| Pupil | |
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| The human eye The pupil is the central transparent area (showing as black). The grey/blue area surrounding it is the iris. The white outer area is the sclera, the central transparent part of which is the cornea. |
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| Schematic diagram of the human eye. |
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina.[1] It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils.[2] In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop. The image of the pupil as seen from outside the eye is the entrance pupil, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea. On the inner edge lies a prominent structure, the collarette, marking the junction of the embryonic pupillary membrane covering the embryonic pupil.
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The iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. In humans the pupil is round, but other species, such as some cats, have slit pupils.[2] The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles. The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye's fibers. This causes the effect in one eye to carry over to the other. If the drug pilocarpine is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibers, conversely, atropine will cause paralysis of accommodation (cycloplegia) and dilation of the pupil. The sympathetic nerve system can dilate the pupil in two ways: by the stimulation of the sympathetic nerve in the neck, or by influx of adrenaline.s.
When bright light is shone on the eye light sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, will send signals to the oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil. This is the pupillary light reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.
The pupil gets wider in the dark but narrower in light. When narrow, the diameter is 3 to 5 millimeters. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 4 to 9 mm. In any human age group there is however considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals. After 25 years of age the average pupil size decreases, though not at a steady rate.[3] At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus. When only one eye is stimulated, both eyes contract equally. The constriction of the pupil and near vision are closely tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.[4]
A condition called bene dilitatism occurs when the optic nerves are partially damaged. This condition is typified by chronically widened pupils due to the decreased ability of the optic nerves to respond to light. In normal lighting, people afflicted with this condition normally have dilated pupils, and bright lighting can cause pain. At the other end of the spectrum, people with this condition have trouble seeing in darkness. It is necessary for these people to be especially careful when driving at night due to their inability to see objects in their full perspective. This condition is not otherwise dangerous.
The pupil dilates in response to extreme emotional situations such as fear, or to contact of a sensory nerve, such as pain. Task-evoked pupillary response is the tendency of pupils to dilate slightly in response to loads on working memory, increased attention, sensory discrimination, or other cognitive loads.[5]
Facial expressions of sadness with small pupils are judged significantly more intensely sad with decreasing pupil size though the brainstem pupillary control Edinger-Westphal nucleus in proportion to a person's pupil size change response to that in another.[6] The greater degree to which a person's pupil dilation mirrors another person's coincides with that person having a greater empathy score.[7]
The sphincter muscle has a parasympathetic innervation, and the dilator has a sympathetic innervation. In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited. The reverse is true, so control of pupil size is controlled by differences in contraction intensity of each muscle.
Certain drugs cause constriction of the pupils, such as alcohol and opioids. Other drugs, such as atropine, LSD, MDMA, mescaline, psilocybin mushrooms, cocaine and amphetamines may cause pupil dilation.
Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic. Dilation of the pupil is mydriasis. Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide.
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