Iris sphincter muscle

Muscle in the eye responsible for constricting the pupil

The iris sphincter muscle (also known as the sphincter pupillae) is a circular muscle located in the iris of the eye. It plays a crucial role in controlling the size of the pupil and, consequently, the amount of light that enters the eye.

Anatomy

The iris sphincter muscle is situated in the stroma of the iris, encircling the pupil. It is composed of smooth muscle fibers that are arranged concentrically around the pupil. The muscle is innervated by the parasympathetic nervous system, specifically by fibers originating from the Edinger-Westphal nucleus.

Function

The primary function of the iris sphincter muscle is to constrict the pupil, a process known as miosis. This action reduces the amount of light entering the eye, which is particularly important in bright lighting conditions. The constriction of the pupil also enhances the depth of field and reduces spherical aberration, improving the clarity of vision.

Physiology

The contraction of the iris sphincter muscle is mediated by the release of the neurotransmitter acetylcholine from the parasympathetic nerve endings. When acetylcholine binds to the muscarinic receptors on the muscle fibers, it triggers a cascade of events leading to muscle contraction. This process is part of the pupillary light reflex, which adjusts the pupil size in response to changes in light intensity.

Clinical Significance

Dysfunction of the iris sphincter muscle can lead to various clinical conditions. For instance, damage to the parasympathetic innervation can result in a dilated pupil that does not constrict properly, a condition known as mydriasis. This can be caused by trauma, certain medications, or neurological disorders.

Related Structures

The iris sphincter muscle works in conjunction with the dilator pupillae muscle, which is responsible for dilating the pupil. The balance between these two muscles determines the pupil size at any given moment.

See Also

• Iris (anatomy)
• Pupil
• Parasympathetic nervous system
• Edinger-Westphal nucleus
• Miosis
• Mydriasis
• Pupillary light reflex