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The iris is the coloured, muscular part of the eye surrounding the pupil. It consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction). The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. It encircles the pupil and functions to constrict the pupil in bright light via the pupillary light reflex. The dilator muscle of the iris expands the opening when it contracts. The size of the pupil is dependent on many factors, including light, emotional state, cognitive load, arousal, and stimulation.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle, pupillary sphincter, pupillary constrictor, circular muscle of iris, sphincter pupillae |
| Location | Part of the eye called the iris |
| Function | Constricts the pupil in bright light or during accommodation |
| Control | Parasympathetic postganglionic fibers, superior cervical ganglion |
| Dimensions | 0.75 mm wide by 0.15 mm thick |
| Innervation | Short ciliary nerves |
| Muscle Type | Smooth muscle |
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What You'll Learn
The iris sphincter muscle
The iris is the coloured, muscular part of the eye surrounding the pupil. It is located in front of the lens and behind the cornea. The iris sphincter muscle, also known as the pupillary sphincter, is a muscle located in the iris. It encircles the pupil and functions to constrict it in bright light via the pupillary light reflex or during accommodation. The muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The sphincter muscle of the iris works in opposition to the dilator muscle of the iris. While the sphincter muscle constricts the pupil in bright light, the dilator muscle expands the opening when it contracts. The dilator muscle is innervated by sympathetic fibres from the superior cervical ganglion, which help promote sphincter muscle relaxation during periods of low light or night vision.
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The ciliary muscle
The iris is the coloured, muscular part of the eye that surrounds the pupil. It consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil, determining how much light reaches the retina.
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The iris consists of two sheets of smooth muscle
The iris is the coloured, muscular structure of the eye that surrounds the pupil. It is located in front of the lens and behind the cornea. The iris consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil, determining how much light reaches the retina.
The sphincter muscle of the iris is circular and constricts the pupil in bright light. It encircles the pupil and functions to constrict it via the pupillary light reflex or during accommodation. The sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. The sphincter muscle contracts the pupil in a circular motion.
The dilator muscle of the iris, on the other hand, expands the pupil when it contracts. The dilator muscles are located on the anterior surface of the iris and project forward. They enlarge the pupil by pulling the iris radially.
The collarette is the thickest region of the iris, where the sphincter and dilator muscles overlap. The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated in reptiles, including birds. In fish, the iris often lacks muscle, resulting in a fixed pupil size.
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The sphincter pupillae and dilator pupillae muscles
The iris is the coloured, muscular part of the eye surrounding the pupil. It is located in front of the lens and behind the cornea. The iris consists of two sheets of smooth muscle with opposing actions: contraction (constriction) and dilation (expansion). These muscles control the size of the pupil, determining how much light reaches the retina.
The dilator pupillae, on the other hand, is the opposing muscle of the sphincter pupillae. It is located on the anterior surface of the iris. The dilator muscle enlarges the pupil by pulling the iris radially. The pupil's diameter changes when the sphincter and dilator muscles constrict or dilate, altering the inner border of the iris.
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The collarette
The iris is the coloured, muscular part of the eye surrounding the pupil. It is responsible for controlling the size of the pupil and, thus, the amount of light that reaches the retina. The iris consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction).
The sphincter pupillae and dilator pupillae muscles develop from optic cup neuroectoderm. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, while the dilator muscle of the iris expands the opening when it contracts. The size of the pupil is dependent on many factors, including light, emotional state, cognitive load, arousal, and stimulation.
The protruding iris collarette is a variation of a normally flat part of the eye called the iris collarette. It is a raised, rippled ring of tissue that encircles the irises in both eyes. This condition is completely harmless and is not associated with any vision problems.
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Frequently asked questions
The muscle that controls the iris is called the iris sphincter muscle, or the pupillary sphincter.
The iris sphincter muscle is located in the coloured part of the eye, encircling the pupil.
The iris sphincter muscle constricts the pupil in bright light, controlling the amount of light that reaches the retina.
The iris sphincter muscle is a smooth muscle.
Yes, the iris also contains the dilator pupillae muscle, which enlarges the pupil by pulling the iris radially.
