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The iris dilator muscle, also known as the dilator pupillae muscle, is a smooth muscle of the eye that runs radially in the iris. It is a ring of contractile cells that contract to widen the pupil, allowing more light to enter the eye. The iris dilator muscle is innervated by the sympathetic nervous system and is stimulated by the fight-or-flight response, causing the pupil to dilate and increase the amount of light reaching the retina. This muscle works in opposition to the pupillary constrictor or iris sphincter muscle, which constricts the pupil in response to bright light.
| Characteristics | Values |
|---|---|
| Name | Iris dilator muscle, pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers, dilator pupillae muscle |
| Type | Smooth muscle |
| Location | Iris of the eye |
| Function | Pupillary dilation, widening the pupil to allow more light to enter the eye |
| Controlled By | Sympathetic nervous system, sympathetic pathways |
| Innervation | Superior cervical ganglion, sympathetic root of ciliary ganglion, internal carotid artery, cavernous sinus, ophthalmic artery, nasociliary nerve, long ciliary nerve |
| Structure | Radial, spokelike arrangement of contractile cells (myoepithelial cells) |
| Related Conditions | Horner syndrome, Adie syndrome, iridoplegia, closed-angle glaucoma |
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What You'll Learn
The iris dilator muscle is a smooth muscle of the eye
The iris dilator muscle, also known as the dilator pupillae muscle, is a smooth muscle of the eye. It is a ring of contractile cells within the iris, arranged radially in a spokelike manner. This muscle is responsible for pupillary dilation, or mydriasis, which occurs when there is insufficient light for the normal function of the eye. During this process, the muscle contracts, widening the pupil and allowing more light to enter the eye.
The iris dilator muscle is innervated by the sympathetic nervous system, specifically by postganglionic sympathetic nerves arising from the superior cervical ganglion. These nerves travel through various structures, including the internal carotid artery, cavernous sinus, and ophthalmic nerve, before reaching the iris dilator muscle. The sympathetic innervation stimulates the contraction of the muscle, leading to pupil dilation.
The dilator pupillae muscle is located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. It is positioned behind the rich connective tissue stroma and neurovasculature of the anterior iris leaf. The contraction of the muscle opposes the action of the pupillary constrictor or iris sphincter muscle, which works to constrict the pupil in bright light conditions.
The sympathetic and parasympathetic systems work together to constantly adjust the pupillary aperture. While the sympathetic system stimulates dilation, the parasympathetic system promotes sphincter muscle relaxation, which is crucial for low-light or night vision. Disruption of the innervation of the iris dilator muscle can lead to conditions such as Horner syndrome, characterized by an abnormally small pupil.
In summary, the iris dilator muscle is a smooth muscle of the eye that plays a vital role in adjusting the pupil size to regulate the amount of light entering the eye. Its function is intricately coordinated with other muscles and systems in the eye to maintain optimal vision in varying light conditions.
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It is made up of contractile myoepithelial cells
The iris dilator muscle, also known as the pupil dilator muscle, radial muscle of iris, radiating fibres, or dilator pupillae muscle, is a smooth muscle of the eye. It is responsible for widening the pupil to allow more light to enter the eye.
The iris dilator muscle is made up of contractile myoepithelial cells. These cells are arranged in a spokelike or circumferential, spoke-like formation, located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. When stimulated, these contractile cells contract, causing the pupil to dilate and let more light into the eye. This process is particularly important during the fight-or-flight reflex, when more light needs to reach the retina.
The dilator pupillae muscle is innervated by the sympathetic nervous system. Specifically, postganglionic sympathetic nerves arise from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. These nerves travel via the internal carotid artery through the carotid canal to the foramen lacerum and then enter the middle cranial fossa above the foramen lacerum. They then travel through the cavernous sinus in the middle cranial fossa and with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure.
From here, the nerves travel with the nasociliary nerve and then the long ciliary nerve. They pierce the sclera and travel between the sclera and choroid to reach the iris dilator muscle. They also pass through the ciliary ganglion and travel in short ciliary nerves to reach the iris dilator muscle. This process allows for the dilation of the pupil, which is critical for night vision or periods of low light.
