Definition
The human eye has two types of photoreceptors that enable us to see. On the one hand there are the rod receptors and on the other hand the cone receptors, which are subdivided again: blue, green and red receptors. These photoreceptors represent a layer of the retina and send a signal to the cells linked to them if they detect light. The cones are used for photopic vision (color vision and daytime vision) and the rods for scotopic vision (perception in darkness).
Structure
The human retina, also called retina, is 200 μm thick and consists of different cell layers. On the outside are the pigment epithelial cells, which are very important for the metabolism of the retina, as they absorb and break down dead photoreceptors and also secreted cell components that are produced during the visual process. Further inwards now follow the actual photoreceptors, which are separated into rods and cones.
What both have in common is that they have an outer member that points in the direction of the pigment epithelium and also has contact with it. This is followed by a thin cilium, which connects the outer and inner members. In the rods, the outer member is a stack of membrane discs, similar to a stack of coins.
In the cones, on the other hand, the outer member consists of membrane folds, so that in longitudinal section the outer member looks like a kind of hair comb, with the teeth representing the individual folds. The cell membrane of the outer member contains the visual dye of the photoreceptors. The dye of the cones is called rhodopsin and consists of a glycoprotein opsin and the 11-cis-retinal, a modification of vitamin A1.
The visual dyes of the cones differ from rhodopsin and from each other by different forms of opsin, but also have the retinal. The visual process consumes the visual dye in the membrane discs and membrane folds and must be regenerated. The membrane disks and folds are constantly being regenerated.
In the process, they migrate from the inner phalanx to the outer phalanx and are ultimately released, absorbed and broken down by the pigment epithelium. A malfunction of the pigment epithelium causes a deposition of cell debris and visual dye, as is the case with retinitis pigmentosa, for example. The inner link is the actual cell body of the photoreceptors and contains the cell nucleus and the cell organelles.
This is where important processes take place, such as the reading of DNA, the production of proteins or cell messenger substances; in the case of the photoreceptors, glutamate is the messenger substance. The inner link runs out thinly and has a so-called receptor foot at the end, through which the cell is connected to so-called bipolar cells (transmitting cells). In the receptor foot, transmitter bubbles with the messenger substance glutamate are stored.
Glutamate is used for signal transmission to the bipolar cells. A special feature of the photoreceptors is that transmitter substance is permanently released in the dark, whereas the release decreases with light incidence. It is therefore not as with other perceptual cells that a stimulus leads to an increased transmitter release.
There are rod and cone bipolar cells, which in turn are connected to ganglion cells, which make up the ganglion cell layer and whose cell extensions ultimately form the optic nerve. There is also a complex horizontal interconnection of retinal cells, which is realized by horizontal cells and amacrine cells. The retina is stabilized by so-called Müller cells, the glial cells of the retina, which span the entire retina and act as a scaffold.
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