Pupil of the Eye: Definition, Anatomy & Function (2023)


Pupil of the Eye: Definition, Anatomy & Function (1)

What is the pupil of the eye?

The pupil of your eye is the black hole in the middle of your iris. Your iris is the colored part of your eye. Muscles in your iris control how big the pupil is. Usually, this happens as a result of how much light is available. This makes the pupil an important part of your vision system.

When you’re in bright light, your pupil constricts and gets smaller. When the light is dim, your pupil dilates and gets bigger. The name of this reaction is the pupillary light response.

The pupil has protection. The cornea is a clear dome over the eye that covers the pupil and provides protection. Another layer of tissue called the conjunctiva also protects the pupil and the whole eye.

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What does the pupil do?

Your pupil lets light into your eye as the muscles of your iris change its shape. The lens in your eye focuses light that passes through your pupil. Light then goes to the back of your eye and hits your retina.

Your retina turns light into electrical signals. Your brain receives these signals and turns them into images. This is how you’re able to see.

Two muscles in your iris control how you pupil opens and closes. The iris dilator muscle opens your pupil from the inside out. It sort of looks like a sun with rays extending out from the middle. The iris sphincter is a circular muscle that controls closing your pupil.

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Nerves that affect the pupil connect to the afferent pathway (the path from your eye to your brain). They also connect to the efferent pathway (the path from your brain to your eye).

The pupil also gives aqueous humor a pathway to the front of your eye. Aqueous humor is a fluid that nourishes your eyes.


What does the pupil look like?

The pupil looks like a black hole. It looks black because light usually stays inside your eye. One of the few times that you might see light coming back out of your eye is in a photograph. That’s why you might see red-looking eyes in pictures taken with a flash. This is light reflecting off the blood vessels in your eyes.

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Conditions and Disorders

What are the common conditions and disorders that affect the pupil?

A variation in pupil size can be an issue itself or a symptom of other conditions. Common conditions of the pupil of the eye include:

Conditions of pupil size and behavior

  • Marcus Gunn pupil: This condition means that your pupils don’t respond normally when an eye care specialist shines a light into your eyes.
  • Anisocoria (uneven pupil size): This is the medical term for one pupil being bigger than the other. It can be normal, temporarily, but it can also be caused by serious conditions.
  • Polycoria: This is the medical term for having more than one working pupil in one eye.
  • Argyll Robertson pupil: This term refers to small pupils that get smaller (constrict) correctly when focusing on nearby objects, but not when they’re exposed to bright light.
  • Miosis (small pupil): The medical name for very small pupils (constricted) is miosis. Small pupils can happen when you’ve had a stroke or when you’ve used opiates, among other reasons. Mydriasis is the name of the opposite condition, having dilated pupils.
  • Adie syndrome: This condition means that your pupil stays dilated and doesn’t react normally to changes in light. It can also make your pupil stay constricted rather than dilated.

Other conditions that affect the pupils

  • Horner syndrome: This rare condition affects the eyes and the face, usually on one side. It’s also called oculosympathetic palsy. Palsy is a word used for nerve paralysis. There are three types of Horner syndrome.
  • Third cranial nerve palsy: The third cranial nerve is the oculomotor nerve. It controls the muscles that move your eye, constrict your pupils, focus your eyes and position your upper eyelid. Palsy of this nerve affects many functions.
  • Traumatic brain injury: These types of brain injuries involve your brain hitting against the skull. One of the most common types is a concussion. One of the first things a provider will check is how your pupils look.
  • Cataracts: Cataracts make the normally clear lenses in your eyes foggy or cloudy. Many people will have cataract surgery. This type of surgery may affect your pupil size. Also, the size of your pupils may affect complications after the surgery.
  • Headaches: Cluster headaches often affect one side of your head, and therefore, one of your eyes/pupils. You may have one eye that waters.

What tests may be done to test the health of the pupil of the eye?