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Pupillary dilation is controlled by the sympathetic nervous system
The iris is made up of two types of muscles: a ring of sphincter muscles that constrict the pupil to prevent excess light from entering the eye, and a set of dilator muscles that expand the pupil to allow more light to enter. The iris dilator muscle, also known as the pupillary dilator, is a smooth muscle of the eye that runs radially in the iris.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. While the sympathetic system stimulates pupillary dilation, the parasympathetic system causes constriction. The size of the pupils at any given time reflects the balance of these forces acting simultaneously. Pupillary dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity.
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The muscle is innervated by postganglionic sympathetic nerves
The iris dilator muscle, also known as the pupil dilator muscle, is a smooth muscle of the eye. It is stimulated by the sympathetic nervous system, which triggers a "'fight-or-flight' response. This response causes the muscle to contract, widening the pupil and allowing more light to enter the eye.
Postganglionic sympathetic nerves play a crucial role in the body's response to threatening or traumatic situations. They are involved in activating the "fight-or-flight" response, which includes physical effects such as faster breathing, increased heart rate, and dilation of pupils. These nerves release neurotransmitters like norepinephrine (noradrenaline) and epinephrine (adrenaline), which act on adrenergic receptors.
The sympathetic nervous system, including postganglionic nerves, regulates many homeostatic mechanisms in the body. It up- and down-regulates functions such as blood glucose levels, body temperature, cardiac output, and immune system function. The balance between the sympathetic and parasympathetic nervous systems helps maintain a stable internal environment.
In summary, the iris dilator muscle is innervated by postganglionic sympathetic nerves, which are integral to the body's response to threatening stimuli and the regulation of various physiological processes.
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The iris sphincter muscle is sympathetically innervated
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris, which is the coloured part of the eye. The iris sphincter muscle is sympathetically innervated, meaning it receives nerve signals from the sympathetic nervous system. This is in contrast to the iris dilator muscle, which is innervated by the parasympathetic nervous system.
The iris sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. The muscle encircles the pupil of the iris and functions to constrict the pupil in response to bright light or during accommodation, such as when reading or focusing on an object up close. By controlling the pupil's diameter, the iris sphincter muscle plays a crucial role in regulating the amount of light that reaches the retina at the back of the eye.
The sympathetic innervation of the iris sphincter muscle is primarily mediated by NA (noradrenaline), which acts on β-adrenoceptors. This activation induces relaxation in the sphincter by activating adenylate cyclase and forming cyclic AMP (cAMP). In some animal species, α-adrenoceptors are also involved, but they mediate contraction instead of relaxation. The sympathetic regulation of the iris dilator muscle is driven by α1B-adrenoceptors, which contribute to muscle contraction.
In terms of the neural pathway involved, the pretectal nuclei project to the Edinger-Westphal nuclei, which then innervate the iris sphincter muscle via the oculomotor nerve. The sympathetic pathways for pupil dilation originate in the hypothalamus and descend through the brainstem and thoracic cord before ascending through the neck and into the orbit to innervate the iris dilator muscle. This innervation causes the muscle to contract and dilate the pupil, allowing more light to enter the eye.
It is important to note that while the iris sphincter muscle is sympathetically innervated in teleosts (a group of ray-finned fish), it is primarily under parasympathetic control in mammals, birds, reptiles, and amphibia. The control of these muscles is reversed in teleosts compared to other vertebrate groups. Additionally, the literature on the innervation of fish iris muscles is often contradictory, and more research is needed to fully understand the complex interactions between the sympathetic and parasympathetic systems in iris muscle control.
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Frequently asked questions
The iris dilator muscle is a smooth muscle of the eye, which runs radially in the iris. It is a dilator, which allows more light to enter the eye by widening the pupil.
The iris dilator muscle is made up of a spokelike arrangement of modified contractile cells called myoepithelial cells.
The iris dilator muscle is innervated by the sympathetic nervous system. When stimulated, the muscle contracts, widening the pupil and allowing more light to enter the eye.
The function of the iris dilator muscle is to increase the size of the pupil, allowing more light to enter the eye. This is particularly important in low-light conditions or during the body's "'fight-or-flight" response.
