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The pupil is an opening in the iris that lets light pass through to the retina. The iris sphincter muscle, or pupillary sphincter, is a muscle in the iris that controls the diameter of the pupil. The iris also contains the dilator pupillae muscle, which dilates the pupil. The iris sphincter muscle is controlled by the parasympathetic nervous system, while the dilator pupillae muscle is controlled by the sympathetic nervous system. The diameter of the pupil changes in response to various brain states, such as arousal, locomotion, emotion, attention, and cognitive load.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle, pupillary sphincter, sphincter pupillae, iris dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers, dilator pupillae muscle |
| Location | In the coloured part of the eye called the iris |
| Controlled by | Parasympathetic fibres that travel from the Edinger-Westphal nucleus with the oculomotor nerve (cranial nerve 3) to the ciliary ganglion |
| Function | Constricts the pupil in bright light via the pupillary light reflex or during accommodation |
| Controlled by | The dilator muscle is under sympathetic control in mammals. The sympathetic fibres course along the spinal cord to T1 to T3, leave in the ventral roots, and run anteriorly with the internal carotid to the cranial cervical ganglion. |
| Function | The dilator muscle produces mydriasis, increasing the size of the pupil to allow more light to enter the eye |
| Size | About 0.75 mm wide and 0.15 mm thick |
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What You'll Learn
The iris sphincter muscle
The function of the iris sphincter muscle is to control the pupil's diameter, allowing the iris to control the amount of light that reaches the back of the eye at the retina. The pupil's diameter fluctuates in association with changes in brain states induced by the neuromodulator systems. The iris sphincter muscle plays a primary role in controlling rapid pupil dilation with the onset of movement.
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The parasympathetic nervous system
The pupil size is controlled by the iris sphincter muscle and the iris dilator muscle, which constrict and dilate the pupil, respectively. 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 sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of 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 sphincter muscle is controlled by the parasympathetic nervous system. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia and become terminated on muscarinic receptors of the muscle fibres. The parasympathetic control of the iris sphincter muscle is well known since muscarinic antagonists were able to block the miosis induced by light exposure. The sphincter is contracted after treatment with ACh or its analogues. The receptors responsible for these actions are mainly of the muscarinic M3 subtype with fewer M2 receptors.
The iris, via the sphincter pupillae, is a part of the accommodative reflex. The accommodative reflex of the eye is an involuntary response when the visual focus switches from an object at a distance to one that is closer. The reflex is controlled by the parasympathetic nervous system and involves three actions: alteration of pupil aperture, lens shape, and convergence. The sphincter pupillae causes the narrowing of the pupil to prevent diverging light rays from the corneal periphery from creating a blurred image.
In summary, the parasympathetic nervous system, through its control of the iris sphincter muscle, plays a vital role in regulating pupil size and adapting to different lighting conditions to optimise vision.
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The accommodative reflex
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. By controlling the pupil's diameter, the iris controls the amount of light that reaches the back of the eye at the retina. The sphincter pupillae causes the narrowing of the pupil in order to prevent diverging light rays from the corneal periphery from creating a blurred image. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The iris dilator muscle, on the other hand, is controlled by the sympathetic nervous system and functions to dilate the pupil.
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Adie syndrome
The pupil size is controlled by the iris sphincter muscle and the iris dilator muscle, which constrict and dilate the pupil, respectively. The iris sphincter muscle is controlled by the Edinger-Westphal nucleus (EWN) and is under the control of the parasympathetic nervous system. On the other hand, the iris dilator muscle is controlled by the superior cervical sympathetic ganglion and is under the control of the sympathetic nervous system.
Young adults aged 25 to 45 (with a mean age of 32 years) are most commonly affected by Adie syndrome, with a female predominance of 2.6:1. The disorder is usually unilateral, affecting one eye, but it can eventually involve both eyes. The affected pupil may be slightly oval and distorted, and it may take several minutes to return to its original size after focusing on a close object. In some cases, the affected pupil will eventually become smaller than the unaffected pupil, a condition known as the "little old Adie" pupil.
There is currently no cure for Adie syndrome, but treatments are available to manage the symptoms. Low-concentration pilocarpine tests can be used to demonstrate cholinergic denervation supersensitivity in the tonic pupil, which is prevalent in 80% of cases. Diagnostic tests for Adie syndrome may involve a multidisciplinary care team, including specialists from various fields, to confirm the diagnosis and rule out other potential causes.
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The iris dilator muscle
The function of the iris dilator muscle is to dilate the pupil, thus allowing more light to enter the eye. When the muscle is stimulated, the contractile cells contract, widening the pupil. This process is known as mydriasis. The iris dilator muscle is considered a major regulator of pupil dilation, particularly in long-lasting pupil dilation.
In addition to endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may also act on the iris dilator muscle, resulting in mydriasis. Prolonged mydriasis can have negative consequences, such as an increased risk of closed-angle glaucoma.
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Frequently asked questions
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, controls pupil size.
The iris sphincter muscle is controlled by parasympathetic fibres that travel from the Edinger-Westphal nucleus with the oculomotor nerve to the ciliary ganglion, where they synapse. The muscle fibres are located near the pupillary margin and encircle the pupil. When stimulated, the muscle constricts the pupil, reducing the amount of light that reaches the retina.
The iris sphincter muscle contracts in response to bright light, a process known as the pupillary light reflex. This reflex is controlled by the parasympathetic nervous system and involves the optic nerve, pretectal nucleus of the midbrain, accessory oculomotor nucleus, and oculomotor nerve. The reflex helps to maintain clear vision by reducing the amount of light that reaches the retina.