An eye care specialist may do tests to tell if your eyes are healthy, including:

  • A comprehensive eye exam: As part of this exam, a provider will shine a light on your pupil to see if it gets bigger. Both pupils should react to the light.
  • Using different types of medicated eye drops to test pupil reaction: Medications include things like pilocarpine and phenylephrine.
  • Tests for other diseases: If your pupils aren’t working the way they should, you might have a health condition that’s affecting them. Some of them may be very serious, like a stroke or a brain injury. A provider may order blood tests or imaging tests.

What are common treatments for conditions involving your pupils?

Treating eye conditions depends on what’s causing them. For instance, if your current medication affects your pupils, a provider may suggest that you stop taking that drug or substitute another medicine.

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Some conditions may require medication. Often, these may be in the form of eye drops or topical preparations (like ointments that you apply to your skin).

Other forms of medications may come in oral form (pills, capsules or liquids). You’ll get some drugs as an injection (a shot) and others through a vein (intravenously or IV).

Providers may treat some eye conditions with eye exercises or surgery.


What can I do to take care of my eyes, including my pupils?

There are things you can do to keep your eyes healthy. These include:

  1. Don’t smoke: Try to quit smoking if you use tobacco.
  2. Eat a diet that’s healthy for you: Check with a provider, but you may want to include vegetables and fruit, especially green leafy vegetables like spinach and kale. You may want to eat fish like tuna, salmon and halibut. You can add in things like walnuts and flax seeds. Remember to drink enough water.
  3. Exercise: Try to move 30 minutes per day at least five days per week. Some people with obesity may have higher risks for eye problems related to other health conditions, like diabetes.
  4. Wear protective eyewear: Protective gear includes sunglasses and safety glasses while you work or participate in certain sports.
  5. Use care if you wear contact lenses: Make sure your hands are clean when you put your lenses in or take them out. Follow manufacturer or provider instructions on how long to wear your lenses and how to clean them.
  6. Rest your eyes: If you use a computer or other screens, or do a lot of reading, use the 20-20-20 rule. This means you look at something 20 feet away for 20 seconds about every 20 minutes to help prevent eye strain.
  7. Get regular eye exams: Check with your provider about how often you need vision screenings or exams. If you have some conditions, like glaucoma or diabetes, you may need more frequent visits.

A note from Cleveland Clinic

Your pupils are important parts of your vision system. Problems with your pupils can point a healthcare provider toward a diagnosis of other illnesses. See an eye care specialist as often as they suggest. It’s also good to do what you can to keep yourself and your eyes healthy.

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Pupil of the Eye: Definition, Anatomy & Function? ›

The pupil, or black dot at the centre of the eye, is an opening through which light can enter the eye. The iris, or coloured part of the eye, surrounds the pupil. It controls how much light enters the eye by changing the size of the pupil.

What is the pupil and its function? ›

Your pupil lets light into your eye as the muscles of your iris change its shape. The lens in your eye focuses light that passes through your pupil. Light then goes to the back of your eye and hits your retina. Your retina turns light into electrical signals.

What are two functions of the pupil? ›

The pupil dilates in dim light and constricts under bright light. This way it protects the lens and retina from damage from high light intensity and also allows us to see clearly under dim conditions.

Which best describes the main function of the pupil? ›

The function of the pupil is clearly that of controlling the amount of light entering the eye, and hence the light reflex.

Where is the pupil function? ›

The pupil is the opening in the center of the iris (the structure that gives our eyes their color). The function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight. Typically, the pupils appear perfectly round, equal in size and black in color.

What is the action of the pupil? ›

Pupils respond to three distinct kinds of stimuli: they constrict in response to brightness (the pupil light response), constrict in response to near fixation (the pupil near response), and dilate in response to increases in arousal and mental effort, either triggered by an external stimulus or spontaneously.

What is the function of the pupil quizlet? ›

Function: it regulates the amount of light that enters the eye with the iris. Pupils get larger when the room gets darker to let more light in.

What is the function of pupil in accommodation? ›

During the accommodation reflex, the pupil constricts to increase the depth of focus of the eye by blocking the light scattered by the periphery of the cornea. The lens then increases its curvature to become more biconvex, thus increasing refractive power.

