Pupil

Generality

The pupil is a "circular opening located in the center of the iris," which allows light to enter the back of the eyeball.

The pupillary hole has a variable diameter: in the dark it dilates, in bright conditions it shrinks. The size of the pupil is controlled by the iris, which, thanks to a system of muscles, responds to stimulation of the autonomic nervous system, thus modulating the amount of light that enters the eye.

Appearance and structure

The pupil is located approximately 3 mm from the vertex of the cornea. Upon observation, this small hole in the center of the iris appears black, as most of the light, which is able to pass through the cornea and crystalline, is absorbed by the tissues inside the eye. A certain amount of light, however, manages to reflect itself and, in particular situations, makes the pupil seem "luminous".
Under normal conditions, the pupil diameter tends to vary between 2 and 5 mm. The pupil dilates to let in more light (mydriasis) and shrinks when it needs to let in less (miosis).

Pupillary constriction and dilation

The iris is made up of a loose connective stroma, rich in pigmented cells, and anteriorly lined with endothelial tissue. In the stroma, a ring of smooth muscle cells forms the constrictor (or sphincter) muscle of the pupil. The posterior face of the iris. instead, it has smooth muscle cells arranged radially, which constitute the dilator muscle of the pupil.

• The cells that make up the constrictor muscle are arranged to form concentric rings around the pupil and when they contract, the pupil diameter decreases, causing constriction (miosis). The activity of the constrictor muscle is regulated by parasympathetic neurons.
• The dilator muscle is organized into a radius and innervated by the orthosympathetic; when it contracts, it causes an increase in pupil diameter (mydriasis). Therefore, the contraction of the radial muscles generates pupil dilation.

Why does the pupil change size without us noticing it?
The pupil's response is an involuntary reflex: in case of intense light, its constriction allows the photosensitive cells of the retina not to be damaged. Conversely, in low light conditions, its expansion serves to capture as much light as possible.

Mydriasis

The word "mydriasis" comes from the Greek "amadros", which means dark, and indicates pupil dilation.
From a physiological point of view, the pupillary hole widens transiently due to the adaptation of the eye to darkness. This reaction can also follow an intense emotion, such as anxiety, excitement or fear.