The ciliary body is a part of the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

that includes the

ciliary muscle 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, the uvea ( vascular layer). It controls accommodation for vie ...

, which controls the shape of the lens, and the ciliary epithelium, which produces the

aqueous humor The aqueous humour is a transparent water-like fluid similar to blood 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 po ...

. The aqueous humor is produced in the non-pigmented portion of the ciliary body. The ciliary body is part of the

uvea The uvea (; derived from meaning "grape"), also called the uveal layer, uveal coat, uveal tract, vascular tunic or vascular layer, is the pigmented middle layer of the three concentric layers that make up an eye, precisely between the inne ...

, the layer of tissue that delivers

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and nutrients to the eye tissues. The ciliary body joins the

ora serrata The ora serrata is the serrated junction between the choroid and the ciliary body. This junction marks the transition from the simple, non-photosensitive area of the ciliary body to the complex, multi-layered, photosensitive region of the retina. ...

of the

choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear o ...

to the root of the iris.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company, 1990.

Structure

The ciliary body is a ring-shaped thickening of tissue inside the

that divides the

posterior chamber The posterior chamber is a narrow space behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes. The posterior chamber consists of small space directly posterior to the iris but ante ...

from the

vitreous body The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the Lens (vision), lens and the retina of the eye, eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as ...

. It contains the

, vessels, and fibrous connective tissue. Folds on the inner ciliary epithelium are called

ciliary processes In the anatomy of the eye, the ciliary processes are formed by the inward folding of the various layers of the choroid, viz. the choroid proper and the lamina basalis, and are received between corresponding foldings of the suspensory ligam ...

, and these secrete aqueous humor into the posterior chamber. The aqueous humor then flows through the iris into the anterior chamber. The ciliary body is attached to the lens by

connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...

called the

Zonule of Zinn The zonule of Zinn () (Zinn's membrane, ciliary zonule) (after Johann Gottfried Zinn) is a ring of fibrous strands forming a zonule (little band) that connects the ciliary body with the crystalline lens of the eye. The Zonular fibers are viscoela ...

(fibers of Zinn). Relaxation of the ciliary muscle puts tension on these fibers and changes the shape of the lens in order to

focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

on the retina. The inner layer is transparent and covers the

, and is continuous from the neural tissue of the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

. The outer layer is highly pigmented, continuous with the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...

, and constitutes the cells of the dilator muscle. This double membrane is often considered continuous with the retina and a rudiment of the embryological correspondent to the retina. The inner layer is unpigmented until it reaches the iris, where it takes on pigment. The retina ends at the

. The space between the ciliary body and the base of the iris is the ciliary sulcus.

Nerve supply

The parasympathetic innervation of the ciliary body is the most clearly understood. Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the

oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements o ...

) and travel through the

ciliary ganglion The ciliary ganglion is a 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 parasympathetic neurons. T ...

. Postsynaptic fibers from the ciliary ganglion form the short ciliary nerves.

Parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

activation of the M3

muscarinic receptors Muscarinic acetylcholine receptors (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-recep ...

causes ciliary muscle contraction, the effect of contraction is to decrease the diameter of the ring of ciliary muscle. The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as reading a book. The ciliary body is also known to receive sympathetic innervation via long ciliary nerves. When test subjects are startled, their eyes automatically adjust for distance vision.

Function

The ciliary body has three functions: accommodation,

production, and

resorption Resorption is the absorption of cells or tissue into the circulatory system, usually by osteoclasts. Types of resorption include: * Bone resorption Bone resorption is resorption of bone tissue, that is, the process by which osteoclasts break ...

, and maintenance of the lens zonules for the purpose of anchoring the lens in place.

Accommodation

Accommodation essentially means that when the ciliary muscle contracts, the lens becomes more convex, generally improving the focus for closer objects. When it relaxes, it flattens the lens, generally improving the focus for farther objects.

Aqueous humor

The ciliary epithelium of the ciliary processes produces

, which is responsible for providing oxygen, nutrients, and metabolic waste removal to the lens and the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

, which do not have their own blood supply. Approximately 80% of aqueous humor production is carried out through active secretion mechanisms (the Na+K+ATPase enzyme creating an osmotic gradient for the passage of water into the posterior chamber) and twenty percent is produced through the ultrafiltration of plasma.

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 ...

affects the rate of ultrafiltration, but not secretion.

Lens zonules

The zonular fibers collectively make up the suspensory ligament of the lens. These provide strong attachments between the ciliary muscle and the capsule of the lens.

Clinical significance

Glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

is a group of ocular disorders characterized by high

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 ...

-associated neuropathies. Intraocular pressure depends on the levels of production and resorption of aqueous humor. Because the ciliary body produces aqueous humor, it is the main target of many medications against

glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

. Its inhibition leads to the lowering of aqueous humor production and causes a subsequent drop in the

. There 3 main types of medication affecting the ciliary body: *

Beta blockers Beta blockers, also spelled β-blockers, are a class of medications that are predominantly used to manage abnormal heart rhythms (arrhythmia), and to protect the heart from a second heart attack after a first heart attack (secondary prevention). ...

, the second most common treatment method for glaucoma, reduce the production of aqueous humor. They are relatively inexpensive and are available in generic form.

Timolol Timolol is a beta blocker medication used either by mouth or as eye drops. As eye drops it is used to treat increased pressure inside the eye such as in ocular hypertension and glaucoma. By mouth it is used for high blood pressure, chest ...

, Levobunolol, and Betaxolol are common beta blockers prescribed to treat glaucoma. *

Alpha-adrenergic agonist Alpha-adrenergic agonists are a class of sympathomimetic agents that selectively stimulates alpha adrenergic receptors. The alpha-adrenergic receptor has two subclasses α1 and α2. Alpha 2 receptors are associated with sympatholytic properties ...

s work by decreasing production of fluid and increasing drainage.

Brimonidine Brimonidine is an α2 agonist medication used to treat open-angle glaucoma, ocular hypertension, and rosacea. In rosacea it improves the redness. It is used as eye drops or applied to the skin. It is also available in the fixed-dose combinatio ...

and

Apraclonidine Apraclonidine (INN), also known under the brand name Iopidine, is a sympathomimetic used in glaucoma therapy. It is an α2 adrenergic receptor agonist and a weak α1 adrenergic receptor agonist. Topical apraclonidine is administered at a conc ...

are two commonly prescribes alpha agonists for glaucoma treatment. Alphagan P uses a purite preservative, which is better tolerated by those who have allergic reactions than the older BAK preservative in other eye drops. Furthermore, less selective alpha agonists such as

epinephrine Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands a ...

may decrease the production of aqueous humor through vasoconstriction of the ciliary body (only for open-angle glaucoma). *

Carbonic anhydrase inhibitors Carbonic anhydrase inhibitors are a class of pharmaceuticals that suppress the activity of carbonic anhydrase. Their clinical use has been established as anti-glaucoma agents, diuretics, antiepileptics, in the management of mountain sickness, g ...

also decrease fluid production. They are available as eye drops ( Trusopt and Azopt) and pills ( Diamox and Neptazane). This may be helpful if using more than one type of eye medication.

References

External links

* * - "Sagittal Section Through the Eyeball" {{Authority control Human eye anatomy