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Pupil
Eye iris.jpg
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.
Schematic diagram of the human eye en.svg
Schematic diagram of the human eye.

The pupil is an opening 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 split 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.

Contents

Control

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 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 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 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 has a hole that is covered with a lens that doesn't fall off.

Optic effects

When bright light is shone on the eye, light sensitive ganglion cells in the retina, containing the pigment melanopsin, 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 4 millimeters. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 5 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 5 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.

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[4].

Facial expressions of sadness with small pupils are judged significantly more intensely sad with decreasing pupil size though people are unaware of pupil size affecting their judgment. A person's own pupil size also mirrors this with them being smaller when viewing sad faces with small pupils. There is no parallel effect when people look at neutral, happy or angry expressions. Brain areas involved in this include those processing social signals in the amygdala, and areas involved in the mirror neuron system such as the left frontal operculum. The degree of empathetic contagion activated the brainstem pupillary control Edinger-Westphal nucleus in proportion to a person's pupil size change response to that in another.[5] The greater degree to which a person's pupil dilation mirrors another person's coincides with that person having a greater empathy score.[6]

Effect of drugs

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 alcohol and opioids. Other drugs, such as atropine, LSD, 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.

See also

Additional images

References

  1. ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
  2. ^ a b Malmström T, Kröger RH (January 2006). "Pupil shapes and lens optics in the eyes of terrestrial vertebrates". J. Exp. Biol. 209 (Pt 1): 18–25. doi:10.1242/jeb.01959. PMID 16354774. 
  3. ^ Aging Eyes and Pupil Size
  4. ^ Beatty, Jackson; Brennis Lucero-Wagoner (2000). "The Pupillary System". in John T. Cacioppo, Gary Berntson, Louis G. Tassinary (eds.). Handbook of Psychophysiology (2 ed.). Cambridge University Press. pp. 142–162. ISBN 052162634X. 
  5. ^ Harrison NA, Singer T, Rotshtein P, Dolan RJ, Critchley HD. (2006). Pupillary contagion: central mechanisms engaged in sadness processing. Soc Cogn Affect Neurosci. 1(1):5-17. PMID 17186063
  6. ^ Harrison NA, Wilson CE, Critchley HD. (2007). Processing of observed pupil size modulates perception of sadness and predicts empathy. Emotion. 7(4):724-9. PMID 18039039

External links


Source material

Up to date as of January 22, 2010
(Redirected to The Pupil article)

From Wikisource

The Pupil
by Henry James

Contents

PD-icon.svg This work is in the public domain in the United States because it was published before January 1, 1923.

The author died in 1916, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 80 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

PUPIL (Lat. pupillus, orphan, minor, dim. of pupas, boy, allied to puer, from root puor peu-, to beget, cf. "pupa," Lat. for "doll," the name given to the stage intervening between the larval and imaginal stages in certain insects), properly a word taken from Roman law for one below the age of puberty (impubes), and not under patria potestas, who was under the protection of a tutor, a ward or minor (see Infant; and Roman Law). The term was thus taken by the Civil Law and Scots Law for a person of either sex under the age of puberty in the care of a guardian. Apart from these technical meanings the word is generally used of one who is undergoing instruction or education by a teacher. In education the term "pupil-teacher" is applied to one who, while still receiving education, is engaged in teaching in elementary schools. The system was introduced into England from Holland about 1840. At first the education which the pupil-teachers received was given at the schools to which they were attached. During the last quarter of the 19th century was developed a system of "pupil-teacher centres" where training and education was given. In 1907 was introduced "bursaries," as an alternative; these enable those intending to become teachers to continue their education at. training colleges or selected schools as "student teachers." (See Education.) A special use of the Lat. feminine diminutive pupilla has been adopted in English and other languages for the central orifice in the iris of the eye, the pupil. The origin of the sense may be found in the parallel use in early English of "baby," referring to small images seen reflected in that part of the eye (see EYE and Vision).


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Simple English

There are different types of pupils:

  • The centre of the eye, which is an opening into the inside, is called pupil
  • A child who is expected to be taught something is a pupil. In American English the more common term is student








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