Ganong's Review of Medical Physiology, 23rd Edition

192 SECTION III Central & Peripheral Neurophysiology

darkness. The amount of rhodopsin in the receptors therefore
varies inversely with the incident light level.

CONE PIGMENTS

Primates have three different kinds of cones. These receptors
subserve color vision and respond maximally to light at wave-
lengths of 440, 535, and 565 nm. Each contains retinene 1 and
an opsin. The opsin resembles rhodopsin and spans the cone
membrane seven times but has a characteristic structure in
each type of cone. The cell membrane of cones is invaginated
to form the saccules, but the cones have no separate intracel-
lular disks like those in rods. The details of the responses of
cones to light are probably similar to those in rods. Light ac-
tivates retinene 1 , and this activates Gt2, a G protein that dif-
fers somewhat from rod transducin. Gt2 in turn activates
phosphodiesterase, catalyzing the conversion of cGMP to 5'-
GMP. This results in closure of Na+ channels between the ex-
tracellular fluid and the cone cytoplasm, a decrease in intra-
cellular Na+ concentration, and hyperpolarization of the cone
synaptic terminals.
The sequence of events in photoreceptors by which incident
light leads to production of a signal in the next succeeding
neural unit in the retina is summarized in Figure 12–15.

RESYNTHESIS OF CYCLIC GMP

Light reduces the concentration of Ca2+ as well as that of Na+
in photoreceptors. The resulting decrease in Ca2+ concentra-
tion activates guanylyl cyclase, which generates more cGMP.
It also inhibits the light-activated phosphodiesterase. Both ac-
tions speed recovery, restoring the Na+ channels to their open
position.

MELANOPSIN

A small number of photoreceptors contain melanopsin rather

than rhodopsin or cone pigments. The axons of these neurons

project to the suprachiasmatic nuclei and the part of the lateral

geniculate nuclei that controls the pupillary responses to light.

When the gene for melanopsin is knocked out, circadian photo-

entrainment is abolished. The papillary light responses are re-

duced, and they are abolished when the rods and cones are also

inactivated. Thus, a part of the pupillary responses and all the cir-

cadian entrainment responses to light–dark changes are con-

trolled by a system separate from the rod and cone systems.

PROCESSING OF VISUAL

INFORMATION IN THE RETINA

In a sense, the processing of visual information in the retina in-

volves the formation of three images. The first image, formed by

the action of light on the photoreceptors, is changed to a second

image in the bipolar cells, and this in turn is converted to a third

image in the ganglion cells. In the formation of the second im-

age, the signal is altered by the horizontal cells, and in the for-

mation of the third, it is altered by the amacrine cells. There is

little change in the impulse pattern in the lateral geniculate bod-

ies, so the third image reaches the occipital cortex.

FIGURE 12–14 Initial steps in phototransduction in rods.
Light activates rhodopsin, which activates transducin to bind GTP. This
activates phosphodiesterase, which catalyzes the conversion of cGMP
to 5'-GMP. The resulting decrease in the cytoplasmic cGMP concentra-
tion causes cGMP-gated ion channels to close.

Outer segment membrane

Rhodopsin Transducin

cGMP

phospho-

diesterase

Disk

Light

GTP

5'-GMP

cGMP-gated

channel

Na+

ECF

Rod outer segment

cGMP

FIGURE 12–15 Sequence of events involved in

phototransduction in rods and cones.

Incident light

Structural change in the

retinene 1 of photopigment

Conformational change

of photopigment

Activation of transducin

Activation of

phosphodiesterase

Decreased

intracellular cGMP

Closure of Na+ channels

Hyperpolarization

Decreased release of

synaptic transmitter

Response in bipolar cells

and other neural elements