Anatomy & Physiology

Anatomy & Physiology Study Guide The ciliary body is a thickened region that begins deep to the junction between the cornea and the sclera. The iris attaches to it anteriorly. The bulk of the ciliary body consists of the ciliary muscle, which is a smooth muscular ring that projects into the interior of the eye. The connective tissue fibers of these ligaments keep the lens posterior to the iris and centered on the pupil. Subsequently, light passing through the pupil also passes through the lens. The choroid is a vascular layer that separates the fibrous and neural tunics posterior to the ora serrata. Protected by the sclera and attached to the outermost layer of the retina, the choroid contains an extensive capillary network that delivers oxygen and nutrients to the retina. The choroid also contains melanocytes, which are especially numerous near the sclera. The Neural Tunic (Retina) The neural tunic, or retina, is the innermost layer of the eye. It is made of a thin, outer layer called the pigmented part, and a thick inner layer called the neural part. Light that passes through the neural part is absorbed by the pigmented part of the retina. In addition to light receptors, the neural part of 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 Achieve Page 192 of 368 ©2018