The rods and cones of the eye - what are they?

2023 Author: Alfred Shackley | [email protected]. Last modified: 2023-05-21 13:14

A person receives information about the world around 90% through the organ of vision. The role of the retina is visual function. The retina consists of specially structured photoreceptors - cones and rods.

Rods and cones are photographic receptors with a high degree of sensitivity; they convert light signals coming from the outside into impulses perceived by the central nervous system - the brain.

When illuminated - during daylight hours - the cones experience an increased load. The rods are responsible for twilight vision - if they are not active enough, night blindness appears.

The cones and rods in the retina have different structures, since their functions are different.

The structure of the human organ of vision

1. The cornea is a transparent membrane with vessels and nerve endings, bordering the sclera, located on the front of the organ of vision.
2. The anterior chamber is between the cornea and the iris and contains intraocular fluid.
3. The iris is the area of the eye with an opening for the pupil. Its structure: muscles that change the diameter of the pupil when the lighting changes and regulate the flow of light.
4. The pupil is an opening through which light passes into the eye.
5. The lens is an elastic transparent lens that can instantly adjust to visual images - change focus to assess the size of objects and the distance to them.
6. The vitreous is an absolute transparent substance of a gel-like consistency, thanks to which the eye has a spherical shape. Performs an exchange function in the organ of vision.
7. Retina - consists of 3 layers, is responsible for vision and color perception, it includes blood vessels, nerve fibers and high sensitivity photoreceptors. It is thanks to this structure of the retina that impulses arrive in the brain, which arise as a result of the perception of light waves of different lengths. Thanks to this ability of the retina, a person can distinguish between primary colors and their many shades. Different types of people have different color sensitivity.
8. The sclera is the outer shell of the eye that forms the cornea.

The organ of vision also includes the vascular part and the optic nerve, which transmit signals received from the outside to the brain. The part of the brain that receives and transforms information is also considered one of the parts of the visual system.

Where are the rods and cones located? Why are they not reflected in the list? These are the receptors of the nerve tissue that make up the retina. Thanks to the cones and rods, the retina receives a picture that is fixed by a section of the cornea and the lens. The impulses transmit the image to the central nervous system, where the information is processed. This process takes place in a matter of fractions of a second - almost instantly.

Most of the sensitive photoreceptors are located in the macula - this is the name of the central region of the retina. The second name for the macula is the macula of the eye. This name was given to the macula because when examining this area, a yellowish tint is clearly visible.

The structure of the outer part of the retina includes pigment, and the inner part contains light-sensitive elements.

Cones in the eye

The cones got their name because they are very similar in shape to the flasks, only very small. In an adult, the retina contains 7 million of these receptors.

Each cone consists of 4 layers:

• outer - membrane discs with a color pigment iodopsin; it is this pigment that provides high sensitivity in the perception of light waves of various lengths;
• the connecting layer - the second layer - the constriction, which allows to form the shape of the sensitive receptor - consists of mitochondria;
• the inner part is the basal segment, the connecting link;
• synaptic area.

Currently, only 2 light-sensitive pigments in the composition of this type of photoreceptors - chlorolab and erythrolab - have been fully studied. The first is responsible for the perception of the yellow-green spectral region, the second - the yellow-red.

Sticks in the eyes

Retinal rods have a cylindrical shape, the length is 30 times the diameter.

The sticks include the following elements:

• membrane discs;
• cilia;
• mitochondria;
• nervous tissue.

The maximum light sensitivity is provided by the pigment rhodopsin (visual purple). He cannot distinguish color shades, but he reacts even to minimal light flashes that he receives from the outside. The receptor of rods is excited even by a flash, the energy of which is only one photon. It is this ability that allows you to see at dusk.

Rhodopsin is a protein from the group of visual pigments and belongs to chromoproteins. It got its second name - visual purple - during research. Compared to other pigments, it stands out sharply with a bright red tint.

There are two components in rhodopsin - a colorless protein and a yellow pigment.

The reaction of rhodopsin to a light beam is as follows: when exposed to light, the pigment decomposes, causing excitation of the optic nerve. In the daytime, the sensitivity of the eye shifts to the blue area, in the nighttime, visual purpura is restored within 30 minutes.

During this time, the human eye adapts to the twilight and begins to more clearly perceive the surrounding information. It is this that can explain that in the dark, over time, they begin to see more clearly. The less light comes in, the more acute twilight vision is.

Cones and rods of the eye - functions

Photoreceptors cannot be considered separately - in the visual apparatus, they form a single whole and are responsible for visual functions and color perception. Without coordinated work of receptors of both types, the central nervous system receives distorted information.

Color vision is provided by the symbiosis of rods and cones. Rods are sensitive in the green part of the spectrum - 498 nm, no more, and then cones with different types of pigment are responsible for perception.

To assess the yellow-red and blue-green ranges, long-wave and medium-wave cones with wide photosensitive zones and inner overlap of these zones are used. That is, photoreceptors react simultaneously to all colors, but to their own they are excited more intensely.

It is impossible to distinguish colors at night, one color pigment can only react to light flashes.

Diffuse biopolar cells in the retina form synapses (the point of contact between a neuron and a cell receiving a signal, or between two neurons) with several rods at once - this is called synaptic convergence.

Increased perception of light radiation is provided by monosynaptic bipolar cells that connect the cones with the ganglion cell. A ganglion cell is a neuron that is located in the retina of the eye and generates nerve impulses.

Together, rods and cones bind amacrylic and horizontal cells, due to which the first information processing takes place in the retina itself. This provides a quick reaction of a person to what is happening around him. Amacrylic and horizontal cells are responsible for lateral inhibition - that is, the excitation of one neuron produces a "calming" effect on the other, which increases the acuity of information perception.

Despite the different structure of photoreceptors, they complement each other's functions. Thanks to their coordinated work, it is possible to get a clear and clear image.