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The iris sphincter muscle, also known as the pupillary constrictor, is a muscle located in the iris of the eye. It is responsible for constricting the pupil in bright light or during accommodation, controlling the amount of light that reaches the retina. The muscle is innervated by parasympathetic postganglionic fibers, which release acetylcholine, acting primarily on the muscarinic acetylcholine receptor of the iris sphincter muscle. This process is known as the pupillary light reflex. Damage to these parasympathetic postganglionic fibers can result in Adie syndrome, characterized by a lack of response to light and a tonically dilated pupil.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle |
| Other names | Pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers, sphincter pupillae |
| Location | Part of the eye called the iris |
| Function | Constricts the pupil in bright light via the pupillary light reflex or during accommodation |
| Control | Controlled by parasympathetic postganglionic fibers releasing acetylcholine acting primarily on the muscarinic acetylcholine receptor (M3) of iris sphincter muscle |
| Innervation | Receives parasympathetic innervation via the short ciliary nerves |
| Innervation origin | Edinger-Westphal nucleus of cranial nerve III |
| Innervation process | Preganglionic fibers travel along the oculomotor nerve (CN III) and synapse on neurons in the ciliary ganglion; postganglionic parasympathetic fibers then enter the eye through the short ciliary nerves |
| Sympathetic innervation | Superior cervical ganglia |
| Sympathetic function | Promote sphincter muscle relaxation, critical for periods of low-light or night vision |
| Related disorders | Adie syndrome (Holmes-Adie syndrome), iridoplegia |
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What You'll Learn
- The iris sphincter muscle is innervated by the short ciliary nerves
- Parasympathetic fibres originate from the Edinger-Westphal nucleus
- Postganglionic parasympathetic fibres enter the eye through the short ciliary nerves
- The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle are intrinsic ocular muscles
- Adie syndrome is a neuro-ophthalmological disorder characterised by a lack of response to light
The iris sphincter muscle is innervated by the short ciliary nerves
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. The iris sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. The iris sphincter muscle encircles the pupil of the iris, constricting it in bright light via the pupillary light reflex or during accommodation.
The parasympathetic fibres that innervate the iris sphincter muscle synapse on muscarinic receptors of the muscle fibres. The reciprocal function of pupillary dilation is controlled by sympathetic innervation from the superior cervical ganglion. These sympathetic fibres promote sphincter muscle relaxation, which is crucial for low-light conditions or night vision.
The iris sphincter muscle is an important component of the vertebrate eye, also found in some cephalopods. It plays a key role in controlling the amount of light that reaches the retina by adjusting the pupil's diameter.
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Parasympathetic fibres originate from the Edinger-Westphal nucleus
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 parasympathetic fibres that serve the iris sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The Edinger-Westphal nucleus is a group of neurons located in the midbrain, specifically in the oculomotor nuclear complex. These parasympathetic fibres arise from preganglionic neurons in the Edinger-Westphal nucleus and travel along the oculomotor nerve (CN III). They then make nicotinic cholinergic synapses with postganglionic neurons in the ciliary ganglion. From there, the postganglionic parasympathetic fibres exit the brain and enter the eye through the short ciliary nerves.
The short ciliary nerves pierce the sclera at the back of the eye and travel forward between the sclera and the choroid layer. Finally, they innervate the iris sphincter muscle, releasing acetylcholine and acting on muscarinic acetylcholine receptors (M3) to cause pupillary constriction. This process is known as the pupillary light reflex and allows the iris to control the amount of light that reaches the retina at the back of the eye.
The iris sphincter muscle is one of two muscles in the iris, both of which are smooth muscles. The other muscle is the pupillary dilator muscle, which works in opposition to the sphincter muscle to control the pupil's diameter. While the sphincter muscle constricts the pupil in bright light, the dilator muscle promotes sphincter muscle relaxation in low-light conditions, allowing for night vision. This reciprocal function is primarily controlled by sympathetic innervation from the superior cervical ganglion, which projects to the dilator muscle via long ciliary nerves.
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Postganglionic parasympathetic fibres enter the eye through the short ciliary nerves
The iris sphincter muscle, also known as the pupillary constrictor or circular muscle of the iris, encircles the pupil of the iris. It is controlled by parasympathetic postganglionic fibres that release acetylcholine, acting on the muscarinic acetylcholine receptor (M3) of the iris sphincter muscle.
The postganglionic parasympathetic fibres that innervate the iris sphincter muscle originate from the ciliary ganglion. The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It is one of four parasympathetic ganglia of the head, the others being the submandibular ganglion, pterygopalatine ganglion, and otic ganglion.
Preganglionic fibres from the Edinger-Westphal nucleus travel along the oculomotor nerve (CN III) and synapse on neurons in the ciliary ganglion. The postganglionic parasympathetic fibres then exit the ciliary ganglion via the short ciliary nerves, which run forward and pierce the sclera at the back of the eye. The short ciliary nerves travel between the sclera and the choroid to innervate the iris sphincter muscle.