What is two pupil called? ›

The primary sign of polycoria is the appearance of two pupils. Other signs and symptoms may include the following: blurred vision in the affected eye.

What is the function of the pupil Wikipedia? ›

The pupil function or aperture function describes how a light wave is affected upon transmission through an optical imaging system such as a camera, microscope, or the human eye.

Is the pupil the most important part of the eye? ›

Cornea: This is one of the most important parts of the eye. It is the dome-shaped surface on the front of the eye that helps to focus light.

What is the most sensitive part of the eye? ›

The macula is located in the middle of this nerve tissue and it is also known as fovea centralis. The macula, which is responsible for high-resolution images, is the most sensitive portion of the retina (mainly cone cells).

What is the function of the pupil why pupil size is important? ›

The pupil controls how much light is let into the eye. It is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision. In the bright sunlight, our pupil shrinks to a very small diameter to allow us to function normally.

What regulates the pupil? ›

The size of the pupil is controlled by the activities of two muscles: the circumferential sphincter muscle found in the margin of the iris, innervated by the parasympathetic nervous system: and the iris dilator muscle, running radially from the iris root to the peripheral border of the sphincter.

What is the difference between the iris and the pupil? ›

The iris is the structure that gives the eye colour. The pupil is the black hole in the middle of the iris. The iris determines the amount of light going into the eye by controlling the size of the pupil. If we see a bright light, the iris contracts and the pupil becomes smaller.

Does the pupil let in light? ›

The pupil is an opening that lets light into your eye. Since most of the light entering your eye does not escape, your pupil appears black. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.

How does the pupil respond to emotion? ›

The processing of emotional signals usually causes an increase in pupil size, and this effect has been largely attributed to autonomic arousal prompted by the stimuli. Additionally, changes in pupil size were associated with decision making during non-emotional perceptual tasks.

How do pupils respond to light? ›

Pupils dilate (get larger) when room light is dimmed. Pupils are round and equal in size, in both bright and dim light. Pupils quickly and symmetrically constrict to a bright light directed into either of the eyes and when the bright light swings between the two eyes. Unequal or sluggish response to light.

What happens to your pupils when? ›

Your pupils get bigger or smaller, depending on the amount of light around you. In low light, your pupils open up, or dilate, to let in more light. When it's bright, they get smaller, or constrict, to let in less light. Sometimes your pupils can dilate without any change in the light.

What is the best definition for the pupil quizlet? ›

What is the best definition for the pupil? It is a circular opening in the iris. The optic disk is where: The optic nerve leaves the eye.

Which function of the is to control the size of the pupil? ›

The size of the pupil is controlled by Iris. Iris contains a group of two smooth muscles attached at the corner of the pupil. They stretch when there is less light and shrink when it is too bright. These muscles are known as sphincter pupillae.

What is the definition of pupil quizlet? ›

pupil. The adjustable opening in the center of the eye through which light enters.

What part of the eye is responsible for accommodation? ›

Ciliary Muscle - The ciliary muscle is a smooth muscle that is shaped like a ring and it's located in the middle of the eyes. It holds the lens with the suspensory ligaments and also adjusts the optical power or shape of the lens during accommodation.

What name is given to the ability of the eye to focus on close objects? ›

The ability of the eye lens to adjust its focal length is called as power of accommodation. It helps us to see both near and distant objects clearly.

What part the eye is important for accommodation? ›

The ciliary muscles are capable of modifying the curvature of the lens and thereby affecting the focal length of the lens. This adjustment in focal length is called the power of accommodation of the eye. Hence, the part of the eye which helps in its accommodation is ciliary muscles.

What is the rarest eye pupil? ›

Two Pupils in One Eye.

It's called polycoria and is one of the rarest conditions in the world. It is often confused with psudopolycoria–a condition in which it appears there are two or more pupils in one eye but the additional “pupils” are simply holes in the iris.

