Sphincter pupillae muscle

The sphincter pupillae muscle is a circular muscle located in the iris of the eye. It is responsible for constricting the pupil, thereby regulating the amount of light that enters the eye.

Structure

The sphincter pupillae muscle is composed of smooth muscle fibers that form a circular band around the pupil. It is innervated by the parasympathetic nervous system, specifically by fibers originating from the Edinger-Westphal nucleus that travel along the oculomotor nerve (cranial nerve III) to the ciliary ganglion, and then via the short ciliary nerves to the muscle.

Function

The primary function of the sphincter pupillae muscle is to constrict the pupil in response to bright light, a reflex known as the pupillary light reflex. This action reduces the amount of light entering the eye, protecting the retina from excessive illumination and improving visual acuity by increasing the depth of focus.

Antagonist

The antagonist of the sphincter pupillae muscle is the dilator pupillae muscle, which functions to dilate the pupil, allowing more light to enter the eye in low-light conditions.

Clinical significance

Dysfunction of the sphincter pupillae muscle can lead to abnormal pupillary responses. For example, damage to the parasympathetic innervation can result in a condition known as mydriasis, where the pupil remains dilated and does not constrict properly in response to light.

See also

• Pupillary light reflex
• Iris (anatomy)
• Oculomotor nerve
• Ciliary ganglion

References

Richard S.,

 Clinical Anatomy by Regions, 
  
 Lippincott Williams & Wilkins, 
 2011, 
  
  
 ISBN 978-0-7817-9426-7,

,

 The Pupil: Anatomy, Physiology, and Clinical Applications, 
 Butterworth-Heinemann, 
 1999, 
 Vol. 1 
 pp. 1-1000,