How big are the human pupils?

The size of the human pupil is relatively variable. One of the most important influencing factors is the brightness of the environment. During the day, the pupil has a diameter of about 1.5 millimeters.

At night or in darkness the pupil widens to a diameter of eight to even 12 millimeters. As a result, the circular area of the pupil varies between 1.8 square millimeters in brightness and over 50 square millimeters in darkness. The maximum pupil opening usually decreases with aging.

Function of the pupil

A narrowing of the pupil causes – similar to a camera – an increase in depth of field. This is especially important when imaging near objects. Accordingly, a reflex narrowing of the pupil occurs during near-accommodation.

In addition, edge rays are faded out when the pupil is narrow, which reduces blur caused by spherical aberration. The dependence of pupil width on brightness ensures that not too much and not too little light falls on the retina. The afference runs via the optic nerve (optic nerve, 2nd cranial nerve), which receives the light stimulus, through numerous stations to the area pretectalis of the midbrain in the brain stem.There the efferent pathway begins, the information is directed to a core area in the midbrain, the nucleus Edinger Westphal on both sides, from where the parasympathetic fibers of the nervus oculomotorius (3rd cranial nerve) are activated, which finally lead to a contraction of the Musculus sphincter pupillae on both sides and thus to a constriction of the pupil.

In the course of the fibers from the eye to the midbrain and back, fibers on the opposite side also partially cross over. Therefore, when one eye is illuminated, not only the pupil of this eye narrows (direct light reaction) but also the pupil of the other eye (consensual light reaction). With the knowledge of the afferent and efferent thigh and the fact that normally both pupils always constrict when illuminated, conclusions can be drawn about the location of the damage in the event of a disorder of the pupillomotor system: If the afferent tract is disturbed (e.g. the optic nerve), neither a direct nor a consensual light reaction will occur when the affected eye is illuminated.

However, when the healthy eye is illuminated, both reactions can be triggered. The diseased eye can therefore not be constricted directly, but it can be consensual. This is called amaurotic pupil rigidity.

If the efferent thigh is disturbed (e.g. the oculomotor nerve), there is no constriction in the affected eye, but there is a consensual constriction of the pupil of the opposite side, because the perception of the light stimulus (afference) is intact, so that the healthy opposite side can constrict itself when light falls on it. If the healthy opposite side is illuminated, the direct light reaction here is intact, but the consensual one on the opposite side is not. The affected eye can therefore neither directly nor consensually narrow.

This is called absolute pupil rigidity. A third disturbance of the pupil reaction is pupillotony. In this case, the pupil of the affected eye is wider in light and narrower in darkness than that of the healthy eye, whereby the light reaction is slower, i.e. expansion in darkness and narrowing in light is delayed.

The cause is a disorder of the parasympathetic fibers in the efferent thigh. If the symptomatology is additionally accompanied by a disturbance of the muscle reflexes in both (especially non-triggerability of the Achilles tendon reflex), the disease is also called Adie syndrome. The testing of the pupil reaction is standard in almost every clinical examination, it also plays an important role in coma and brain death diagnostics.