Visual perception in the retina
For us to be able to see, the light must reach the retina in the back of the eye.It first falls through the cornea, pupil and lens, then crosses the vitreous body behind the lens and must first penetrate the entire retina itself before it reaches the places where it can trigger an effect for the first time. The cornea and lens are part of the (optical) refractive apparatus, which ensures that light is refracted correctly and that the overall image is displayed accurately on the retina. Otherwise the objects would not be perceived sharply.
This is the case, for example, with myopia or hyperopia. The pupil is an important protective device that regulates the incidence of light by expanding or contracting. There are also drugs that override this protective function.
This is necessary, for example, after operations, when the pupil needs to be immobilized for some time in order to better promote the healing process. Once the light has penetrated the retina, it hits cells called rods and cones. These cells are sensitive to light.
They possess receptors (“light sensors”) that are bound to a protein, more precisely to a G protein, the so-called transducin. This particular G protein is bound to another molecule, rhodopsin. It consists of a vitamin A part and a protein part, the so-called opsin.
A light particle that encounters such rhodopsin changes its chemical structure by straightening a previously broken chain of carbon atoms. This simple change in the chemical structure of rhodopsin now makes interaction with the transducin possible. This also changes the structure of the receptor in such a way that an enzyme cascade is activated and signal amplification occurs.
In the eye this leads to an increased negative electrical charge on the cell membrane (hyperpolarization), which is passed on as an electrical signal (transmission of vision). The uvula cells are located in the point of sharpest vision, also called yellow spot (macula lutea) or in professional circles fovea centralis. There are 3 types of cones, which differ in that they react to light of a very specific wavelength range.
There is the blue, green and red receptor. This covers the color range visible to us. The other colors result mainly from the simultaneous, but differently strong activation of these three cell types.
Genetic variations in the blue, green and red receptors can lead to the different color blindness. The rod cells are mainly found in the peripheral area (periphery) around the fovea centralis. Rod cells do not have receptors for different color ranges. However, they are much more light sensitive than cones. Their function is to enhance contrast and vision in darkness (night vision) or in low light (twilight vision).
