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 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]