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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 peripheral vision.


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.



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