Dominic Stone
The pupil is an opening in the iris of the eye that allows light to pass through to the retina. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. It is not the pupil itself, but it encircles the pupil and controls its diameter by contracting and expanding it. The iris sphincter muscle is innervated by the parasympathetic nervous system, while the iris dilator muscle, which works in opposition to the sphincter muscle, is innervated by the sympathetic nervous system.
| Characteristics | Values |
|---|---|
| What is it? | The pupil is not a muscle, but the iris has two muscles: the iris sphincter muscle and the iris dilator muscle. |
| Function | The iris sphincter muscle constricts the pupil in bright light or during accommodation. The iris dilator muscle increases the size of the pupil to allow more light to enter the eye. |
| Control | The iris sphincter muscle is controlled by the parasympathetic nervous system. The iris dilator muscle is controlled by the sympathetic nervous system. |
| Innervation | The iris sphincter muscle receives innervation from the Edinger-Westphal nucleus of cranial nerve III. The iris dilator muscle receives innervation from the superior cervical ganglion. |
| Size | The iris sphincter muscle is about 0.75 mm wide and 0.15 mm thick. |
| Composition | Both muscles are composed of smooth muscle fibres. |
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What You'll Learn
- The iris sphincter muscle encircles the pupil and constricts it in bright light
- The iris dilator muscle is a smooth muscle that enlarges the pupil
- The sympathetic nervous system stimulates the iris dilator muscle
- The iris sphincter and dilator muscles control pupil size
- Adie syndrome is a disorder distinguished by a lack of response to light and a dilated pupil
The iris sphincter muscle encircles the pupil and constricts it in bright light
The pupil is not a muscle, but muscles do play a key role in its function. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris. The iris is a circular, pigmented diaphragm that lies in front of the lens of the eye. The iris sphincter muscle fibres are located near the pupillary margin and slightly anterior to the pigmented epithelium of the iris.
The iris sphincter muscle encircles the pupil of the iris and controls the amount of light that reaches the back of the eye (the retina) by constricting the pupil in bright light via the pupillary light reflex, or during accommodation. This is known as miosis. The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, which originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia and is terminated on muscarinic receptors of the muscle fibres.
The iris sphincter muscle works in conjunction with the iris dilator muscle, or dilator pupillae, to control the diameter of the pupil. The iris dilator muscle is a smooth muscle of the eye, running radially in the iris, and is innervated by the sympathetic nervous system. When stimulated, the muscle contracts, widening the pupil and allowing more light to enter the eye. This process is known as midriasis and occurs during the "'fight-or-flight' reflex". The sympathetic and parasympathetic systems work in opposition to constantly adjust the size of the pupil.
The iris sphincter and dilator muscles are both smooth muscles and are found in vertebrates and some cephalopods. In humans, the iris sphincter muscle is about 0.75 mm wide and 0.15 mm thick.
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The iris dilator muscle is a smooth muscle that enlarges the pupil
The pupil is not a muscle, but the iris, the coloured part of the eye, contains two muscles that control the size of the pupil: the iris sphincter muscle and the iris dilator muscle. The iris sphincter muscle is a circular muscle that constricts the pupil in bright light or during accommodation. The iris dilator muscle, on the other hand, is a smooth muscle that enlarges the pupil.
The iris dilator muscle, also known as the pupillary dilator, radial muscle of the iris, or radiating fibres, is a muscle of the eye that runs radially in the iris. It consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When these cells are stimulated, they contract, pulling the iris radially and widening the pupil to allow more light to enter the eye. This process is known as pupil dilation or midriasis and occurs when there is insufficient light for the normal function of the eye or during heightened sympathetic activity, such as in the "fight-or-flight reflex".
The iris dilator muscle is innervated by the sympathetic system, which releases noradrenaline, acting on α1-receptors. The sympathetic pathways for pupil dilation begin with first-order cell bodies in the hypothalamus, with axons that traverse the brainstem to the thoracic cord level, where the second-order cell bodies reside. These axons then enter the paravertebral sympathetic chain and course over the lung apex to the superior cervical ganglion, where the final third-order cell body originates. The third-order sympathetic axons then ascend as a plexus surrounding the internal carotid artery to enter the cavernous sinus before passing through the superior orbital fissure into the orbit to innervate the iris dilator muscles.
The iris dilator muscle works in opposition to the iris sphincter muscle to constantly adjust the size of the pupil. While the dilator muscle enlarges the pupil, the sphincter muscle contracts it in a circular motion, controlling the amount of light that reaches the back of the eye at the retina. This reciprocal function is primarily controlled by sympathetic innervation, which promotes sphincter muscle relaxation, allowing for periods of low-light or night vision.
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The sympathetic nervous system stimulates the iris dilator muscle
The pupil is an optical element that controls the amount of light that reaches the retina by acting as an aperture stop. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle 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.
