Schematic diagram of the human eye
|Artery||long posterior ciliary arteries|
The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes.1 It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor.2 The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, 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 ora serrata. The ciliary body is part of the uveal tract— the layer of tissue that provides most of the nutrients in the eye. It extends from the ora serrata to the root of the iris.
There are three sets of ciliary muscles in the eye, the longitudinal, radial, and circular muscles. They are near the front of the eye, above and below the lens. They are attached to the lens by connective tissue called the zonule of Zinn, and are responsible for shaping the lens to focus light on the retina. The ciliary body receives parasympathetic innervation from the oculomotor nerve.
The ciliary body has three functions: accommodation, aqueous humor production and the resorption, and maintenance of the lens zonules. It also anchors the lens in place. 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. One of the essential roles of the ciliary body is also the production of the aqueous humor, which is responsible for providing most of the nutrients for the lens and the cornea and involved in waste management of these areas.
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 affects the rate of ultrafiltration, but not secretion.3
Glaucoma is a group of ocular disorders characterized by high intraocular pressure-associated neuropathies.4 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. Its inhibition leads to the lowering of aqueous humor production and causes a subsequent drop in the intraocular pressure. There 3 main types of medication affecting the ciliary body:56
- Beta blockers, 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 and Apraclonidine are two commonly prescribes alpha agonists for glaucoma treatment. Because Alphagan P bas purite preservative, alpha agonists are especially for those who have allergic reaction to preservatives in other eye drops. Furthermore, less selective alpha agonists such as [epinephrine] may decrease the production of aqueous humor through vasoconstriction of the ciliary body (only for open-angle glaucoma).
- Carbonic anhydrase inhibitors 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.
- Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.
- Lang, G. Ophthalmology: A Pocket Textbook Atlas, 2 ed.. Pg. 207. Ulm, Germany. 2007.
- Murgatroyd, Harry, and Jane Bembridge. "Intraocular Pressure." Continuing Education in Anaesthesia, Critical Care & Pain 8.3 (2013): 100-03. Oxford Journals. Web. 11 Dec. 2013.
- Casson, RJ; Chidlow G., Wood JP, Crowston JG, Goldberg I. (2012). "Definition of glaucoma: clinical and experimental concepts". Clin Exp Ophthalmol 40 (4): 341–9. doi:10.1111/j.1442-9071.2012.02773.x. PMID 22356435.
- "Glaucoma Medications and Their Side Effects." Glaucoma Research Foundation. N.p., n.d. Web. 11 Dec. 2013. <http://www.glaucoma.org/treatment/glaucoma-medications-and-their-side-effects.php>.
- "Medication Guide." Glaucoma Research Foundation. N.p., n.d. Web. 11 Dec. 2013. <http://www.glaucoma.org/treatment/medication-guide.php>.
- Ciliary+body at eMedicine Dictionary
- BU Histology Learning System: 08011loa
- Atlas image: eye_1 at the University of Michigan Health System - "Sagittal Section Through the Eyeball"