Pupil

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Pupil
Eye iris.jpg
The pupil is the central opening of the iris on the inside of the eye, which normally appears black. The grey/blue or brown area surrounding the pupil is the iris. The white outer area of the eye is the sclera. The central outermost transparent colorless part of the eye (through which we can see the iris and pupil) is the cornea.
Schematic diagram of the human eye en.svg
Cross-section of the human eye, showing the position of the pupil.
Details
Part of Eye
System Visual system
Identifiers
Latin pupilla. (plural: pupillae)
MeSH D011680
TA98 A15.2.03.028
TA2 6754
FMA 58252
Anatomical terminology

The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. [1] It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.[ citation needed ] The size of the pupil is controlled by the iris, and varies depending on many factors, the most significant being the amount of light in the environment. The term "pupil" was coined by Gerard of Cremona. [2]

Contents

In humans, the pupil is circular, but its shape varies between species; some cats, reptiles, and foxes have vertical slit pupils, goats and sheep have horizontally oriented pupils, and some catfish have annular types. [3] 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.

Function

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. This is known as the pupillary light reflex.

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.

Effect of light

The pupil diameter can vary greatly due to various factors (primarily the pupillary light reflex), from constriction to as small as 2 mm, to dilation larger than 8 mm in some individuals, though the maximal dilation also varies substantially by individual and decreases with age Pupillary light reflex.jpg
The pupil diameter can vary greatly due to various factors (primarily the pupillary light reflex), from constriction to as small as 2 mm, to dilation larger than 8 mm in some individuals, though the maximal dilation also varies substantially by individual and decreases with age

The pupil gets wider in the dark and narrower in light. When narrow, the diameter is 2 to 4 millimeters. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8 mm. However, in any human age group there is 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. [4] [5] At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus. 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. [6]

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.[ citation needed ]

Clinical significance

Effect of drugs

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.

Certain drugs cause constriction of the pupils, such as opioids. [7] Other drugs, such as atropine, LSD, MDMA, mescaline, psilocybin mushrooms, cocaine and amphetamines may cause pupil dilation. [8] [9]

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.

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.

Diseases

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 affected by 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.

Size

The size of the pupil (often measured as diameter) can be a symptom of an underlying disease. Dilation of the pupil is known as mydriasis and contraction as miosis.

A human adult exhibiting voluntary control over his iris muscles, which grants him the ability to dilate and constrict his pupils on command Voluntary pupil dilation.gif
A human adult exhibiting voluntary control over his iris muscles, which grants him the ability to dilate and constrict his pupils on command
Pupil dilated naturally to 9 mm due to dim light. The subject is an extreme case, as most individuals are not able to naturally dilate their pupils to that extent Human eye in dim light.jpg
Pupil dilated naturally to 9 mm due to dim light. The subject is an extreme case, as most individuals are not able to naturally dilate their pupils to that extent
Pupil constriction can be in response to negative emotional states Human eye in bright light.jpg
Pupil constriction can be in response to negative emotional states

Not all variations in size are indicative of disease however. In addition to dilation and contraction caused by light and darkness, it has been shown that solving simple multiplication problems affects the size of the pupil. [10] The simple act of recollection can dilate the size of the pupil, [11] however when the brain is required to process at a rate above its maximum capacity, the pupils contract. [12] There is also evidence that pupil size is related to the extent of positive or negative emotional arousal experienced by a person. [13]

Myopic individuals have larger resting and dark dilated pupils than hyperopic and emmetropic individuals, likely due to requiring less accommodation (which results in pupil constriction). [14]

Some humans are able to exert direct control over their iris muscles, giving them the ability to manipulate the size of their pupils (i.e. dilating and constricting them) on command, without any changes in lighting condition or eye accommodation state. [15] However, this ability is likely very rare and its purpose or advantages over those without it are unclear.

Animals

The W-shaped pupil of the cuttlefish expanding when the lights are turned off.

Not all animals have circular pupils. Some have slits or ovals which may be oriented vertically, as in crocodiles, vipers, cats and foxes, or horizontally as in some rays, flying frogs, mongooses and artiodactyls such as elk, red deer, reindeer and hippopotamus, as well as the domestic horse. Goats, sheep, toads and octopus pupils tend to be horizontal and rectangular with rounded corners. Some skates and rays have crescent shaped pupils, [16] gecko pupils range from circular, to a slit, to a series of pinholes, [17] and the cuttlefish pupil is a smoothly curving W shape. Although human pupils are normally circular, abnormalities like colobomas can result in unusual pupil shapes, such as teardrop, keyhole or oval pupil shapes.

