302 Chapter 10
Figure 10.41 The effects of light on the retina. ( a ) In the dark, the continuous dark current (1) depolarizes the
photoreceptors and causes them to (2) release inhibitory neurotransmitter at their synapses with bipolar cells. ( b ) In the light, (3) cGMP
declines (due to its conversion to GMP), stopping the dark current and hyperpolarizing the photoreceptors. As a result, (4) the release
of inhibitory neurotransmitter is stopped. Because they are not inhibited in the light, (5) bipolar cells release excitatory neurotransmitter
at their synapses with ganglion cells so that the ganglion cell axons are stimulated to produce action potentials.
Inner
segmentOuter
segmentRodIn DarkDark currentNa+/K+
pumpcGMPNa+Na+K+K+K+^ channelBipolar cell does
not stimulate
ganglion cell(a) (b)Release of inhibitory
neurotransmitter
continuous in the darkNa+^ channel kept
open by cGMPNa+K+K+Inner
segmentOuter
segmentRodBipolar
cellBipolar
cellGanglion
cellGanglion
cellIn LightDark current stopscGMPGMPNa+/K+^ pumps continueLoss of cations causes
rod to become
hyperpolarized,
inhibiting its release
of neurotransmitterNo inhibitory
neurotransmitterRelease of excitatory
neurotransmitter
(glutamate) stimulates
ganglion cellDecline in cGMP closes
Na+^ channel
12345