The name is absent

62

tocurrent. During a light response, the photocurrent, which is actually a shutting off of the
inward dark current, causes hyperpolarization of the cell membrane. The dark current’s in-
stantaneous I-V relation is nearly flat throughout the rod’s physiological voltage range, from
-20 to -80 mV, in both light and darkness (Baylor and Nunn, figure 6) [18]. This property
means that from the standpoint of the rod, the photocurrent acts as a current source whose
magnitude depends on light, and not on the membrane potential. The consequence, which
may seem Conterintuitive, is that although the photocurrent is mediated by a closing of the
ion channels carrying the dark current, the voltage-independence of the current through
these channels means that it does not contribute to a membrane conductance change dur-
ing a light response. There may, however, be a slow conductance change associated with the
voltage dependance of the Na-Ca-K exchange pump. [60] It is important to note that unlike
the rod, the cone dark current I-V relation is not flat, and therefore cones do undergo a
conductance decrease when exposed to light. [112]

With the dark current ruled out as a source of conductance change, we conclude that
the lack of observed net conductance change during a rod light response is likely due to the
coordinated counterbalancing of h and Kx conductances, as we show with our simulation
(figure 4.2). While the opposite conductance changes by Ih and I_Kx were first investigated
some time ago [87, 103], the question remains as to what, if any, advantage these comple-
mentary changes would confer during a light response. One theory is that the two different
conductance changes are a consequence of having two separate mechanisms (⅛_c and 7⅛)
for filtering small and large signals. [19] Alternatively, we propose that the answer to this

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