What is it called when one pupil is always bigger than the other? ›

Anisocoria is unequal pupil size. The pupil is the black part in the center of the eye.

Can you see without pupils? ›

Can a person see without pupils? There are only two ways you could have no pupils: One is to have no iris. Since the pupil is the opening in your iris,if you have no iris, you cannot, by definition, have a pupil either (see aniridia). You could still see, but would be very sensitive to bright lights and glare.

Does eye size change with age? ›

People's eyes stop growing in length by the age of 20 or 21, when they reach about 24 millimeters. The weight of the eyes' lenses continues to increase over time. Scientists say that eyes grow rapidly after birth.

Why is the eye called a pupil? ›

This reflection looks something like a little doll. The part of the eye that seems to hold this image was thus called "pupilla" (literally, "little doll") in Latin. "Pupil," the English word for this part of the eye, derives from "pupille," the Middle French descendent of the Latin word.

Is pupil a muscle? ›

The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the colored part of the eye called the iris.

What is the advantage of pupil? ›

The pupil is a circular opening in the center of the iris that allows light to enter the eye.

What color is the human eye most sensitive to? ›

As mentioned previously, cones are composed of three different photo pigments that enable color perception. This curve peaks at 555 nanometers, which means that under normal lighting conditions, the eye is most sensitive to a yellowish-green color.

What is the most sensitive eye color? ›

Lighter colored eyes like blue, hazel and green have less of a pigment called 'melanin' than brown eyes do. Melanin helps protect the retina from UV damage and blue light, putting those with blue eyes at a higher risk of developing UV-related eye damage.

What color is the human eye least sensitive to? ›

The measurement of the light wave is in wavelength. The figure 1 shows the relationship between eye sensitivity and colour wavelength and colour luminosity. From figure 1, green proves to be the most sensitive to our eyes while red and violet proves to be the least.

What your pupils say about you? ›

Our pupils respond to more than just the light. They indicate arousal, interest or mental exhaustion. Pupil dilation is even used by the FBI to detect deception.

What causes pupil to constrict? ›

In normal conditions, the pupils change size to let in the right amount of light. In the dark, they open wider or dilate to let in more light; in bright light, they get smaller or constrict to prevent too much light from getting in.

What affects pupil size? ›

The colorful part of your eye (iris) controls the size of your pupil with tiny muscles. In bright light, your pupils will get smaller to prevent light from entering. In the dark, pupils get larger to allow more light in.

What part of your nervous system controls the pupil? ›

The autonomic nervous system supplies (innervates) organs, like your eyes. The oculomotor nerve is the third cranial nerve (CN III). It allows movement of the eye muscles, constriction of the pupil, focusing the eyes and the position of the upper eyelid.

Does the brain control the pupils? ›

A growing body of anatomical and functional work has identified key roles for three brain regions in funneling cognitively relevant information to brainstem circuits that control the pupil: the pretectal olivary nucleus (PON), intermediate layers of the superior colliculus (SCi), and locus coeruleus (LC).

What might a slow pupil reaction indicate? ›

When the pupil is involved in a oculomotor nerve palsy, it is fixed and dilated (or minimally reactive). A partially dilated pupil which reacts sluggishly to light suggests a relative pupil-sparing CN III palsy.

Can you have an eye without iris or pupil? ›

What is aniridia? Aniridia is a serious and rare genetic eye disorder that affects the colored part of the eye (iris). Aniridia means lack of an iris. With this condition, the iris is partly or fully gone.

Do pupils dilate in fear? ›

Fear or Surprise

Additionally, the pupils will dilate if a person is frightened or excited due to the natural adrenalin response of the body.

Does the pupil dilate or the iris? ›

Pupil dilation occurs when the opening in the center of your iris grows bigger to let in more light. Under normal circumstances, pupils can dilate to let in more light or in response to a variety of stimuli. During an eye exam, a doctor will administer eye drops to increase the size of a patient's pupils.

