The center of the iris of the eye that allows light to enter the retina.

Appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.

In humans the pupil is round.

It is the eye’s aperture and its appearance is magnified by the cornea.

On its inner edge lies the collarette, marking the junction of the embryonic pupillary membrane covering the embryonic pupil.

A contractile structure, consisting mainly of smooth muscle, surrounds and the pupil. and it allows light through the pupil, while the iris regulates the amount of light by controlling the size of the pupil.

The iris contains two groups of intrinsic eye smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae.

Sphincter pupillae contraction of the iris constricts the size of the pupil.

The dilator pupillae of the iris, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract.

The sphincter muscle has a parasympathetic innervation.

The dilator has a sympathetic innervation.

The sympathetic system begins in the hypothalamus, descends through the brain stem into the cervical cord to synapse in the ciliospinal center of Budge-Waller at the C8-T1 level.

The second-order neuron exits the C8-T1 nerve root, travels over the lung apex, and ascends to the superior cervical ganglia with the carotid artery, and the third-order neuron leaves the superior cervical ganglia to ascend with the internal carotid artery through the cavernous sinus, where fibers destined for the pupil dilator and the Mueller muscle of the eyelid travel with the trigeminal nerve.

The parasympathetic fibers begin in the Edinger-Westphal subnucleus of cranial nerve III in the midbrain, and fibers destined for the iris sphincter travel with the oculomotor (cranial III) nerve.

The pupil provides a visible example of the neural feedback control in the body.

This is subserved by a balance between the antagonistic sympathetic and parasympathetic divisions of the autonomic nervous system.

In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited.

Pilocarpine constricts pupils, and allows accomodation due to parasympathetic action of the circular muscles.

Atropine impairs accomodation of the eye and dilates the pupil.

Sympathetic discharge dilates the pupil by stimulating sympathetic nerves in the neck and by influx of adrenaline.

The sensory pathway of the eye has partial crossover of nerve fibers which allows its affect of one eye to carry over to the other, therefore when one eye is stimulated, both eyes contract equally.

When light enters the eye light sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, 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 the circular iris sphincter muscle contracts, it reduces the size of the pupil: This is the pupillary light reflex, which is a test of brainstem function.

Pupils enlarge in darkness, and narrow with exposure to light.

Physical examination of pupil size in light, pupil size in the dark, pupil reactivity to light and dark help to evaluate the pupil

Pupil reactivity is graded subjectively on a scale of 0 to 4.

The narrowed pupil has a diameter of 3-4 mm, while a dilated pupil can reach 9mm.

After age 25 years pupil size decreases.

Intensified oscillation of pupil size is ref2242ed to as hippus.

The pupil constricts with near sight vision.

When the optic nerves or partially damaged, the pupils become chronically dilated because the ability of the optic nerves to respond to light is diminished and this condition is called bene dilitasm.

People with bene dilatism have normally dilated pupils in normal lighting, but bright light exposure can cause pain.

Bene dilatism Is associated with visual impairment in darkness.

Pupils dilate when a person sees a object of interest.

Pupil dilatation occurs under emotional distress such as fear or pain, and there is a tendency for pupils to dilate when using cognitive functions.

Drugs that cause constriction of the pupils include: alcohol, opioids, atropine, LSD, mescaline, psilocybin mushrooms, and cocaine.

Pupil dilation may be caused by amphetamines.

Pupillary constriction is ref2242ed to as miosis and pupillary dilation as mydriasis.

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