Pupil

Synonyms in a broader sense

visual hole

Definition

The pupil forms the black center of the colored iris. It is through this iris that light enters the eye and travels to the retina, where it leads to the signal transduction that is responsible for the creation of a visual impression. The pupil is variable in size. The pupil reflex is a very important functional test in the clinic.

Anatomy & Physiology

The pupil can change its size, this is called pupillomotor function. It can narrow up to 1. 5 mm, which is then called miosis (Greek), its dilation up to 8 mm is called mydriasis (Greek).

Two muscles are responsible for pupillomotor function: both are inner eye muscles. Each muscle needs an innervation by a nerve so that it can be “controlled”. In the case of the muscles for pupillomotor function, these are nerves of the autonomic nervous system.

It is roughly divided into two parts, the sympathetic nervous system and the parasympathetic nervous system. A characteristic of this part of our nervous system is that we cannot or hardly can control it at all. This is also true for the pupil width.

The presence or absence of light is primarily responsible for the width. If a lot of light falls on the pupil, the sphincter pupillae muscle is activated. This is done via the parasympathetic nervous system and the pupil becomes narrow.

If it is dark, the pupil becomes dilated and the pupil widens. This leads to an activation of the musculus dilatator pupillae, which is innervated by the sympathetic nervous system. But besides light as the main initiator for a change in pupil width, other factors also play a role.

A classic example is a dilation of the pupil when confronted with a person to whom one is inclined. Mydriasis can also occur during excitement and fear. This is due to the fact that in these situations the sympathetic nervous system is activated, which is not only responsible for the eye but also attacks the rest of the body, it becomes especially active in situations of increased readiness to act.

A classic example from the time of our ancestors is the “tiger in the bush”, at whose sight the sympathetic nervous system is activated and thus optimally prepares the person for the upcoming escape. Opposite happens with the parasympathetic nervous system, he becomes more active in situations in which one has come to rest.

• The Musculus sphincter pupillae causes a narrowing of the pupil
• While the musculus dilatator pupillae causes an enlargement.

The pupil width also changes with accommodation (close up), here a miosis occurs, with the opposite view into the distance the pupil will dilate.

Normally, both pupils are equally wide (isocoria). If one pupil is significantly wider or narrower than the other, this is called anisocoria. Anisocoria can occur, for example, in the case of increased intracranial pressure (e.g. due to bleeding after craniocerebral trauma or brain tumors) or in the context of Horner’s syndrome, which is classically characterized by the triad of miosis (narrow pupil), ptosis (drooping upper eyelid) and enopthalmos (sunken eyeball).