The retina is the light-sensitive portion of the eye that contains (1) the cones, which are responsible for color vision, and (2) the rods, which are mainly responsible for black and white vision and vision in the dark. When either rods or cones are excited, signals are transmitted first through successive layers of neurons in the retina itself and, finally, into optic nerve fibers and the cerebral cortex. The purpose of this chapter is to explain the mechanisms by which the rods and cones detect light and color and convert the visual image into optic nerve signals.
Anatomy and Function of the Structural Elements of the Retina
Layers of the Retina. Figure 50-1 shows the functional components of the retina which are arranged in layers from the outside to the inside as follows: (1) pigmented layer, (2) layer of rods and cones projecting to the pigment, (3) outer nuclear layer containing the cell bodies of the rods and cones, (4) outer plexiform layer, (5) inner nuclear layer, (6) inner plexiform layer, (7) ganglionic layer, (8) layer of optic nerve fibers, and (9) inner limiting membrane.
After light passes through the lens system of the eye and then through the vitreous humor, it enters the retina from the inside (see Figure 50-1); that is, it passes first through the ganglion cells and then through the plexiform and nuclear layers before it finally reaches the layer of rods and cones located all the way on the outer edge of the retina. This distance is a thickness of several hundred micrometers; visual acuity is decreased by this passage through such nonhomogeneous tissue. However, in the central foveal region of the retina, as discussed subsequently, the inside layers are pulled aside to decrease this loss of acuity.
Foveal Region of the Retina and Its Importance in Acute Vision. The fovea is a minute area in the center of the retina, shown in Figure 50-2, occupying a total area a little more than 1 square millimeter; it is especially capable of acute and detailed vision. The central fovea, only 0.3 millimeter in diameter, is composed almost entirely of cones; these cones have a special structure that aids their detection of detail in the visual image. That is, the foveal cones have especially long and slender bodies, in contradistinction to the much fatter cones located more peripherally in the retina. Also, in the foveal region, the blood vessels, ganglion cells, inner nuclear layer of cells, and plexiform layers are all displaced to one side rather than resting directly on top of the cones. This allows light to pass unimpeded to the cones.
Rods and Cones. Figure 50-3 is a diagrammatic representation of the essential components of a photoreceptor (either a rod or a cone). As shown in Figure 50-4, the outer segment of the cone is conical in shape. In general, the rods are narrower and longer than the cones, but this is not always the case. In the peripheral portions of the retina, the rods are 2 to 5 micrometers in diameter, whereas the cones are 5 to 8 micrometers in diameter; in the central part of the retina, in the fovea, there are rods, and the cones are slender and have a diameter of only 1.5 micrometers.
To the right in Figure 50-3 are labeled the major functional segments of either a rod or a cone: (1) the outer segment, (2) the inner segment, (3) the nucleus, and (4) the synaptic body. The light-sensitive photochemical is found in the outer segment. In the case of the rods, this is rhodopsin; in the cones, it is one of three "color"
Layers of retina.
j Pigmented layer
Outer nuclear layer
Outer plexiform layer
Inner nuclear layer j Pigmented layer
Outer nuclear layer
Outer plexiform layer
Inner nuclear layer
} Inner plexiform layer
Ganglion cell I Ganglion cell layer
To optic nerve
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