(anatomy) The smooth muscle of the ciliary body.
Ciliary muscle
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(¦sil·ē′er·ē ′məs·əl)
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n
A tiny smooth muscle at the junction of the cornea and sclera, consisting of two groups of fibers: circular fibers, which exert parasympathetic control through the oculomotor nerve and the ciliary ganglion, and radial fibers, which exert sympathetic control. Ciliary muscles are responsible for accommodation for far vision through flattening of the lens.
| Medical Dictionary: ciliary muscle |
n.
A smooth muscle of the ciliary body of the eye, consisting of circular fibers and radiating fibers, and whose action changes the shape of lens in the process of accommodation.
| Wikipedia: Ciliary muscle |
| Ciliary muscle | |
|---|---|
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| The choroid and iris. (Ciliary muscle is labeled near top.) | |
| Latin | musculus ciliaris |
| Gray's | subject #225 1011 |
| Origin | |
| Insertion | |
| Artery | Long posterior ciliary arteries |
| Nerve | Oculomotor nerve (parasympathetics) |
| Actions | accommodation |
The ciliary muscle is a smooth muscle in the eye that controls the eye's accommodation for viewing objects at varying distances and regulates the flow of aqueous humour through Schlemm's canal.
Contents |
Mode of action
The ciliary fibers have circular, longitudinal and radial orientations.[1]
The circular ciliary muscle fibers affect zonular fibers in the eye (fibers that suspend the lens in position during accommodation), enabling changes in lens shape for light focusing. When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibers (fibers that hold or flatten the lens). This release of tension of the zonular fibers causes the lens to become more spherical, adapting to short range focus. The other way around, relaxation of the ciliary muscle causes the zonular fibers to become taut, flattening the lens, increasing long range focus.
Contraction and relaxation of the longitudinal fibers, which insert into the trabecular meshwork in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impeding aqueous humour flow into the canal of Schlemm.[2]
Innervation
Contraction of the lens happens when there is parasympathetic activation of the M3 muscarinic receptors on the ciliary muscles. The parasympathetic signal is carried by cranial nerve III (the oculomotor nerve) synapsing on the ciliary ganglion. This leads to contraction of the ciliary muscles, a consequent reduction in the size of the ciliary body, and a lessening of the tension on the lens, hence allowing the lens to spring back into a more spherical shape to accommodate for close vision.
Relaxation occurs by sympathetic activation of the beta-2 receptors,which results in a increase in zonular tension. This causes the surfaces of the lens to flatten and thus increases the focal distance.[3]
The ciliary muscle receives both parasympathetic and sympathetic innervation, but the adrenergic tone is dominant.[3]
See also
Additional images
 The arteries of the choroid and iris. The greater part of the sclera has been removed. |
 Iris, front view. |
References
- ^ Riordan-Eva Paul, "Chapter 1. Anatomy & Embryology of the Eye" (Chapter). Riordan-Eva P, Whitcher JP (2008). Vaughan & Asbury's General Ophthalmology (17th ed.). McGraw-Hill. http://www.accessmedicine.com/content.aspx?aID=3088500.
- ^ Salmon John F, "Chapter 11. Glaucoma" (Chapter). Riordan-Eva P, Whitcher JP (2008). Vaughan & Asbury's General Ophthalmology (17th ed.). McGraw-Hill. http://www.accessmedicine.com/content.aspx?aID=3089104.
- ^ a b Hardman JG, Limbird LE, Gilman AG (2006). "Table 6-1". Goodman & Gilman's The Pharmacological Basis of Therapeutics (11th Edition ed.). New York: McGraw-Hill. pp. 143-145. ISBN 0-07-142280-3.
External links
- Ciliary+muscle at eMedicine Dictionary
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 | Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more |
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