Logan Steele
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris, the coloured part of the eye. The iris sphincter muscle is responsible for constricting the pupil, which is the dark aperture in the centre of the iris. The muscle fibres are located near the pupillary margin and encircle the pupil. The sphincter pupillae is stimulated by the parasympathetic nervous system, which causes the muscle to contract and decrease in diameter, controlling the amount of light that reaches the retina.
| Characteristics | Values |
|---|---|
| Name | Iris sphincter muscle, pupillary sphincter, pupillary constrictor, circular muscle of iris, sphincter pupillae muscle |
| Location | In the iris, the coloured part of the eye |
| Structure | A circular ring of smooth muscle fibres |
| Function | Constricts the pupil in bright light (pupillary light reflex) or during accommodation |
| Controlled by | Parasympathetic nerve fibres |
| Dimensions | 0.75 mm wide and 0.15 mm thick |
| Blood supply | Anterior and long posterior ciliary arteries |
| Drainage | Minor venous circle in the iris, which empties into the vorticose veins |
| Disorders | Adie syndrome, accommodative iridoplegia, complete iridoplegia, reflex iridoplegia (Argyll Robertson pupil), Horner syndrome, oculomotor nerve aberrancy |
Explore related products
What You'll Learn
The iris sphincter muscle
Disorders such as Adie syndrome, or Holmes-Adie syndrome, are characterised by a lack of response to light and a tonically dilated pupil due to damage to the postganglionic fibres of the parasympathetic innervation to the eye, resulting in an inability to constrict the iris sphincter muscle. Iridoplegia, or accommodative iridoplegia, refers to the inability of the pupil to constrict during accommodation. This condition can be caused by direct trauma to the orbit, inflammation, or even the use of certain eyedrops.
Unlocking the Mystery of Hip Muscles
You may want to see also
Explore related products
Pupillary light reflex
The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye. This helps the eye adapt to various levels of lightness or darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis), while a lower intensity of light causes the pupil to dilate (mydriasis). Thus, the pupillary light reflex regulates the intensity of light entering the eye.
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 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. By controlling the pupil's diameter, the iris controls the amount of light that reaches the back of the eye at the retina.
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 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 pupillary light reflex is measured based on a 0 to 4+ gradient that considers the magnitude and speed of the light response. A normal, healthy adult patient is expected to have a 4+ response, which indicates a brisk, large response. A 3+ grading indicates a moderate response, 2+ is a small, slowed response, 1+ represents a tiny/just visible response, and 0 indicates unresponsive pupils.
There are four types of pupillary light reflexes, based on the terminology of absolute laterality (left versus right) and relative laterality (same side versus opposite side, ipsilateral versus contralateral, direct versus consensual). The direct pupillary reflex is the pupillary response to light that enters the ipsilateral (same) eye, while the consensual pupillary reflex is the response of a pupil to light that enters the contralateral (opposite) eye.
Flexing and Muscle Growth: Is There a Link?
You may want to see also
Explore related products
Pupil dilation and constriction
The pupil is the dark aperture in the center of the iris, the coloured part of the eye. Pupil dilation and constriction are reflexive events that occur in response to light conditions. The iris sphincter muscle, also known as the pupillary sphincter, sphincter pupillae, or pupillary constrictor, is responsible for constricting the pupil. This muscle is located in the iris and encircles the pupil. It is composed of smooth muscle fibres and is controlled by the parasympathetic nervous system. When stimulated by bright light, the iris sphincter muscle contracts, reducing the diameter of the pupil and limiting the amount of light that reaches the retina at the back of the eye. This process is known as the pupillary light reflex or miosis.
The parasympathetic innervation of the iris sphincter muscle originates from the Edinger-Westphal nucleus of cranial nerve III. The signal is transmitted through the short ciliary nerves to the muscarinic receptors of the muscle fibres. The reciprocal process of pupil dilation is controlled by the sympathetic nervous system, which acts through the superior cervical ganglion to stimulate the dilator muscle of the iris. The sympathetic and parasympathetic systems work in opposition to constantly adjust the size of the pupil.
The size of the pupil can vary with age and lighting conditions. For individuals aged 20 to 80 years, pupil size typically ranges from 2 to 4 mm in bright light and between 4 and 8 mm in low light. The pupil's ability to dilate and constrict is essential for maintaining clear vision and protecting the retina from excessive light exposure.
In lower animals, such as amphibians and cephalopods, the pupil's dilation and constriction may be influenced by direct light stimulation of the sphincter muscle or through extensive autonomic innervation. For example, in frogs and toads, sympathetic inhibition of the sphincter muscle is believed to cause pupil dilation, as there is no structural evidence of a dilator muscle in these species.
Training Your Posture Muscles: A Guide to Better Alignment
You may want to see also
Explore related products
Adie syndrome
The syndrome manifests unilaterally in 80% of cases, typically affecting one eye, although the other eye may eventually become affected as well. The pupil in the affected eye constricts slowly in bright light and when focusing on objects close at hand. Over time, the affected pupil, initially larger than the unaffected pupil, may become smaller, a condition known as "little old Adie" pupil. This change can be observed under a microscope, where some sectors of the sphincter muscles constrict while others do not, resembling the movement of an earthworm.
The diagnosis of Adie syndrome is primarily clinical, and a low-concentration pilocarpine test can be useful to demonstrate cholinergic denervation supersensitivity in the affected pupil. Treatment focuses on managing the symptoms and improving the patient's quality of life.
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is responsible for constricting the pupil. It is located in the iris, the coloured part of the eye, and encircles the pupil. The muscle fibres are controlled by parasympathetic nerve fibres, which cause the pupil to constrict in response to bright light or during accommodation. This process is known as the pupillary light reflex and helps control the amount of light that reaches the retina.
Loosening Wrist Muscles: Simple Techniques for Quick Relief
You may want to see also
Explore related products
Iridoplegia
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye, the iris. It encircles the pupil and functions to constrict it in bright light via the pupillary light reflex or during accommodation. This controls the amount of light that reaches the retina.
The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction (miosis) and accommodation. The parasympathetic fibres that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture.
The Core's Contralateral Rotators: Uncover Your Abdominal Muscles' Power
You may want to see also
Frequently asked questions
The muscle that constricts the pupil is called the iris sphincter muscle, or the pupillary sphincter, sphincter pupillae, or pupillary constrictor.
The iris sphincter muscle is a muscle in the part of the eye called the iris. It is a circular muscle, about 1 millimeter wide, that encircles the pupil.
The iris sphincter muscle is controlled by the parasympathetic nervous system, which causes the muscle to contract and decrease in diameter. This contraction constricts the pupil, limiting the amount of light that reaches the retina.
The function of the iris sphincter muscle is to constrict the pupil in response to bright light, via the pupillary light reflex, or during accommodation. By controlling the pupil's diameter, the iris regulates the amount of light that reaches the retina at the back of the eye.
Yes, there are several conditions that can impact the function of the iris sphincter muscle. Adie syndrome, also known as Holmes-Adie syndrome, is a disorder characterized by a lack of response to light and a pupil that remains tonically dilated. Argyll Robertson pupils are another example, where small, irregular pupils exhibit limited light response and prompt dilation during distance viewing.
