Photoreceptors

Photoreceptors, sensitive apparatuses of animals, serving for the perception of light stimulation. In the animal world, a gradual complication in the structure and function of photoreceptors can be noted. The most primitive form of them is sometimes considered special "sensitive spots," so-called eye spots of certain protozoa, for example, flagellates (Euglena and others), representing _chus a pigment accumulation. The presence of a "sensitive spot" creates for the animal in question the possibility of primitive reactions to light exposure, i.e., it determines the manifestation of phototaxis. The gradual complication of photoreceptors is expressed in the appearance of special structures, obviously significantly facilitating the process of perceiving light stimulation and transmitting it to nerve centers. For all photoreceptors, starting with the most primitive, the presence of a special brush-like structure, represented by a series of perpendicular threads to the surface, is characteristic. The appearance of various complications of this original structure leads in arthropods and vertebrates to the most complex formations, characterized not only by the ability to determine degrees of illumination and direction of light, but in the most perfect cases also to obtain with the help of photoreceptors an image of the external environment similar to a photograph. In the simplest case (for example, in turbellarians) we have to deal with an isolated sensitive cell, as it were, enclosed by a pigment cell and continuing at the other end into a nerve process. In other animals of the same group (for example, in Planaria) we encounter certain complications—the photoreceptor represents a collection of a large number (over 150) sensitive and pigment cells (fig. 1 and 2). With further complication of photoreceptors in invertebrates (for example, in some worms, mollusks, arthropods) refracting media (lenses) appear, ensuring the possibility of accommodation—the photoreceptors acquire the character of a chamber, i.e., become capable of transmitting a more or less clear image. Photoreceptors reach the highest development in vertebrates—the eye, pre- za- senting features of significant convergent similarity with the corresponding organs of invertebrates, despite significant differences in origin and structure (for example, in contrast to the eyes of arthropods

Fig. 2. Inverted cup-shaped pigment cell of the ciliated worm Planaria gonocephala: 1-epidermis; 2-pigment cell; 3-visual cells, from which cones with a brush-like border extend into the pigment cup. (After Hesse.)

za vertebrates "inverted," i.e., they have an external location of photosensitive elements) (fig. 3 and 4). These latter, i.e., photoreceptor eyes, are represented by 2 kinds of sharply modified nerve cells, perceiving stimulation—rods and cones. Despite the significant difference in the external form of these cells, common to them is the sensitive segment—the outer part directly perceiving light stimulation, which from the cell is transmitted to the visual-Ris 3. Development of the eye in annelids. A-Ranzania; B- Syllis: 1-cuticle; 2-epidermis; 3 - threads of indifferent epithelial cells; 4- sensitive cells; 5 -indifferent cells; 6, 7- rods of sensitive cells; C-Nereis: 1~ filling mass; 2-threads of indifferent cells; 3-epidermis; 4~ indifferent cells; 5- sensitive cells. (After Hesse.) n departments of the brain through two systems of nerve cells (bipolar and ganglion nerve cells). For photoreceptors, as generally for all receptors, a very high degree of sensitivity to one specific adequate, in this case light, stimulation is characteristic. The mechanism of photoreceptor operation cannot be considered at the present time sufficiently clarified. Almost all researchers agree that the first stage of light action is the photochemical reaction occurring in the photoreceptors. The substrate of this reaction is a special substance that easily decomposes in light—visual purple, or rhodopsin_u easily detectable in the rods of animals that have been in darkness for some time. A series of studies, especially detailed works by Hecht on the photosensitive organs of ascidians and mollusks, showed that the photochem. reaction is by its nature bimo-. lecular and reduces to the breakdown of the photosensitive substance into 2 components. In the reaction of photoreceptors to light exposure, 2 periods should be distinguished: 1) the period of sensitization, i.e., the direct action of light, and 2) the la-tent period, i.e., the period of processing and conduction of stimulation.

From the point of view of Hecht's positions, adaptation (getting used to light

Fig. 4. Retina of the bream (Abramis brama).. A-eye in light; B-eye in darkness: 1-black pigment; 2-ellipsoid of cone; 3, 1O -cones; 4-lipoid of cone; 5-guanine (tapetum); 6- rods; 7-nucleus of cone; 8- membrana limi-tans externa; 9-outer granular layer (nuclei of rods); 11-nuclei of pigment epithelium. (From Frisch.)

and darkness) is explained, which reduces to the reversibility of the breakdown reaction of the photosensitive substance (see also Vision, Light perception, Color perception).

S. Zalkid-PHOTOSENSITIZATION, the ability to increase the sensitivity of an object to those rays of the visible spectrum to which, under ordinary conditions, this object either little or not at all reacts. The phenomenon of photosensitization was first studied by Raab (1899) and Tappeiner (1904), who found that fluorescent substances (acridine, eosin, erythrosin) are much more poisonous for protozoa and bacteria in light _g than in darkness, then introduced into the practice of phototherapy by Dreyer, Jesionek (Dreyer, Je-sionek) and others. Dreyer used photosensitization with a solution of erythrosin with local illumination by a voltaic arc in the treatment of lupus, Jesionek-in various skin diseases. Later, as a photosensitizer, tripaflavin (hydrochloride of diaminomethyl-acridine) was used b. ch., as the least toxic. A weak solution is administered intravenously with subsequent local illumination of the desired area of the body with a Finzen apparatus or other source of light. 5-10-15 injections are given, up to 0D6 g of dry substance pro injectione. Wessely introduced this method into otolaryngology,.. other authors-g. obr. in the treatment of lupus.. Photosensitization did not find wide application, t. c, has a number of negative aspects: some toxicity of sensitizers, coloring of the skin?.. and tissues, difficulty in dosing and hence the danger of skin damage and frequent general side effects, b. ch., dyspeptic nature. No exact explanation of photosensitization has yet been found. Tappeiner's school believed that fluorescent substances act on living cells, and light only enhances their action. Dreyer and others believe that sensitizers themselves are harmless, but only enhance the absorption of radiant energy by tissues. Different substances usually sensitize to certain parts of the spectrum, namely to those that are absent in their own spectrum. Therefore, it is believed that they only increase the biological action of rays absorbed by the given substance. However, photosensitization cannot be explained by one such adsorp-«j, since sensitivity can be increased to all parts of the spectrum. In other cases, non-fluorescent substances can also act as sensitizers, and photosensitization is caused by the hyperemia they cause (iodine, brilliant green) or their photochemical decomposition with the formation of poisonous products (decomposition of iodides). In experiments with lower organisms, photosensitization is achieved b. ch. in the presence of oxygen and the use of easily oxidizing substances as sensitizers. The most powerful sensitizers are natural plant and animal pigments: chlorophyll, hematoporphyrin and bile pigments (Hausmann). Since these substances participate in the normal metabolism of the body, their significance for the biological action of radiant energy is understandable. In some pathological conditions associated with increased production of these pigments, the body becomes especially sensitive to light, and the picture of the disease strongly resembles experimental photo-«ensitization. It is quite obvious that under the concept of photosensitization, phenomena often different in their chemical and biological nature are described.