The ciliary body is a part of the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

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, uvea (vascular layer). It controls accommodation for viewing ob ...

, 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 ciliary body. The ciliary body is part of the uvea, the layer of tissue that delivers

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

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 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 anteri ...

from the

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

. It contains the

, vessels, and fibrous connective tissue. Folds on the inner ciliary epithelium are called ciliary processes, and these secrete aqueous humor into the posterior chamber. The aqueous humor then flows through the pupil 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, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue ...

called the

zonular fibers 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. These fibers are sometimes collec ...

(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, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

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

, and is continuous from the neural tissue of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. The outer layer is highly pigmented, continuous with the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ...

, and constitutes the cells of the

dilator muscle The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike a ...

. 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

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. 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 enteric nervous system is sometimes considered part o ...

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. 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, aqueous humor production and resorption, 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 aqueous humor, which is responsible for providing oxygen, nutrients, and metabolic waste removal to the lens and the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

, which do not have their own blood supply. Eighty percent 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 Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response fu ...

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 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 re ...

is a group of ocular disorders characterized by high

intraocular pressure Intraocular pressure (IOP) is the fluid pressure inside the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response fu ...

-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 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 re ...

. 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, and to protect the heart from a second heart attack after a first heart attack ( secondary prevention). They are a ...

, 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, Levobunolol, and Betaxolol are common beta blockers prescribed to treat glaucoma. * Alpha-adrenergic agonists work by decreasing production of fluid and increasing drainage.

Brimonidine Brimonidine is a 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. Common side effects when used in the eyes include itchiness, re ...

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

also decrease fluid production. They are available as eye drops (

Trusopt Dorzolamide, sold under the brand name Trusopt among others, is a medication used to treat high pressure inside the eye, including in cases of glaucoma. It is used as an eye drop. Effects begin within three hours and last for at least eight hours ...

and Azopt) and pills (

Diamox Acetazolamide, sold under the trade name Diamox among others, is a medication used to treat glaucoma, epilepsy, altitude sickness, periodic paralysis, idiopathic intracranial hypertension (raised brain pressure of unclear cause), urine al ...

and

Neptazane Methazolamide (trade name Neptazane) is a potent carbonic anhydrase inhibitor. It is indicated in the treatment of increased intraocular pressure (IOP) in chronic open-angle glaucoma and secondary glaucoma. Also it is used preoperatively in acu ...

). 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