retina(redirected from Retin-A)
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the internal membrane of the eye, which converts light stimulation into nervous excitation and effects the primary processing of visual signals. The retina lines the fundus of the eye and covers the ciliary body and the internal surface of the iris; thus, three distinct parts of the retina are distinguished—the pars optica retinae (consisting of ten layers), the pars ciliaris retinae, and the pars iridica retinae.
In vertebrates, including man, the retina is formed during embryonic development from the optic vesicles (primary eye rudiments), which are paired lateral evaginations of the rudimentary forebrain. By means of invagination of the distal wall, each optic vessel is converted into an optic cup. The external layer of the optic cup, which is formed by the optic-vesicle wall closest to the brain, develops into pigmented epithelium, whose cells contain granules of the pigment melanin. The external layer of the optic cup is adjacent to the internal surface of the vascular tunic. Cytoplasmic processes (outgrowths) extend from the cells of the vascular tunic toward photoreceptor cells.
The remaining retinal layers develop from the inner wall of the optic cup. The outer nuclear layer consists of the nuclear parts of the photoreceptors, whose processes extend beyond the external limiting membrane toward the cell processes of the pigmented epithelium; the external limiting membrane is a thickened layer of the endings of the cell processes of Müller’s fibers. In the outer molecular layer, centrad photoreceptor processes form synapses with the dendrites of the cells of the inner nuclear layer. The latter layer contains the nuclear parts of amacrine neurons (which lack axons) and bipolar and horizontal neurons. It also contains glial elements (Müller’s fibers), which perform supportive and trophic functions in the retina.
The inner molecular layer is formed by the centrad processes of bipolar neurons, which come into contact with the neuronal dendrites of the ganglion cell layer. The ganglion cell layer consists of multipolar ganglion cells, whose axons form layers of nerve fibers; the optic nerve consists of ganglion cells. The ganglion cell layer also contains efferent fibers that extend from the brain’s visual area to the retina. The internal limiting membrane, which is the innermost retinal layer, is formed (as is the external limiting membrane) by the cell processes of Müller’s fibers.
A distinct oval elevation called the optic disk is found on the posterior surface of the optic part of the retina. The point of exit of the optic nerve from the retina forms the blind spot. In the central part of the fundus of the eye and not far from the blind spot is the macula lutea, within which there is a central depression that contains the maximum number of cone cells per unit of retinal surface. This is the area of keenest vision on the retina. Photoreceptor cells transmit signals to cells of the inner nuclear layer, which in turn direct visual signals to the ganglion cells. Information reaches the brain’s visual area along the centrad processes of ganglion cells.
Signals are conducted along the retina in various ways. For example, in the macula lutea, signals are conducted from cone cells to bipolar cells in a predominantly isolated manner; each bipolar cell communicates with a ganglion cell, which ensures the greatest keenness of vision in this area. Toward the retinal periphery, the concentration of cones decreases and the number of rods increases; here, several rod and cone cells are in contact with a bipolar cell and several bipolar cells are in contact with a ganglion cell. When compared with the isolated conduction of information, this type of aggregation results in stronger visual signals being sent by ganglion cells to the brain and in increased photosensitivity. The human retina contains approximately 7 million cone cells and 75–150 million rod cells.
In many vertebrate animals, including fish, amphibians, and birds, fluctuations in the illumination of the photoreceptors is regulated by the retinomotor reaction, which coordinates the movements of pigmented granules in the cell processes of pigmented epithelium and the expansion or contraction of the processes of rods and cones perpendicularly to the retinal surface. At twilight or in complete darkness, pigment granules gather in the nuclear parts of pigmented-epithelium cells, whereas the cell processes, which are deprived of pigment granules, remain transparent. The external segment of the rods moves nearer the external limiting membrane, and the external segment of the cones moves between the cell processes of the pigmented epithelium. When illumination increases most pigment granules migrate into the cell processes of the pigmented epithelium, and the external segments of the rods move outward and arrange themselves between the processes. At the same time, the cones move closer to the external limiting membrane, where previously, under weak illumination, the external processes of the rods had been located.
The movements of the external processes of rods and cones during the retinomotor reaction are caused by the contractility of the myoid. As a result of the retinomotor reaction, rods, which are more photosensitive than cones, screen themselves with pigment granules from excessive excitation by light.
REFERENCESFiziologiia sensornykh sistem, part 1: Fiziologiia zreniia. Leningrad, 1971. (Rukovodstvo po fiziologii.)
Keidel, W. D. Fiziologiia organov chuvstv, part 1. Moscow, 1975. (Translated from German.)
O. G. STROEVA