What is the retina of the eye?
The retina is a nerve tissue and the innermost of the three wall layers of the eyeball. It extends from the edge of the pupil to the exit point of the optic nerve. Its task is to perceive light: the retina registers the optical light impulses that enter the eye and converts them into electrical signals, which are then transmitted to the brain via the optic nerve.
The structure of the retina
The retina is divided into two sections – an anterior and a posterior section.
Anterior retinal section
The anterior part of the retina (pars caeca retinae) covers the back of the iris and the ciliary body. It contains no photoreceptors (photoreceptors) and is therefore insensitive to light.
The boundary between the posterior retinal segment and the ciliary body runs along the posterior edge of the ciliary body. This transition has the shape of a jagged line and is known as the ora serrata.
Posterior section of the retina
The posterior retinal section (pars optica retinae) lines the entire back of the eye, i.e. the inside of the posterior eyeball. It has light-sensitive photoreceptors:
Pigment epithelium (stratum pigmentosum)
The monolayer pigment epithelium (stratum pigmentosum) lies on the inside of the middle layer of the eye and thus borders on the choroid. It has elongated brown pigment granules and extends to the photoreceptors in the stratum nervosum. The main function 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 layer of the retina, contains the first three neuron types of the visual pathway, which are connected in series. From the outside in, these are
- photoreceptor cells (rods and cones)
- bipolar cells
- ganglion cells
Other cell types (horizontal cells, Müller cells, etc.) are also found in the stratum nervosum.
The cell bodies of the three neuron types (rod and cone cells, bipolar cells, ganglion cells) are arranged in layers. This results in a total of ten layers that make up the stratum nervosum of the retina.
Rods and cones
The rods and cones share the tasks of light perception:
- Rods: The approximately 120 million rods in the eye are responsible for seeing at dusk and for black and white vision.
- Cones: The six to seven million cones are less sensitive to light and enable us to see colors during the day.
Cones and rods are in direct contact with neuronal switch cells via synapses, which end at the optic ganglion cells. Several sensory cells end at a ganglion cell.
Yellow spot and optic pit
The so-called “yellow spot” (macula lutea) is a roundish 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 contains only 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 visual pit is the place of sharpest vision on the retina.
As the distance from the fovea increases, the proportion of cones in the retina decreases.
Blind spot
The processes of the ganglion cells gather at a point in the area of the posterior fundus of the eye. At the so-called “blind spot” (papilla nervi optici), the nerve endings leave the retina and emerge from the eye in a bundle as the optic nerve. It transmits the light signals from the retina to the visual center in the brain.
As there are no light-sensing cells in this part of the retina, vision is not possible in this area – hence the name “blind spot”.
The function of the retina
What problems can the retina cause?
The retina of the eye can be affected by various diseases and injuries. Some examples:
- Macular degeneration: The retina suffers damage in the area of the macula (yellow spot). Older people are most frequently affected (age-related macular degeneration, AMD).
- Retinal detachment: The retina detaches from the back of the eye. Without treatment, those affected go blind.
- Retinal artery occlusion: Rarely, blood clots enter the retinal artery or one of its side branches and block the blood flow. This manifests itself as sudden unilateral blindness or visual field loss (scotoma).
- Diabetic retinopathy: Untreated or poorly controlled diabetes mellitus (diabetes) damages the smallest blood vessels in the retina. This leads to a lack of oxygen and the death of photoreceptors in the retina. Visual impairment and blindness are the possible consequences.
- Retinopathy of prematurity: In premature babies with a birth weight of less than 2500 grams, the retinal vessels are still developing. Oxygen disrupts this process, causing the immature vessels to close and then proliferate.
- Retinitis pigmentosa: This term refers to a group of genetic retinal diseases in which the light-sensing cells gradually die off.
- Injuries: For example, an eye contusion can lead to a tear in the ora serrata – the boundary between the anterior and posterior sections of the retina.
