Victor Forge
The iris is the coloured part of the eye, unique to each individual. It consists of two sheets of smooth muscle with opposing functions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil, determining how much light reaches the retina. The iris sphincter muscle, also known as the pupillary sphincter, 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 iris is composed of smooth muscle fibres in mammals and amphibians, but in reptiles, including birds, it is composed of striated muscle.
| Characteristics | Values |
|---|---|
| Number of muscles in the iris | 2 |
| Muscle type | Smooth muscle |
| Muscle function | Dilation (expansion) and contraction (constriction) |
| Muscle control | Parasympathetic nervous system and various other neural and humoral systems |
| Muscle dimensions | 0.75 mm wide by 0.15 mm thick |
| Muscle fibers | Located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris |
| Muscle contraction | Contraction of the iris sphincter muscle and/or the iris dilator muscle controls the size of the pupil |
| Pupil size | Ranges from less than 2 mm in diameter to as large as 9 mm in diameter |
| Muscle innervation | Sympathetic and parasympathetic innervation work in opposition to constantly adjust the pupillary aperture |
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What You'll Learn
The iris sphincter 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 and, consequently, the amount of light that reaches the retina.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupil's aperture. While the parasympathetic system causes pupillary constriction, the sympathetic system promotes sphincter muscle relaxation, which is crucial for low-light conditions or night vision. Adie syndrome, also known as Holmes-Adie syndrome, is a disorder characterised by a lack of response to light and a dilated pupil. It results from damage to the postganglionic fibres of the parasympathetic innervation, leading to an inability to constrict the iris sphincter muscle.
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The iris dilator muscle
The iris is the coloured muscular part of the eye that surrounds the pupil. The iris dilator muscle, also known as the pupillary dilator, radial muscle of iris, radiating fibres, or dilator pupillae muscle, is a smooth muscle of the eye. It runs radially in the iris and is responsible for dilation or expansion.
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The ciliary muscle
The ciliary body is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. It includes the ciliary muscle and the finger-like ciliary processes. The ciliary processes are attached to the lens via zonular fibres. The layers of ciliary muscle are described differently in the literature, but the most common classification divides this muscle into three separate layers: the outermost longitudinal layer, the middle radial layer, and the innermost circular layer.
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The role of the iris in vertebrates and cephalopods
The iris is a thin, annular structure in the eye of most mammals and birds. It is the coloured muscular part of the eye surrounding the pupil. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the retina. The iris sphincter muscle constricts the pupil in bright light, while the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour.
The vertebrate eye comprises tissues from different embryonic origins. The lens and the cornea are derived from the surface ectoderm, while the retina and the epithelial layers of the iris and ciliary body are derived from the anterior neural plate. The iris plays a key role in visual function, regulating the amount of light entering the eye and falling on the retina. It is also involved in the circulation of the aqueous humour and, therefore, functions in regulating intraocular pressure.
The eyes of cephalopods, such as octopuses, squid, and potentially cuttlefish, have been compared to vertebrate eyes for the past 140 years as an example of convergent evolution, where both types of organisms have independently evolved similar traits. The cephalopod eye is a camera-type eye, consisting of an iris, a circular lens, vitreous cavity (eye gel), pigment cells, and photoreceptor cells that translate light from the light-sensitive retina into nerve signals. The iris of the cephalopod is not a structure of the inner eye but lies in front of or on top of the posterior chamber. It consists of five cell layers, including a layer of muscles and collagen strands.
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Disorders of the iris muscle
The iris consists of two sheets of smooth muscle that control the size of the pupil by dilation (expansion) and contraction (constriction). 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.
Essential iris atrophy is a rare, progressive disorder characterised by a displaced and/or distorted pupil, patchy areas of degeneration (atrophy) on the iris, and/or holes in the iris. This disorder can lead to secondary glaucoma and vision loss. Chandler's syndrome is another rare eye disorder in which the endothelium, the single layer of cells lining the inner surface of the cornea, proliferates, causing corneal edema, distortion of the iris, and unusually high pressure in the eye (glaucoma). Cogan-Reese syndrome is an extremely rare disorder characterised by loss of iris tissue and the development of small wart-like growths on the iris, also resulting in increased pressure within the eye (glaucoma) and corneal swelling (edema).
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Frequently asked questions
The iris is the coloured part of the eye. It consists of two sheets of smooth muscle that control the size of the pupil by dilating (expanding) and contracting.
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 constricts it in bright light.
The iris dilator muscle, also known as the dilator pupillae, is a set of muscles that pull the iris radially to enlarge the pupil, pulling it in folds.
The muscles in the iris control the pupil, helping you see clearly and adjust to changes in lighting. They work without you thinking about them to control the size of your pupil.
Iridoplegia is the paralysis of the sphincter muscle of the iris. It often results from direct trauma to the eye, inflammation, or even from eyedrops.
