PHOTORECEPTORS

From Big Medical Encyclopedia

PHOTORECEPTORS (Greek phos, photos light + lat. to accept recipere) — the cells which are specifically reacting to light. T. happen two types — unspecialized, to the Crimea the single visual cells disseminated through a body (e.g., at worms, a lancelet), and specialized, i.e. visual cells of eyes of vertebrate and backboneless animals belong. T. contain a light-sensitive rhodopsin (see Rhodopsins) and carry out process of photoreception — transformation of energy of the absorbed light in a nervous signal (see Sight).

The retina of an eye of vertebrata and the person contains two types F. — sticks and flasks (see the Retina). Sticks are capable to answer with excitement absorption even of one light quantum and are responsible for twilight (scotopic) sight. Flasks are approximately a hundred times less sensitive, than sticks, and are F. day (fotogshchesky) vision. Thanks to various spectral response of a flask provide color perception (see. Color sight).

Morphologically sticks and flasks represent the cells extended in length consisting of several specialized parts: an outside segment, a connecting eyelash, an internal segment and nr e with and N and Friday and h e from whom the termination. The ultrastructural organiza


a tion of an outside segment of sticks and flasks significantly differs. In sticks the outside segment is formed by a set of the closed disks, to-rye are separated from a cellular (plasmatic) membrane and as if float in cytoplasm. В: olbochka the segment is formed by not closed disks representing deep embolies of a plasma membrane; the intra disk space is reported by a narrow crack with the extracellular environment. Thickness of a disk apprx. 15 nanometers, distance between disks also apprx. 15 nanometers. Between the membranes forming a disk there is a narrow crack about one nanometer, edges at edge of a disk extends, forming a so-called loop with an internal diameter about several nanometers. Thus, in cytoplasm of an outside segment of a stick the closed intra disk space is formed, the structure of contents to-rogo is other than cytoplasm and has high sensitivity to changes of osmotic pressure out of a disk.

Biosynthesis of molecular components of a photoreceptor membrane of a disk (proteins and lipids) happens in an internal segment F.; then in sticks they it is transported through a connecting eyelash to the basis of an outside segment where the «new» disk is formed. In flasks updating of membranes of outside segments happens on the mechanism of molecular substitution: the proteins and lipids which are again synthesized in an internal segment are evenly distributed on an outside segment. In process of movement along an outside segment disks from-shnurovyvayutsya. The apical fragments containing «old» disks break off and englobed by cells of a pigmental epithelium. Process of their exfoliating submits to a circadian rhythm (in morning and day time tops of receptors of twilight sight — sticks, in evening and night — flasks break off).

The kernel of flasks large, usually adjacent to a basal part of an internal segment, a kernel of sticks a little from it is removed.

Presynaptic nerves of sticks (a so-called sferul) and flasks are filled with numerous sinap-tichesky bubbles (see the Synapse), the concentrating hl. obr. along long axis F. Dendriga of neurons of the second order (horizontal and bipolar cells), as a rule, deeply press in presynaptic nerves.

After full adaptation to darkness permeability of a plasma membrane of an outside segment for ions of sodium becomes very high (109 ions of sodium in 1 sec.);


at threshold osveshchennost sodium current decreases (to 10" ions of sodium in 1 sec.) by one photo isomerization, t. page decolouration of one molecule of rhodopsin (see). In response to one photoisomerization in a stick there is a potential about 1 mV, in a flask — several tens microvolts. Absorption of one light quantum one molecule of rhodopsin leads to emergence in cytoplasm of an outside segment of a large number of molecules of intracellular intermediaries — calcium ions, a cyclic guanozinmo-nofosfat (tsGMF), etc., blocking from within permeability of a membrane to ions of sodium. In outside segments F. two main are made fiziol. process — phototransduction and adaptation. Phototransduction — the complex, multicomponent process which is beginning with a photochemical isomerization a retina of la in a molecule of a rhodopsin and coming to an end with reduction of an ionic conduction of a plasma membrane of an outside segment therefore on it there is a giperpo-lyarizatsionny electric potential — so-called late receptor potential. This potential is also that nervous signal, to-ry F extends lengthways. to presynaptic nerves and through a synoptic mediator it is transmitted to neurons of the second order — to horizontal and bipolar cells of a retina.

