance plot-
ted as Z(f) = ∣V(∕)∣∕∣∕(∕)∣. This analysis demonstrated that HCN channels cause a peak
in the electrical impedance of both rod and cone photoreceptors at around 4 Hz, an effect
equivalent to a bandpass filter (figure 3.5 A2 and B2, black traces).

Application of 100 μM ZD 7288, which selectively blocks HCN channels [65, 94], was
found to abolish the bandpass filter response (figure 3.5 A2 and B2, red traces). The fre-
quency responses with HCN channels blocked with ZD7288 are those of a passive mem-
brane, and equivalent to a low-pass filter. These results reveal that the bandpass filter effect
seen when HCN channels are active arises because of a combination of an ∕⅛-mediated high-
pass filter, and a Iowpass filter formed from the membrane resistance and capacitance. In
other words HCN channels serve to lower the membrane impedance at low frequencies,
which means the cell will be less sensitive to inputs at these frequencies.

From a theoretical perspective, a passive rod network would be modeled electrically with
cells represented by the parallel combination of a resistor and capacitor, coupled to a neigh-
boring cell of the same configuration by a resistor. This is the same as the cable equation
(see section 5.4). Because the network has only parallel resistances and capacitances and no

<<   43   44   45   46   47   48   49   >>

More intriguing information