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The pupillary constrictor muscle, also known as the iris sphincter muscle, is a circular muscle found in the iris of the eye. It is responsible for constricting the pupil in response to bright light, thus regulating the amount of light that reaches the retina. This process is known as the pupillary light reflex. The muscle is innervated by the parasympathetic nervous system, which causes the muscle to contract and decrease in diameter, resulting in pupil constriction. This reflex is important for maintaining clear vision in various lighting conditions.
| Characteristics | Values |
|---|---|
| Muscle type | Smooth muscle |
| Muscle shape | Circular ring |
| Muscle width | 1 millimeter |
| Muscle thickness | 0.15 millimeters |
| Muscle location | Pupillary zone of stromal layer of the iris |
| Muscle function | Constricts the pupil (miosis) |
| Muscle innervation | Parasympathetic nervous system |
| Nerve fibers | Postganglionic, short ciliary nerves |
| Nerve origin | Edinger-Westphal nucleus |
| Nerve pathway | Oculomotor nerve, ciliary ganglion |
| Stimulus for constriction | Bright light, accommodation |
| Stimulus for dilation | Insufficient light, sympathetic activity |
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What You'll Learn
The parasympathetic nervous system stimulates the muscle
The stimulation of the sphincter pupillae muscle by the parasympathetic nervous system is a result of the release of acetylcholine, which acts on muscarinic acetylcholine receptors, specifically the M3 receptor. This stimulation causes the muscle to contract, resulting in the constriction of the pupil. The parasympathetic nerve fibres that innervate the sphincter pupillae originate in the Edinger-Westphal nucleus, which is located near the oculomotor nerve nucleus. These nerve fibres then travel along the oculomotor nerve and synapse at the ciliary ganglion. From there, postganglionic fibres reach the sphincter pupillae via the short ciliary nerves.
The pupillary light reflex is a response that occurs when the eyes are exposed to bright light, and the amount of light reaching the retina needs to be reduced to maintain clear vision. This reflex involves the optic nerve (CN II), the pretectal nucleus of the midbrain, the accessory oculomotor nucleus, and the oculomotor nerve (CN III). When the retinal photoreceptors detect bright light, the optic nerve sends a neural impulse to the pretectal nucleus, which then stimulates the accessory oculomotor nucleus. This, in turn, innervates and constricts the sphincter pupillae muscle via the short ciliary nerves, resulting in miosis and reduced pupil size.
It is important to note that the sphincter pupillae muscle works in conjunction with the dilator pupillae muscle, which is responsible for dilating the pupil. The dilator pupillae muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline. When the sympathetic system is stimulated, it causes the dilation of the pupil, allowing more light to enter the eye. This dilation response is often associated with the ""fight-or-flight" reflex, where increased light input to the retina is necessary.
In summary, the parasympathetic nervous system stimulates the sphincter pupillae muscle, leading to its contraction and the constriction of the pupil. This response is crucial for maintaining appropriate light levels reaching the retina and is part of the pupillary light reflex. The parasympathetic innervation of the sphincter pupillae muscle involves a complex neural pathway that ensures precise control over the size of the pupil in response to changing light conditions.
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The muscle is controlled by parasympathetic postganglionic fibres
The iris sphincter muscle, also known as the pupillary sphincter, pupillary constrictor, circular muscle of iris, or sphincter pupillae, is a muscle located in the iris of the eye. It is responsible for constricting the pupil, thus limiting the amount of light that reaches the retina. This process is known as miosis.
The pupillary constrictor muscle is innervated by the parasympathetic nervous system, which stimulates the muscle to contract and decrease in diameter. Specifically, the muscle is controlled by parasympathetic postganglionic fibres releasing acetylcholine, acting primarily on the muscarinic acetylcholine receptor (M3) of the iris sphincter muscle. This receptor is a type of cholinergic receptor, and drugs that have cholinergic effects can stimulate the muscle, resulting in miosis.
The parasympathetic nerve fibres that innervate the pupillary constrictor muscle originate in the Edinger-Westphal nucleus, which is located near the oculomotor nerve nucleus. These fibres travel along the oculomotor nerve before synapsing at the ciliary ganglion. Postganglionic fibres then arrive at the sphincter pupillae via the short ciliary nerves.
The pupillary constrictor muscle works in opposition to the pupillary dilator muscle, which is responsible for increasing the size of the pupil to allow more light to enter the eye. The pupillary dilator muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline. When an individual is presented with a threatening stimulus that activates the fight-or-flight response, this innervation contracts the pupillary dilator muscle and dilates the pupil.
The pupillary light reflex is a response to bright light, where the amount of light that reaches the retina must be decreased to maintain clear vision. This reflex arc includes the optic nerve, pretectal nucleus of the midbrain, accessory oculomotor nucleus, and oculomotor nerve. When the retinal photoreceptors detect light, a neural impulse is sent to the pretectal nucleus, which then stimulates the accessory oculomotor nucleus. This, in turn, innervates and constricts the sphincter pupillae via the short ciliary nerves.