What is it called when your pupils shrink? ›

Miosis of the eye refers to having small or constricted pupils. The condition is also called pupillary miosis. Pupils are the black circles in the middle of your eyes. With miosis, the muscles of your iris (the colored part of your eye) tighten around your pupil.

Do pupils react if blind? ›

Retinal function will influence the pupil responses, so if there is a severe retinopathy causing blindness, then the pupils will be unreactive.

What happens to the pupil at night? ›

In dim light the pupils dilate (open wider) so more light can enter. Switch to bright light and the pupils automatically contract. This is the result of a nerve signal generated in the back of the eye triggering the muscles in the iris.

What does it mean when your pupils are dilated? ›

The pupils are constantly changing. They enlarge (dilate) in darker conditions to allow more light to enter the eye, and they get smaller (constrict) in brighter conditions to minimize the amount of light that enters the eye.

What part of the brain controls pupil size? ›

The pupillary dilation pathway is a sympathetically driven response beginning in the hypothalamus and ending with the contraction of the dilator pupillae muscle.

Why do pupils appear black? ›

Pupil absorbs all the light that falls on it and allows them to go in the eye so it looks black. No light is reflected back from it. Therefore the colour of pupil is black.

What happens if the pupil is damaged? ›

Blown pupil symptoms

The most obvious symptom of blown pupils is having one or both pupils that appear abnormally dilated (large) and do not react to light. Other symptoms associated with this include: Light sensitivity (photophobia). Headache.

What do you call eyes without pupils? ›

Aniridia is a serious and rare genetic eye disorder. The iris is partly or fully gone, often in both eyes. It can also affect other parts of the eye. Your child may have certain problems from birth, such as increased light sensitivity. Other eye problems may develop later, such as cataracts or glaucoma.

Can a damaged pupil be repaired? ›

Iris surgeries come in the form of iris repair (iridoplasty) or an iris prosthesis. Iris repair often involves the use of sutures inside the eye to reshape the iris to its original shape, re-creating a round pupil. Sometimes the surgeon may cut some of the existing iris to help improve the appearance.

What causes pupils not to react to light? ›

Problems that can cause a pupil not to constrict to light exposure include traumatic injury to the muscles of the iris that control the pupil, inflammation inside the eye that causes the iris to become sticky and to adhere to the lens, and problems that result in severe vision loss in an eye such as a retinal ...

What are small pupils a symptom of? ›

Small pupils can be due to bright light, an emotional response, or looking at something far away. There are six health risks that can cause pinpoint pupils, or miosis. These include substance abuse, prescription drugs, environmental toxins, diseases, Horner syndrome, and trauma to the eye or brain.

What does it mean when your pupils are constricted? ›

In bright light, your pupils get smaller (constrict) to limit the amount of light that enters. In the dark, your pupils get bigger (dilate). That allows more light in, which improves night vision.

Why do doctors check pupils? ›

Detection of pupil problems are critical in detecting some brain and eye conditions that could potentially lead to vision loss or even death. Contact an eye doctor near you who can conduct pupil tests to reveal if you have any underlying eye conditions.

What nerve controls the pupil? ›

The oculomotor nerve (the third cranial nerve; CN III) has three main motor functions: Innervation to the pupil and lens (autonomic, parasympathetic)

What emotions cause pupils to constrict? ›

These findings support the notion that pupil constriction in the context of intense sadness may function to counteract blurry vision. Pupil size, like emotional tears, appears to have acquired value as a social signal in this context.

What is the rarest eyes? ›

Gray: The Rarest Eye Color

With this change, gray now tops the list as the rarest eye color. There's not much information on gray-colored eyes.1 In studies, gray and blue have historically been combined.

Why do pupils go white? ›

The appearance of a white pupil is never a normal condition and requires immediate evaluation by specialists trained in ophthalmology. A cloudy cornea or cataract may be mistaken for a white pupil, but usually a white pupil is due to infection or disease.

What is the most common eye color? ›

Brown, which is the most common eye color in the world. Green, which is the least common eye color. Only 9% of people in the United States have green eyes. Hazel, a combination of brown and green.


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