The pupillary reflex describes the involuntary adaptation of the eye to changing light conditions. The width of the pupil changes reflectively with incident light. This reflex is controlled by the parasympathetic nervous system and plays an important role in visual acuity and in the protection of the retina.
- If the environment is very bright, the light stimulus is correspondingly high and the pupil diameter decreases (miosis).
- If the light stimulus is small, i.e. in dark conditions, the pupil widens (mydriasis).
Function
The pupillary reflex serves to quickly adapt the eye to the prevailing light conditions. As soon as a person comes out of the dark into the bright, he is first blinded and can only perceive his surroundings to a limited extent. On the other hand, if you come from a bright environment, you will perceive your surroundings in the dark very poorly.
In order to prevent this condition from lasting for a long time, various adaptation mechanisms have developed in the course of evolution, which allow people to react quickly to changing light conditions. Of these adaptation mechanisms, the pupil reflex is the fastest. Furthermore, the pupillary reflex serves to protect the retina.
Pain in the area of the eyes can occur under strong light incidence. The body reacts to this with a narrowing of the pupil. This constriction greatly reduces the amount of light that reaches the retina.
This natural protective mechanism reduces the pain and the risk of damage to the retina. Like any reflex, the pupillary reflex also has a reflex arc, which consists of a part that goes to the brain and a part that goes away from the brain. A relatively large number of anatomical structures are involved in the process of the pupillary reflex.
These include nerves as well as the muscles of the eye. Roughly speaking, the pupil is narrowed in the case of strong light incidence so that the amount of incident light is reduced. The strong incidence of light is converted into electrical impulses on the retina and transmitted via the optic nerve to the central nervous system.
The perceiving structures of the eye are called rods and cones. These cells are the sensory cells of the eye and have different tasks. The rods are primarily responsible for the perception of light-dark vision and are therefore more important for the pupillary reflex than the cones.
It is in these cells that the conversion into electrical signals takes place. Before the signals reach the optic nerve, they are bundled and processed by intermediate cells. This increases sensitivity.
These intermediate cells are connected to the optic nerve and transmit the signals in a bundled form. The nerve cells of the optic nerve now follow different anatomical structures all the way to the brain stem. This is where an area is located that processes the incoming signals and then relays them.
Some of these signals are transmitted to the cerebrum. However, this part is of no importance for the pupillary reflex. The part of the reflex arc described so far is assigned to the part leading to the brain.
In the area of the brain stem, the area praetectalis, the second part of the reflex arc begins. Depending on light conditions, signals are sent back to the eye via one of the two parts of the autonomic nervous system. These signals are transmitted either via a brain nerve, the oculomotor nerve, or other nerve fibers. Under strong light conditions, the signals reach a muscle, which causes the pupil to narrow. In low light conditions, the signals reach a muscle that causes the pupil to dilate.
All articles in this series: