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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, running radially in the iris. The muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, which acts on α1-receptors. When stimulated, the muscle contracts, widening the pupil and allowing more light to enter the eye. This is known as mydriasis. Conversely, when the circular muscle contracts, and the radial muscle relaxes, the pupil constricts, a process known as miosis.
| Characteristics | Values |
|---|---|
| Name | Iris dilator muscle, dilator pupillae muscle, pupil dilator muscle, radial muscle of iris, radiating fibres |
| Type of muscle | Smooth muscle |
| Location | Circumferential, spoke-like arrangement in the mid-periphery of the posterior leaf of the iris |
| Innervation | Sympathetic nervous system, postganglionic sympathetic nerves |
| Function | Pupillary dilation (mydriasis) |
| Response | Contraction of muscle, dilation of pupil, increased light entering the eye |
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What You'll Learn
- The iris dilator muscle is innervated by the sympathetic nervous system
- Pupillary dilation is controlled by the dilator pupillae
- The iris dilator muscle is a smooth muscle of the eye
- The iris sphincter is controlled by the parasympathetic nervous system
- The iris dilator muscle is also known as the dilator pupillae muscle
The iris dilator muscle is innervated by the sympathetic nervous system
The iris dilator muscle, also known as the dilator pupillae muscle, is a smooth muscle of the eye. It is composed of smooth muscle bundles, 3–4 μm thick, radially located on the outer part of the iris. The muscle is made up of a spokelike arrangement of modified contractile cells called myoepithelial cells.
The sympathetic nervous system stimulates the myoepithelial cells of the iris dilator muscle to contract, leading to pupillary dilation (mydriasis). This widening of the pupil allows more light to enter the eye. The sympathetic nervous system acts by releasing noradrenaline, which acts on α1-receptors. This response is particularly important when presented with a threatening stimulus that activates the fight-or-flight response, as the increased light reaching the retina can improve an individual's ability to respond to the threat.
It is worth noting that the iris sphincter muscle, which also controls pupil size, is primarily under parasympathetic control. This muscle contracts during a constriction response (miosis), causing the pupil to narrow. The parasympathetic nervous system and sympathetic nervous system work together to regulate pupillary response, which can range in size from 1.5 mm to 8 mm.
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Pupillary dilation is controlled by the dilator pupillae
Pupillary dilation, or mydriasis, is the widening of the pupil. This dilation is caused by the contraction of the smooth cells of the radial muscle, controlled by the sympathetic nervous system (SNS). The radial muscle is also known as the dilator pupillae muscle or the iris dilator muscle.
The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, and their contraction facilitates pupillary dilation. The muscle is located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. The muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, which acts on α1-receptors. This stimulation causes the muscle to contract and dilate the pupil, allowing more light to enter the eye.
The pupillary dilation pathway is a sympathetically driven response beginning in the hypothalamus and ending with the contraction of the dilator pupillae muscle. This response can be triggered by any physical or emotional stress that activates the autonomic sympathetic nervous system, which is mediated by the hypothalamus. The dilation pathway is a three-neuron pathway, with the first-order neuron beginning in the hypothalamus and descending through the midbrain to synapse onto the spinal cord's ciliospinal center of Budge. The second-order neuron then exits the spinal cord and ascends through the thorax to the superior cervical ganglion. The third-order postganglionic neurons travel along the periarterial carotid plexus through the cavernous sinus and enter the orbit through the short and long ciliary nerves. These nerves then synapse on the dilator pupillae muscle, causing pupillary dilation.
It is important to note that the pupil constrictor muscle, controlled by the parasympathetic nervous system, is also involved in pupillary dilation. The constrictor muscle is the primary controller of rapid pupil dilation upon brain arousal, while the dilator pupillae muscle is involved in long-lasting pupil dilation. The pupil diameter is also influenced by changes in luminance, with dilation occurring in low light conditions to allow more light to enter the eye.
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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 located in the outer part of the iris and is responsible for dilating the pupil. The muscle is composed of smooth muscle bundles, 3-4 μm thick, which are radially located on the outer part of the iris. The iris dilator muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, stimulating α1-receptors. This stimulation causes the muscle to contract and dilate the pupil, allowing more light to enter the eye.
