Logan Steele
The ciliary muscle is an intrinsic muscle of the eye that controls the eye's ability to accommodate objects at varying distances by changing the shape of the lens. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle contracts and relaxes to change the thickness and curvature of the lens, thereby altering the eye's power of focus at different distances. This process is known as accommodation. The ciliary muscle receives parasympathetic innervation from the short ciliary nerves, which arise from the ciliary ganglion and are part of the oculomotor nerve.
| 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) |
| Location | Within the ciliary body of the eye |
| Shape | Triangle-shaped region of smooth muscle fibers |
| Function | Controls the shape of the lens, regulates the flow of aqueous humor into Schlemm's canal, and changes the shape of the lens within the eye |
| Innervation | Parasympathetic innervation from short ciliary nerves, arising from the oculomotor nerve via the ciliary ganglion |
| Blood Supply | Branches of the ophthalmic artery |
| Accommodation | Controls the eye's ability to focus at different distances |
| Glaucoma Treatment | Muscarinic receptor agonists cause contraction of the ciliary muscles, facilitating drainage of aqueous humor and reducing intraocular pressure |
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What You'll Learn
Ciliary muscle contraction
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 controls the accommodation of the lens for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. The ciliary muscle does not change the size of the pupil, which is carried out by the sphincter pupillae muscle and dilator pupillae.
The ciliary muscle is composed of a series of muscle fibres arranged in longitudinal, circular, and radial directions. These fibres can contract to change the shape of the lens and provide different focal lengths. The ciliary muscle is the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. The ciliary body is a 5-6mm wide ring of tissue extending from the scleral spur anteriorly to the ora serrata posteriorly.
When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibres (fibers that hold or flatten the lens). The release of tension of the zonular fibres causes the lens to become more spherical, adapting to short-range focus. Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens, increasing the focal distance, and adapting to long-range focus.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion receives preganglionic fibres via the oculomotor nerve (cranial nerve III) that originate from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. The ciliary muscle is also involved in the treatment of open-angle glaucoma (OAG) and closed-angle glaucoma (CAG). Muscarinic receptor agonists, such as pilocarpine, cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork and facilitating drainage of the aqueous humour into the canal of Schlemm, ultimately decreasing intraocular pressure.
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Ciliary muscle relaxation
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 responsible for controlling accommodation for viewing objects at varying distances and regulating the flow of aqueous humour into Schlemm's canal. The ciliary muscle also changes the shape of the lens within the eye but not the size of the pupil, which is controlled by the sphincter pupillae muscle and dilator pupillae.
The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. It occupies the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. The ciliary body itself is a 5-6mm wide ring of tissue extending from the scleral spur anteriorly to the ora serrata posteriorly.
When the ciliary muscle contracts, it pulls itself forward, moving the frontal region towards the axis of the eye. This releases the tension on the lens caused by the zonular fibres, allowing the lens to become more spherical and adapt to short-range focus.
Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens and increasing the focal distance, which improves long-range focus. This process is essential for the eye to change power and focus at different distances, known as "accommodation". The ciliary muscle's relaxation and contraction are controlled by the oculomotor nerve.
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Ciliary muscle innervation
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 controls the eye's accommodation for viewing objects at varying distances and regulates the flow of aqueous humour 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 ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion is a parasympathetic ganglion that is located behind the eye and contains about 2,500 neurons. The ciliary ganglion receives preganglionic fibres via the oculomotor nerve (cranial nerve III) that originate from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. 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 (the oculomotor nerve) and travel through the ciliary ganglion via the postganglionic parasympathetic fibres, which travel in the short ciliary nerves and supply the ciliary body and iris.
The ciliary muscle, together with the sphincter pupillae, functions mainly via the parasympathetic nerve fibres of the oculomotor nerve (CN III). Contraction of the ciliary muscle loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision. It also regulates the pore size of the trabecular meshwork. The ciliary muscle alters the tightness 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.
There is some evidence that the ciliary muscle also receives innervation from the sympathetic fibres of the autonomic nervous system (ANS). These fibres are thought to provide inhibitory impulses that inhibit the accommodation reflex.
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Ciliary muscle fibres
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 controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour 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 ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The outermost layer of the ciliary muscle is the longitudinal layer, which is situated adjacent to the loose connective tissue of the ciliary body. The middle layer is the radial layer of muscle, which represents the transition from the longitudinal to the circular muscle layer. The innermost muscle layer is the circular layer, which functions as a sphincter of the eye. This layer is typically well-developed in hypermetropic (long-sighted) eyes, but rudimentary or absent in myopic (short-sighted) eyes.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion receives preganglionic fibres via the oculomotor nerve (cranial nerve III) that originate from the Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. There is also evidence that the ciliary muscle receives innervation from the sympathetic fibres of the autonomic nervous system (ANS).
The ciliary muscle contracts and relaxes to change the shape of the lens, enabling the eye to focus at different distances. When the ciliary muscle contracts, it pulls itself forward, releasing the tension on the lens caused by the zonular fibres. This causes the lens to become more spherical, adapting to short-range focus. Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens and increasing the focal distance for long-range focus.
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Ciliary muscle and glaucoma treatment
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 controls the accommodation of objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. The ciliary muscle 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 ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The ciliary muscle functions mainly through the parasympathetic nerve fibres of the oculomotor nerve (CN III). Contraction of the ciliary muscle loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision. It also regulates the pore size of the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye.
Glaucoma is a group of ocular disorders characterised by high intraocular pressure-associated neuropathies. Intraocular pressure depends on the levels of production and resorption of aqueous humour. The ciliary body produces aqueous humour, so it is the main target of many medications against glaucoma. In angle-closure glaucoma, the iris presses against the meshwork and inhibits drainage of the aqueous humour out of the anterior chamber of the eye, causing a rapid increase in intraocular pressure.
Both open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) can be treated with muscarinic receptor agonists, such as pilocarpine, that cause the ciliary muscle to contract and lead to the opening of the trabecular meshwork. Once the pores in the trabecular meshwork have opened, the aqueous humour can flow through the canal of Schlemm, decreasing intraocular pressure. Beta-blockers are the second most common treatment method for glaucoma, as they reduce the production of aqueous humour. They are relatively inexpensive and available in generic form. Timolol, Levobunolol, and Betaxolol are common beta-blockers prescribed to treat glaucoma.
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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 middle layer, the uvea (vascular layer).
The ciliary muscle controls 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 contracts and relaxes to change the thickness and curvature of the lens, enabling the eye to change power and focus at different distances.
The ciliary body is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. It includes the ciliary muscle and the finger-like ciliary processes.
When the ciliary muscle contracts, it pulls itself forward, releasing tension on the lens caused by the zonular fibres, allowing the lens to become more spherical and increasing its power to refract light for near vision.
