The pupil is significantly involved in the visual process. It regulates the incidence of light on the retina and is thus involved in the formation of the visual impression. Through the process of stimulus processing, it adapts to the prevailing light conditions.
What is the pupil?
In the eye, the pupil is visible as a black circle and forms the opening of the iris. It is a recess of the iris tissue. The pupil is also referred to as the eye hole. The term is derived from the Latin word “pupilla”, which means “little doll”. The reason for this is the reduced self-reflection in the eye of the opposite person, which was perceived as a dolly. The size of the pupil is conditioned by the incidence of light and its angle.
Anatomy and structure
The diameter of the pupil varies from 1.5 to 8-12 millimeters. Outwardly, it is covered by the anterior chamber of the eye and the cornea. Inside the eye, behind the pupil, lies the lens. It is controlled by the inner eye musculature: the pupil constrictor (Musculus sphincter pupillae) and the pupil dilator (Musculus dilatator pupillae). Ring- and fan-shaped muscles behind the visual aperture are responsible for its widening. Muscle contraction and adjustment of pupil size occurs unconsciously and is dependent on ambient brightness. This adjustment is called the pupillary reflex. Conscious control of pupil width is not possible. It is subject to various factors.
Function and tasks
Together with the iris, the pupil acts as the eye’s iris mechanism. They control the light falling on the retina. Thus, the iris and pupil are involved in the first step of stimulus reception. In the eye, the light is further processed as a stimulus. The retina passes it on to the optic nerve, from where the information is transmitted to the brain. In the pupillary reflex, information is transmitted to the central nervous system on the one hand (afferent) and the corresponding muscles are triggered on the other (efferent). Usually the pupils are of the same size. This is due to crossing nerve fibers that lead from the midbrain to the eyes. Brightness shrinks the pupils, darkness dilates them. The change in brightness is perceived by the retina, but it can only get used to it slowly. The pupil takes over the regulation. In medicine, the widening of the pupil is called mydriasis, while the narrowing is also called miosis. Both terms originate from the Greek. The constriction of the pupil, also called parasympathetic innervation, is a process of the autonomic nervous system. It is sometimes responsible for the recovery and regeneration of the body. Similar to a camera, constriction of the pupil causes an increase in depth of field. When constricted, peripheral rays are blocked out, preventing blurry images. The opposite sympathetic innervation, i.e. dilatation, triggers an increase in the organism’s performance. An example of this is pupil dilation in darkness. This process enables a higher receptivity of the sparsely available light. In addition to its primary function, the pupil also indicates emotions. For example, the pupils dilate in fear, disgust, or joy. These aspects depend on the autonomic nervous system, which responds to emotional state. A new study looks at reading decisions based on changes in pupil size. In a procedure known as pupillometry, doctors measure this size using an infrared camera. This can be used to measure a person’s emotional stress on a computer. But drug use, medication and various diseases also have an effect on this. Taking drugs such as heroin narrows the pupil, while cannabis and LSD, for example, enlarge it. For this reason, physicians often check the pupillary reflex during physical examinations. Depending on the symptoms, a doctor measures their diameter and responsiveness. He also checks whether both pupils react equally to stimuli and whether they are the same size.
Diseases
Diseases that are reflected in pupil size are divided into afferent and efferent diseases. The term “afferent” refers to signal transmission to the brain, while “efferent” names transmission from the brain to the organ. Damage to the retina and the associated disease is afferent.Such damage leads to transmission problems of the collected impressions. Therefore, the pupil adjusts incorrectly. Reasons for this are externally inflicted injuries, diabetes or glaucoma. Another possibility is detachment of the retina. Another afferent disease is damage to the optic nerve. Rarely, external influences are responsible for this. Pathological changes in the cerebral vessels or pressure on the optic nerve triggered by tumors can cause such damage. Inflammations such as multiple sclerosis are also possible causes. Pupillary reactions are often the result. Efferent disorders are triggered by muscles or their nerves. For example, external injuries or Lyme disease can affect the eye muscles. The same effects are seen in multiple sclerosis and diabetes. Pupillotonia is a disorder of the parasympathetic innervation. The mostly harmless disorder triggers a different size regulation of the pupils. Finally, Horner’s syndrome also affects pupil adjustment. This is a nerve damage caused by the failure of the sympathetic nervous system. Unilateral miosis with retracted eyeball or drooping eyelid is the result. Local pill disorders may also result from congenital malformations or age-related degenerative changes. One congenital malformation of the eye is the congenital absence of the iris (aniridia).
