Logan Steele
The iris is the coloured, muscular part of the eye surrounding the pupil. It is made up of two sheets of smooth muscle with opposing actions: the sphincter muscle, which constricts the pupil in bright light, and the dilator muscle, which expands the pupil by pulling the iris radially. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is located near the pupillary margin and encircles the pupil. The dilator muscle of the iris, on the other hand, is innervated by sympathetic fibres and controlled by the parasympathetic nervous system.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle, pupillary sphincter, sphincter pupillae |
| Location | In the colored part of the eye called the iris |
| Function | Controls the size of the pupil by contracting or relaxing, thus controlling the amount of light that reaches the back of the eye at the retina |
| Dimensions | 0.75 mm wide and 0.15 mm thick |
| Innervation | Parasympathetic innervation via the 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. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. The muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
Ghrelin has been shown to interfere with the kinetics of the iris sphincter muscle, promoting relaxation. This has been observed in both basal and pre-contracted muscles, as well as in rabbit models.
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The dilator muscle
The iris consists of two sheets of smooth muscle with opposing functions: dilation (expansion) and contraction (constriction). The dilator muscle of the iris, or the dilator pupillae, is responsible for expanding the pupil when it contracts, in contrast to the sphincter muscle, which constricts the pupil. The dilator muscle enlarges the pupil by pulling the iris radially. The sphincter and dilator muscles overlap at the iris collarette, which is the thickest portion of the iris.
The dilator pupillae and sphincter pupillae muscles develop from the optic cup neuroectoderm, along with the iris epithelium. The dilator muscle fibres are located anterior to the pigmented epithelium of the iris. The dilator muscle is part of the accommodative reflex, which is an involuntary response when the visual focus switches from a distant object to a closer one.
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The role of ghrelin
The iris is the coloured, muscular part of the eye surrounding the pupil. It consists of two sheets of smooth muscle with opposing functions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and, consequently, the amount of light that 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.
Ghrelin is a hormone produced and released by the stomach. It is often referred to as the "hunger hormone" because it signals the brain when the stomach is empty, indicating that it is time to eat. Ghrelin levels increase between meals and decrease when the stomach is full. This hormone also plays a role in increasing food intake and promoting fat storage. It stimulates the release of growth hormones from the pituitary gland and is involved in insulin release. Additionally, ghrelin helps protect cardiovascular health and may have a role in mediating memory and stress.
Ghrelin levels are influenced by food intake and body weight. They tend to be higher in individuals who restrict their calorie intake, such as those on restrictive diets or suffering from anorexia nervosa. Conversely, people with obesity often exhibit lower ghrelin levels. However, it is important to note that ghrelin levels are typically lower in individuals with higher body weights compared to lean individuals, suggesting that ghrelin may not be the sole cause of obesity.
Furthermore, ghrelin has been found to interfere with the kinetics of the iris sphincter muscle. It induces a decrease in muscle tension and promotes relaxation, particularly in pre-contracted muscles. This effect has been observed in both rat and rabbit eyes, indicating that it is not species-specific. The ghrelin-induced tension decrease in the iris dilator muscle is dependent on GHSR-1a activation. Additionally, ghrelin triggers the activation of various signalling pathways, and its association with the induction of cyclooxygenase and prostaglandin release has been established.
In summary, ghrelin is a hormone that plays a crucial role in signalling hunger, regulating food intake, and promoting fat storage. It has additional functions related to growth hormone release, insulin regulation, and cardiovascular protection. Moreover, ghrelin influences the kinetics of the iris sphincter muscle, contributing to muscle relaxation through mechanisms involving GHSR-1a activation and the induction of cyclooxygenase and prostaglandin release.
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Sympathetic and parasympathetic systems
The iris consists of two sheets of smooth muscle that control the size of the pupil by expanding or contracting it. 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 sympathetic and parasympathetic nervous systems play a crucial role in controlling the iris and pupil size. The vertebrate eye receives innervation from both the parasympathetic and sympathetic ganglia. The parasympathetic system controls the constriction of the pupil, while the sympathetic system controls pupil dilation. These two systems work in tandem to regulate the amount of light that reaches the retina at the back of the eye.
The ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris, controlling ocular accommodation and pupil constriction. On the other hand, the superior cervical ganglion neurons project to the dilator pupillae muscle of the iris, controlling pupil dilation. The sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The sympathetic and parasympathetic pathways are considered the "'core'" of neural regulation of ocular homeostasis. They also influence ocular blood flow, aqueous humour production, and intraocular pressure (IOP). The parasympathetic system innervates the eye through postganglionic fibres originating from neurons in the ciliary and pterygopalatine ganglia, while the sympathetic system uses postganglionic fibres from the superior cervical ganglion.
In summary, the sympathetic and parasympathetic systems have distinct roles in regulating the iris and pupil size, with the parasympathetic system constricting the pupil and the sympathetic system dilating it. These systems work together to ensure proper visual function and maintain the health of the eye.
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Iris colour and eye colour
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 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 or sphincter pupillae, encircles the pupil and constricts it in bright light, while the dilator muscle of the iris expands the opening when it contracts. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscles enlarge the pupil by pulling the iris radially. The collarette, or the thickest region of the iris, is where these two muscles overlap.
The amount of pigment in the iris determines eye colour. With very little pigment, eyes appear blue, while increased pigment results in shades of deep brown to black. The outer border of the iris does not change size during constriction or dilation; only the inner border, where the constricting muscle is located, changes the pupil's diameter.
In addition to light, several factors influence the size of the pupil, including emotional state, cognitive load, arousal, and stimulation. The pupil size can range from less than 2 mm to as large as 9 mm in diameter, although there is individual variation and a decrease in size with age.
Heterochromia is an uncommon ocular condition in humans where one iris differs in colour from the other (complete heterochromia) or where a part of one iris is a different colour (partial or sectoral heterochromia). It is often associated with ocular diseases such as chronic iritis or diffuse iris melanoma but can also occur as a normal variant.
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Frequently asked questions
The iris is the coloured, muscular part of the eye surrounding the pupil.
The iris consists of two sheets of smooth muscle with contrary actions: the iris sphincter muscle (also known as the pupillary sphincter or sphincter pupillae) and the iris dilator muscle.
The sphincter muscle constricts the pupil in bright light, while the dilator muscle expands the pupil by pulling the iris radially.
The sphincter and dilator muscles overlap at the thickest region of the iris, known as the collarette. The sympathetic and parasympathetic systems work in opposition to constantly adjust the size of the pupil.
