Pupil is a component of the eye which is located in the middle of the iris. The primary function of the pupil is to manage the amount of light that enters the eye. It appears to be black in color because the light entering the tissue is largely soaked up by the tissues inside the eye. The reflection of the pupil when seen from the exterior is called the entrance pupil and it does not match up to the location and the size of the physical pupil since it is magnified manifold by the cornea.

Whenever there is sudden bright light gleaming on the eye, it consequently constricts, this mechanism is called the borebiblical reflex. Also, the pupil of a person tends to dilate if something of their interest catches the eye.

The oculomar nerve is the nerve responsible for the dilation of the pupil. It is when this nerve contracts that the size of the pupil is reduced.

The iris is a structure with the ability to shrink, tighten or become narrower and is majorly composed of smooth muscles which envelop the pupil. The light enters through the pupil but it is the iris that controls the amount of light to be let inside by adjusting the size of the pupil. There are two groups of smooth muscles that constitute the iris- one group of muscles being circular in nature called sphincter pupille and the other group being radical in nature called dilator pupillae. The sphincter pupillae controls the increase or decrease in the pupil size by contracting itself. As for the dilator pupillae, they get stimulated by the sympathetic nerve and in turn cause the dilation of the iris.

The pupil can be dilated by the sympathetic nerve system in two ways- one by innervating the sympathetic nerve present in the neck and second by the incursion of adrenaline. The pupil reacts to light and gets wide or narrow accordingly. When there is little or no light the pupil gets wider whereas, in the presence of light the pupil becomes narrower. The diameter of the pupil when it is narrow in the light is usually three to four millimeter, whereas that in the dark is four to five millimeter.

There are a certain drugs that lead to the constriction of the pupil such as alcohol, marijuana and opioids. Other drugs such as LSD, cocaine, psilocybin mushroom, atropine and amphetamines may cause the dilation of the pupil. The constriction of the pupil is also termed as miosis and the substances that trigger a miosis are called meiotic substances.