Ciliary body | |
---|---|
Details | |
Part of | Eye |
System | Visual system |
Artery | long and short posterior ciliary arteries |
Identifiers | |
Latin | corpus ciliare |
MeSH | D002924 |
TA98 | A15.2.03.009 |
TA2 | 6765 |
FMA | 58295 |
Anatomical terminology |
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 ciliary body. [1] The ciliary body is part of the uvea, the layer of tissue that delivers oxygen and nutrients to the eye tissues. The ciliary body joins the ora serrata of the choroid to the root of the iris. [2]
The ciliary body is a ring-shaped thickening of tissue inside the eye that divides the posterior chamber from the vitreous body. It contains the ciliary muscle, 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. [3]
The ciliary body is attached to the lens by connective tissue called the Zonule of Zinn (fibers of Zinn). Relaxation of the ciliary muscle puts tension on these fibers and changes the shape of the lens in order to focus light on the retina.
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 space between the ciliary body and the base of the iris is the ciliary sulcus. [4]
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) and travel through the ciliary ganglion. Postsynaptic fibers from the ciliary ganglion form the short ciliary nerves. Parasympathetic 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. [5] The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as reading a book. [6]
The ciliary body is also known to receive sympathetic innervation via long ciliary nerves. [7] When test subjects are startled, their eyes automatically adjust for distance vision. [8]
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 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.
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, 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 affects the rate of ultrafiltration, but not secretion. [9]
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.
Glaucoma is a group of ocular disorders characterized by high intraocular pressure-associated neuropathies. [10] 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: [11] [12]
Glaucoma is a group of eye diseases that lead to damage of the optic nerve, which 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 sight" because the loss of vision usually occurs slowly over a long period of time. A major risk factor for glaucoma is increased pressure within the eye, known as intraocular pressure (IOP). It is associated with old age, a family history of glaucoma, and certain medical conditions or medications. The word glaucoma comes from the Ancient Greek word γλαυκός, meaning 'gleaming, blue-green, gray'.
The iris is a thin, annular structure in the eye in most mammals and birds, responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by the iris.
The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Relatively long, thin fiber cells make up the majority of the lens. These cells vary in architecture and are arranged in concentric layers. New layers of cells are recruited from a thin epithelium at the front of the lens, just below the basement membrane surrounding the lens. As a result the vertebrate lens grows throughout life. The surrounding lens membrane referred to as the lens capsule also grows in a systematic way ensuring the lens maintains an optically suitable shape in concert with the underlying fiber cells. Thousands of suspensory ligaments are embedded into the capsule at its largest diameter which suspend the lens within the eye. Most of these lens structures are derived from the epithelium of the embryo before birth.
Eye surgery, also known as ophthalmic surgery or ocular surgery, is surgery performed on the eye or its adnexa. Eye surgery is part of ophthalmology and is performed by an ophthalmologist or eye surgeon. The eye is a fragile organ, and requires due care before, during, and after a surgical procedure to minimize or prevent further damage. An eye surgeon is responsible for selecting the appropriate surgical procedure for the patient, and for taking the necessary safety precautions. Mentions of eye surgery can be found in several ancient texts dating back as early as 1800 BC, with cataract treatment starting in the fifth century BC. It continues to be a widely practiced class of surgery, with various techniques having been developed for treating eye problems.
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 posterior chambers of the eye, and is not to be confused with the vitreous humour, which is located in the space between the lens and the retina, also known as the posterior cavity or vitreous chamber. Blood cannot normally enter the eyeball.
The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object, comprising coordinated changes in vergence, lens shape (accommodation) and pupil size. It is dependent on cranial nerve II, superior centers (interneuron) and cranial nerve III. The change in the shape of the lens is controlled by ciliary muscles inside the eye. Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation. The reflex, controlled by the parasympathetic nervous system, involves three responses: pupil constriction, lens accommodation, and convergence.
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image. Accommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, uvea. It controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm's canal. 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 muscle and dilator pupillae.
