Why do my eyes cross involuntarily?

What is the pupillary reflex?

The pupillary reflex describes the involuntary adjustment of the eye to changing lighting conditions. The width of the pupil changes reflexively with incident light.

  • If the environment is very bright, the light stimulus is correspondingly high and the pupil diameter is reduced (Miosis).
  • If the light stimulus is small, i.e. in dark conditions, the pupil dilates (Mydriasis).

This reflex is controlled by the central nervous system's parasympathetic nervous system and plays an important role in visual acuity and in protecting the retina.

You might also be interested in the following article: Retina of the eye

 

 

What do we have a pupillary reflex for?

The pupillary reflex is used for rapid adjustment of the eye to the prevailing lighting 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 this Protection of the Netzhaut. 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 the pain and the risk of damage to the Retina.

How does the pupillary reflex work?

Like every reflex, the pupillary reflex has one 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 eyes are called Chopsticks and cones designated. These cells are the sensory cells of the eye and have different tasks.
The rod are primarily responsible for the perception of light-dark vision and are therefore more important for the pupillary reflex than the cones. In these cells the Conversion into electrical signals instead of. 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 theOptic nerve now follow various anatomical structures up 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.

By doing Area of ​​the brain stem, the Area pretectalis, the second part of the reflex arc begins. Depending on the light conditions, signals are transmitted via one of the two parts of the vegetative nervous system back to the eye Posted. These signals are conducted either 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 theBrain stem

How can you test the pupillary reflex?

The Examination of the pupillary reflex is one of the standard examinations in neurology. The pupillary reflex can be controlled by means of a Flashlight examination be tested.

One eye is illuminated and the reaction of both eyes is examined.

  • Due to the incidence of the flashlight, the pupils shrink, one speaks of one direct pupillary reaction. Due to interconnections in the optic nerve, in healthy conditions not only the illuminated eye reacts, but also the one on the opposite side with a narrowing of the pupils. One speaks of one consensual or indirect pupillary response. Both eyes should have the same width, this is called isokor.

If there are deviations, one speaks of a 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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What causes disorders of the pupillary reflex?

At Disorders of the pupillary reflex a distinction is made between damage that affects the afferent thigh concern, i.e. the nerves transmit information from the retina to the brain, from those who have the efferent thigh concern, i.e. carry information from the brain to the eye muscles.

  • Afferent thigh damage mostly affect parts of the optic nerve (optic nerve). During the examination one can find a disturbed direct pupillary reaction, ie. If the affected eye is illuminated, there is no constriction of the pupil, while if the healthy eye is illuminated in both, a constriction occurs. Causes for this can be injuries, inflammations or tumors in the area of ​​the optic nerve, but also cerebral haemorrhages and multiple sclerosis.
  • Damage to the efferent thigh concern the motor nerve, which is responsible for the innervation of the muscles responsible for the pupillary reaction (oculomotor nerve). A disruption in this area becomes noticeable in the absence of a direct or consensual pupillary reaction in the affected eye. Causes for this can be inflammation, injuries or tumors in the area of ​​the oculomotor nerve, but also a lack of oxygen.

Read more about the topic here:Injury to the visual pathway

How do drugs affect the pupillary reflex?

Drugs and other medicines develop their effect by inhibiting or activating sympathetic or parasympathetic nerves in the CNS. The pupillary reaction is also innervated via such fibers.

During the Sympathetic leads to a dilatation (mydriasis) of the pupil, it occurs when the Parasympathetic nervous system constriction (miosis) of the pupil.

  • Drugs like opiate and nicotine activate the Parasympathetic nervous system. As a result, they trigger relaxation, anxiety relief and pain relief in the body. In addition, they also lead to a narrowing of the pupil. With an overdose of opiates, patients often have a maximally small pupil, which is why one speaks of a pin-sized pupil.
  • Other drugs like Amphetamines, speed, ecstasy, cocaine etc. lead to activation of the Sympathetic. The intoxicating effect manifests itself in increased euphoria, increased concentration, increased self-confidence, increased libido, etc. Side effects include a dilated pupil, which is often noticeable very quickly during police checks.

Further interesting information on this topic can be found at:Which drugs or drugs affect the pupil?

Also read the article:Consequences of drugs.

How does the pupillary reflex change in MS?

In the 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.

  • This damage makes itself through the Lack of direct pupillary response noticeable in the affected eye.
  • However, the consensual pupillary reaction in the illuminated contralateral eye persists.

Often, damage to the pupil reflex is not the only symptom in MS patients; as a rule, they also suffer from it as the disease progresses Double vision due to ocular muscle paresis and other visual disturbances.

What is the convergence reaction?

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 initiated, whereby the amount of incident light is regulated.
Furthermore, muscle activity leads to a Change of lens shape. All of this leads to better vision of nearby objects.

What is the indirect pupillary reflex?

The indirect or consensual pupillary reflex describes the reaction of one eye to the illumination of the eye on the other 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 of 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.

additional Information

You can find more information on the subject of the pupil reflex here:

Quality assurance by: Dr. Nicolas Gumpert | Last change: 10.04.2021