The iris dilator muscle, also known as the pupillary dilator or dilator pupillae, is a smooth muscle of the eye, running radially in the iris. The iris dilator muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, which acts on α1-receptors. The sympathetic nervous system is the major source of innervation through the stimulation of α1, α2 and β2 adrenergic receptors. The sympathetic fibres help promote sphincter muscle relaxation, which is critical for periods of low light or night vision.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. The iris sphincter and dilator muscles control pupil size. Pupil dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity, for example, in the fight-or-flight reflex. When the α-receptors are stimulated, the dilator smooth muscle contracts to cause pupillary dilation, and conversely, when the α-receptors are blocked, the dilator smooth muscle relaxes, causing pupillary miosis.
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The iris sphincter and dilator muscles control pupil size
The pupil is not a muscle; however, its size is controlled by two muscles in the iris: the iris sphincter and dilator muscles. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye called the iris. The muscle fibres are located near the pupillary margin and encircle the pupil. When stimulated by bright light, the iris sphincter muscle contracts the pupil in a circular motion, reducing its diameter and controlling the amount of light that reaches the retina. This process is known as the pupillary light reflex or miosis.
The other muscle involved in pupil size regulation is the iris dilator muscle, also known as the pupillary dilator or dilator pupillae. This muscle is composed of smooth muscle bundles, 3-4 μm thick, radially located on the outer part of the iris. The iris dilator muscle is innervated by the sympathetic nervous system, which stimulates the release of noradrenaline. When stimulated, the muscle fibres contract, causing the pupil to dilate or enlarge. This response is particularly evident during periods of low light or during the "'fight-or-flight' response, when increased light sensitivity is required.
The sphincter and dilator muscles work in opposition to constantly adjust the pupil's diameter, a process known as the pupillary reflex. The iris collarette is the thickest portion of the iris, where these two muscles overlap. While the collarette is typically flat, it can be more prominent in some individuals. The function of these muscles is involuntary and reflexive, ensuring that the eye receives the optimal amount of light for normal vision.
The iris sphincter muscle receives parasympathetic innervation from the Edinger-Westphal nucleus of cranial nerve III via the short ciliary nerves. In contrast, the iris dilator muscle is innervated by postganglionic sympathetic nerves arising from the superior cervical ganglion. These nerves travel through various structures, including the internal carotid artery, cavernous sinus, and superior orbital fissure, before reaching the iris dilator muscle.
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Adie syndrome is a disorder distinguished by a lack of response to light and a dilated pupil
Adie syndrome, also known as Holmes-Adie syndrome, is a rare neurological disorder that affects the pupil of the eye. The pupil is typically larger than normal (dilated) and does not respond well to direct light. The disorder is characterised by poor or absent tendon reflexes and impaired sweating. The syndrome is caused by damage to the postganglionic fibres of the parasympathetic nervous system, which controls the constriction of the pupil in response to light. This damage is often the result of a viral or bacterial infection that causes inflammation.
The pupil is the black circular opening in the centre of the iris, which allows light to enter the eye. It is not a muscle itself, but its size is controlled by two muscles in the iris: the sphincter muscle and the dilator muscle. The sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris, which encircles the pupil. When stimulated by the parasympathetic nervous system, it contracts the pupil in a circular motion, making the pupil appear smaller. This occurs in bright light or when focusing on nearby objects, reducing the amount of light that enters the eye.
The dilator muscle, on the other hand, is a smooth muscle that runs radially in the iris. It is stimulated by the sympathetic nervous system, which acts by releasing noradrenaline. When stimulated, the dilator muscle enlarges the pupil by pulling the iris radially, allowing more light to enter the eye. This occurs in low-light conditions, when focusing on faraway objects, or when a person is excited or experiences a threatening stimulus (fight-or-flight response).
In individuals with Adie syndrome, the pupil appears dilated because the sphincter muscle is unable to constrict the pupil properly. This is due to damage to the postganglionic fibres that supply the sphincter muscle. As a result, the pupil does not respond well to bright light and remains larger than normal. However, the pupil in Adie syndrome may still show a response to accommodation, or near response, slowly constricting when focusing on nearby objects. This is because the tonic pupil becomes supersensitive to acetylcholine during the reinnervation process, resulting in a strong and sustained pupillary response after constriction.
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Frequently asked questions
The pupil is an opening in the iris of the eye that allows light to pass through to the retina.
No, the pupil is not a muscle. However, the size of the pupil is controlled by two muscles in the iris: the sphincter pupillae and the dilator pupillae.
The sphincter pupillae and dilator pupillae are smooth muscles that work in opposition to change the diameter of the pupil. The sphincter pupillae constricts the pupil, while the dilator pupillae dilates it. These muscles function reflexively to adjust the amount of light that reaches the retina.
