The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer,

uvea The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye. History and etymolog ...

( vascular layer). It controls accommodation for viewing objects at varying distances and regulates the flow of

aqueous humor The aqueous humour is a transparent water-like fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the posteri ...

into

Schlemm's canal Schlemm's canal is a circular lymphatic-like vessel in the eye. It collects aqueous humor from the anterior chamber and delivers it into the episcleral blood vessels. Canaloplasty may be used to widen it. Structure Schlemm's canal is an endot ...

. It also changes the shape of the lens within the eye but not the size of the pupil which is carried out by the

sphincter pupillae 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 ...

muscle and dilator pupillae.

Structure

Development

The ciliary muscle develops from mesenchyme within the choroid and is considered a cranial

neural crest Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...

derivative.Dudek RW, Fix JD (2004). "Eye" (chapter 9). ''Embryology - Board Review Series'' (3rd edition, illustrated). Lippincott Williams & Wilkins. p. 92. , . Books.Google.com. Retrieved on 2010-01-17 from https://books.google.com/books?id=MmoJQWsJteoC.

Nerve supply

The ciliary muscle receives parasympathetic fibers from the

short ciliary nerves The short ciliary nerves are nerves of the orbit around the eye. They are branches of the ciliary ganglion. They supply parasympathetic and sympathetic nerve fibers to the ciliary muscle, iris, and cornea. Damage to the short ciliary nerve ma ...

that arise from the

ciliary ganglion The ciliary ganglion is a bundle of nerve parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympath ...

. 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. The postganglionic parasympathetic innervation arises from the ciliary ganglion. Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the oculomotor nerve) and travel through the

via the postganglionic parasympathetic fibers which travel in the

and supply the

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...

and iris. Parasympathetic activation of the M3

muscarinic receptors Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-rec ...

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. The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as reading a book.

Function

Accommodation

The ciliary fibers have circular (Ivanoff), longitudinal ( meridional) and radial orientations. According to

Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (31 August 1821 – 8 September 1894) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The Helmholtz Associatio ...

's theory, the circular ciliary muscle fibers affect zonular fibers in the eye (fibers that suspend the

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

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, 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.

Trabecular meshwork pore size

Contraction and relaxation of the longitudinal fibers, which insert into the

trabecular meshwork The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the ...

in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impeding

aqueous humour The aqueous humour is a transparent water-like fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the poster ...

flow into the canal of Schlemm.

Clinical significance

Glaucoma

Open-angle

glaucoma Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...

(OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists (e.g.,

pilocarpine Pilocarpine is a medication used to reduce pressure inside the eye and treat dry mouth. As eye drops it is used to manage angle closure glaucoma until surgery can be performed, ocular hypertension, primary open angle glaucoma, and to bring abo ...

), which cause rapid

miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...

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 Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated t ...

.Le, Tao T.; Cai, Xumei; Waples-Trefil, Flora. "QID: 22067". USMLERx. MedIQ Learning, LLC. 2006–2010. 13 January 201
Usmlerx.com

History

Etymology

The word ''ciliary'' had its origins around 1685–1695. The term '' cilia'' originated a few years later in 1705–1715, and is the

Neo-Latin New Latin (also called Neo-Latin or Modern Latin) is the revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy ...

plural The plural (sometimes abbreviated pl., pl, or ), in many languages, is one of the values of the grammatical category of number. The plural of a noun typically denotes a quantity greater than the default quantity represented by that noun. This de ...

of ''cilium'' meaning

eyelash An eyelash (also called lash) (Latin: ''Cilia'') is one of the hairs that grows at the edge of the eyelids. It grows in one layer on the edge of the upper and lower eyelids. Eyelashes protect the eye from debris, dust, and small particles and p ...

. In

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

, ''cilia'' means upper

eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyel ...

and is perhaps a

back formation In etymology, back-formation is the process or result of creating a new word via inflection, typically by removing or substituting actual or supposed affixes from a lexical item, in a way that expands the number of lexemes associated with the ...

from ''supercilium'', meaning

eyebrow An eyebrow is an area of short hairs above each eye that follows the shape of the lower margin of the brow ridges of some mammals. In humans, eyebrows serve two main functions: first, communication through facial expression, and second, preven ...

. The suffix ''-ary'' originally occurred in loanwords from

Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English p ...

(''-arie''),

Old French Old French (, , ; Modern French: ) was the language spoken in most of the northern half of France from approximately the 8th to the 14th centuries. Rather than a unified language, Old French was a linkage of Romance dialects, mutually intelligib ...

(''-er'', ''-eer'', ''-ier'', -''aire'', -er), and Latin (''-ārius''); it can generally mean "pertaining to, connected with", "contributing to", and "for the purpose of". Taken together, ''cili(a)-ary'' pertains to various anatomical structures in and around the eye, namely the

and annular suspension of the

lens of the eye The lens, or crystalline lens, is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it ca ...

."ciliary," in Dictionary.com Unabridged. Source location: Random House, Inc. Reference.com. Retrieved on 2010-01-16 from http://dictionary.reference.com/browse/ciliary.

Additional images

File:Gray873.png, The arteries of the choroid and iris. The greater part of the sclera has been removed. File:Gray878.png, Iris, front view.

References

External links

Lens, zonule fibers, and ciliary muscles—SEM
{{DEFAULTSORT:Ciliary Muscle Muscles of the head and neck Human eye anatomy