The lens capsule is a component of the globe of the eye. It is a clear elastic basement membrane similar in composition to other basement membranes in the body. The capsule is very thick basement membraneand the thickness varies in different areas on the lens surface and with the age of the animal. It and is composed of various types of fibers such as collagen IV laminin etc. and these help it stay under constant tension. The capsule is attached to the surrounding eye by numerous suspensory ligaments and in turn suspends the rest of the lens in an appropriate position. As the lens grows throughout life so must the capsule. Due to the shape of the capsule the lens naturally tends towards a rounder or more globular configuration, a shape it must assume for the eye to focus at a near distance. Tension on the capsule is varied to allow the lens to subtly change shape to allow the eye to focus in a process called accommodation.
Ocular hypertension is the presence of elevated fluid pressure inside the eye, usually with no optic nerve damage or visual field loss.
The zonule of Zinn is a ring of fibrous strands forming a zonule that connects the ciliary body with the crystalline lens of the eye. These fibers are sometimes collectively referred to as the suspensory ligaments of the lens, as they act like suspensory ligaments.
Pigment dispersion syndrome (PDS) is an eye disorder that can lead to a form of glaucoma known as pigmentary glaucoma. It takes place when pigment cells slough off from the back of the iris and float around in the aqueous humor. Over time, these pigment cells can accumulate in the anterior chamber in such a way that they begin to clog the trabecular meshwork, which can in turn prevent the aqueous humour from draining and therefore increases the pressure inside the eye. A common finding in PDS are central, vertical corneal endothelial pigment deposits, known as Krukenberg spindle. With PDS, the intraocular pressure tends to spike at times and then can return to normal. Exercise has been shown to contribute to spikes in pressure as well. When the pressure is great enough to cause damage to the optic nerve, this is called pigmentary glaucoma. As with all types of glaucoma, when damage happens to the optic nerve fibers, the vision loss that occurs is irreversible and painless.
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.
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 anterior to the lens. The posterior chamber is part of the anterior segment and should not be confused with the vitreous chamber.
Glaucoma is a group of diseases affecting the optic nerve that results in vision loss and is frequently characterized by raised intraocular pressure (IOP). There are many glaucoma surgeries, and variations or combinations of those surgeries, that facilitate the escape of excess aqueous humor from the eye to lower intraocular pressure, and a few that lower IOP by decreasing the production of aqueous humor.
A spasm of accommodation is a condition in which the ciliary muscle of the eye remains in a constant state of contraction. Normal accommodation allows the eye to "accommodate" for near-vision. However, in a state of perpetual contraction, the ciliary muscle cannot relax when viewing distant objects. This causes vision to blur when attempting to view objects from a distance. This may cause pseudomyopia or latent hyperopia.
Pseudoexfoliation syndrome, often abbreviated as PEX and sometimes as PES or PXS, is an aging-related systemic disease manifesting itself primarily in the eyes which is characterized by the accumulation of microscopic granular amyloid-like protein fibers. Its cause is unknown, although there is speculation that there may be a genetic basis. It is more prevalent in women than men, and in persons past the age of seventy. Its prevalence in different human populations varies; for example, it is prevalent in Scandinavia. The buildup of protein clumps can block normal drainage of the eye fluid called the aqueous humor and can cause, in turn, a buildup of pressure leading to glaucoma and loss of vision. As worldwide populations become older because of shifts in demography, PEX may become a matter of greater concern.
Canine glaucoma refers to a group of diseases in dogs that affect the optic nerve and involve a loss of retinal ganglion cells in a characteristic pattern. An intraocular pressure greater than 22 mmHg (2.9 kPa) is a significant risk factor for the development of glaucoma. Untreated glaucoma in dogs leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness.
Uveitis–glaucoma–hyphaema (UGH) syndrome, also known as Ellingson syndrome, is a complication of cataract surgery, caused by intraocular lens subluxation or dislocation. The chafing of mispositioned intraocular lens over iris, ciliary body or iridocorneal angle cause elevated intraocular pressure (IOP) anterior uveitis and hyphema. It is most commonly caused by anterior chamber IOLs and sulcus IOLs but, the condition can be seen with any type of IOL, including posterior chamber lenses and cosmetic iris implants.
Hypotony maculopathy is maculopathy due to very low intraocular pressure known as ocular hypotony. Maculopathy occurs either due to increased outflow of aqueous humor through angle of anterior chamber or less commonly, due to decreased aqueous humor secretion by ciliary body.
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