Photosensitive apparatus of the eye is lined with a back wall of the eyeball and occupies 72% of the area of its inner surface. It is called the RETINA. Retin-a has the shape of a plate thickness of approximately a quarter of a millimeter and consists of 10 layers.
In its origin the retina is the forward part of the brain: in the process of development of the embryo retina is formed from the eye of bubbles, which are protrusions of the front wall of the primary brain bubble. The main of its layers is a layer of light - sensitive cells called PHOTORECEPTORS. They are of two kinds: STICKS and CONES. Such names they received due to their shape:
|Rods and cones.
|Chopsticks (gray) and cones (purple) in the retina
(photo by Steve Gschmeissner, Science Photo Library)
Sticks in each eye approximately 125-130 million. They are characterized by high sensitivity to light and work in low light, that is responsible for twilight vision. However, the sticks are not able to distinguish colors, and with their help we see in black and white. They contain the visual pigment RHODOPSIN.
Sticks are located throughout the retina, except for the center, so thanks to them detected objects in the peripheral visual field.
Cones are much smaller than sticks around 6-7 million in the retina of each eye. Cones provide color vision, but they are 100 times less sensitive to light than rods. Therefore, color vision - the daytime and in the dark, when work only sticks, people can not distinguish colors. Cones are much better than sticks, perceive rapid movement.
The pigment in the cones, to whom we owe a color vision, called IODOPSIN. Sticks are "blue", "green" and "red", depending on the wavelength of light, which they mostly absorb.
Cone cells are located mainly in the center of the retina called the macula (also known as the MACULA). The minimum thickness of the retina (0.05-0.08 mm) and missing all layers except the layer of cones. The macula has a yellow color due to the high content of yellow pigment. Yellow spot man sees best: all the light information reaching the area of the retina is transmitted fully and without distortion, with maximum clarity.
Retin-a person arranged unusual: it seems to be inverted. Layer of the retina from light-sensitive cells is not the front side of the vitreous body, as you would expect, and the rear side of the choroid. To reach the rods and cones, the light must first Wade through 9 of the other layers of the retina.
Between the retina and the choroid is the pigment layer containing a black pigment called melanin. This pigment absorbs the light coming through the retina, and does not allow it to bounce back scatter within the eye. The albino - people with congenital absence of melanin in all cells of the body at high illumination light within the eyeball is reflected in all directions by the surface of the retina. As a result, a single discrete spot light, which is normally opened only a few sticks or cones, reflected everywhere and excite many receptors. So the albino visual acuity is rarely higher than 0.2 0.1 if the norm of 1.0.
The structure of the retina
Under the influence of light rays in the photoreceptors occurs photochemical reaction is the decomposition of visual pigments. The result of this reaction, energy is released. This energy is in the form of an electrical signal is transmitted to the intermediate cells of BIPOLAR (also called interneuron or inserted neurons), and then to GANGLION CELLS, which generate nerve impulses and nerve fibers send them to the brain.
Each cone is connected through a bipolar cell with a single ganglion cell. But the signals sticks, going to the ganglion cells are the so-called convergence: one bipolar cell is connected a few sticks, it sums up their signals and transmits them to one ganglion cell. Convergence allows to increase the light sensitivity of the eye and the sensitivity of peripheral vision to movements, whereas in the case of cones no summation allows you to increase visual acuity, but the sensitivity kolbochnye" vision lowered.
Nerve fibers from the entire retina is collected into a single optic nerve in a particular area of the retina BLIND SPOT. It is located in the place where the optic nerve exits the eye, and is called so, because everything that falls on this area disappears from the field of view of a person (details here and here).
Along the optic nerve image information from the retina reaches the brain and processed there, so that we see the final picture of the world.
More: cerebral part of the visual system (visual analyzer)
Now much is known about the mechanisms of the visual system, but you need to be honest and admit that modern science still does not know until the end how the brain copes with the daunting task of converting the electrical signals of the retina in the visual scene, as we perceive - with the complexity of form, depth, motion, and color. But the study goes on, and, hopefully, science in the future will solve all the mysteries of the visual analyzer and will be able to use them in practice in medicine, Cybernetics, and other areas.