The name is absent

64

ther (figure 4.3, red arrow), and with low membrane impedance (high conductance), signals
dissipate more readily. If Iχ_x was absent and only I_h was present in rods, not only would
high-pass filtering of input signals be reduced, but signals would dissipate more quickly due
to unopposed h conductance increase (figure 4.3). If only I_Kx was present, then the propa-
gation of signals in the network would be weighted to favor larger responses that completely
turn off the Kx conductance. By having both h and Kx conductance, the cell achieves a high
degree of filtering of input signals while minimizing the distortion of signal propagation in
the network that would be a consequence of membrane conductance change.

We demonstrate that I_h filters the light response even when I_Kx, ICa and other potas-
sium currents are blocked. This is further evidence that both currents are necessary for fil-
tering of input signals to rods. While it has been previously shown I_h and Iχ_x cause opposite
conductance changes during a light response, the potential advantage of these complemen-
tary conductance changes has been unclear. Results from our membrane model of the rod
show that the conductance changes from I_h and I_Kx do largely cancel one another, and
that the time course of this net conductance change depends on the stimulus flash intensity.
Further, we propose a potential advantage of the complementary conductances may be to
maximize the amount of filtering by voltage gated channels while minimizing any pertur-
bation of signal spread in the rod network. This would allow the cell to optimally spread
signals from illuminated cells into adjacent rods for better use of the synapses between rods
and bipolar cells.

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