Harrison's Internal Medicine Chapter 29. Disorders of the Eye The Human Visual System The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to aid in the guidance of behavior. The act of seeing begins with the capture of images focused by the cornea and lens upon a light-sensitive membrane in the back of the eye, called the retina. The retina is actually part of the brain, banished to the periphery to serve as a transducer for the conversion of patterns of light energy into neuronal signals. Light is absorbed by photopigment in. | Chapter 029. Disorders of the Eye Part 1 Harrison s Internal Medicine Chapter 29. Disorders of the Eye The Human Visual System The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to aid in the guidance of behavior. The act of seeing begins with the capture of images focused by the cornea and lens upon a light-sensitive membrane in the back of the eye called the retina. The retina is actually part of the brain banished to the periphery to serve as a transducer for the conversion of patterns of light energy into neuronal signals. Light is absorbed by photopigment in two types of receptors rods and cones. In the human retina there are 100 million rods and 5 million cones. The rods operate in dim scotopic illumination. The cones function under daylight photopic conditions. The cone system is specialized for color perception and high spatial resolution. The majority of cones are located within the macula the portion of the retina serving the central 10 of vision. In the middle of the macula a small pit termed the fovea packed exclusively with cones provides best visual acuity. Photoreceptors hyperpolarize in response to light activating bipolar amacrine and horizontal cells in the inner nuclear layer. After processing of photoreceptor responses by this complex retinal circuit the flow of sensory information ultimately converges upon a final common pathway the ganglion cells. These cells translate the visual image impinging upon the retina into a continuously varying barrage of action potentials that propagates along the primary optic pathway to visual centers within the brain. There are a million ganglion cells in each retina and hence a million fibers in each optic nerve. Ganglion cell axons sweep along the inner surface of the retina in the nerve fiber layer exit the eye at the optic disc and travel through the optic nerve optic chiasm and optic tract to reach targets in the brain. The majority of .
