The name is absent

39

series capacitance, it can act only as a low pass filter. Changes in passive membrane resis-
tance by blocking I_h can only cause a shift in the cutoff frequency of the Iowpass filter, and
not the highpass filter characteristic observed in these experiments.

From an experimental standpoint, although our chirped current injection experiments
were carried out in the whole mount retina, we were able to observe the same bandpass effect
in cone photoreceptors as in rods. Salamander cones have been shown to have much weaker
coupling to adjacent rods than rod-rod coupling, which would mean that if the bandpass fil-
ter effect were from the network, the effect should be much less for cones than rods. Because
we instead observe a very similar bandpass filtering effect in cones and rods, it is likely that
the effect comes from the electrical properties of HCN channels in individual cells, than
from the coupled network. We believe that the bandpass filtering observed in this and other
studies [115] is a result of active voltage gated conductances, such as I_h, and not the coupled
rod network.

3.3.3 Equivalent circuit

With the realization that the electrical characteristics of the cell are equivalent to the com-
bination of an HCN mediated high-pass filter and a low-pass filter from the cell membrane,
an equivalent circuit can be constructed (figure 3.6 C). The voltage response of a rod and
cone due to the I_h current at given input current can be modeled as a linear time dependent
circuit [33, 34, 87,103]. This circuit model has been studied before, and demonstrates what
is referred to as the ’’inductive” property of the photoreceptors due to I_h [30,33,34,87,103].

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