The retinal is a multilayered structure that lines the inside
of the back of the eye. Light rays are focused on the retina by
the cornea and lens. When they reach the retina, they cause chemical reactions
in the deepest layer of the retina, the rods
and cones (photoreceptors). This creates an impulse that is
transmitted through the middle layer of the retina (the bipolar cells)
to the inner portion (the ganglion cells) (Figure 31–1).
The ganglion cells then travel and coalesce in the posterior portion
of the eye to form to the optic nerve, which transmits the impulses
to the brain. The retina is analogous to the film in a camera, in
that it senses and changes in reaction to light.
Photomicrograph of retina. (A) Retinal pigment epithelium.
(B) Photoreceptors, (C) Bipolar cells, (D) Ganglion cells. (Photo
contributed by Morton Smith, MD.)
The cones are responsible for discriminating fine visual detail
and color. They are concentrated in the macula,
which is the central portion of the retina between the vascular
arcades. At the center of the macular is the fovea, visible as a
focal area of increased pigment (Figure 31–2). This is
the area of highest visual discrimination. It is used when reading
or watching objects. The rods are more sensitive to dim light. They
are concentrated in the peripheral retina. They are primarily responsible for
Normal posterior retina and optic nerve. The arrow points
to normal fovea.
On a molecular level, the perception of light is based on chemical
reactions that occur in the outer layer of the photoreceptors. In
the rods, photons are absorbed by rhodopsin molecules. This causes
a reaction that results in release of glutamate (a neurotransmitter),
which initiates a sequence of cellular connections that ultimately
stimulates the ganglion cells and is transmitted to the brain via
the optic nerve. The cones have a similar response, but there are
3 separate opsin molecules that respond to different wavelengths
of light. The apparent color of an object results from central processing
of the relative inputs from these 3 types of cones.
The retina is highly metabolically active. The rods and cones
are nourished by the retinal pigment epithelium, which lies between
the retina and the choroidal blood vessels. The inner portion of
the retina receives its blood supply from the blood vessels that
line the retina. The retina itself is transparent. The red reflex
that is visible during ophthalmoscopy and in photographs results
from light reflecting off the blood supply within the choroid.
The vitreous is the normally clear substance that fills the posterior
portion of the eye between the retina and the lens.