Anatomy & Physiology I and II

Anatomy & Physiology Study Guide the retina contains supporting cells and neurons that perform preliminary processing and integration of visual information. The retina contains several layers of cells. The layer closest to the pigmented part of the retina contains cells that detect light called photoreceptors . Two types of photoreceptors are within the eye: rods and cones. Rods do not discriminate among colors of light. With sensitivity to light, rods allow for vision in dimly lit rooms, at twilight, and in the pale moonlight. Cones provide color vision. Three types of cones are present, and their stimulation in various combinations provides the perception of different colors. Cones produce sharper, clearer images than rods do, but a more intense light is required for cones to function. The outer surface of the retina does not have rods and cones distributed evenly across it. Approximately 125 million rods form a broad band around the periphery of the retina. The density of rods gradually decreases as one moves from the periphery toward the retinal center. In contrast, most of the roughly six million cones are concentrated in the area where a visual image arrives after it passes through the cornea and lens. This region, which is known as the macula lutea , contains no rods. The highest concentration of cones occurs at the center of the macula lutea, an area called the fovea . The fovea is the site of sharpest vision. Rods and cones synapse with roughly six million neurons called bipolar cells, which in turn synapse within the layer of neurons called ganglion cells. Horizontal and amacrine cells can facilitate or inhibit communication between photoreceptors and ganglion cells, thereby altering the sensitivity of the retina. The effect is comparable to adjusting the contrast on a television screen. These cells play an important role in the adjustment of the eye to dim or brightly lit environments. The Optic Disc Axons from an estimated one million ganglion cells converge on the optic disc, a circular region just medial to the fovea. From this point, the axons turn, penetrate the wall of the eye, and proceed toward the diencephalon. The blood vessels that serve the retina are the central retinal artery and central retinal vein, which pass through the center of the optic nerve and emerge on the surface of the optic disc. The optic disc has no photoreceptors or other structures typical of the rest of the retina. Because light striking this area goes unnoticed, the optic disc is commonly called the blind spot. The Chambers of the Eye The eye contains a large posterior cavity, or vitreous chamber, and a smaller anterior cavity. The anterior cavity is subdivided into the anterior chamber, which extends from the cornea to the iris, and a posterior chamber, between the iris and the ciliary body and lens. The fluid aqueous humor fills the anterior and posterior chambers of the eye. Aqueous humor is a fluid that circulates within the anterior cavity, passing from the posterior to the anterior chamber through the pupil. It also freely diffuses through the vitreous body and across the surface of the retina. Aqueous humor forms through active secretion by epithelial cells of the ciliary body’s ciliary processes. It supplies an important route for nutrient and waste transport and forms a fluid cushion. The eye is filled with fluid, and the fluid pressure in the aqueous humor helps to maintain the shape of the eye. ©2018 Achieve Test Prep Page 192 of 367