Located in the outer layer of the retina. Rods and cones are similar in structure; they consist of four sections:
1. The outer segment is a photosensitive area where light energy is converted into receptor potential. The outer segment is filled with membranous disks formed by the plasma membrane. The rods contain 600 - 1000 disks in each outer segment, which are flattened membrane sacs arranged like a column of coins. Cones have fewer membrane disks; they represent folds of the plasma membrane.
2. Constriction - a place where the outer segment is almost completely separated from the inner segment by invagination of the outer membrane. The connection between the two segments is carried out through the cytoplasm and a pair of cilia passing from one segment to another.
3. The inner segment is an area of active metabolism filled with mitochondria that supply energy for vision processes and polyribosomes on which proteins involved in the formation of membrane discs and visual pigment are synthesized. This is where the core is located.
4. Synaptic region - the place where the cell forms synapses with bipolar cells. Diffuse bipolar cells can form synapses with several rods. This phenomenon, called synaptic convergence, reduces visual acuity but increases the eye's light sensitivity. Monosynaptic bipolar cells connect one cone to one ganglion cell, which provides better visual acuity than rods. Horizontal cells and amacrine cells link together a number of rods or cones. Thanks to these cells, visual information undergoes certain processing even before leaving the retina. These cells are also involved in lateral inhibition.
There are more rods in the retina than cones - 120 million and 6 - 7 million, respectively. Thin, elongated rods measuring 50x3 microns are evenly distributed throughout the entire retina, except for the central fovea, where elongated conical cones measuring 60x1.5 microns predominate. Since the cones in the central fovea are very densely packed (150 thousand per square mm), this area is distinguished by high visual acuity. Rods are more sensitive to light and respond to weaker lighting. Rods contain only one visual pigment, cannot distinguish colors, and are used primarily for night vision. Cones contain three visual pigments that enable color recognition and are used primarily in daylight. Rod vision is less sharp because the rods are less densely packed and the signals from them converge, but this is what provides the high sensitivity necessary for night vision.
Photoreceptor combines two different complexes in its structural and functional organization. External part The photoreceptor cell facing the pigment epithelium includes lipoprotein structures containing the visual pigment - rhodopsin, which absorbs light quanta. An increase in the area of the receptor membrane in the outer segment discs, which contain receptive proteins, increases sensitivity to light. The opposite pole of the cell ends in a complex synaptic structure corresponding to similar synapses in neurons, and transmits information about the perception of visual signals to the next nerve cells in the chain. For the structure and function of photoreceptors, which were not specifically studied in this work, see next. reviews: Kolmer, Polyak, Walls, Pedler, Ostrovsky, Cohen, . Baburina, Baburina and Beltadze, Stell, Vinnikov, Rodieck, Lychakov, Podugolnikova and Maksimov, Howardovsky, Byzov, Zak, Bochkin and Ostrovsky.
IN receptor cell light stimuli are converted into receptor potential.
Under the influence of the latter, the release of a mediator changes, which acts on nerve ending sensory neuron of the second order and causes the appearance of a postsynaptic potential in it.
Photoreceptors have been studied for over a hundred years. However, significant advances in understanding the structure and function of rods and cones have been associated with the last few decades, with the advent of electron microscopy. Only at the ultrastructural level did it become clear that the membrane disks of rods are arranged in stacks, separated from the outer plasma membrane, while in cones the outer plasma membrane forms folds, connecting to each disk on one side (Fig. 2, a).
Stacks of disks are constantly renewed, the upper stoics periodically move outward, where they are phagocytosed pigment epithelium. The process of disc rejection is associated with the daily rhythm of illumination and in the retinal cones of some fish, reptiles, and birds occurs immediately after dark. In the rods of many vertebrates, the membranes are rejected at the beginning of the photoperiod [Baburina, Beltadze, 1983].
Connecting leg, containing 9 pairs of fibrils, connects the outer and inner segments of the photoreceptor. In the outer part of the internal segment, a closely located cluster of mitochondria forms an ellipsoid (Fig. 2, a). The oil droplet observed in the cones of some vertebrates is visible among the mitochondria. Other organelles of the inner segment are the paraboloid (glycogen granules) and the myoid.
The synaptic endings of rods and cones form specialized connections with the terminals of the dendrites of bipolar cells, the terminals of the dendrites and axons of horizontal cells (Fig. 2, b; 3).
These synapses vary in location and design and can be invaginating, semi-invaginating, or superficial. Invaginating synapses are formed in dyads and triads, in which the central process is usually a bipolar dendrite located directly under the synaptic ribbon, surrounded by synaptic vesicles, and the terminals of the dendrites of horizontal cells are located on the sides (see Fig. 2, b; 3). At the synaptic terminal of the rod, only a few terminals of dendrites of second-order neurons are observed. Synaptic terminals of cones, as a rule, are much more complex, larger and include many triads grouped around synaptic ribbons. The details of the synaptic connections of bipolars and horizontal cells with photoreceptor terminals vary significantly among different vertebrates.
