Where is the retina?

The retina is derived from the Latin word “rete” meaning “net”. The retina is the innermost, light-sensitive tissue layer of the eye. The optics of the eye focuses the image of our field of vision on the retina in two dimensions. This image is processed in the retina and sent by nerve impulses along the optic nerve to the visual cortex. In the visual cortex, information from the two eyes is combined and three-dimensional vision occurs. When the light comes to the eye, it reaches the retina through the pupil opening in the middle of the iris, the lens and the vitreous cavity, and the image information focused on the retina is converted into electrochemical information and transmitted to the brain.

The retina consists of several layers of neurons connected by synapses. The retina lies over the pigment epithelial cells, which contain dense pigment and are in contact with the choroid layer. The primary photosensitive cells in the retina are photoreceptor cells, which are of two types: rods and cones. The image information falling on the retina is received by the photoreceptors, processed and transmitted to the retinal nerve fibers with the help of other neural cells in the retina. The visual information coming to the retinal nerve fibers is transmitted to the brain through the optic nerve channel formed by the nerve fibers. Rods from photoreceptors are mainly more sensitive to light, working in dim light and providing monochromatic vision. Cone photoreceptors operate in well-lit conditions and are responsible for high-acuity vision and color perception, used for tasks such as reading.

Light hitting the retina initiates a series of chemical and electrical events. Triggered nerve impulses are sent to various visual centers of the brain via optic nerve fibers. Neural signals from rods and cones are processed by other neurons. The processed information turns into action potentials in ganglion cells and is transmitted by retinal ganglion cell axons that form the optic nerve.

The retina is considered a part of the central nervous system (CNS) and is actually the only part of the brain that can be directly visualized. Just as the brain is isolated from the vasculature via the blood-brain barrier, the retina is likewise protected by the blood-retina barrier.

While creating an image on the retina, light detection takes place primarily in the photoreceptors; It is then transmitted to the bipolar cells, then to the photosensitive ganglion cells, and finally along the optic nerve. At each synaptic stage there is also a connection between horizontal and amacrine cells.

The entire retina contains about 7 million cones and 130 million rods. The optic disc, which is part of the retina, is located in the papilla where the optic nerve fibers leave the eye. Because the optic nerve head lacks photoreceptors, it is responsible for the invisible area called the “blind spot” in our visual field.
Although there are more than 130 million retinal receptors, there are only about 1.2 million nerve fibers (axons) in the optic nerve. This indicates a large amount of preprocessing in the retina. Fovea produces the most accurate information. Although the fovea occupies approximately 0.01% of the visual field (less than 2° visual angle), approximately 10% of the axons in the optic nerve are reserved for the fovea. An important part of the information in the visual center of the brain is devoted to the processing of information coming from the macula.
As it is seen, the retina layer, which takes the first image of the outside world with very complex mechanisms and works with photochemical reactions, is the most valuable region of our eye, while the most valuable region of the retina layer is the macula region.
Retinal diseases that affect the macula cause deterioration of central vision. In diseases where the macula remains healthy but the surrounding retina deteriorates, the peripheral visual field is impaired. In diseases in which the entire retina, including the macula, is affected, deterioration occurs in our entire vision.
Retinal Diseases include many diseases that can be classified as vascular retinal diseases, degenerative retinal diseases, retinal tumors or intraocular tumors in which the retina is affected, infectious retinal diseases, immunological retinal diseases, retinal tears and detachments and genetic origin retinal diseases. prof. Dr. Umit INAN