Structure and Function
The following topics relating to the structure and function of the retina appear below:
Photoreceptor Cells
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Photoreceptor cells include rods and cones and are uniquely located towards the posterior aspect of the retinal sublayers, further away from the pupil where light enters the eye. Rods are more sensitive in dim light (scotopic vision) and reside in the periphery of the retina. Cones are more sensitive in daylight (photopic vision) and capture wavelengths of colored light. Cones localize in the center of the retina at the fovea. There are approximately 6 million cones and often more than 100 million rods within the retina.[1] There exist three types of cones including tritans, deutrans, and protans, named for detecting short, medium, and long wavelengths, respectively. In terms of sensing colored light, each type of cone cell can respectively characterize as detecting blue, green, and red wavelengths. The overlap of detectable wavelength spectrums between the three types of cones results in the visible light spectrum perceived by humans. Rod cells contain rhodopsin, which is a light-sensitive pigment made of retinal that allows for the absorption of photons. Retinal is vitamin A aldehyde, making vitamin A an essential dietary component for the facilitation of the phototransduction pathway. Vitamin A deficiency is a significant risk factor for blindness in young children and remains prominent in under-developed regions, including South Asia and sub-Saharan Africa.[2]
Layers of the Retina
The retina, more specifically, subdivides into ten distinct layers that are described in order from the innermost layers closer to the pupil to the layers further towards the posterior and periphery of the eyeball:
Macula
The macula, also called the macula lutea for its yellowish pigmented appearance, makes up the most sensitive area of the retina, offering the highest visual acuity. It is found temporally from the optic disc upon fundoscopic examination. Lutein and zeaxanthin are carotenoids that make up the macular pigments and produce the yellow coloring. These macular pigments are known to have anti-inflammatory and blue-light filtering properties.[12] Dietary supplementation of lutein and zeaxanthin has been shown to increase pigment density and is associated with reduced risk of diabetic retinopathy in adults and retinopathy of prematurity in infants born pre-term.[13] In the center of the macula is an avascular depression called the fovea, which contains a high concentration of cones.
The macula further subdivides into the following sequentially-smaller concentric zones that characteristically show decreasing rod density and fewer layers of cells covering the photoreceptor cells:
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