Logan Steele
The iris is a thin, annular structure in the eye that is responsible for controlling the diameter and size of the pupil, and thus the amount of light that reaches the retina. The iris itself contains two muscles: the sphincter muscle and the dilator muscle. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscles enlarge the pupil by pulling the iris radially. The iris sphincter muscle is also known as the pupillary sphincter or sphincter pupillae.
| Characteristics | Values |
|---|---|
| Name | Iris Sphincter Muscle |
| Other Names | Pupillary Sphincter, Pupillary Constrictor, Circular Muscle of Iris, Circular Fibers, Sphincter Pupillae |
| Location | Part of the eye called the iris |
| Function | Constricts the pupil in bright light or during accommodation |
| Control | Parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of the iris sphincter muscle |
| Dimensions | About 0.75 mm wide and 0.15 mm thick |
| Composition | Smooth muscle |
| Blood Supply | Anterior ciliary arteries, long posterior ciliary arteries, and anastomotic connections from the anterior choroid |
| Innervation | Short ciliary nerves |
| Related Disorders | Adie Syndrome, Iridoplegia |
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What You'll Learn
- The iris sphincter muscle is located in the coloured part of the eye
- The iris controls the amount of light that reaches the retina
- The iris consists of smooth muscle in mammals and amphibians
- The iris sphincter muscle is innervated by the short ciliary nerves
- The iris sphincter muscle and the dilator muscle overlap at the collarette
The iris sphincter muscle is located in the coloured part of the eye
The iris is a thin, annular structure in the eye of most mammals and birds. It is responsible for controlling the diameter and size of the pupil, thereby controlling the amount of light that reaches the retina. The iris is also what gives our eyes their colour. The word "iris" is derived from the Greek word for "rainbow", which is quite fitting given the many colours of this eye part.
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye, the iris. The muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. The iris sphincter muscle encircles the pupil of the iris, constricting it in bright light via the pupillary light reflex or during accommodation.
The iris sphincter muscle and the iris dilator muscle work together to control the size of the pupil. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscles enlarge the pupil by pulling the iris radially. The iris collarette is the thickest portion of the iris, where the sphincter and dilator muscles overlap. The collarette is typically flat, but in some cases, it can be quite prominent.
The iris sphincter muscle is innervated by the short ciliary nerves, which originate from the Edinger-Westphal nucleus of cranial nerve III. The parasympathetic fibres that serve the sphincter muscle make nicotinic cholinergic synapses on neurons in the ciliary ganglion. This results in pupillary constriction, or miosis, and accommodation. The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture.
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The iris controls the amount of light that reaches the retina
The iris is a thin, annular structure in the eye in most mammals and birds. It is the coloured part of the eye and is responsible for controlling the size of the pupil, and therefore the amount of light that reaches the retina. The pupil is the eye's aperture, while the iris is the diaphragm.
The iris consists of two layers: the front pigmented fibrovascular layer, known as the stroma, and, behind that, the pigmented epithelial cells. The stroma is connected to the iris sphincter muscle, which contracts the pupil in a circular motion, and the dilator muscles, which pull the iris radially to enlarge the pupil. The iris sphincter muscle encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex. The iris sphincter and dilator muscles overlap at the thickest region of the iris, called the collarette.
The size of the pupil is dependent on many factors, including light intensity, emotional state, cognitive load, arousal, and stimulation. The pupil opening gets bigger or smaller in response to the intensity of light entering the eye. When it is dark, our pupils get bigger to let in more light, and when it is light, our pupils contract to let in less light. The iris controls the amount of light that reaches the retina by controlling the pupil's diameter.
The light that enters the eye through the pupil is then focused by the lens onto the retina, a light-sensitive layer of tissue at the back of the eye. The cells in the retina absorb and convert the light to electrochemical impulses, which are then sent to the brain to be interpreted as images.
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The iris consists of smooth muscle in mammals and amphibians
The iris is a thin, annular structure in the eye in most mammals and birds. It is responsible for controlling the diameter and size of the pupil, and thus the amount of light that reaches the retina. The iris consists of two layers: the front pigmented fibrovascular layer, known as the stroma, and, behind the stroma, pigmented epithelial cells. The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds.
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 sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of 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. By controlling the pupil's diameter, the iris controls the amount of light that reaches the back of the eye at the retina. The iris is a circular, pigmented diaphragm that lies anterior to the lens in a coronal plane.
The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). Many fish do not have iris muscles, and as a result, their irises are unable to dilate and contract, so the pupil always remains a fixed size. The iris collarette is the thickest portion of the iris, where the sphincter and dilator muscles overlap. While the collarette is typically flat, this structure can be rather prominent in some cases.
The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction (miosis) and accommodation. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia become terminated on muscarinic receptors of the muscle fibres. In mammals, contraction of the iris sphincter muscle is brought about by the action of acetylcholine on muscarinic receptors.
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The iris sphincter muscle is innervated by the short ciliary nerves
The iris is a thin, annular structure in the eye in most mammals and birds. It is responsible for controlling the size of the pupil and, thus, the amount of light that reaches the retina. The iris is usually strongly pigmented, with colours typically ranging from brown, hazel, green, grey, and blue. The iris consists of two layers: the front pigmented fibrovascular layer, known as the stroma, and the pigmented epithelial cells behind the stroma.
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 muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. The iris sphincter muscle encircles the pupil of the iris and functions to constrict 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 back of the eye at the retina.
The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction (miosis) and accommodation. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia and are terminated on muscarinic receptors of the muscle fibres. The postganglionic fibres travel in the short ciliary nerves, through the choroid, to the sphincter pupillae, where they release acetylcholine, causing the sphincter to contract and reduce the size of the pupil.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. The sympathetic fibres help promote sphincter muscle relaxation, which is critical for periods of low light or night vision.
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The iris sphincter muscle and the dilator muscle overlap at the collarette
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 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 two muscles: the sphincter muscle (sphincter pupillae) and the dilator muscle (dilator pupillae).
The iris sphincter muscle is a muscle located in the coloured part of the eye called 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 fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The dilator muscles, on the other hand, enlarge the pupil by pulling the iris radially. The iris sphincter and dilator muscles work in tandem to control the size of the pupil. The size of the pupils is dependent on several factors, including light, emotional state, cognitive load, arousal, and stimulation. The pupil size can range from less than 2 mm in diameter to as large as 9 mm in diameter, with considerable variation between individuals and a decrease in size with age.
The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. It is a vestige of the coating of the embryonic pupil. The collarette is typically flat, but in some cases, it can be prominent. This is the region where the iris sphincter muscle and the dilator muscle overlap. Radial ridges extend from the periphery to the pupillary zone, supplying the iris with blood vessels.
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Frequently asked questions
The iris is a thin, annular structure in the eye in most mammals and birds. It is responsible for controlling the size of the pupil and the amount of light that reaches the retina.
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. It encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex.
The iris controls the size of the pupil by contracting the iris sphincter muscle and/or the iris dilator muscle. The size of the pupil is dependent on factors including light, emotional state, cognitive load, arousal, and stimulation.
