At the center of the iris of the eye is a hole known as the pupil and it allows light to enter the retina. The color of the pupil appears to be black as most of the light enters the pupil which is absorbed by the retina. In animals like cats, have a slit pupil unlike human which is round.
As per the structure of the eye, the pupil from the outside is the entrance to pupil which does not correspond to the size and location of the physical pupil as it is enlarged by the cornea. The inner edge is a prominent structure, the collarette which distinguished the embryonic papillary membrane that covers the embryonic pupil.
The iris consists of a smooth muscle that surrounds the pupil. It is a contractile structure. The pupil is a medium through which the light enters the eye and at the same time the iris controls the amount of light by regulating the size of the pupils.
There are two groups of smooth muscles in the iris- a circular group known as sphincter papillae and the second one is the radial group known as the dilator papillae. Now when there is a contraction in the sphincter papillae, the iris decreases the size of the pupil. Here the dilator papilla along with the sympathetic nerves from the superior cervical ganglion forces the pupil to dilate when they contract.
When bright light strikes the eye, sensitive cells in the retina which include cone and rods photoreceptors and melanopsin ganglion cells pass signals to the oculomotor nerve. This also includes the parasympathetic part of the eye. When this muscle contracts, the size of the pupil reduces. This is known as papillary light reflex.
The pupil expands in the dark and contracts in bright light. After a person is 25 years old, the size of the pupil decreases and does not remain steady. At this point the pupils are not completely still and may lead to oscillation which becomes worse and this is known as hippus.
There are many complications if the optic nerves get partially damaged and one such condition is known as bene dilitatism. In this the pupil chronically widens as there is a decrease in the ability of the optic nerves to react to the light. When the light is normal, people suffering from this condition have dilated pupils and any bright streak of light can cause severe pain. On the other side of the spectrum people with this problem are unable to see in darkness. This condition is dangerous only in the night and not otherwise.
When a person is in emotional situation like fear, the pupil dilates in response or contracts in pain. Any task evoked papillary response effects the pupil as it tends to dilate. There is a similar response in the pupil due to increase attention, loads on working memory and sensory discrimination.
The pupil dilates as a result of effect of drugs also. There are certain drugs that may cause constriction of the pupils like opioids and alcohol. Drugs like LDS, mescaline, atropine, MDMA, cocaine, psilocybin and amphetamines can cause the pupil to dilate.