The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, and their contraction facilitates pupillary dilation. The muscle fibres course radially through the iris, giving it a spoke-like appearance. The muscle is located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium.
The iris dilator muscle is one of two muscles of the iris, the other being the sphincter muscle. These muscles work in opposition to each other to control the size of the pupil. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscle enlarges the pupil by pulling the iris radially. The iris collarette is the thickest portion of the iris, where the two muscles overlap.
The iris dilator muscle is primarily regulated by the sympathetic nervous system through adrenergic receptors, particularly the α1 receptors. However, it also receives some innervation from the parasympathetic nervous system. The sympathetic nervous system stimulates the muscle to contract, while the parasympathetic nervous system causes it to relax. This reciprocal function is critical for adjusting the pupil size in response to changing light conditions.
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The iris sphincter is controlled by the parasympathetic nervous system
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris, the coloured part of the eye. The iris sphincter is controlled by the parasympathetic nervous system (PSNS), which causes the muscle to contract and relax.
The parasympathetic nervous system stimulates the iris sphincter muscle, causing it to contract and decrease in diameter. This contraction leads to the constriction of the pupil, known as miosis. Miosis occurs when the circular muscle controlled by the PSNS contracts, and the radial muscle relaxes. Miosis is important for preventing diverging light rays from the corneal periphery from creating a blurred image. It also helps to control the amount of light that reaches the retina at the back of the eye.
The parasympathetic fibres that innervate the iris sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. These fibres travel along the oculomotor nerve and synapse at the ciliary ganglion. From there, the postganglionic fibres reach the iris sphincter muscle via the short ciliary nerves.
The iris sphincter muscle works in conjunction with the dilator pupillae muscle, also known as the iris dilator muscle, to regulate the size of the pupil. While the iris sphincter muscle constricts the pupil, the dilator pupillae muscle dilates it. The dilator pupillae muscle is located radially within the iris and is innervated by the sympathetic nervous system (SNS). When stimulated, the SNS causes the contraction of the dilator pupillae muscle, leading to the dilation of the pupil, or mydriasis.
The interaction between the parasympathetic and sympathetic nervous systems allows for the constant adjustment of the pupillary aperture, ensuring the appropriate amount of light reaches the retina.
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The iris dilator muscle is also known as the dilator pupillae muscle
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 made up of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye. The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibres project from the superior cervical ganglion to join the carotid plexus. These fibres then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle.
The iris dilator muscle is one of two muscles of the iris, the other being the iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae. The iris sphincter muscle is 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. 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 contraction of the dilator pupillae muscle facilitates pupillary dilation (mydriasis). Mydriasis is the widening of the pupil and may be caused by adrenaline, anticholinergic agents, stimulant drugs such as MDMA, cocaine, and amphetamines, and some hallucinogenics such as LSD. Mydriasis can also be caused by endogenous sympathetic stimulation and sympathomimetic drugs. Prolonged significant mydriasis can increase the risk of closed-angle glaucoma.
The iris dilator muscle is sometimes referred to as a radial muscle of iris or radiating fibres because its fibres course radially through the iris. The English name "dilator pupillae muscle" is derived from the Latin expression "musculus dilatator pupillae", which can be traced back to Roman antiquity. The Latin expression "dilatare" means "to dilate" or "to spread out".
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Frequently asked questions
The radial muscle of the pupil, also known as the iris dilator muscle, is a smooth muscle of the eye that runs radially in the iris.
The radial muscle of the pupil controls the amount of light entering the eye and the depth of field of the eye. When the radial muscle contracts, the pupil dilates, allowing more light to enter the eye.
The radial muscle of the pupil is innervated by the sympathetic nervous system.
The radial muscle of the pupil is stimulated to contract by adrenaline and other substances, such as stimulant drugs and hallucinogenics.
The opposite of the radial muscle of the pupil is the circular muscle, also known as the iris sphincter muscle. This muscle is controlled by the parasympathetic nervous system and causes the pupil to constrict when it contracts.