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The pupillary light reflex
The reflex arc of the pupillary light reflex involves the optic nerve (CN II), the pretectal nucleus of the midbrain, the accessory oculomotor nucleus, and the oculomotor nerve (CN III). The process begins with the retinal photoreceptors detecting light and sending a neural impulse through the optic nerve to the pretectal nucleus. The pretectal nucleus then stimulates the accessory oculomotor nucleus, which innervates and constricts the sphincter pupillae muscle via the short ciliary nerves. This causes the pupil to constrict, reducing the amount of light entering the eye.
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The role of the oculomotor nerve
The oculomotor nerve is the third cranial nerve (CN III). It is the chief motor nerve to the ocular and extraocular muscles. It provides motor and parasympathetic innervation to some of the structures within the bony orbit. The oculomotor nerve originates from the oculomotor nucleus and the Edinger-Westphal nucleus within the midbrain of the brainstem. It exits the brainstem near the midline at the base of the midbrain, just caudal to the mammillary bodies.
The oculomotor nerve has two primary functions. Firstly, it constricts the pupil (miosis) by innervating the smooth muscle (sphincter pupillae) near the pupil. This causes the pupil to decrease in diameter, reducing the amount of light entering the eye. Secondly, it innervates the ciliary muscles. The ciliary muscle changes the shape of the lens during accommodation, making it more spherical and thus more adapted to short-range vision.
The oculomotor nerve divides into superior and inferior branches in the anterior part of the cavernous sinus. The superior branch innervates the superior rectus and the levator palpabrae superioris. The levator palpabrae superioris raises the upper eyelid. Sympathetic fibres travel with the superior branch of the oculomotor nerve and innervate the superior tarsal muscle, which helps to keep the eyelid elevated. The inferior branch provides motor innervation to the medial rectus, inferior rectus, and inferior oblique. It also supplies pre-ganglionic parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles.
The oculomotor nerve can be susceptible to pathologies, and impairments may interfere with normal vision. Damage to the oculomotor nerve or any of its branches can lead to oculomotor nerve palsy (third nerve palsy). This can cause pupillary dilatation, resulting in anisocoria, symptomatic glare in bright light, and accommodation deficit that may cause blurred vision for near objects. Treatment for oculomotor nerve palsy typically involves medical management of systemic predisposing factors and conservative measures in the acute phase, followed by surgical intervention if necessary.
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The role of the iris dilator muscle
The iris dilator muscle, also known as the pupil dilator muscle, radial muscle of iris, radiating fibres, or musculus dilatator pupillae, is a smooth muscle of the eye. It is one of the two muscles of the iris, the other being the iris sphincter muscle. The iris dilator muscle fibres course radially through the iris, with a spoke-like arrangement of modified contractile cells called myoepithelial cells.
The iris dilator muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, acting on α1-receptors. This innervation contracts the muscle and dilates the pupil, allowing more light to enter the eye. The sympathetic fibres that innervate the iris dilator muscle originate from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. They travel via the internal carotid artery through the carotid canal to the foramen lacerum and then enter the middle cranial fossa above the foramen lacerum. Subsequently, they traverse the cavernous sinus in the middle cranial fossa and accompany the ophthalmic artery in the optic canal or the ophthalmic nerve through the superior orbital fissure. They then travel with the nasociliary nerve and the long ciliary nerve, pierce the sclera, and pass between the sclera and choroid to reach the iris dilator muscle.
The iris dilator muscle is involved in the pupillary light reflex, which occurs when the eyes are exposed to bright light, and the amount of light reaching the retina needs to be reduced to maintain clear vision. This reflex arc includes the optic nerve, the pretectal nucleus of the midbrain, the accessory oculomotor nucleus, and the oculomotor nerve. When the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus, which then stimulates the accessory oculomotor nucleus. This, in turn, innervates and constricts the sphincter pupillae via the short ciliary nerves.
The iris dilator muscle is also influenced by certain medications, such as tranquilizers, bronchodilators, antidepressants, and vasoconstrictors. These drugs can act on the iris dilator muscle through a sympathomimetic effect, causing pupil dilation. Conversely, parasympathomimetic drugs have an opposing effect, constricting the pupil and increasing the pupillary block.
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Frequently asked questions
The pupillary constrictor muscle is also known as the iris sphincter muscle, or the pupillary sphincter muscle. It is a circular muscle found in the iris of the eye, which encircles the pupil.
The muscle is stimulated by the parasympathetic nervous system, which causes the muscle to contract and decrease in diameter, constricting the pupil. This is known as miosis, and it reduces the amount of light reaching the retina.
The pupillary constrictor muscle is innervated by the parasympathetic system. Parasympathetic nerve fibres originate in the Edinger-Westphal nucleus and travel along the oculomotor nerve before synapsing at the ciliary ganglion.
