It is known that the pupil changes as soon as it is confronted with high or low light exposure. The effect occurs, for example, when someone comes out of bright daylight into a dark room. In this way, the eye always adapts to its environment. This is the pupillary reflex, also known as light or dark adaptation, which occurs whenever the eye needs to protect the retina, also known as the retina, from excessive light exposure. The reflex takes place unconsciously and is also used in the medical field. A standard diagnostic procedure in emergencies is the pupil test. This is done by using a flashlight or a pupillometer to test how the eye reacts. Since the pupillary reflex is controlled by the brain, this allows for diagnosis of brain activity and consciousness to better assess the patient’s condition.
What is the pupillary reflex?
The pupillary reflex, also known as light or dark adaptation, occurs whenever the eye needs to protect the retina, also called the retina, from too much light. The pupil is a visual opening of the eye through which light enters the interior of the eye. The visible change in the size of the pupil when light enters it, is the reflex of the iris. Involved in the pupillary reflex are the third cerebral and optic nerves. In the retina, stimulus reception takes place. In this process, the pupil can constrict or dilate and regulate the incident light through the iris muscles. With varying illumination, the eye continues to try to produce images. The size of the pupil is thereby adjusted by the iris, like a camera aperture, to the prevailing light conditions. This happens as soon as the photoreceptors of the retina perceive 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 visual aperture is extremely narrowed in strong light conditions, which is referred to 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. In this process, 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 connected to form the reflex. When we speak of innervation, we are talking about the supply of nerves to organs or tissues. The pupil dilates due to the sympathetic innervation of the dilatator pupillae muscle. This muscle lies on the pigment leaflet of the iris and serves as an antagonist of the sphincter pupillae muscle, which in turn is responsible for causing the pupil to constrict. In this case, parasympathetic innervation takes place. The sphincter pupillae muscle is located in the posterior part of the iris stroma and has reticular fibers. In this case, the reflex of the iris normally occurs in both eyes simultaneously, even when light enters only one of the two pupils.
Function and task
The retina is equipped with different light-sensitive cells, which in turn respond to different spectral ranges. Therefore, the eye can not only distinguish between light and dark, but also perform a natural white balance. Thus, the constant change in the color temperature of an environment is hardly noticed by the sighted person. The pupil does not only react with a reflex when light falls on it. The pupil also dilates or constricts when drugs or medications are taken, so the pupillary reflex can tell us a lot about the state of consciousness of the person concerned. For example, the pupillary reflex is also severely impaired when a person has sustained severe head injuries. In comatose states or by the onset of clinical death, pupillary response no longer occurs. If the reflex on either pupil fails, it may also be due to a brain tumor or cerebral hemorrhage.
Diseases and conditions
Pupillary reflex disorders come in afferent and efferent forms. Afferent disorders of the pupillary reflex are disorders that involve signal transmission from the eye to the brain. Efferent disorders involve the reverse pathway, a disturbed signal transmission from the brain to the eye. In afferent disorders, e.g.B. the optic nerve is damaged, then no immediate pupillary reaction takes place as soon as light is shone into the affected eye. Similarly, if the efferent limb is impaired, pupillary constriction no longer takes place. This can be the case, for example, if there is damage to the third cranial nerve, which is also responsible for the movement of the eyeball, among other things. Damage to the retina, in turn, leads to an incorrect reaction of the pupil width, since the transmission of the received light stimuli no longer takes place. If the optic nerve is damaged, the pupil no longer responds adequately to changes in light stimuli. This can occur with pathological changes in the cerebral vessels, likewise with tumors that lie on the optic nerve or in the vicinity and exert pressure there. Likewise, such damage occurs in the disease multiple sclerosis. Efferent disorders can also disturb the respective muscles and nerves. Muscles make pupil adjustments, and nerves supply these muscles. If a disorder is present, the pupils are unequal, and medicine refers to this as anisocoria. For example, the right pupil may be dilated while the left is constricted or normal. There are also disorders of the muscles that regulate pupil width. This can be caused by external injury or by diseases such as diabetes or Lyme disease. Parasympathetic innervation, on the other hand, is usually disturbed when nerve damage is present. In medicine, this is referred to as pupillotonia. Here, too, the pupils can be variably dilated. The cause is a misdirected innervation of the pupillary muscle. If sympathetic innervation is disturbed, it is Horner’s syndrome, which usually occurs unilaterally. Symptoms include miosis, a drooping eyelid, or an eyeball that is retracted far into the orbit. This is then referred to as enophthalmos.
