The retina is a light-sensitive tissue that is found in the inside surface of the back of the eye. The eye captures light and creates an image of the visual world on the retina. The retina’s function can be compared to film in a camera. As the light hits the retina, it starts a chain reaction of chemical and electrical signals that trigger nerve impulses to be sent to various visual centers of the brain. These brain centers then interpret the signals as visual images.
The cells that make up the retina consist of three basic types: photoreceptor cells, neuronal cells, and glial cells. Photoreceptor cells are known as cones and rods. Cones work best in bright conditions and provide color vision. Rods function in dim light and provide black-and-white vision. There are three types of cones; each one perceives different wavelengths or colors of light and each one contains a different colored visual pigment. These pigments are called the red, blue, or green visual pigments. The center of the retina contains mostly cones while rods dominate the outer portions of the retina. The highest density of cones is at the center of the fovea—the center of the retina is the macula and the center of the macula is the fovea. There are no rods at the center of the fovea.
Neural cells include bipolar cells, ganglion cells, horizontal cells, and amacrine cells. Bipolar cells connect the photoreceptors to the ganglion cells. Ganglion cells have dendrites that connect with bipolar cells. Horizontal cells connect bipolar cells with each other. And finally, amacrine cells connect bipolar and ganglion cells with each other. Glial cells are scattered between and within the axons of the ganglion cells in the retina and optic nerve. These supporting cells of the retina include Müller cells, astrocytes, and microglial cells.
The retina comprises 10 different cell layers. The inner surface of the retina is next to the vitreous of the eye, which is the glass-like portion of the eye. The outermost layer of the retina, the retinal pigment epithelium, is attached to the choroid. Starting from the inner surface, the first layer is the inner limiting membrane. This basement membrane consists of Müller cells, which serve as support cells for the neurons of the retina. The second layer is the nerve fiber layer, which contain axons of the ganglion cell nuclei. The third layer is the ganglion cell layer. It comprises the nuclei of retina ganglion cells and axons of the optic nerve. The fourth layer is the inner plexiform layer and contains the synapse between the dendrites of the retinal ganglion cells and cells of the inner nuclear layer. The fifth layer is the inner nuclear layer. This layer is made up of three types of cells: bipolar cells, horizontal cells, and amacrine cells. The sixth layer is the outer plexiform layer that consists of the synapses between the dendrites of horizontal cells from the inner nuclear layer and the rods and cones of the outer nuclear layer. In the macular region, this is known as the fiber layer of Henle. The seventh layer is the outer nuclear layer and contains the cell bodies of rods and cones. The eighth layer is the external limiting membrane. This layer separates the inner segment of the rods and cones from their nucleus. The ninth layer is the photoreceptor layer, also known as the layer of rods and cones or Jacob’s membrane. As its name implies, it comprises both rods and cones. Lastly, the tenth and outermost layer is the pigmented layer or retinal pigment epithelium. This layer is filled with densely packed pigmented hexagonal cells.
The main function of the retina is to convert light into neural signals. This involves four basic processes: photoreception, transmission to bipolar cells, transmission to ganglion cells, and transmission along the optic nerve. Damage to any of these cells or processes can cause different visual problems including blindness.
In photoreception, light passes through the inner layers of the retina to reach the rods and cones. The photoreceptors contain a photopigment, which captures individual photons of light and turns them into neural signals. The rods and cones transfer the light and relay the signal to their cell bodies and out to their axons. These axons then contact the dendrites of both bipolar cells and horizontal cells. Horizontal cells are parallel interneurons that help with signal processing. The bipolar cells then pass the signal from photoreceptors to their axons. In the inner plexiform layer, bipolar axons contact ganglion cell dendrites and amacrine cells. The final step involves transmission along the optic nerve. The ganglion cells send their axons through the nerve fiber layer and meet at the center of the retina. This forms the optic nerve. The ganglion cell axons leave the eye, making up the optic nerve. These axons then travel along with the signal all the way to the lateral geniculate nucleus in the brainstem.
Macular degeneration is a major disorder of the retina and describes a group of diseases characterized by the loss of vision in the center of the visual field caused by macula cell death. It is mainly age related and usually comes in two forms: wet and dry. In the dry form, cellular debris builds up between the retina and the choroid, which may cause the retina to become detached. Retinal detachment involves the retina peeling away from the choroid. In the more severe wet form, blood vessels form behind the retina, which may also cause the retina to become detached. Retinal detachment may also be caused by trauma, such as a blow to the eye or head. When a retina begins to detach, the patient’s visual field may have one or more signs such as “floaters,” bright flashes of light, or a black curtain over a portion of the field of vision. Retinal detachment can be repaired if it is done quickly before the retina is devoid of oxygen. If the retina does lose too much oxygen, its cells will die, causing blindness. An ophthalmologist can repair retinal detachments or tears with a laser.
Retinopathy is a disorder caused by damage to the blood vessels that supply the retina. High blood pressure or hypertension can cause damage, which leads to hypertensive retinopathy. Diabetes mellitus can also cause damage that leads to diabetic retinopathy. Retinoblastoma is a cancer of the retina. It most commonly affects young children, but can occur rarely in adults.
Mario J. Perez
See also: Optic Nerve; Retinopathy; Visual Perception; Visual System
Kolb, Helga, Ralph Nelson, Eduardo Fernandez, & Bryan Jones (Eds.). (2011). Webvision: The organization of the retina and visual system. Retrieved from http://webvision.med.utah.edu/book/
Machemer, Robert, & Georg Michelson. (2012). The atlas of ophthalmology: Online multimedia database. Retrieved from http://www.atlasophthalmology.com/atlas/frontpage.jsf