Logan Steele
The pupil is the black opening in the centre of the iris, which is the coloured part of the eye. The pupil lets light into the eye, and the lens of the eye focuses this light onto the retina, which turns it into electrical signals that are sent to the brain. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle in the iris that encircles the pupil. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscle, or iris dilator muscle, enlarges the pupil by pulling the iris radially.
| Characteristics | Values |
|---|---|
| Name of the muscle | Iris sphincter muscle, also known as the pupillary sphincter, pupillary constrictor, circular muscle of iris, or circular fibers |
| Location | In the iris, the coloured part of the eye |
| Function | Controls the amount of light that reaches the retina by constricting the pupil in bright light or during accommodation |
| Controlled by | Parasympathetic fibers that travel from the Edinger-Westphal nucleus with the oculomotor nerve (cranial nerve 3) to the ciliary ganglion |
| Innervation | Receives parasympathetic innervation from the postganglionic fibers of the ciliary ganglion, called short ciliary nerves, which are branches of the oculomotor nerve (CN III) |
| Antagonistic muscle | Iris dilator muscle |
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What You'll Learn
The iris sphincter muscle
The vertebrate eye is made up of tissue from various embryonic origins. The lens and the cornea originate from the surface ectoderm. The retina and the epithelial layers of the iris and ciliary body originate from the anterior neural plate. Within the stroma of the iris, the sphincter pupillae and dilator pupillae muscles develop from the optic cup neuroectoderm, along with the iris epithelium.
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The iris dilator muscle
The pupil is the black opening in the centre of the iris, the coloured part of the eye. The iris dilator muscle, also known as the dilator pupillae muscle, is one of the two muscles in the iris that control the size of the pupil. The other muscle is the iris sphincter muscle.
The function of the iris dilator muscle is to increase the size of the pupil, which allows more light to enter the eye. This is particularly important in low-light conditions or during the night. The sympathetic and parasympathetic systems work together to constantly adjust the size of the pupil and control the amount of light that reaches the retina.
In certain conditions, such as Horner syndrome, the iris dilator muscle may be affected. Horner syndrome is a rare condition that affects the eyes and face, usually on one side. It can cause the pupil to remain constricted instead of dilated, impacting the function of the iris dilator muscle.
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How pupil size changes with light
The pupil is the black opening in the middle of the iris, which is the coloured part of the eye. The iris sphincter muscle, also known as the pupillary sphincter, is a muscle located in the iris. The iris sphincter muscle encircles the pupil and controls the amount of light that reaches the retina, which is the light-sensitive layer at the back of the eye.
The pupil changes size in response to different lighting conditions. In bright light, the pupil constricts to reduce the amount of light entering the eye. This process is known as the pupillary light reflex. The iris sphincter muscle contracts the pupil in a circular motion, causing it to constrict and reduce the amount of light entering the eye.
In dim light, the pupil dilates to allow more light to enter the eye so that you can see better. This is called mydriasis. The pupil can range in diameter from 1.5 mm to 8 mm. The dilator muscles in the iris enlarge the pupil by pulling the iris radially.
The size of the pupil can also be influenced by factors other than light, such as emotions, certain medications, and mental stimuli. For example, the pupil may dilate when you are fearful, angry, in pain, or in love. Additionally, the pupil may dilate when you find a task difficult or mentally taxing.
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Pupil issues and related diseases
The pupil is the black opening in the middle of the iris, which is the coloured part of the eye. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. It controls the pupil's diameter, thereby determining the amount of light that reaches the retina. Pupil issues can indicate certain diseases.
Anisocoria, or uneven pupil size, may be indicative of a serious problem, such as a torn or blocked blood vessel in the neck, a brain aneurysm, or third nerve palsy. It can also be caused by a reaction to certain topical dilating medications. Physiologic anisocoria, on the other hand, is a harmless condition found in up to 20% of people, where slight differences in pupil size do not affect the reaction to light.
Adie's syndrome, also known as Adie's tonic pupil, is a condition where the pupil stays dilated and does not react normally to changes in light. This can also cause the pupil to remain constricted. Treatment options include Pilocarpine eye drops to constrict the affected pupil, and in some cases, neurosurgery may be required.
Horner syndrome, also called oculosympathetic palsy, is a rare condition that affects the eyes and face, usually on one side. It can be caused by an underlying issue, in which case treatment is necessary.
Third cranial nerve palsy involves the oculomotor nerve, which controls the muscles that move the eye, constrict the pupils, and focus the eyes. It can cause eyelid drooping and affect the ability to move the eye normally. Treatment may involve surgery to realign the eyes or correct eyelid drooping.
Traumatic brain injuries, such as concussions, can also cause pupil issues, and examining the pupils is often one of the first steps in diagnosis.
Optic nerve diseases, such as glaucoma or ischemic optic neuropathy, impair pupillary responses to bright light and can be assessed using chromatic pupillometry methods.
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The iris and the vertebrate eye
The iris is a thin, annular structure in the eye of most mammals and birds. It is responsible for controlling the size of the pupil and, consequently, the amount of light that reaches the retina. The pupil is the black opening in the centre of the iris, which gets bigger or smaller in response to changes in light. The iris consists of two layers: the front pigmented fibrovascular layer, known as the stroma, and the pigmented epithelial cells behind the stroma. The stroma is connected to the sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and the dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil. The iris sphincter muscle encircles the pupil and constricts it in bright light via the pupillary light reflex. In lower animals, the muscle cells are photosensitive, causing iris action without brain input. In humans, the iris sphincter muscle is controlled by parasympathetic postganglionic fibres releasing acetylcholine.
The vertebrate eye has evolved from the eyes of ancient vertebrates, such as the hagfish and lamprey. The hagfish eye lacks a lens, iris, cornea, or intra- or extraocular muscles, while the lamprey eye has five extraocular muscles, unlike the eyes of jawed vertebrates, which have six. The evolution of the highly contractile iris has allowed vertebrates to adjust light levels. The vertebrate eye is made up of tissue from various embryonic origins. The lens and cornea originate from the surface ectoderm, while the retina and epithelial layers of the iris and ciliary body originate from the anterior neural plate.
The colour of the iris is determined by the quantity of melanin pigment in the iris, with brown eyes having more melanin than blue or green eyes. The colour is also influenced by variable amounts of eumelanin and pheomelanin, as well as the texture, fibrous tissue, and blood vessels within the iris stroma.
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Frequently asked questions
The pupil is the black opening in the middle of the coloured part of your eye (iris). The pupil lets light into your eye as the muscles of your iris change its shape.
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. The sphincter muscle fibres are located near the pupillary margin and they encircle the pupil of the iris.
The sphincter muscle contracts the pupil in a circular motion, controlling the pupil's diameter and reducing the amount of light that reaches the back of the eye at the retina.
Yes, there are two muscles in the iris that control the pupil. The other muscle is the dilator pupillae, which enlarges the pupil by pulling the iris radially.
