Iris sphincter muscle

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Iris sphincter muscle
Iris, front view. (Muscle visible but not labeled.)
The upper half of a sagittal section through the front of the eyeball. ("Sphincter of pupil" labeled near bottom-center.)
Latin Musculus sphincter pupillae
encircles iris[1]
encircles iris[1]
long posterior ciliary arteries
short ciliary nerves
Actions constricts pupil
iris dilator muscle
Gray's p.1013
TA A15.2.03.029
FMA FMA:49157
Anatomical terms of muscle

The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

Comparative Anatomy[edit]

It is found in vertebrates and some cephalopods.

General Structure[edit]

Initially, all the myocytes are of the smooth muscle type but, later in life, most cells are of the striated muscle type.[2]

Its dimensions are about 0.75 mm wide by 0.15 mm thick.

Mode of Action[edit]

In humans, it functions to constrict the pupil in bright light (pupillary reflex) or during accommodation.


It is controlled by parasympathetic fibers that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves.

See also[edit]


  1. ^ a b Gest, Thomas R; Burkel, William E. "Anatomy Tables - Eye." Medical Gross Anatomy. 2000. University of Michigan Medical School. 5 Jan. 2010 <>.
  2. ^ Muscarinic and Nicotinic Synaptic Activation of the Developing..

External links[edit]