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The ciliary muscle (TA: musculus ciliaris) is located within the ciliary body of the eye. It acts to facilitate lens accommodation for near vision, and receives parasympathetic innervation from short ciliary nerves, arising from the oculomotor nerve via the ciliary ganglion.
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The ciliary muscle forms a triangle-shaped region of smooth muscle fibers within the anterior aspect of the ciliary body, located anterior to the choroid and posterior to the iris. The fibers of the ciliary muscle are (largely academically) divided into longitudinal, radial, and circular groups, in order from most peripheral to most central 1. From the ciliary muscle project thin processes attaching to a complex network of zonular fibers, which, in turn, attach to the lens capsule 1.
The ciliary muscle receives parasympathetic innervation from short ciliary nerves projecting from the ciliary ganglion. The preganglionic parasympathetic fibers originate in the Edinger-Westphal nuclei located in the midbrain at the level of the superior colliculus anterolateral to the aqueduct of Sylvius, traveling to the ciliary ganglion via the oculomotor nerve 2,3.
Some researchers suggest a small sympathetic component may also exist, acting to inhibit accommodation 4.
Ciliary muscle contraction facilitates lens accommodation. All three fiber groups contract synergistically, initiating a complex mechanism that simultaneously pulls the anterior border of the choroid forward and causes a release of tension of the zonular fibers at the lens equator. This reduced zonular tension, in turn, increases the convexity of the lens, which increases its refractive power for near vision 1,3.
An additional function of ciliary muscle contraction is to increase the pore diameter of the trabecular meshwork, aiding the drainage of aqueous humor into the canal of Schlemm 1.
accommodation is a key component of the ocular near response, which consists of pupillary constriction, miosis, lens accommodation, and eye convergence 2
cycloplegia is the term given to ciliary muscle paralysis causing loss of lens accommodation
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