| Ciliary muscle | |
|---|---|
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| Details | |
| Pronunciation | UK:/ˈsɪliəri/,US:/ˈsɪliɛri/[2] |
| Origin | 1) longitudinal fibers →scleral spur; 2) circular fibers → encircle root of iris[1] |
| Insertion | 1) longitudinal fibers → ciliary process, 2) circular fibers → encircle root of iris[1] |
| Artery | Long posterior ciliary arteries |
| Vein | Vorticose veins |
| Nerve | Short ciliary Parasympathetic fibers in theoculomotor nerve (CN-III) synapse in theciliary ganglion. Parasympathetic postganglionic fibers from theciliary ganglion travel throughshort ciliary nerves into the ocular globe. |
| Actions | 1)Accommodation, 2) regulation oftrabecular meshwork pore sizes |
| Identifiers | |
| Latin | musculus ciliaris |
| TA98 | A15.2.03.014 |
| TA2 | 6770 |
| FMA | 49151 |
| Anatomical terms of muscle | |
Theciliary muscle is anintrinsic muscle of theeye formed as a ring ofsmooth muscle[3][4] in the eye's middle layer, theuvea (vascular layer). It controlsaccommodation for viewing objects at varying distances and regulates the flow ofaqueous humor intoSchlemm's canal. It also changes the shape of the lens within the eye but not the size of the pupil[5] which is carried out by thesphincter pupillae muscle anddilator pupillae.
The ciliary muscle,pupillary sphincter muscle andpupillary dilator muscle sometimes are calledintrinsic ocular muscles[6] orintraocular muscles.[7]
The ciliary muscle develops frommesenchyme within thechoroid and is considered a cranialneural crest derivative.[8]
The ciliary muscle receives parasympathetic fibers from theshort ciliary nerves that arise from theciliary ganglion. The parasympathetic postganglionic fibers are part of cranial nerve V1 (Nasociliary nerve of the trigeminal), while presynaptic parasympathetic fibers to the ciliary ganglia travel with the oculomotor nerve.[9] The postganglionic parasympathetic innervation arises from the ciliary ganglion.[10]
Presynaptic parasympathetic signals that originate in theEdinger-Westphal nucleus are carried by cranial nerve III (theoculomotor nerve) and travel through theciliary ganglion via the postganglionic parasympathetic fibers which travel in theshort ciliary nerves and supply theciliary body and iris.Parasympathetic activation of the M3muscarinic receptors causes ciliary muscle contraction. The effect of contraction is to decrease the diameter of the ring of ciliary muscle causing relaxation of the zonule fibers, the lens becomes more spherical, increasing its power to refract light for near vision.[citation needed]
The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as reading a book.[11]
The ciliary fibers have circular (Ivanoff),[12] longitudinal (meridional) and radial orientations.[13]
According toHermann von Helmholtz's theory, the circular ciliary muscle fibers affectzonular fibers in theeye (fibers that suspend thelens in position duringaccommodation), 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. Conversely, relaxation of the ciliary muscle causes the zonular fibers to become taut, flattening the lens, increasing thefocal distance,[14] increasing long range focus. Although Helmholtz's theory has been widely accepted since 1855, its mechanism still remains controversial. Alternative theories of accommodation have been proposed by others, including L. Johnson, M. Tscherning, and especially Ronald A. Schachar.[3]
Contraction and relaxation of the longitudinal fibers, which insert into thetrabecular meshwork in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impedingaqueous humour flow into thecanal of Schlemm.[15]
Open-angleglaucoma (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists (e.g.,pilocarpine), which cause rapidmiosis and contraction of the ciliary muscles, opening the trabecular meshwork, facilitating drainage of the aqueous humour into the canal of Schlemm and ultimately decreasingintraocular pressure.[16]
The wordciliary had its origins around 1685–1695.[17] The termcilia originated a few years later in 1705–1715, and is theNeo-Latinplural ofcilium meaningeyelash. InLatin,cilia means uppereyelid and is perhaps aback formation fromsupercilium, meaningeyebrow. The suffix-ary originally occurred in loanwords fromMiddle English (-arie),Old French (-er,-eer,-ier, -aire, -er), and Latin (-ārius); it can generally mean "pertaining to, connected with", "contributing to", and "for the purpose of".[18] Taken together,cili(a)-ary pertains to various anatomical structures in and around the eye, namely theciliary body and annular suspension of thelens of the eye.[19]
| Fibrous tunic (outer) |
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| Uvea / vascular tunic (middle) |
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| Retina (inner) |
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| Anatomical regions of the eye |
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