There may be differences in pupil shape even between closely related animals. In felids, there are differences between small- and large eyed species. The domestic cat (Felis sylvestris domesticus) has vertical slit pupils, its large relative the Siberian tiger (Panthera tigris altaica) has circular pupils and the Eurasian lynx (Lynx lynx) is intermediate between those of the domestic cat and the Siberian tiger. A similar difference between small and large species may be present in canines. The small red fox (Vulpes vulpes) has vertical slit pupils whereas their large relatives, the gray wolf (Canis lupus lupus) and domestic dogs (Canis lupus familiaris) have round pupils.[ citation needed ]

Evolution and adaptation

One explanation for the evolution of slit pupils is that they can exclude light more effectively than a circular pupil.[ citation needed ] This would explain why slit pupils tend to be found in the eyes of animals with a crepuscular or nocturnal lifestyle that need to protect their eyes during daylight. Constriction of a circular pupil (by a ring-shaped muscle) is less complete than closure of a slit pupil, which uses two additional muscles that laterally compress the pupil. [18] For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans. [19] However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5 mm in the tarsier) and the rectangular pupils of many ungulates which do not close to a narrow slit in bright light. [20] An alternative explanation is that a partially constricted circular pupil shades the peripheral zones of the lens which would lead to poorly focused images at relevant wavelengths. The vertical slit pupil allows for use of all wavelengths across the full diameter of the lens, even in bright light. [3] It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, breaking up the circular outline of the eye. [21]

Activity pattern and behavior

In a study of Australian snakes, pupil shapes correlated both with diel activity times and with foraging behavior. Most snake species with vertical pupils were nocturnal and also ambush foragers, and most snakes with circular pupils were diurnal and active foragers. Overall, foraging behaviour predicted pupil shape accurately in more cases than did diel time of activity, because many active-foraging snakes with circular pupils were not diurnal. It has been suggested that there may be a similar link between foraging behaviour and pupil shape amongst the felidae and canidae discussed above. [21]

A 2015 study [22] confirmed the hypothesis that elongated pupils have increased dynamic range, and furthered the correlations with diel activity. However it noted that other hypotheses could not explain the orientation of the pupils. They showed that vertical pupils enable ambush predators to optimise their depth perception, and horizontal pupils to optimise the field of view and image quality of horizontal contours. They further explained why elongated pupils are correlated with the animal's height.

Society and culture

The pupil plays a role in eye contact and nonverbal communication. The voluntary or involuntary enlargement or dilation of the pupils indicates cognitive arousal, interest in the subject of attention, and/or sexual arousal. On the other hand, when the pupil is voluntarily or involuntarily contracted, it could indicate the opposite - disinterest or disgust. Exceptionally large or dilated pupils are also perceived to be an attractive feature in body language. [23]

In a surprising number of unrelated languages, the etymological meaning of the term for pupil is "little person". [24] [25] This is true, for example, of the word pupil itself: this comes into English from Latin pūpilla, which means "doll, girl", and is a diminutive form of pupa, "girl". (The double meaning in Latin is preserved in English, where pupil means both "schoolchild" and "dark central portion of the eye within the iris".) [26] This may be because the reflection of one's image in the pupil is a minuscule version of one's self. [24] In the Old Babylonian period (c. 1800-1600 BC) in ancient Mesopotamia, the expression "protective spirit of the eye" is attested, perhaps arising from the same phenomenon.

The English phrase apple of my eye arises from an Old English usage, in which the word apple meant not only the fruit but also the pupil or eyeball. [27]

See also

Related Research Articles

<span class="mw-page-title-main">Iris (anatomy)</span> Colored part of an eye

The iris is a thin, annular structure in the eye in most mammals and birds, responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by the iris.

A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the human body, some microscopically small, in particular the millions of precapillary sphincters. Sphincters relax at death, often releasing fluids and faeces.

<span class="mw-page-title-main">Mydriasis</span> Excessive dilation of the pupil

Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drug. It may also be of unknown cause.

<span class="mw-page-title-main">Oculomotor nerve</span> Cranial nerve III, for eye movements

The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation. The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of eye movement.

<span class="mw-page-title-main">Pupillary light reflex</span> Eye reflex which alters the pupils size in response to light intensity

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict, whereas a lower intensity of light causes the pupil to dilate. Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

<span class="mw-page-title-main">Miosis</span> Excessive constriction of the pupil

Miosis, or myosis, is excessive constriction of the pupil. The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.

<span class="mw-page-title-main">Horner's syndrome</span> Facial disorder due to damage of the sympathetic nerves

Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion of the sympathetic trunk. It is characterized by miosis, partial ptosis, apparent anhidrosis, with apparent enophthalmos.

<span class="mw-page-title-main">Ciliary ganglion</span> Bundle of nerves, parasympathetic ganglion

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. These neurons supply the pupillary sphincter muscle, which constricts the pupil, and the ciliary muscle which contracts to make the lens more convex. Both of these muscles are involuntary since they are controlled by the parasympathetic division of the autonomic nervous system.

