Ciliary muscle
Template:Short description Template:Infobox muscle The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle<ref name="Kleinmann">Template:Cite journal</ref><ref name="Schachar">Schachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) Template:Cite book Template:ISBN.</ref> in the eye's middle layer, the uvea (vascular layer). It controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm's canal. It also changes the shape of the lens within the eye but not the size of the pupil<ref>Template:Cite journal</ref> which is carried out by the sphincter pupillae muscle and dilator pupillae.
The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles<ref>Template:Cite journal</ref> or intraocular muscles.<ref>Template:Cite book</ref>
StructureEdit
DevelopmentEdit
The ciliary muscle develops from mesenchyme within the choroid and is considered a cranial neural crest derivative.<ref name="dudekrw">Template:Cite book</ref>
Nerve supplyEdit
The ciliary muscle receives parasympathetic fibers from the short ciliary nerves that arise from the ciliary 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.<ref>Template:Cite book</ref> The postganglionic parasympathetic innervation arises from the ciliary ganglion.<ref>Template:Cite journal</ref>
Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the oculomotor nerve) and travel through the ciliary ganglion via the postganglionic parasympathetic fibers which travel in the short ciliary nerves and supply the ciliary body and iris. Parasympathetic activation of the M3 muscarinic 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.Template:Citation needed
The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as reading a book.<ref>Template:Cite book</ref>
FunctionEdit
AccommodationEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The ciliary fibers have circular (Ivanoff),<ref>Template:Cite book</ref> longitudinal (meridional) and radial orientations.<ref>Riordan-Eva Paul, "Chapter 1. Anatomy & Embryology of the Eye" (Chapter). Template:Cite book AccessMedicine.com Template:Webarchive</ref>
According to Hermann von Helmholtz's theory, 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. Conversely, relaxation of the ciliary muscle causes the zonular fibers to become taut, flattening the lens, increasing the focal distance,<ref name="goodman">Template:Cite book</ref> 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.<ref name=Kleinmann/>
Trabecular meshwork pore sizeEdit
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.<ref>Salmon John F, "Chapter 11. Glaucoma" (Chapter). Template:Cite book AccessMedicine.com Template:Webarchive</ref>
Clinical significanceEdit
GlaucomaEdit
Open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists (e.g., pilocarpine), which cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork, facilitating drainage of the aqueous humour into the canal of Schlemm and ultimately decreasing intraocular pressure.<ref name="Le">Le, Tao T.; Cai, Xumei; Waples-Trefil, Flora. "QID: 22067". USMLERx. MedIQ Learning, LLC. 2006–2010. 13 January 2010 Usmlerx.com Template:Webarchive</ref>
HistoryEdit
EtymologyEdit
The word ciliary had its origins around 1685–1695.<ref>"cilia", Unabridged. Source location: Random House, Inc. Reference.com. Retrieved on 2010-01-16 from http://dictionary.reference.com/browse/cilia.</ref> The term cilia originated a few years later in 1705–1715, and is the Neo-Latin plural of cilium meaning eyelash. In Latin, cilia means upper eyelid and is perhaps a back formation from supercilium, meaning eyebrow. The suffix -ary originally occurred in loanwords from Middle 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".<ref>Dictionary.com, "-ary", in The American Heritage Dictionary of the English Language, Fourth Edition. Source location: Houghton Mifflin Company, 2004. Reference.com. Retrieved on 2010-01-16 from http://dictionary.reference.com/browse/-ary.</ref> Taken together, cili(a)-ary pertains to various anatomical structures in and around the eye, namely the ciliary body and annular suspension of the lens of the eye.<ref>"ciliary," in Dictionary.com Unabridged. Source location: Random House, Inc. Reference.com. Retrieved on 2010-01-16 from http://dictionary.reference.com/browse/ciliary.</ref>
Additional imagesEdit
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The arteries of the choroid and iris. The greater part of the sclera has been removed.
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Iris, front view.