Dominic Stone
The ciliary muscle is an intrinsic muscle of the eye that facilitates the accommodation reflex and enables us to view objects at varying distances. The ciliary muscle is located within the ciliary body of the eye and is innervated by the parasympathetic nerve fibers of the oculomotor nerve (CN III). The ciliary body is a part of the eye that includes the ciliary muscle and the ciliary epithelium, which produces the aqueous humor. The ciliary muscle controls the shape of the lens, and its contraction facilitates lens accommodation for near vision. The contraction of the ciliary muscle also aids in the treatment of glaucoma by decreasing intraocular pressure.
| Characteristics | Values |
|---|---|
| Innervation | Parasympathetic fibers from the short ciliary nerves |
| Nerve Origin | Ciliary ganglion |
| Pre-synaptic Nerve Pathway | Edinger-Westphal nucleus to oculomotor nerve (CN III) |
| Post-ganglionic Nerve Pathway | Short ciliary nerves |
| Post-synaptic Nerve Pathway | Cranial nerve V1 (Nasociliary nerve of the trigeminal) |
| Receptor | M3 muscarinic receptors |
| Effect of Contraction | Decreased diameter of ciliary muscle ring, relaxation of zonular fibers, increased lens convexity, and improved near vision |
| Effect of Relaxation | Taut zonular fibers, flattened lens, improved long-range focus |
| Additional Functions | Increase pore diameter of trabecular meshwork, aid drainage of aqueous humor, treat glaucoma |
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What You'll Learn
- The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens
- It receives parasympathetic innervation from the short ciliary nerves, arising from the oculomotor nerve
- The ciliary muscle contraction facilitates lens accommodation for near vision
- The ciliary body, which includes the ciliary muscle, produces aqueous humour
- Glaucoma is treated with muscarinic receptor agonists that cause the contraction of the ciliary muscle
The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens
The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The outermost layer is the longitudinal muscle layer, followed by the radial layer, and finally, the innermost layer is the circular layer (Müller's muscle). The ciliary muscle acts on the lens by changing its shape, a process known as accommodation. This is achieved through the contraction and relaxation of the ciliary muscle, which affects the tension on the zonular fibres that suspend the lens. When the ciliary muscle contracts, it pulls itself forward, releasing the tension on the zonular fibres and allowing the lens to become more spherical, increasing its power to refract light for near vision. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range focus.
The ciliary muscle receives its innervation from the parasympathetic nervous system. Specifically, it is supplied by the short ciliary nerves, which arise from the ciliary ganglion. The parasympathetic postganglionic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal). Additionally, the presynaptic parasympathetic fibres to the ciliary ganglia travel with the oculomotor nerve (CN III). Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction, leading to a decrease in the diameter of the ciliary muscle ring and subsequent relaxation of the zonular fibres.
The ciliary muscle plays a crucial role in the accommodation reflex, which is essential for focusing on objects located near the eye. The contraction of the ciliary muscle increases the convexity of the lens, improving the focus for closer objects. This reflex is often targeted by medications for the treatment of glaucoma, a group of ocular disorders characterised by high intraocular pressure. By inhibiting the ciliary body's production of aqueous humour, medications can help reduce intraocular pressure and manage glaucoma.
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It receives parasympathetic innervation from the short ciliary nerves, arising from the oculomotor nerve
The ciliary muscle is an intrinsic muscle of the eye that facilitates accommodation for near vision. It is located within the ciliary body of the eye, between the anterior border of the choroid and iris. The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves, which arise from the oculomotor nerve via the ciliary ganglion. The parasympathetic postganglionic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal). Presynaptic parasympathetic fibres to the ciliary ganglia travel with the oculomotor nerve. The postganglionic parasympathetic innervation arises from the ciliary ganglion.
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 fibres from the ciliary ganglion form the short ciliary nerves. Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction. This contraction decreases the diameter of the ring of ciliary muscle, causing relaxation of the zonule fibres. As a result, the lens becomes more spherical, increasing its power to refract light and improving focus for closer objects.
The ciliary muscle, together with the sphincter pupillae, is primarily instructed by the parasympathetic nerve fibres of the oculomotor nerve (CN III). The contraction of the ciliary muscle loosens the zonular fibres, increasing the convexity of the lens and inducing accommodation for near vision. It also regulates the pore size of the trabecular meshwork, aiding in the drainage of aqueous humour into the canal of Schlemm.