The ciliary ganglion contains many more nerve fibres directed to the ciliary muscle than to the constrictor pupillae, with some sources estimating a ratio of 20:1. This is because the ciliary muscle is much larger than the constrictor pupillae. The ciliary ganglion contains postganglionic parasympathetic neurons that supply the ciliary muscle and the pupillary sphincter muscle. However, 95% of the neurons in the ciliary ganglion innervate the ciliary muscle due to its much larger size.
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The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle are intrinsic ocular muscles
The ciliary muscle, pupillary sphincter muscle, and pupillary dilator muscle are intrinsic ocular muscles, also known as intraocular muscles. These muscles are found within the eye structure and are responsible for adjusting the shape of the lens and the size of the pupil. They are distinct from extraocular muscles, which are located outside the eye and are responsible for controlling the eye's external movement.
The ciliary muscle is an intrinsic muscle of the eye that forms a ring of smooth muscle in the eye's middle layer, known as the uvea or vascular layer. It controls accommodation for viewing objects at varying distances by altering the shape of the lens. This muscle receives parasympathetic fibres from the short ciliary nerves, which arise from the ciliary ganglion. The contraction of the ciliary muscle releases tension on the lens caused by the zonular fibres, allowing the lens to become more spherical and adapt to short-range focus. Conversely, relaxation of the ciliary muscle causes the zonular fibres to flatten the lens, increasing the focal distance and improving long-range focus.
The pupillary sphincter muscle and pupillary dilator muscle, also known as the sphincter pupillae and dilator pupillae, respectively, work together to control the size of the pupil. The pupillary sphincter muscle constricts the pupil in response to bright light or during accommodation, while the pupillary dilator muscle functions to dilate the pupil. These muscles are innervated by parasympathetic fibres that release acetylcholine, acting primarily on the muscarinic acetylcholine receptor (M3) of the iris sphincter muscle.
The iris sphincter muscle, also known as the pupillary sphincter muscle, encircles the pupil and functions as a constrictor. It is found in vertebrates and some cephalopods, with dimensions of approximately 0.75 mm in width and 0.15 mm in thickness. In humans, it responds to bright light or accommodation by constricting the pupil, a response known as the pupillary light reflex. In lower animals, the muscle cells themselves are photosensitive, allowing iris action without brain input.
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Adie syndrome is a neuro-ophthalmological disorder characterised by a lack of response to light
Adie syndrome, also known as Holmes-Adie syndrome, is a rare neuro-ophthalmological disorder characterised by a lack of response to light. It is a neurological disorder that affects the pupil of the eye and the autonomic nervous system. The pupil is typically larger than normal (mydriasis) and slow to react or non-reactive to direct light (tonic pupil). The affected pupil constricts slowly in bright light, and in some cases, it does not constrict at all. The pupil may also be slightly ovally distorted.
The syndrome is frequently observed in females with impaired sweating and absent knee or ankle jerks. It is caused by damage to the postganglionic fibres of the parasympathetic innervation of the eye, which is usually a result of a viral or bacterial infection causing inflammation. The infection damages neurons in the ciliary ganglion, located in the posterior orbit, which controls eye constriction. Adie syndrome can also cause problems with autonomic control due to damage to the dorsal root ganglia of the spinal cord.
The iris sphincter muscle, also known as the pupillary sphincter, is a muscle in the iris of the eye. It encircles the pupil and constricts it in bright light, a response known as the pupillary light reflex. This muscle is controlled by parasympathetic postganglionic fibres releasing acetylcholine, which acts on the muscarinic acetylcholine receptor of the iris sphincter muscle. In Adie syndrome, the pupil's poor response to light is due to damage to these postganglionic fibres.
The diagnosis of Adie syndrome involves a thorough clinical evaluation, including a detailed patient history and a complete eye examination by an ophthalmologist. An eye doctor may use diluted pilocarpine eye drops to test the pupil's reaction. Pilocarpine causes the pupil to constrict, and in individuals with Adie syndrome, the affected pupil will constrict in response to this drug, whereas a normal pupil will not. CT and MRI scans may also be useful in diagnostic testing.
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Frequently asked questions
The pupillary constrictor muscle, also known as the iris sphincter muscle, is a muscle in the iris of the eye. It constricts the pupil in bright light or during accommodation.
The pupillary constrictor muscle is innervated by the short ciliary nerves, which are controlled by parasympathetic postganglionic fibres releasing acetylcholine.
The function of the pupillary constrictor muscle is to constrict the pupil in response to bright light, controlling the amount of light that reaches the retina.
The pupillary constrictor and dilator muscles work in opposition to constantly adjust the pupil's aperture. The constrictor muscle narrows the pupil in bright light, while the dilator muscle, innervated by sympathetic fibres, promotes muscle relaxation and pupil dilation in low-light conditions.
Yes, the pupillary constrictor muscle can become paralysed in a condition known as iridoplegia, often resulting from direct trauma to the orbit, inflammation, or even eyedrops.