Photoreceptors are interconnected Electron microscopic studies revealed gap junctions between them. They are found between the red rods of the toad, and in the retina of the axolotl and mammals. The morphology of gap junctions between photoreceptors differs significantly in various types vertebrates [Davydova, 1983] by the level of location of contacts, by the types of receptors between which there are connections, by their length, etc. It has been established that photoreceptors of the same type connected to each other by contacts, for example, cones with the same spectral sensitivity or rods, detect and electrical communication [Byzov, 1984]. Although, as a rule, contacts are observed between receptors of the same type, connections have also been found between receptors various types. For example, in the retina of the frog (Rana pipiens), three contacts were found in serial sections of the red rod - with another red rod, with a single cone and with the main member of the double cone. A single cone contacts three red rods. Gap junctions have been found between receptors of different types and in the retina of the mammal cat; for example, a thin long extension of the cone synaptic stalk forms a connection with the rod spherule. The authors of this finding believe that the interaction of the rod and cone systems in some predominantly rod retinas in mammals occurs already at entry level processing of visual signals.
Light microscopy allows you to observe even at the level of photoreceptors more complex structure in lower vertebrates compared to mammals. In many species of vertebrates, not only single cones are observed, but also double cones (Fig. 1, A, B), which are absent in mammals (Fig. 1, C). In birds and turtles, as mentioned above, no less than six different types of cones are found. According to L.V. Zueva, the color vision system of reptiles and birds consists of four or even more receivers and may be superior in abilities to the three-component human color vision system.
Three types of retinal photoreceptors have been described: rods, cones, and pigment-containing ganglion cells.
Receptor department visual analyzer.
Previously (during the 200-year history of eye research) it was believed that the receptor department of the visual analyzer (visual sensory system) consists of two types of photoreceptors, but now we must talk about three types of retinal photoreceptors:
1. Cones(there are 6-7 million of them): they need high illumination, they have different sensitivity to different spectrums (wavelengths), provide color vision, and contain the pigment iodopsin.
2. Sticks(there are 110-120 million of them): they work in low light, have very high sensitivity, but do not distinguish colors and do not produce a sharp image, they contain the pigment rhodopsin (“visual purple”).
These two types of photoreceptors are located in the receptor layer of the retina perpendicular to the direction of the light beam (in columns). Moreover, they are, one might say, indecently turned to the light with their rears.
But relatively recently, photoreceptors of the third type were discovered in the retina:
3. Melanopsin-containing retinal ganglion cells (RGCs) , or intrinsically photosensitive retinal ganglion cells (ipRGCs): they are only 2% of retinal ganglion cells, they react to light, but do not produce visual images, they contain the pigment melanopsin, which is very different from rod rhodopsin and cone iodopsin. Nerve pathways These ganglion (ganglionic) cells conduct light excitation from the retina to the hypothalamus in three different ways.
Rods and cones contain light-sensitive pigments. Both pigments are based on modified vitamin A. If there is not enough vitamin A, then visual perception suffers, because there are not enough “blanks” for the production of visual pigment.
Rods have a light absorption maximum in the region of 500 nm.
Cones, unlike rods, come in three types:
1.
“Blue” (short wave - S) - 430-470 nm. There are 2% of them total number cones.
2.
“Green” (medium wave - M) – 500-530 nm. There are 32% of them.
3.
“Red” (long wavelength - L) – 620-760 nm. There are 64% of them.
Each type of photoreceptor uses a different type of visual pigment. Interestingly, in the 2000s, huge variability was discovered in the ratio of red and green cones in different people. The standard ratio given above is 1:2, but it can be as high as 1:40 when comparing different people. Yet the brain compensates for these differences, and people with different ratios of red to green cones can name colors of the same wavelength in the same way.
Photochemical processes in the eye proceed economically: even in bright light, only a small part of the pigment disintegrates. In sticks it is only 0.006%. In the dark, the pigments are restored.
Rhodopsin is a rod pigment.
Iodopsin is a pigment of red cones.
Iodopsin is restored 530 times faster than rhodopsin, therefore, with a lack of vitamin A, the vision of the rods is primarily affected, or twilight vision.
The photoreceptor layer lies on a layer of pigment cells that contain the pigment fuchsin. It absorbs light and provides clarity of vision.
A distinctive feature of photoreceptors is not depolarization, but hyperpolarization in response to stimulation.
We can say that the action of light “damages” the photoreceptor, destroys its protein, and it stops working normally and falls into an inhibited state.
The photochemical “fragility” of retinal photoreceptor cells and pigment epithelial cells to photodamage is associated with the following factors:
1)
the presence in them of photosensitizers that effectively absorb light,
2)
sufficiently high partial pressure of oxygen,
3)
the presence of easily oxidized substrates, primarily polyunsaturated ones fatty acids in the composition of phospholipids.
