Logan Steele
The pupillary sphincter muscle, also known as the iris sphincter muscle or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris. It is a circular muscle, about 1 millimeter wide, that encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle, pupillary sphincter, pupillary constrictor, circular muscle of iris, sphincter pupillae |
| Location | Pupillary zone of stromal layer of the iris |
| Function | Constricts the pupil in bright light (pupillary light reflex) or during accommodation |
| Controlled by | Parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle |
| Diameter | About 1 millimeter |
| Thickness | 0.15 millimeters |
| Muscle type | Smooth muscle |
| Innervation | Short ciliary nerves, long ciliary nerves |
| Blood supply | Anterior and long posterior ciliary arteries |
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What You'll Learn
- The pupillary sphincter muscle is located in the iris, the coloured part of the eye
- It is a circular muscle that encircles the pupil
- It contracts the pupil in bright light via the pupillary light reflex
- It is controlled by parasympathetic postganglionic fibres
- It is also called the iris sphincter muscle or sphincter pupillae
The pupillary sphincter muscle is located in the iris, the coloured part of the eye
The pupillary sphincter muscle, also known as the iris sphincter muscle or sphincter pupillae, is located in the iris, the coloured part of the eye. It is a circular muscle, about 1 millimetre wide and 0.15 millimetres thick, that encircles the pupil of the iris. This muscle is responsible for constricting the pupil in response to bright light, a process known as the pupillary light reflex. By controlling the pupil's diameter, the iris regulates the amount of light that reaches the retina at the back of the eye.
The pupillary sphincter muscle is composed of smooth muscle fibres, and its contraction is stimulated by the parasympathetic nervous system. When the eyes are exposed to bright light, the amount of light falling onto the retina needs to be reduced to maintain clear vision. This is achieved through the pupillary light reflex, a reflex arc that involves the optic nerve, the pretectal nucleus of the midbrain, the accessory oculomotor nucleus, and the oculomotor nerve.
The pupillary sphincter muscle receives parasympathetic input via the short ciliary nerves, which originate from the Edinger-Westphal nucleus of the oculomotor nerve. The activation of these nerves leads to the constriction of the sphincter pupillae, resulting in a decrease in the pupil's diameter. Additionally, drugs with cholinergic effects, such as pilocarpine, bethanechol, and metoclopramide, can stimulate the smooth muscle of the sphincter pupillae, causing pupil constriction or miosis.
The sphincter pupillae is located in the stromal layer of the iris, cushioned within the connective tissue of the stroma. It is composed of six to eight circles of smooth muscle fibres, with nerves and blood vessels supplying each fibre. The outer margin of the sphincter pupillae is related to the myoepithelial cells of the dilator pupillae muscle, which works in opposition to the sphincter muscle to enlarge the pupil by pulling the iris radially.
The pupillary sphincter muscle is not only found in humans but also in other vertebrates and some cephalopods. In lower animals, such as amphibians, the muscle cells themselves are photosensitive, allowing for iris action without brain input. Studies on newts, salamanders, and frogs have provided insights into the innervation and function of the iris sphincter muscle in these species.
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It is a circular muscle that encircles the pupil
The pupillary sphincter muscle, also known as the iris sphincter muscle or sphincter pupillae, is a circular muscle that encircles the pupil. It is located in the coloured part of the eye, known as the iris. The muscle fibres are positioned near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The pupillary sphincter muscle is a smooth muscle that contracts the pupil in a circular motion, controlling the amount of light that reaches the retina at the back of the eye. This is known as the pupillary light reflex. When the eyes are exposed to bright light, the sphincter pupillae contracts to decrease the amount of light that falls onto the retina, maintaining clear vision.
The muscle is composed of six to eight circles of smooth muscle fibres, with nerves and blood vessels supplying each fibre. The arterial sources for the sphincter pupillae are the anterior and long posterior ciliary arteries, which are branches of the ophthalmic artery. These arteries anastomose at the level of the ciliary body and reach the sphincter pupillae through two successive arterial circles.
The pupillary sphincter muscle is controlled by the parasympathetic nervous system, which causes the muscle to decrease in diameter as it contracts. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. In contrast, sympathetic innervation helps promote sphincter muscle relaxation, which is crucial for low-light conditions or night vision.
