Pupil

Pupil

Overview
The pupil is a hole located in the center of the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 that allows light to enter the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. It appears black because most of the light entering the pupil is absorbed by the tissues
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 inside the eye. In humans the pupil is round, but other species, such as some cats
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...

, have slit pupils. In optical terms, the anatomical pupil is the eye's aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 and the iris is the aperture stop. The image of the pupil as seen from outside the eye is the entrance pupil
Entrance pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...

, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

.
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Encyclopedia
The pupil is a hole located in the center of the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 that allows light to enter the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. It appears black because most of the light entering the pupil is absorbed by the tissues
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 inside the eye. In humans the pupil is round, but other species, such as some cats
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...

, have slit pupils. In optical terms, the anatomical pupil is the eye's aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 and the iris is the aperture stop. The image of the pupil as seen from outside the eye is the entrance pupil
Entrance pupil
In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil...

, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

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

Controlling


The iris is a contractile structure, consisting mainly of smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

, 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
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...

, have slit pupils. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae
Iris sphincter muscle
The iris sphincter muscle is a muscle in the part of the eye called the iris...

, 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.

Optic effects


When bright light is shone on the eye light sensitive cells in the retina, including rod and cone photoreceptors and melanopsin
Melanopsin
Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...

 ganglion cells, will send signals to the oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

, specifically the parasympathetic part coming from the Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

, 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. At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus
Hippus
Hippus, also known as pupillary athetosis, is spasmodic, rhythmic Hippus, also known as pupillary athetosis, is spasmodic, rhythmic Hippus, also known as pupillary athetosis, is spasmodic, rhythmic (Hippus, also known as pupillary athetosis, is spasmodic, rhythmic (...

. 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.

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.

Psychological effects


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.

Facial expressions of sadness
Sadness
Sadness is emotional pain associated with, or characterized by feelings of disadvantage, loss, despair, helplessness, sorrow, and rage. When sad, people often become outspoken, less energetic, and emotional...

 with small pupils are judged significantly more intensely sad with decreasing pupil size though the brainstem pupillary control Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

 in proportion to a person's pupil size change response to that in another. The greater degree to which a person's pupil dilation mirrors another person's coincides with that person having a greater empathy
Empathy
Empathy is the capacity to recognize and, to some extent, share feelings that are being experienced by another sapient or semi-sapient being. Someone may need to have a certain amount of empathy before they are able to feel compassion. The English word was coined in 1909 by E.B...

 score.

Effect of drugs


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
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 and opioids. Other drugs, such as atropine
Atropine
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade , Jimson weed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects...

, LSD
LSD
Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed and open eye visuals, synaesthesia, an...

, MDMA, mescaline
Mescaline
Mescaline or 3,4,5-trimethoxyphenethylamine is a naturally occurring psychedelic alkaloid of the phenethylamine class used mainly as an entheogen....

, psilocybin mushrooms, cocaine
Cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...

 and amphetamines may cause pupil dilation.

Another term for the constriction of the pupil is miosis
Miosis
Miosis is the constriction of the pupil of the eye to two millimeters or less...

. Substances that cause miosis are described as miotic. Dilation of the pupil is mydriasis
Mydriasis
Mydriasis is a dilation of the pupil due to disease, trauma or the use of drugs. Normally, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day...

. Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide
Tropicamide
Tropicamide is an anticholinergic used as a mydriatic. In Russia, it is used to some degree as an inexpensive recreational drug...

.

See also

  • Pupillary response
    Pupillary response
    Pupillary response or dilation of the pupil is a physiological response that varies the size of the pupil of the eye via the iris dilator muscle...

  • Pupil function
  • Dilated fundus examination
    Dilated fundus examination
    Dilated fundus examination is a diagnostic procedure that employs the use of mydriatic eye drops to dilate or enlarge the pupil in order to obtain a better view of the fundus of the eye. Once the pupil is dilated, examiners often use specialized equipment such as an ophthalmoscope or fundus...

  • Eye contact
    Eye contact
    Eye contact is a meeting of the eyes between two individuals.In human beings, eye contact is a form of nonverbal communication and is thought to have a large influence on social behavior. Coined in the early to mid-1960s, the term has come in the West to often define the act as a meaningful and...

  • Horner's syndrome
    Horner's syndrome
    Horner's syndrome is the combination of drooping of the eyelid and constriction of the pupil , sometimes accompanied by decreased sweating of the face on the same side; redness of the conjunctiva of the eye is often also present...

  • Mydriasis
    Mydriasis
    Mydriasis is a dilation of the pupil due to disease, trauma or the use of drugs. Normally, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day...

  • Synechia (eye)
  • Anisocoria
    Anisocoria
    -Causes:In the absence of the iris or eyeball proper, anisocoria is usually the result of a defect in efferent nervous pathways controlling the pupil traveling in the oculomotor nerve or the sympathetic pathways...

  • Adie's pupil
  • Argyll Robertson pupil
    Argyll Robertson pupil
    Argyll Robertson pupils are bilateral small pupils that constrict when the patient focuses on a near object , but do not constrict when exposed to bright light . They are a highly specific sign of neurosyphilis. In general, pupils that “accommodate but do not react” are said to show light-near...

  • Light Near dissociation
  • Marcus Gunn Pupil
    Marcus Gunn pupil
    Marcus Gunn pupil is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils constrict less when a bright light is swung from the unaffected eye to the affected eye...


External links

- "Sagittal Section Through the Eyeball" - "Sagittal Section Through the Eyeball"