The name is absent

67

tionally, an increase in pH buffering capacity in the extracellular solution appears to reduce
the CSRFA in cone light responses [55].

Both the hemichannel and proton feedback theories cause feedback through a modula-
tory effect on the cone calcium current. However, it is unclear if the observed cone depolar-
ization during a surround response could be due entirely to an increase in calcium current.
Others have proposed that the effect on the calcium current must have a secondary effect
on a calcium activated chloride current in order to cause a significant depolarization [68].
To assess the plausibility of these two theories, we built a model of the cone photoreceptor
using data from the known cone ion channels and photocurrent. A block diagram for this
model is shown in figure 4.4 A. This model includes sub-models of the photocurrent [97],
h-current I_h [13], voltage gated calcium current Ic_a> internal calcium concentration [Ca]_i,
and calcium activated chloride current Ici(Ca) [12,68]. The flash response for the model is
shown in figure 4.4 B. The model for the feedback begins with an external effector causing a
left (negative) shift in the activation curve for the voltage gated calcium channels. It there-
fore accommodates both the hemichannel and proton feedback models. The leftward shift
causes an increase in calcium current (figure 4.5 B), which over time causes an increase in
internal calcium concentration (figure 4.5 C). The calcium current by itself causes a slight
depolarization in the membrane (figure 4.5 A), but the main effect comes from a gating of
a calcium activated chloride current. With E_cι i∏ the ^cone at -43 mV, hyperpolarization
of the cone membrane potential with light causes gated the chloride current to be an in-
ward current (figure 4.5 D), causing a depolarization in the cone membrane (figure 4.5 A),

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