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The ciliary muscle contraction facilitates lens accommodation for near vision
The ciliary muscle is an intrinsic muscle of the eye that participates in the accommodation reflex. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle occupies the biggest portion of the ciliary body, which lies between the anterior border of the choroid and iris. It is composed of smooth muscle fibers oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle controls accommodation for viewing objects at varying distances. It changes the shape of the lens within the eye, adapting to short-range focus. 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 fibers, which hold or flatten the lens. As a result, the lens becomes more spherical, increasing its power to refract light for near vision. This process is known as accommodation, where the ciliary muscle excursion relieves lens tension, allowing it to return to its more convex shape.
The ciliary muscle receives parasympathetic fibers from the short ciliary nerves that arise from the ciliary ganglion. Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the oculomotor nerve). Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction. The contraction decreases the diameter of the ring of ciliary muscle, causing the relaxation of the zonular fibers and the increased convexity of the lens. This induces accommodation for near vision.
The ciliary muscle, along with the sphincter pupillae, functions primarily through the parasympathetic nerve fibers of the oculomotor nerve (CN III). The contraction of the ciliary muscle loosens the zonular fibers, increasing the convexity of the lens and improving near vision. This mechanism is essential for the eye's ability to focus on objects at varying distances.
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The ciliary body, which includes the ciliary muscle, produces aqueous humour
The ciliary body is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. It is composed of several unique structures, including the ciliary muscle, ciliary processes, ciliary vessels, and ciliary epithelia. The ciliary muscle is the biggest part of the ciliary body and is responsible for changing the shape of the lens to facilitate near vision. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary body is attached to the lens by zonular fibres, which are tiny fibrous cords. The contraction of the ciliary muscle loosens these zonular fibres, increasing the convexity of the lens and inducing accommodation for near vision. This process also aids in the drainage of aqueous humour into the canal of Schlemm. The ciliary body is the main target of many medications against glaucoma, as its inhibition leads to a decrease in intraocular pressure.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves, which arise from the ciliary ganglion. The ciliary ganglion receives signals from the Edinger-Westphal nucleus via the oculomotor nerve (CN III). 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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Glaucoma is treated with muscarinic receptor agonists that cause the contraction of the ciliary muscle
Glaucoma is a neurodegenerative disease that causes progressive neuronal death. It is a leading cause of blindness, with primary angle-closure glaucoma (PACG) being a common form of the disease. The traditional treatment approach for glaucoma has been to lower intraocular pressure (IOP) by inhibiting aqueous humour secretion (inflow). However, newer therapeutic approaches focus on increasing the outflow of aqueous humour.
The ciliary muscle is an intrinsic muscle of the eye that plays a crucial role in the accommodation reflex. It is innervated by the parasympathetic nerve fibres of the oculomotor nerve (CN III). The ciliary muscle receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion. Activation of the M3 muscarinic receptors causes contraction of the ciliary muscle.
Glaucoma is treated with muscarinic receptor agonists, such as pilocarpine, which stimulate the contraction of the ciliary muscle. This contraction opens the trabecular meshwork, allowing the aqueous humour to flow through the canal of Schlemm, ultimately reducing intraocular pressure. The protective effect of pilocarpine against neurotoxicity in retinal neurons is mediated through the activation of muscarinic receptors, specifically the M1 subtype.
While the M3 subtype is believed to be the primary mediator of the IOP-lowering effects of muscarinic agonists, recent research suggests that other subtypes may also play a role. A novel agonist, AGN 199170, which has no activity on the M3 subtype, was found to lower IOP in a monkey glaucoma model. This indicates that muscarinic agonists targeted at subtypes other than M3 may be effective in reducing IOP without causing ciliary muscle contraction and its associated ocular side effects.
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Frequently asked questions
The ciliary muscle is an intrinsic smooth muscle of the eye that occupies the largest part of the ciliary body. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial and circular.
The ciliary muscle controls accommodation for viewing objects at varying distances. It changes the shape of the lens within the eye but not the size of the pupil. When the ciliary muscle contracts, it loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision.
The ciliary muscle receives parasympathetic innervation from short ciliary nerves, which arise from the oculomotor nerve via the ciliary ganglion. The ciliary ganglion then gives off short ciliary nerves (postganglionic fibres) that innervate the ciliary body. The ciliary muscle is dually innervated by the autonomic nervous system. Parasympathetic stimulation activates the muscle for contraction, while sympathetic innervation likely has an inhibitory effect.
