The name is absent

56

and cones, and demonstrated that this increases the amplitude and duration of rod and cone
light responses. When considered in the frequency domain, HCN block reveals the low-
pass filter characteristic of the photocurrent [14]. This low-pass characteristic comes from
the slowness of the photocurrent, whose gating depends on a complex cascade of molecular
interactions. In contrast, the opening of voltage gated channels such as HCN channels de-
pends on the motion of charged voltage sensors and their interactions with the pore, which
is generally a much faster process [77, 76]. In normal physiological conditions, HCN chan-
nels reduce response amplitudes at low frequencies, flattening the frequency response of the
cell to light stimuli [14]. This flattening allows the voltage response to be frequency indepen-
dent over a wider range of stimulus frequencies, enabling the synapse to avoid saturation at
low frequencies while still passing higher frequency signals. From a signal processing stand-
point, this compensatory effect by HCN channels is analogous to high-pass filtering. Other
studies previously described high-pass filtering in the rod network and postulated that it
could be a way for the network to increase the signal to noise ratio for transient signals by
spreading them over a larger area [33,87,103]. By dividing transient signals into networked
rods, multiple parallel rod to bipolar cell synapses can be used, increasing the signal-to-noise
ratio at the bipolar cell layer. In addition to their role in the rod network, other studies have
also shown how high-pass filtering due to 7⅛ is important in increasing the speed of the the
individual photoreceptor light response [49, 30].

Although HCNl channels appear to be major players in the rod and cone light response,
they are not the only source of filtering by voltage-gated ion channels. Another current called

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