Ciliary muscle
Overview
 
The ciliary muscle is a ring of striated smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 in the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

's middle layer (vascular layer) that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal
Schlemm's canal
Schlemm's canal, also known as canal of Schlemm or the scleral venous sinus, is a circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream via the anterior ciliary veins....

. It changes the shape of the lens within the eye not the size of the pupil which is carried out by the sphincter pupillae muscle. The muscle has parasympathetic and sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 innervation.
The word ciliary had its origins around 1685–1695.
Encyclopedia
The ciliary muscle is a ring of striated smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 in the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

's middle layer (vascular layer) that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal
Schlemm's canal
Schlemm's canal, also known as canal of Schlemm or the scleral venous sinus, is a circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream via the anterior ciliary veins....

. It changes the shape of the lens within the eye not the size of the pupil which is carried out by the sphincter pupillae muscle. The muscle has parasympathetic and sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 innervation.

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 plural
Plural
In linguistics, plurality or [a] plural is a concept of quantity representing a value of more-than-one. Typically applied to nouns, a plural word or marker is used to distinguish a value other than the default quantity of a noun, which is typically one...

 of cilium meaning eyelash
Eyelash
An eyelash or simply lash is one of the hairs that grow at the edge of the eyelid. Eyelashes protect the eye from debris and perform some of the same function as whiskers do on a cat or a mouse in the sense that they are sensitive to being touched, thus providing a warning that an object is near...

. In Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

, cilia means upper eyelid
Eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

 and is perhaps a back formation from supercilium, meaning eyebrow
Eyebrow
The eyebrow is an area of thick, delicate hairs above the eye that follows the shape of the lower margin of the brow ridges of some mammals. Their main function is to prevent sweat, water, and other debris from falling down into the eye socket, but they are also important to human communication and...

. The suffix -ary originally occurred in loanwords from Middle English
Middle English
Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 (-arie), Old French
Old French
Old French was the Romance dialect continuum spoken in territories that span roughly the northern half of modern France and parts of modern Belgium and Switzerland from the 9th century to the 14th century...

 (-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 ciliary body
Ciliary body
The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor. The inner layer is transparent and covers...

 and annular suspension of the lens of the eye.

Embryologic development

Forms from the mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

 within the choroid
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...

 and considered a cranial neural crest
Neural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....

 derivative.

Accommodation

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

According to Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...

's theory, the circular ciliary muscle fibers affect zonular fibers
Zonule of Zinn
The zonule of Zinn is a ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye....

 in the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 (fibers that suspend the lens
Lens (anatomy)
The 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. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 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 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 Ronald A. Schachar.

Trabecular meshwork pore size

Contraction and relaxation of the longitudinal fibers, which insert into the trabecular meshwork
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 tissue is spongy and lined by trabeculocytes; it allows fluid to drain into a set of...

 in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impeding aqueous humour
Aqueous humour
The aqueous humour is a clear, gelatinous fluid similar to plasma, but containing low-protein concentrations. It is secreted from the ciliary epithelium, a structure supporting the lens. It is located in the space between the lens and the cornea...

 flow into the canal of Schlemm.

Innervation

The ciliary muscle receives both parasympathetic and sympathetic fibers from the ciliary ganglion
Ciliary ganglion
The ciliary ganglion is a parasympathetic ganglion located in the posterior orbit. It measures 1–2 millimeters in diameter and contains approximately 2,500 neurons. Preganglionic axons from the Edinger-Westphal nucleus travel along the oculomotor nerve and form synapses with these cells...

 called short ciliary nerves
Short ciliary nerves
The branches of the ciliary ganglion are the short ciliary nerves.These are delicate filaments, from six to ten in number, which arise from the forepart of the ganglion in two bundles connected with its superior and inferior angles; the lower bundle is the larger.They run forward with the ciliary...

. These postganglionic fibers are part of cranial nerve III (Oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

).
Postsynaptic sympathetic signals that originate in the superior cervical ganglion
Superior cervical ganglion
The superior cervical ganglion , the largest of the cervical ganglia, is placed opposite the second and third cervical vertebræ. It contains neurons that supply sympathetic innervation to the face....

 are carried by the nasociliary nerve or directly extend from the internal carotid plexus
Internal carotid plexus
The internal carotid plexus is situated on the lateral side of the internal carotid artery, and in the plexus there occasionally exists a small gangliform swelling, the carotid ganglion, on the under surface of the artery....

 and pass through the ciliary ganglion. Sympathetic (adrenergic
Adrenergic
An adrenergic agent is a drug, or other substance, which has effects similar to, or the same as, epinephrine . Thus, it is a kind of sympathomimetic agent...

) activation of the muscle's beta-2 receptors result in relaxation and increase in ciliary body size.

Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

 are carried by cranial nerve III (the oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

) and synapse on the ciliary ganglion
Ciliary ganglion
The ciliary ganglion is a parasympathetic ganglion located in the posterior orbit. It measures 1–2 millimeters in diameter and contains approximately 2,500 neurons. Preganglionic axons from the Edinger-Westphal nucleus travel along the oculomotor nerve and form synapses with these cells...

. Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction and consequent reduction in the size of the ciliary body.

The adrenergic
Adrenergic
An adrenergic agent is a drug, or other substance, which has effects similar to, or the same as, epinephrine . Thus, it is a kind of sympathomimetic agent...

 tone is dominant over the parasympathetic tone.

Role in the treatment of glaucoma

Open-angle glaucoma
Glaucoma
Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

 (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists (e.g., pilocarpine
Pilocarpine
Pilocarpine is a parasympathomimetic alkaloid obtained from the leaves of tropical American shrubs from the genus Pilocarpus. It is a non-selective muscarinic receptor agonist in the parasympathetic nervous system, which acts therapeutically at the muscarinic acetylcholine receptor M3 due to its...

), which cause rapid miosis
Miosis
Miosis is the constriction of the pupil of the eye to two millimeters or less...

 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
Intraocular pressure 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 from glaucoma...

.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK