The human retina is a highly organized structure, consisting of alternate layers of cell bodies and synaptic processes. Despite its compact size and apparent simplicity when compared with nervous structures such as the cerebral cortex, the retina has a remarkably sophisticated level of processing power. Visual processing of the retina is elaborated upon by the brain, and the perception of color, contrast, depth, and form occurs in the cortex.
The anatomy of the retina is presented in Chapter 1. Figure 1-17 shows the major cell types and identifies the layers of this tissue. Division of the retina into layers composed of groups of similar cells permits the clinician to localize a function or functional disturbance to a single layer or group of cells. Processing of retinal information proceeds from the photoreceptor layer through the ganglion cell axon to the optic nerve and brain.
The retina is the most complex of the ocular tissues. In order to see, the eye must perform as an optical instrument, as a complex receptor, and as an effective transducer. Rod and cone cells in the photoreceptor layer are capable of transforming light stimulus into a nerve impulse that is conducted by the nerve fiber layer of the retina through the optic nerve and ultimately to the occipital visual cortex. The macula is responsible for the best visual acuity and for color vision, and most of its photoreceptor cells are cones. In the central fovea, there is a nearly 1:1 relationship between the cone photoreceptor, its ganglion cell, and the emerging nerve fiber, and this ensures the most acute vision. In the peripheral retina, many photoreceptors are coupled to the same ganglion cell, and a more complex system of relays is necessary. The result of such an arrangement is that the macula is used primarily for central and color vision (photopic vision) while the remaining retina, which is populated mostly by rod photoreceptors, is utilized primarily for peripheral and night (scotopic) vision.
The rod and cone photoreceptors are located in the avascular outermost layer of the sensory retina and are the site of the chemical reaction initiating the visual process. Each rod photoreceptor cell contains rhodopsin, which is a photosensitive visual pigment formed when opsin protein molecules combine with 11-cis retinal. As a photon of light is absorbed by rhodopsin, 11-cis retinal is immediately isomerized to its all-trans form. Rhodopsin is a membrane-bound glycolipid that is partially embedded in the double membrane disks of the photoreceptor outer segment. Peak light absorption by rhodopsin occurs at approximately 500 nm, which is 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 a variety of opsin proteins.
Scotopic vision is mediated entirely by the 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 it contains photopigments that absorb specific wavelengths and selectively reflect or transmit certain wavelengths of light within the visible spectrum (400-700 nm). Daylight vision is mediated primarily by cone photoreceptors, twilight by a combination of cones and rods, and night vision by the rod photoreceptors.
The examination of the retina is described in Chapter 2 and depicted in Figures 2-12, 2-13, 2-14, 2-15, 2-16, 2-17 and 2-18. The retina can be examined with a direct or indirect ophthalmoscope or with a slitlamp (biomicroscope) and contact or handheld biconvex lens. With these instruments, the skilled observer is clinically able to dissect the layers of the retina in order to determine the type, level, and extent of retinal disease. Fundus photography and fluorescein angiography ( Figures 2-27, 2-28 and 2-29) are useful adjuncts to the clinical examination; photography allows pictorial documentation for future comparison, and angiography provides the vascular detail needed for laser treatment of retinal diseases.
The clinical application of visual electrophysiologic and psychophysical tests is described in Chapter 2. Such tests may be helpful in establishing the diagnosis of certain disease entities.
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