The structure and topography of a molecule of rhodopsin in a photoreceptor membrane of a disk, and also mechanisms of svetozavisimy regulation of maintenance of tsGMF are studied (see. Guanylic acid) in an outside segment of a stick F. Soviet scientists (Yu. L. Ovchinnikov and sotr.) the full amino-acid sequence (primary structure) of rhodopsin is established and the model of an arrangement of its polypeptide chain in a membrane in the form of seven is offered and - sleep - ralnykh the «columns» crossing lipidic a bisly membrane. The enzymatic cascade of strengthening of a weak light signal is described:

the light quantum is isomerized by a retina, then there are conformational reorganizations in a proteinaceous part of a molecule of rhodopsin. At one of stages of a photolysis konformatsionno the changed transmembrane protein rhodopsin connects primembranny protein — so-called GTF-svyazyvayushchy protein (G-protein, or transdutsin). Being in a complex with metarhodopsin II, this protein exchanges guanine riboside-diphosphate (GDF) on guanozintrifo-sfat (GTF). Metarhodopsin II manages to activate several hundred molecules GTF-svyazyvayushchego of protein. Further the complex of this protein with GTF is chipped off from metarhodopsin II, diffuses and interacts in a membrane with other primembran-ny protein — enzyme phosphodiesterase (FDE). FDE activated thus catalyzes disintegration of tsGMF, thereby reducing its concentration in mezhd claim space. Fotofosfori l and rova-ny rhodopsin at a stage of metarhodopsin II it is capable to regulate these reactions. Falling of concentration of tsGMF in an outside segment F. leads to hyperpolarization F. (i.e. to reduction of number of open natrium channels of a plasma membrane). Besides, lighting causes strengthening of free calcium in cytoplasm of an outside segment. In turn, calcium blocks natrium channels. Interaction between svetozavisimy systems of exchange of tsGMF and regulation of ion concentration of calcium in cytoplasm of an outside segment F. has, apparently, crucial importance in mechanisms of phototransduction and F. Predpolagayetsya's adaptation that molecular defects in system of regulation peak liches ky nucleotides in F. are the cornerstone of a serious hereditary illness of eyes — a pigmental retinitis (see).

Arisen in an outside segment F. hyper polarizing potential of an elektrotonicheska (bezimiuls-but) extends to presynaptic nerves F. also reduces the speed of allocation from it the depolarizing mediator. Hyperpolarization of a monosinapticheska connected with F is a consequence of it. horizontal and bipolar cells. The chemical nature of a mediator in a photoreceptor synapse is not finalized (most likely it is the glutamate or aspartate).

Electric communication between separate F is found. in a retina of vertebrata. Are connected among themselves only F. identical type. Thanks to such electric sincytium the receptor potential generated in one F. as if blurs on next. According to a number of researchers, as a result the area (spot) of summation for sticks reaches to dia. 150 microns, for flasks it is approximately twice less.

The data obtained during the studying photochemical biochemical and fiziol. processes in F., allow to look in a new way at a pathogeny of a number of a serious illness of a retina, are important for development of new diagnostic methods of eye diseases, nek-ry questions of hygiene of sight, especially in conditions of production. Understanding of mechanisms of normal and pathological processes in F. important not only for physiology of sight and medicine,

but also for the equipment (development of technical means of transformation, transfer and processing of information). Bibliography: The island with to and y M. A.

Photoreceptor cells, M., 1978; Physiology of touch systems, p.1. Physiology of sight, under the editorship of G. V. Gershuni, L., 1971; Evolutionary physiology, under the editorship of I. G. Kostya on, the p. 2, L., 1983; Methods in enzymology, v. 81, Biomembranes, ed. by L. Packer, N. Y., 1982.

M. A. Ostrovsky.

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