<span class="mw-page-title-main">Iris dilator muscle</span> Smooth muscle of the eye

The iris dilator muscle, is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.

<span class="mw-page-title-main">Iris sphincter muscle</span> Muscle in the eye which constricts the pupil

The iris sphincter muscle is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

<span class="mw-page-title-main">Adie syndrome</span> Neurological disorder

Adie syndrome, also known as Holmes–Adie syndrome, is a neurological disorder characterized by a tonically dilated pupil that reacts slowly to light but shows a more definite response to accommodation. It is frequently seen in females with absent knee or ankle jerks and impaired sweating.

<span class="mw-page-title-main">Anisocoria</span> Unequal size of the eyes pupils

Anisocoria is a condition characterized by an unequal size of the eyes' pupils. Affecting up to 20% of the population, anisocoria is often entirely harmless, but can be a sign of more serious medical problems.

Surprise is a brief mental and physiological state, a startle response experienced by animals and humans as the result of an unexpected event. Surprise can have any valence; that is, it can be neutral/moderate, pleasant, unpleasant, positive, or negative. Surprise can occur in varying levels of intensity ranging from very-surprised, which may induce the fight-or-flight response, or little-surprise that elicits a less intense response to the stimuli.

<span class="mw-page-title-main">Relative afferent pupillary defect</span> Medical condition

A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil, is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils dilate when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.

<span class="mw-page-title-main">Stroma of iris</span> Connective and vascular tissue of the iris

The stroma of the iris is a fibrovascular layer of tissue. It is the upper layer of two in the iris.

Polycoria is a pathological condition of the eye characterized by more than one pupillary opening in the iris. It may be congenital or result from a disease affecting the iris. It results in decreased function of the iris and pupil, affecting the physical eye and visualization.

<span class="mw-page-title-main">Oculomotor nerve palsy</span> Medical condition

Oculomotor nerve palsy or oculomotor neuropathy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements. Damage to this nerve will result in an inability to move the eye normally. The nerve also supplies the upper eyelid muscle and is accompanied by parasympathetic fibers innervating the muscles responsible for pupil constriction. The limitations of eye movement resulting from the condition are generally so severe that patients are often unable to maintain normal eye alignment when gazing straight ahead, leading to strabismus and, as a consequence, double vision (diplopia).

<span class="mw-page-title-main">Pupillary response</span> Physiological response that varies the size of the pupil

Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.

The eye is made up of the sclera, the iris, and the pupil, a black hole located at the center of the eye with the main function of allowing light to pass to the retina. Due to certain muscle spasms in the eye, the pupil can resemble a tadpole, which consists of a circular body, no arms or legs, and a tail.

<span class="mw-page-title-main">Roots of the ciliary ganglion</span>

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged into three roots of the ciliary ganglion, which join enter the posterior surface of the ganglion.

References

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  14. ↑ Cakmak, Hasan Basri; Cagil, Nurullah; Simavlı, Hüseyin; Duzen, Betul; Simsek, Saban (February 2010). "Refractive Error May Influence Mesopic Pupil Size". Current Eye Research. 35 (2): 130–136. doi:10.3109/02713680903447892. ISSN   0271-3683. PMID   20136423. S2CID   27407880.
  15. ↑ Eberhardt, Lisa V.; Grön, Georg; Ulrich, Martin; Huckauf, Anke; Strauch, Christoph (2021-10-01). "Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI". International Journal of Psychophysiology. 168: 33–42. doi:10.1016/j.ijpsycho.2021.08.001. ISSN   0167-8760.
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  21. 1 2 Brischoux, F., Pizzatto, L. and Shine, R. (2010). "Insights into the adaptive significance of vertical pupil shape in snakes". Journal of Evolutionary Biology. 23 (9): 1878–85. doi: 10.1111/j.1420-9101.2010.02046.x . PMID   20629855.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ↑ Banks, Martin S.; Sprague, William W.; Schmoll, Jürgen; Parnell, Jared A. Q.; Love, Gordon D. (2015). "Why do animal eyes have pupils of different shapes?". Science Advances. 1 (7): e1500391. Bibcode:2015SciA....1E0391B. doi: 10.1126/sciadv.1500391 . PMC   4643806 . PMID   26601232.
  23. ↑ Tombs, Selina; Silverman, Irwin (2004-07-01). "Pupillometry: A sexual selection approach". Evolution and Human Behavior. 25 (4): 221–228. doi:10.1016/j.evolhumbehav.2004.05.001. ISSN   1090-5138.
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  26. ↑ "pupil, n.2.", Oxford English Dictionary Online, 3rd. edn (Oxford University Press, 2007).
  27. ↑ apple, n.", Oxford English Dictionary Online, 3rd ed. (Oxford University Press, 2008), § 6 B.
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