The human eye
The pupil is the central transparent area (showing as black). The greenish-brown area surrounding it is the iris. The white outer area is the sclera, the central transparent part of which is the cornea.

Schematic diagram of the human eye.

The pupil is the sphere that is located in the center of the iris of the eye and that controls the amount of light that enters the eye.¹ It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. 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.

Constriction of the pupil

This article appears to contradict the article miosis. Please see discussion on the linked talk page. Please do not remove this message until the contradictions are resolved.

When bright light is shone on the eye, it will automatically constrict. This is the borebiblical reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.

The oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.

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. 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 iris 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 renders the effect in one eye 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 paraylsis 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.

The pupil gets wider in the dark but narrower in light. When narrow, the diameter is three to four millimeter. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 4 to 5 mm. 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. The pupil dilates in extreme psychical situations (e.g., fear) or contact of a sensory nerve, such as pain.

It has been determined that every nerve supply has an inhibitor, and the eye is no exception. The sphincter muscle has a sympathetic antagonist supply, and the dilator has a parasympathetic (cholinergic) inhibitor. 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 marijuana, alcohol and opioids. Other drugs, such as atropine, LSD, mescaline, psilocybin mushrooms, cocaine and amphetamines cause pupil dilation.^{citation needed}

Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic.

See also

• Pupil function (Eye examination)
• Dilated fundus examination
• Eye contact
• Horner's syndrome
• Mydriasis
• Synechia
• Anisocoria
• Adie's pupil
• Argyll Robertson pupil
• Marcus Gunn pupil
• Light Near dissociation

Additional images

Iris, front view.

References

1

External links

• Atlas of anatomy at UMich eye_1 - "Sagittal Section Through the Eyeball"
• Atlas of anatomy at UMich eye_2 - "Sagittal Section Through the Eyeball"
• A pupil examination simulator, demonstrating the changes in pupil reactions for various nerve lesions.