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The iris dilator muscle, also known as the pupil dilator muscle, is a smooth muscle of the eye that runs radially in the iris. It is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, causing the muscle to contract and dilate the pupil, allowing more light to enter the eye. The sympathetic fibres that innervate the dilator muscle arise from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. The iris sphincter and dilator muscles work together to control pupil size, with the sphincter muscle receiving parasympathetic innervation.
| Characteristics | Values |
|---|---|
| Type | Smooth muscle of the eye |
| Function | Widens the pupil to allow more light to enter the eye |
| Innervation | Sympathetic nervous system |
| Stimulus | Threatening stimulus that activates the fight-or-flight response |
| Contraction | Involuntary |
| Controlled by | Cholinergic parasympathetic control from fibres within the oculomotor nerve |
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What You'll Learn
The iris dilator muscle
The dilator muscle is innervated more specifically by postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of ciliary ganglion. From there, they travel via the internal carotid artery through the carotid canal to foramen lacerum. They then enter the middle cranial fossa above foramen lacerum, travel through the cavernous sinus in the middle cranial fossa and then travel with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure. From there, they travel with the nasociliary nerve and then the long ciliary nerve. They then pierce the sclera, travel between sclera and choroid to reach the iris dilator muscle. They will also pass through ciliary ganglion and travel in short ciliary nerves to reach the iris dilator muscle.
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The sympathetic nervous system
In the context of the dilator muscle, the sympathetic nervous system plays a crucial role in pupil dilation. The dilator muscle, specifically the iris dilator muscle or pupillary dilator, is a smooth muscle of the eye that runs radially in the iris. It consists of a spoke-like arrangement of contractile cells called myoepithelial cells. When these cells are stimulated by the sympathetic nervous system, they contract, causing the pupil to dilate and allowing more light to enter the eye. This dilation of the pupil is particularly important in low-light conditions to enhance vision.
The sympathetic innervation of the dilator muscle is quite intricate. Postganglionic sympathetic nerves arise from the superior cervical ganglion, which is part of the sympathetic chain, and travel through various structures, including the internal carotid artery, the cavernous sinus, and the ophthalmic artery. These nerves release noradrenaline, which acts on α1-receptors, leading to the contraction of the dilator muscle and subsequent pupil dilation.
It is worth noting that the sympathetic innervation of the dilator muscle is not limited to humans. In mammals, birds, reptiles, and amphibia, the dilator muscle is primarily innervated by sympathetic fibres. However, in fish, the control of the iris muscles is reversed, with the sphincter muscles being sympathetically innervated instead.
In summary, the sympathetic nervous system's role in innervating the dilator muscle is essential for pupil dilation, particularly in response to changing light conditions or during heightened sympathetic activity, such as the "fight-or-flight" response. This complex process involves a precise interplay between neural signals and muscular contractions, ultimately serving to enhance visual perception when needed.
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The parasympathetic nervous system
It is important to note that the parasympathetic control of the iris sphincter muscle varies across different species. For instance, in mammals, birds, reptiles, and amphibia, the iris sphincter muscle is predominantly under parasympathetic control or may even lack innervation altogether. In contrast, in fish, the control of these muscles is reversed, with the iris sphincter muscles being sympathetically innervated.
The interaction between the parasympathetic nervous system and the iris dilator muscle is a complex process influenced by various factors. While the dilator muscle itself is primarily innervated by the sympathetic nervous system, the overall regulation of pupil size involves a delicate balance between sympathetic and parasympathetic pathways. This interplay ensures that the pupil can dilate or constrict appropriately in response to changing light conditions and other stimuli, contributing to the body's remarkable ability to adapt to its environment.
In summary, while the iris dilator muscle is primarily innervated by the sympathetic nervous system, the parasympathetic nervous system plays a crucial role in regulating pupil size by controlling the iris sphincter muscle. This complex interplay between the two systems allows for precise adjustments to the amount of light entering the eye, demonstrating the body's intricate ability to maintain homeostasis and respond to external stimuli.
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The dilator pupillae muscle
The sympathetic innervation of the dilator pupillae muscle originates from postganglionic sympathetic nerves arising from the superior cervical ganglion. These nerves travel with the ophthalmic artery, forming long ciliary nerves that supply the dilator pupillae muscle. This process allows for the coordination of pupil size in response to light conditions and threatening stimuli.
In contrast to the dilator pupillae muscle, the pupillary constrictor or sphincter muscle works to decrease the size of the pupil. This muscle is innervated by the parasympathetic system, originating in the Edinger-Westphal subnucleus of the third cranial nerve in the midbrain. The balance between the sympathetic and parasympathetic systems regulates pupil size, ensuring the appropriate amount of light reaches the retina.
The terms "dilator pupillae" and "musculus dilatator pupillae" are derived from Latin. "Dilator" means "to spread out," while "pupillae" is the genitive case of "pupilla," meaning "of the pupil." The full Latin term, "musculus dilatator pupillae", can be translated as "little mouse that spreads out the pupil," reflecting the ancient understanding of anatomy.
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The iris sphincter muscle
The reciprocal function of pupil dilation is primarily controlled by the sympathetic system, which acts by releasing noradrenaline, stimulating α1-receptors. In response to a threatening stimulus that activates the fight-or-flight response, this innervation contracts the muscle and dilates the pupil, allowing more light to reach the retina. The sympathetic fibres for pupil dilation originate from the superior cervical ganglion and travel through the internal carotid artery to the cavernous sinus, passing through the superior orbital fissure into the orbit to innervate the iris dilator muscle.
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Frequently asked questions
The iris dilator muscle, also known as the pupil dilator muscle, is a smooth muscle of the eye that runs radially in the iris.
The iris dilator muscle contracts and dilates the pupil, allowing more light to enter the eye. This is particularly useful when there is insufficient light for the normal function of the eye.
The iris dilator muscle is innervated by the sympathetic nervous system. More specifically, it is innervated by postganglionic sympathetic nerves arising from the superior cervical ganglion.
When presented with a threatening stimulus that activates the fight-or-flight response, the iris dilator muscle contracts and dilates the pupil, temporarily letting more light reach the retina.
