61
nels and the photocurrent numerically (see Appendix A.3 for model parameters). The model
was stimulated with five flashes of light of increasing intensity, and the time courses of the
voltage, h, and Kx conductances at each flash intensity were evaluated. During a light re-
sponse the voltage (figure 4.2 A) causes an increase in h conductance and a decrease in Kx
conductance (figure 4.2 B). These complimentary conductance changes tend to counterbal-
ance one another during the flash response, resulting in a reduced net conductance change
whose amplitude is time dependent (figure 4.2 B, green traces). With large stimuli, the faster
response kinetics of Ih cause a small transient conductance increase, followed by a longer
lived conductance decrease due to Iκx∙ Smaller stimuli cause a more synchronous activation
of Ih and IKx (figure 4.2 B, green traces). In our model, the net conductance change due to
both currents deviates no more than 0.3 nS from the resting level, whereas each individual
conductance changes by nearly 0.6 nS. Although the conductance increase by ∕⅛ and de-
crease by IKx are not perfectly synchronized, together they halve the maximal conductance
change of one current individually.
4.1.3 Discussion
It has been observed using current pulse injection that, in contrast to cones, rods do not
undergo an appreciable conductance change during a light response. [17] One explanation
for this observation was that during a light response, an increase in h conductance counter-
acts the conductance decrease from the photocurrent. [17] This hypothesis does not account
for the then unknown Kx conductance, and overlooks an important property of the pho-