What are retinal cells

Retina

The structure of the retina

The retina is divided into two sections - an anterior section and a posterior section.

Anterior retinal segment

The front part of the eye retina (Pars caeca retinae) covers the back of the iris and the radiating body (ciliary body). It does not contain photoreceptors (photoreceptors) and is therefore insensitive to light.

The border between the rear part of the retina runs along the rear edge of the ciliary body. This transition is in the shape of a jagged line and is called a Ora serrata designated.

Posterior retinal section

The posterior part of the retina (Pars optica retinae) lines the entire fundus, i.e. the inside of the back of the eyeball. He owns light-sensitive photoreceptors:

This part of the retina is made up of two leaves: The outer leaf (towards the outside of the eyeball) consists of a pigment epithelium (Stratum pigmentosum). The inner leaf (towards the middle of the eyeball) consists of the light-sensitive layer with the photoreceptors (Stratum nervosum). There is a capillary gap between the two. The two leaves are fused together in only two places - in the area of ​​the ora serrata and in the area of ​​the exit point of the optic nerve.

Pigment epithelium (stratum pigmentosum)

The single-layer pigment epithelium (stratum pigmentosum) lies on the inside of the middle skin of the eye and thus borders the choroid. It has elongated brown pigment granules and extends to the photoreceptors in the stratum nervosum. The main task of the epithelium is to supply the photoreceptors with oxygen and nutrients (via the blood).

Light-sensitive layer (stratum nervosum)

The stratum nervosum, the inner leaf of the eye retina, houses the first three consecutive neuron types of the visual pathway. From the outside in, these are:

  • Photoreceptor cells (rods and cones)
  • bipolar cells
  • Ganglion cells

In addition, there are other cell types in the stratum nervosum (horizontal cells, Müller cells, etc.).

The cell bodies of the three types of neurons (rod and cone cells, bipolar cells, ganglion cells) are arranged in layers. This results in a total of ten layers that build up the stratum nervosum of the retina.

Chopsticks and cones

The rods and cones share the tasks of light perception:

  • rod: The around 120 million rods in the eye are responsible for seeing in twilight and black-and-white vision.
  • Cones: The six to seven million cones are less sensitive to light and allow you to see colors during the day.

The sensory cells border directly on the pigment epithelium and are overlaid by the other nerve cells of the inner retinal layer (bipolar cells, ganglion cells). The incident light must first penetrate these inner ones before it reaches the sensitive photoreceptors.

Cones and rods are in direct contact via synapses with neuronal switching cells that end at the optic ganglion cells. Several sensory cells end at a ganglion cell.

Yellow spot and pit of vision

The so-called “yellow spot” (Macula lutea) is a rounded region in the middle of the retina, in which the light-sensitive sensory cells are particularly dense. In the center of the "yellow spot" there is a depression - called the visual pit or central pit (fovea centralis). It only contains cones as photoreceptors. The overlying cell layers (ganglion cells, bipolar cells) are shifted to the side so that incident light rays fall directly on the cones. This is why the pit of vision is the point of sharpest vision on the retina.

With increasing distance from the fovea, the proportion of cones in the retina decreases.

Blind spot

The appendages of the ganglion cells collect at one point in the area of ​​the back of the eye. At the so-called “blind spot” (papilla nervi optici), the nerve endings leave the retina and emerge from the eye in bundles as an optic nerve. It forwards the light signals from the retina to the visual center in the brain.

Since there are no light sensory cells at this point in the retina, no vision is possible in this area - hence the name "blind spot".