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10.1. RETINA & RETINAL DISORDERS
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The human retina is the most complex of the ocular tissues with a highly organized structure. It receives the visual image, produced by the optical system of the eye, and converts the light energy into an electrical signal, which undergoes initial processing and is then transmitted through the optic nerve to the visual cortex, where the structural (form, color, and contrast) and spatial (position, depth, and motion) attributes are perceived. The anatomy of the retina is described in Chapter 1, Figure 1–17 showing its layers. Function and functional disturbance in the retina often can be localized to a single layer or a single cell type.
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Rod and cone photoreceptors are responsible for the initial transformation, by the process of phototransduction, of light stimuli into the nerve impulses that are conducted through the visual pathways to the visual cortex. These photoreceptors are arranged such that there is an increased density of cones in the center of the macula (fovea), decreasing to the periphery, and a higher density of rods in the periphery. In the foveola, there is a nearly 1:1 relationship between each cone photoreceptor, its ganglion cell, and the emerging nerve fiber, whereas in the peripheral retina, many photoreceptors connect to the same ganglion cell. The fovea is responsible for good spatial resolution (visual acuity) and color vision, both requiring high ambient light (photopic vision) and being best at the foveola, while the remaining retina is utilized primarily for motion, contrast, and night (scotopic) vision.
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The rod and cone photoreceptors are located in the avascular outermost layer of the sensory retina. Each rod photoreceptor cell contains rhodopsin, a photosensitive visual pigment embedded in the double-membrane disks of the photoreceptor outer segment. It is made up of two components, an opsin protein combined with a chromophore. The opsin in rhodopsin is scotopsin, which is formed of seven transmembrane helices. It surrounds the chromophore, retinal, which is derived from vitamin A. When rhodopsin absorbs a photon of light, 11-cis retinal is isomerized to all-trans retinal and eventually to all-trans retinol. The resulting configurational change initiates a secondary messenger cascade. Peak light absorption by rhodopsin occurs at approximately 500 nm, which is in the blue-green region of the light spectrum. Spectral sensitivity studies of cone photopigments have shown peak wavelength absorption at 430, 540, and 575 nm for blue-, green-, and red-sensitive cones, respectively. The cone photopigments are composed of 11-cis retinal bound to other opsin proteins than scotopsin.
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Night (scotopic) vision is mediated entirely by rod photoreceptors. With this dark-adapted form of vision, varying shades of gray are seen but colors cannot be distinguished. As the retina becomes fully light-adapted, the spectral sensitivity of the retina shifts from a rhodopsin-dominated peak of 500 nm to approximately 560 nm, and color sensation becomes evident. An object takes on color when ...