The name is absent

52

show that in acting as a highpass filter, HCN channels reduce the time needed for rods and
cones to respond to encode visual information in the optimal operating conditions for each
cell type—dim light for rods, and brighter light for cones. One explanation for this effect
is the difference in the photocurrents of each cell. Rods, which have a slow but high gain
photocurrent, operate within the active voltage range for HCN channels at dim light inten-
sities, but at brighter light intensities their photocurrent saturates, and is no longer able to
be filtered by HCN channels. On the other hand, cones have a faster but lower gain pho-
tocurrent, which does not utilize the operational range of HCN channels unless the cell is
stimulated with brighter light (figure 3.9). Therefore, although the biophysical and electrical
properties of HCN channels are similar in rods and cones, the channels’ effect on rod and
cone light responses is specific to the distinct function of these photoreceptors.

Despite its importance in shaping the rod and cone light response, Д is not the only
ionic current that plays a role in bandpass filtering in rod photoreceptors, ⅛τ, a potassium
current, also plays a role in shaping the rod light response [19,73], but its role in cones is still
unclear [13]. /ʌ-ʃ, similar to ∕⅛, exerts a depolarizing force on the membrane potential when
the membrane is hyperpolarized. Traditionally, Iχx is thought to be the primary mediator
of high-pass filtering of small signals in rods because it is active around the dark membrane
potential, while I_h is involved with filtering larger signals [19]. Contrary to these beliefs, we
show that although ∕⅛ is minimally active at the rod dark membrane potential, it still affects
the rod light response in dim conditions (figure 3.9). Therefore, it appears that both I_h and
I_κx are important in accelerating dim light responses.

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