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The ciliary muscle is an intrinsic muscle of the eye that is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It is an important part of the eye that controls aspects of near and far vision. The ciliary muscle controls the tightening and loosening of the zonular fibres by contracting and relaxing, thereby altering the shape of the lens and leading to the adaptation to near or far vision.
| Characteristics | Values |
|---|---|
| Definition | An intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). |
| Shape | Elongated and triangular. |
| Location | Beneath the anterior sclera just posterior to the limbus. |
| Function | Controls accommodation for viewing objects at varying distances, regulates the flow of aqueous humor into Schlemm's canal, and changes the shape of the lens within the eye. |
| Composition | Consists of nonpigmented epithelial cells (NPE), pigmented epithelial cells (PE), and the ciliary muscle. |
| Blood Supply | Various branches of the ophthalmic artery, including the central artery of the retina, short and long posterior ciliary arteries, and anterior ciliary arteries. |
| Innervation | Innervated by short ciliary nerves that arise from the ciliary ganglion, a parasympathetic ganglion located behind the eye. |
| Contraction | Activation of M3 muscarinic receptors causes contraction, reducing the diameter of the ciliary muscle ring. |
| Treatment of Glaucoma | Muscarinic receptor agonists cause contraction of the ciliary muscles, facilitating drainage of aqueous humor and decreasing intraocular pressure. |
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What You'll Learn
Ciliary muscle controls near and far vision
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It is an important part of the eye that controls aspects of near and far vision.
The ciliary muscle controls the tightening and loosening of the zonular fibres by contracting and relaxing, thereby altering the shape of the lens and leading to the adaptation to near or far vision. The ciliary muscle, along with the ciliary body, produces aqueous humour and stabilises the lens. The ciliary body is located behind the iris of the eye and is shaped like a ring. It has pleats or ridges that flare out, called ciliary processes. These ciliary processes secrete aqueous fluid and point towards the iris. The ciliary body's zonular fibres provide support to the lens in the eye.
The ciliary muscle also contracts and relaxes its longitudinal fibres to increase and decrease the size of the pore in the trabecular meshwork. The trabecular meshwork is responsible for aqueous humour drainage in the anterior portion of the eye. Changing the pore size can impede or facilitate aqueous humour flow in the canal of Schlemm. The ciliary muscle receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion. The parasympathetic postganglionic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal).
Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (oculomotor nerve) and travel through the ciliary ganglion via the postganglionic parasympathetic fibres. These fibres travel in the short ciliary nerves and supply the ciliary body and iris. 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, causing relaxation of the zonule fibres. The lens becomes more spherical, increasing its power to refract light for near vision. The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as when reading a book.
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Ciliary body makes aqueous fluid
The ciliary muscle is an intrinsic muscle of the eye, formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle, together with the ciliary epithelium (CE) and the ciliary processes, constitute the ciliary body (CB). The ciliary epithelium is the site of aqueous humour secretion, with the ciliary body producing aqueous humour in the eye's posterior chamber.
The ciliary body is a part of the uvea, the layer of tissue that delivers oxygen and nutrients to the eye tissues. It is a ring-shaped thickening of tissue inside the eye that divides the posterior chamber from the vitreous body. The ciliary body joins the ora serrata of the choroid to the root of the iris. The ciliary muscle controls the shape of the lens and is involved in viewing objects at varying distances. It does this by contracting and relaxing its longitudinal fibres, which in turn alter the tightness of the zonular fibres that attach to the lens.
The ciliary epithelium is comprised of two polarized neuroepithelial cell layers, pigmented (PE) and non-pigmented (NPE), apposing each other by their respective apical plasma membranes. The non-pigmented portion of the ciliary body is where aqueous humour is produced. The ciliary processes are protrusions on the internal surface of the ciliary body, which contain blood capillaries that secrete aqueous humour. The ciliary processes are linear folds projecting from the ciliary body into the space behind the iris, where the zonular fibres and ciliary muscle attach to the eyeball.
The aqueous humour is a clear liquid that fills the anterior and posterior chambers of the eye, in front of the vitreous humour. The aqueous humour is a slightly alkaline, low-viscosity ocular fluid formed by the epithelial cells of the ciliary body. It provides nourishment and oxygen to the avascular tissues of the eye, such as the cornea and the lens. It also removes waste products, blood, macrophages, and other debris from the cornea and lens, while maintaining the eyeball's shape and pressure. The aqueous humour flows in and out of the front of the eye, with the balance between secretion and reabsorption regulating the volume and pressure of the intraocular fluid.
