phodiesterase and activates it. The phosphodiesterase then interacts
with cGMP (cyclic guanosine monophosphate), hydrolyzing it to non-
cyclic GMP. The interaction of cGMP with certain ion channels in the
cell membrane keeps these channels open. When the concentration
of cGMP in the cell decreases, these ion channels close, the membrane
potential and thus the cell excitability changes, and this changes the
amount of neurotransmitter being released at the synapse between
the photoreceptor cell and other cells in the retina.
A result of this particular G-protein coupling is enormous amplifi-
cation. One photon of visible light interacting with a single rhodopsin
in arod cell produces an activated rhodopsin that can in turn bind
with as many as one hundred G-proteins, one after another, activating
them. Each G-protein can then activate a cGMP phosphodiesterase
enzyme, and each phosphodiesterase can hydrolyze hundreds of mol-
ecules of cGMP. The result is that a single photon of light may produce
—in a mere second of time—an intracellular decrease of more than
ten thousand molecules of cGMP. This has the effect of closing many,
many ion channels. Thus, a single photon of light can have significant
impact on arod cell’s signaling to other cells in the retina. This enables
the detection of very dim levels of light.
There are three major layers of cells in the retina: photoreceptor cells,
bipolar cells, and ganglion cells (Fig. 14.6). Rods and cones form
synapses with bipolar cells, and bipolar cells form synapses with
ganglion cells. Neural signals thus flow from the photoreceptors to the
bipolar cells to the ganglion cells, and thence to the brain. The axons
of the ganglion cells bundle together to form the optic nerve, which
exits the eyeball at the blind spot. While each retina has more than
one hundred million photoreceptor cells, it has only about one million
ganglion cells; thus, one million axons make up each optic nerve. This