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It contracts the pupil in bright light via the pupillary light reflex
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye, called the iris. The iris sphincter muscle is responsible for constricting the pupil in bright light via the pupillary light reflex. This reflex is a response to the amount of light that falls onto the retina, which needs to be decreased to maintain clear vision.
The pupillary light reflex is a result of the contraction of the sphincter pupillae, a circular muscle located in the pupillary zone of the stromal layer of the iris. The muscle encircles the pupil of the iris, and its fibres are located near the pupillary margin, slightly anterior to the pigmented epithelium of the iris. The muscle itself consists of six to eight circles of smooth muscle fibres, with nerves and blood vessels supplying each fibre.
The contraction of the sphincter pupillae is controlled by the parasympathetic nervous system, which causes the muscle to decrease in diameter as it contracts. This process is stimulated by the release of acetylcholine, which acts on muscarinic acetylcholine receptors in the iris sphincter muscle. In lower animals, the muscle cells themselves are photosensitive, causing iris action without brain input. In contrast, sympathetic innervation helps promote sphincter muscle relaxation, which is critical for periods of low light or night vision.
The pupillary light reflex involves a reflex arc that includes the optic nerve, the pretectal nucleus of the midbrain, the accessory oculomotor nucleus, and the oculomotor nerve. Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus then stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves.
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It is controlled by parasympathetic postganglionic fibres
The pupillary sphincter muscle, also known as the iris sphincter muscle or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris. The muscle fibres are located near the pupillary margin and encircle the pupil of the iris. The pupillary sphincter muscle is responsible for constricting the pupil in bright light via the pupillary light reflex, or during accommodation. By controlling the pupil's diameter, the iris controls the amount of light that reaches the retina at the back of the eye.
The pupillary sphincter muscle is controlled by parasympathetic postganglionic fibres. These fibres release acetylcholine, which acts primarily on the muscarinic acetylcholine receptor (M3) of the iris sphincter muscle. The parasympathetic nervous system stimulates the muscle, causing it to contract and decrease in diameter. This process is known as miosis.
The parasympathetic fibres that control the pupillary sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia and is terminated on the muscarinic receptors of the muscle fibres. This action of acetylcholine on muscarinic receptors is observed in mammals, turtles, and birds, although the specific mechanisms may vary between species.
In contrast, sympathetic innervation promotes sphincter muscle relaxation, which is crucial for periods of low-light or night vision. The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. Disruption of the parasympathetic postganglionic fibres, as seen in Adie syndrome, can result in an inability to constrict the iris sphincter muscle, leading to a tonically dilated pupil.
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It is also called the iris sphincter muscle or sphincter pupillae
The iris sphincter muscle, also called the iris sphincter, pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibres, or sphincter pupillae, is a muscle located in the coloured part of the eye, the iris. It encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation.
The iris sphincter muscle is responsible for the constriction of the pupil, which is controlled by parasympathetic nerve fibres. The pupil is the dark aperture in the centre of the iris, the coloured part of the eye. The diameter of the pupil is dynamic and is regulated by a pair of antagonistic muscles found in the iris: the sphincter pupillae and the dilator pupillae. Both of these muscles are smooth muscles, functioning beyond our control. Their function is to change the diameter of the pupil during two reflexive events.
The sphincter pupillae is a band of smooth muscle encircling the pupil margin. It is located in the pupillary zone of the stromal layer of the iris, attaching to and encircling the pupillary margin of the iris. The muscle itself consists of six to eight circles of smooth muscle fibres, between which are found the nerves and blood vessels that supply each fibre. The outer margin of the sphincter pupillae is related to the myoepithelial cells of the dilator pupillae muscle.
The pupillary light reflex happens when the eyes are exposed to bright light, and the amount of light that falls onto the retina needs to be decreased to maintain clear vision. Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves.
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Frequently asked questions
The pupillary sphincter muscle, also known as the iris sphincter muscle or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris.
The pupillary sphincter muscle controls the pupil's diameter, constricting it in bright light via the pupillary light reflex or during accommodation.
The pupillary sphincter muscle is stimulated by the parasympathetic nervous system, causing it to contract and decrease in diameter. This results in miosis, or the constriction of the pupil.
The pupillary sphincter muscle is about 1 millimeter wide and 0.15 millimeters thick. It consists of six to eight circles of smooth muscle fibers, with nerves and blood vessels located between each fiber.