The ciliary body is targeted by many medications against glaucoma, as its inhibition leads to a lowering of aqueous humour production and a subsequent drop in intraocular pressure.
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Ciliary muscle controls the shape of the lens
The ciliary muscle is an intrinsic muscle of the eye, formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It is located behind the iris, the coloured part of the eye. The ciliary muscle is an important part of the eye that controls aspects of near and far vision.
The ciliary muscle controls the shape of the lens. It does this by contracting and relaxing, thereby altering the shape of the lens and leading to adaptation to near or far vision. The muscle changes the tightness of the zonular fibres by contracting and relaxing, which are suspensory ligaments that attach to the lens.
The ciliary body, which includes the ciliary muscle, makes aqueous fluid, which nourishes the lens and cornea. The ciliary body also helps the lens focus. The ciliary body consists of non-pigmented epithelial cells, pigmented epithelial cells, and the ciliary muscle, which together produce aqueous humour and stabilise the lens. The ciliary epithelium is the site of aqueous humour secretion. It is comprised of two polarised neuroepithelial cell layers, pigmented and non-pigmented, which oppose each other by their respective apical plasma membranes.
The ciliary muscle also contracts and relaxes its longitudinal fibres to increase and decrease the size of the pore in the trabecular meshwork. The trabecular meshwork is responsible for aqueous humour drainage in the anterior portion of the eye. Changing the pore size can impede or facilitate aqueous humour flow in the canal of Schlemm.
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Ciliary muscle is located in the middle layer of the eye
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary body, which includes the ciliary muscle, is located behind the iris, the coloured part of the eye. The iris, ciliary body, and choroid are connected and make up the uvea. The ciliary body is shaped like a ring and has ridges or pleats called ciliary processes, which secrete aqueous fluid and point towards the iris.
The ciliary muscle is elongated and triangular in shape. It is located beneath the anterior sclera, just posterior to the limbus. The ciliary muscle controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It 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 ciliary muscle, along with the ciliary body, produces aqueous humour and stabilizes the lens. The ciliary muscle controls the tightness of the zonular fibres by contracting and relaxing, thereby altering the shape of the lens and leading to adaptation to near or far vision. The ciliary muscle contracts and relaxes its longitudinal fibres to increase and decrease the size of the pore in the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye.
The ciliary muscle is an important part of the eye that controls aspects of near and far vision. The ciliary muscle receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion. The parasympathetic postganglionic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal).
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Ciliary muscle contracts and relaxes
The ciliary muscle is an intrinsic muscle of the eye, formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle contracts and relaxes, altering the tightness of the zonular fibres that are attached to the lens. The zonular fibres are like suspensory ligaments, connecting the ciliary muscle to the lens. When the ciliary muscle contracts, the zonular fibres relax, and the lens becomes more spherical, improving near vision. This change in lens shape is essential for focusing on objects at varying distances.
The contraction of the ciliary muscle is triggered by the activation of M3 muscarinic receptors, which are acetylcholine receptors formed by G-protein complexes. This activation causes a reduction in the diameter of the ciliary muscle ring, allowing the zonular fibres to loosen their grip on the lens. The ciliary muscle also has longitudinal fibres that can contract and relax to alter the size of the pore in the trabecular meshwork. By adjusting this pore size, the ciliary muscle can regulate the drainage of aqueous humour in the anterior portion of the eye, specifically in the canal of Schlemm.
The ciliary muscle is part of the ciliary body, which also includes the ciliary processes and ciliary muscles. The ciliary body is located behind the iris, the coloured part of the eye, and it is not visible. The ciliary processes are protrusions or folds on the internal surface of the ciliary body, and they contain blood capillaries that secrete aqueous humour. This aqueous humour, a clear fluid, nourishes the lens and cornea, and its constant outflow from the anterior chamber of the eye helps maintain intraocular pressure.
The ciliary body plays a crucial role in controlling the shape of the lens. It works in conjunction with the iris to ensure that light reaches the correct spot on the retina. By contracting and relaxing, the ciliary muscle enables the eye to adapt to near or far vision. This adaptive capability is vital for maintaining clear vision and overall eye health.
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Frequently asked questions
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's vascular layer, known as the uvea.
The ciliary muscle controls the tightening and loosening of the zonular fibres by contracting and relaxing, thereby altering the shape of the lens and leading to adaptation to near or far vision.
The ciliary muscle controls accommodation for viewing objects at varying distances. When the ciliary muscle contracts, the lens becomes more spherical, increasing its power to refract light for near vision.
The ciliary muscle is part of the ciliary body, which produces aqueous humour (or aqueous fluid) to nourish the lens and cornea, and to regulate intraocular pressure.