That is why, during the evolution of the visual organs of vertebrates and invertebrates, a fairly reliable system of protection against the danger of photodamage was formed (Ostrovsky, Fedorovich, 1987). This system includes constant renewal of the light-sensitive outer segments of visual cells, a set of antioxidants and the optical media of the eye as light filters, where the lens plays a key role.
Currently, photoreceptors are divided into 2 groups: ciliary (derivatives of cells with a flagellum) and rhabdomeric (derivatives of cells without a flagellum). In both cases, the visual pigment is included in the photoreceptor membrane, and in all types of receptor cells they have a similar chemical nature and are called rhodopsins.
Photoreceptors are located in inner layer retina - photosensitive layer. In humans, visual receptors are ciliary, represented by two types - rods and cones.
There are about 6 million cones, they are located in the central part of the retina and are responsible for color vision. There are much more rods - about 120 million, they are located on the periphery of the retina and are responsible for black and white vision.
Cones provide vision in daylight (photonic), while rods provide vision in clear night conditions (scotopic). At dusk, both types of photoreceptors are equally busy, providing mesopic vision. With photopic vision, maximum acuity and temporal resolution of rapidly changing figures are observed. With scotopic vision, there is a functional color blindness, (“all cats are grey”).
When moving from a lighted room to a dark one, vision drops to almost zero, but gradually it recovers, adapting to the low light intensity in environment (tempo adaptation). As it develops dark adaptation visual acuity improves.
A process opposite to tempo adaptation, developing during the transition from dark room in bright light is called light adaptation.
However, the new adaptation lasts about 30 minutes, while the light adaptation takes only 15-60 seconds.
All types of photoreceptors transmit information about the perception of a light quantum to the central nervous system not using nerve impulse, but by electrotonic means.
Light quanta are absorbed in receptors by specialized molecules from the class of carotenoids - chromolipoproteins.
The spectrum absorbing part of the molecule - the chromophore - is represented by vitamin A aldehydes, or retinals. When retinal binds to opsin, rhodopsin is formed with an absorption maximum of 500 nm (hence its other name - visual purple).
When a photon is absorbed, rhodopsin undergoes a bleaching or bleaching reaction (loss of color by the molecule). This releases energy that forms electric current in receptor cells, which thus transmit information about the light quantum to the central nervous system.
In addition to photoreceptors, the retina contains pigment and glial cells, as well as cells of four classes of nerve cells - bipolar, horizontal, ganglion and amacrine.
Pigment cells provide photoreceptors - rods and cones - with rhodopsin, glial cells perform a supporting function.
Bipolar cells transmit information from photoreceptors to horizontal and amacrine cells. In turn, amacrine cells are synaptically connected with horizontal and ganglion cells, to which the nerve impulse is transmitted. The processes of ganglion cells form the optic nerve.
The transmission of nerve impulses from photoreceptors to bipolar and ganglion cells is main path information flows into the central nervous system, and from photoreceptors to horizontal and amacrine cells - lateral, providing lateral inhibition.
Ganglion cells combine to form receptive fields that may partially or completely overlap. Information from them comes through type C fibers.
The photoreceptors in the human retina include 3 types of cones (each type is excited by light of a certain wavelength), which are responsible for color vision, and one type of rods, which is responsible for twilight vision. In the human retina there are 110 ÷ 125 million rods and 4 ÷ 7 million cones.
Table illustrating the differences between rods and cones (based on the book Principles of Neuronal Science by Eric Kandel)
| Sticks | Cones |
|---|---|
| Used for night vision (in low light conditions) | Used for daytime vision (in high light conditions) |
| Highly sensitive; perceive also diffused light | Not very sensitive to light; react only to direct light |
| Damage causes nyctalopia (hemeralopia) | Damage causes blindness, day blindness, achromatopsia |
| Low visual acuity | High visual acuity; better spatial resolution |
| None in the fovea | Concentrated in the central fovea |
| Slow response to light | Rapid response to light, can perceive faster changes in the stimulus |
| Have more pigment than cones | Have less pigment |
| Membrane discs are not directly attached to the cell membrane | Membrane discs are attached to the outer membrane |
| 20 times more than cones in number. | |
| One type of photosensitive pigment | Three types of photosensitive pigments in humans |
| Wed. Achromatic vision | Wed. Color vision |
In vertebrates, there are horizontal connections between photoreceptors of the same type (for example, between cones with the same sensitivity), and in some cases between receptors of different types. No connections between rods have been found in the primate retina. Despite this, photoreceptors respond to their illumination as if there were connections between them. When one receptor is illuminated, it hyperpolarizes. If there were no connections between photoreceptors, then such an effect would produce the only reacted photoreceptor of the human retina. However, experiments show that neighboring receptors are also hyperpolarized. A likely explanation for this paradox is that the cones of the fovea are very densely packed, and the change membrane potential one photoreceptor flows to neighboring ones.
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