35
the voltage response of an actual rod photoreceptor at multiple light intensities (figure 3.4
A, B). By using actual data for the photocurrent (figure 3.4 D) and comparing the model
response to the actual voltage response, we made every effort to make our model reflect not
only the voltage response of a photoreceptor, but the contribution of individual ionic cur-
rents. Through the simulation, we were able to predict the time course and magnitude of
the Ih current during the voltage response of a rod. Using our data for the single channel
conductance, we were also able to estimate the number of HCN channels open at any given
instant in time during a typical flash light response. The model predicts an opening proba-
bility Po = .02 at the dark membrane potential, which corresponds to an average of about 44
open HCN channels. This means that in darkness and at very dim light intensities, only a few
HCN channels contribute to the resting potential. On the other hand, in response to a bright
flash, HCN channels reach a peak open probability of Po =.65, which corresponds to 1430
open HCN channels (figure 3.4 C). The model also shows that once they are opened by the
fast initial hyperpolarization during the light response, HCN channels are slow to close after
the sharp ’’nose” in the voltage réponse. This finding uncovers a potential advantage for the
extremely low single channel conductance observed by our NSFA studies in photoreceptors.
Namely, at a low opening probability, as in darkness and dim light, greater numbers of low
conductance channels spontaneously opening and closing (as found with photoreceptor ∕⅛)
would cause less noisy fluctuations in the membrane potential than small numbers of high
conductance channels. This means that a low single HCN channel conductance could help
improve the signal-to-noise ratio of rods in dim light.
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