The pupillary reflex describes the involuntary adjustment of the eye to changing light conditions. The width of the pupil changes reflexively with incident light.
This reflex is controlled by the parasympathetic nervous system and plays an important role in visual acuity and the protection of the retina.
You might also be interested in the following article: Retina of the eye
The pupil reflex is used to quickly adapt the eye to the prevailing light conditions. As soon as a person comes out of the dark into the light, they are initially blinded and can only perceive their surroundings to a limited extent. On the other hand, you perceive your surroundings very poorly in the dark if you come from a bright environment.
So that this state does not last long, various adaptation mechanisms have developed in the course of evolution that allow people to react quickly to changed lighting conditions. Of these adaptation mechanisms, the pupillary reflex is the fastest.
In addition, the pupillary reflex serves to protect the retina. Strong light can cause pain in the eye area. The body reacts to this by constricting the pupil. This narrowing greatly reduces the amount of light that reaches the retina. This natural protective mechanism reduces pain and the risk of damage to the retina.
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Like every reflex, the pupillary reflex has a reflex arc, which consists of a part leading to the brain and a part leading away from the brain. A relatively large number of anatomical structures are involved in the process of the pupillary reflex. In addition to nerves, this also includes the muscles of the eye.
To put it roughly, the pupil is narrowed when there is strong incidence of light, so that the amount of incident light is reduced. The strong incidence of light is converted into electrical impulses on the retina and passed on to the central nervous system via the optic nerve. 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 and dark vision and are therefore more important for the pupil reflex than the cones. The conversion into electrical signals takes place in these cells. Before the signals reach the optic nerve, they are bundled and processed by interconnected cells. This increases the sensitivity. These intermediate cells are connected to the optic nerves and transmit the signals in bundled form.
The nerve cells of the optic nerve now follow various anatomical structures down to the brain stem. Here is an area that processes the incoming signals and then forwards them. Part of it is passed on to the cerebrum. However, that 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 that Area pretectalis, the second part of the reflex arc begins. Depending on the light conditions, signals are sent back to the eye via one of the two parts of the vegetative nervous system. These signals are either conducted via a cranial nerve, the oculomotor nerve, or other nerve fibers.
In strong light, the signals reach a muscle, which leads to the constriction of the pupil. In low light conditions, the signals reach a muscle, which leads to an enlargement of the pupil.
Learn more about how the Brain stem.
The examination of the pupillary reflex is one of the standard examinations in neurology. The pupillary reflex can be tested with a flashlight examination.
One eye is illuminated and the reaction of both eyes is examined.
If there are deviations, one speaks of an anisocoria. Usually, during the examination, the doctor examines each eye individually, ie. in each case the illuminated eye is checked for the direct pupil reaction and the non-illuminated eye for the consensual reaction. Often one hand is held between the eyes so that the other eye does not get any light from the flashlight.
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In the case of disorders of the pupillary reflex, a distinction is made between damage that affects the afferent thigh, i.e. the nerves that transmit information from the retina to the brain, and those that affect the efferent thigh, i.e. those that carry information from the brain to the eye muscles.
Read more about the topic here: Injury to the visual pathway.
Drugs and other drugs work by inhibiting or activating sympathetic or parasympathetic nerves in the CNS. The pupillary reaction is also innervated via such fibers.
While the sympathetic nervous system leads to a widening (mydriasis) of the pupil, activation of the parasympathetic nervous system leads to a narrowing (miosis) of the pupil.
Further interesting information on this topic can be found at: Which drugs or drugs affect the pupil?
Also read the article: Consequences of drugs.
Multiple sclerosis is a chronic inflammatory disease of the central nervous system in which the myelin sheaths in the nerves break down. The symptoms are very diverse and the causes have not yet been fully clarified. However, it is assumed that the demyelination of the nerve fibers in MS leads to damage in the area of the afferent leg, i.e. the optic nerve.
Often, damage to the pupillary reflex is not the only symptom of MS patients; as a rule, they also suffer from double vision as a result of eye muscle paresis and other visual disturbances.
The term convergence reaction describes the reflexive process of the eye when the focus changes from a distance to a nearby object. On the one hand, this leads to a convergence movement of the eyes. This means that the pupils of both eyes are directed towards the center line of the head. On the other hand, a constriction of the pupils is initiated, whereby the amount of incident light is regulated.
Furthermore, muscle activity leads to a change in the shape of the lens. All of this leads to better vision of nearby objects.
The indirect or consensual pupillary reflex describes the reaction of one eye to the illumination of the eye on the opposite side. If one eye is illuminated with a flashlight, in healthy conditions, both the illuminated and the non-illuminated eye will narrow the pupils.
This is due to an interconnection in the optic nerve, in which fibers from one eye cross to the opposite side in the so-called optic chiasm. Thus, each side of the responsible brain stem area receives the information from both eyes. Accordingly, there is a consensual reaction to light stimulation.