Pupillary reflex

It is known that the pupil changes when exposed to high or low light exposure. The effect occurs z. For example, if someone comes from bright daylight into a dark room. In this way, the eye always adapts to its surroundings. It is the Pupillary reflex, which is also called Light or dark adaptation and always occurs when the eye has to protect the retina, also known as the retina, from excessive exposure to light.

The reflex takes place unconsciously and is also used in the medical field. A standard diagnosis in emergencies is the pupil test. This is done with a flashlight or a pupillometer to test how the eye reacts. Since the pupil reflex is controlled by the brain, a diagnosis can be made based on brain activity and consciousness and the patient's condition can be better assessed.

What is the pupillary reflex?

The pupillary reflex, which is also known as light or dark adaptation, always occurs when the eye has to protect the retina, also known as the retina, from excessive incidence of light.

The pupil is an opening in the eye through which light penetrates into the interior of the eye. The visible change in size of the pupil when exposed to light is the reflex of the iris. The third cerebral nerve and the optic nerve are involved in the pupillary reflex. The stimulus is picked up in the retina. The pupil can narrow or widen and regulate the incident light through the iris muscles.

With different lighting, the eye continues to try to generate images. The size of the pupil is adapted to the prevailing light conditions by the iris, like a camera screen. This happens as soon as the photoreceptors in the retina sense light. The retina is the sensory area of ​​the eye and is used to perceive all light stimuli. It has a seeing and a blind part.

During the incidence of light, the pupil can never be completely closed, instead the eye hole is extremely narrowed in strong light conditions, which is known as miosis. Conversely, when the pupil dilates, it is mydriasis.

These processes take place biochemically in the sensory cells, which in turn are the cones and rods of the retina. Gamma cells transmit the information that light is incident via the optic nerve to the core area of ​​the midbrain, where in turn the fibers are interconnected to form a reflex.

When speaking of innervation, it is about the supply of the organs or tissue with nerves. The sympathetic innervation of the dilator pupillae muscle dilates the pupil. This muscle lies on the pigment sheet of the iris and acts as an antagonist to the sphincter pupillae muscle, which in turn is responsible for narrowing the pupil. In this case, parasympathetic innervation takes place. The sphincter pupillae muscle lies in the rear part of the iris stroma and has lattice-like fibers. The reflex of the iris usually runs in both eyes simultaneously, even if the light only falls into one of the two pupils.

Function & task

The retina is provided with different light-sensitive cells, which in turn react to different spectral ranges. Therefore, the eye can not only differentiate between light and dark, but also perform a natural white balance. The constant change in the color temperature of an environment is barely perceived by sighted people.

The pupil not only reacts with a reflex when exposed to light. Even when taking drugs or medication, the eye hole expands or narrows, so the pupillary reflex can tell a lot about the state of consciousness of the person concerned.

The pupillary reaction is also severely disturbed, for example, when a person has sustained severe head injuries. In a comatose state or when clinical death occurs, there is no longer any pupillary reaction. If the reflex fails on one of the two pupils, it can also be a brain tumor or cerebral hemorrhage.

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Illnesses & ailments

Disorders of the pupillary reflex exist in afferent and efferent forms. Afferent diseases of the pupil are disorders that affect the transmission of signals from the eye to the brain. Efferent diseases affect the opposite way, a disturbed signal transmission from the brain to the eye.

In afferent disorders z. If, for example, the optic nerve is damaged, there is no immediate pupillary reaction as soon as the affected eye is illuminated. Likewise, there is no more pupillary constriction when the efferent leg is disturbed. This can be B. be the case with damage to the third cranial nerve, the u. a. is also responsible for the movement of the eyeball.

Damage to the retina, in turn, leads to a faulty reaction of the pupil size, since the transmission of the received light stimuli no longer takes place. If the optic nerve is damaged, the pupil no longer reacts adequately to changed light influences. This can occur with pathological changes in the cerebral vessels, also with tumors that are located on or near the optic nerve and exert pressure there. Such damage also occurs in multiple sclerosis.

Efferent disorders can also disrupt specific muscles and nerves. The muscles adjust the pupils, the nerves supply these muscles. If there is a disorder, the pupils are unequal, which medicine speaks of anisocoria. For example, the right pupil may be dilated while the left is narrowed or normal. There are also disorders of the muscles that regulate the pupil size. This can be caused by external injuries or by diseases such as diabetes or Lyme disease.

The parasympathetic innervation, in turn, is mostly disturbed when there is nerve damage. In medicine, it is referred to as pupillotonia. Here, too, the pupils can be dilated differently. The cause is a misdirected innervation of the pupillary muscle.

If the sympathetic innervation is disturbed, it is Horner's syndrome, which usually occurs on one side. Symptoms are miosis, a drooping eyelid or an eyeball that is pulled far into the eye socket. Then there is talk of an enophthalmos.