Victor Forge
The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens, facilitating accommodation for near vision. It is located within the ciliary body and is innervated by the autonomic nervous system. The ciliary muscle receives parasympathetic innervation from the short ciliary nerves that arise from the ciliary ganglion. The ciliary ganglion receives preganglionic fibers 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 also receives sympathetic innervation from the autonomic nervous system, which likely has an inhibitory effect. The contraction of the ciliary muscle loosens the zonular fibers, increasing the convexity of the lens and improving focus for closer objects. This process is targeted by medications for glaucoma, which aim to decrease intraocular pressure by facilitating the drainage of aqueous humor.
| Characteristics | Values |
|---|---|
| Type | Intrinsic muscle of the eye |
| Shape | Ring-shaped |
| Location | Middle layer of the eye (uvea/vascular layer) |
| Function | Controls the shape of the lens, enabling changes in lens shape for light focusing |
| Innervation | Parasympathetic innervation from the short ciliary nerves (postganglionic fibers) that arise from the ciliary ganglion |
| Nerve Supply | Branches of the ophthalmic artery |
| Fibres | Smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular |
| Accommodation | Controls accommodation for viewing objects at varying distances |
| Glaucoma Treatment | Muscarinic receptor agonists cause contraction of the ciliary muscle, opening the trabecular meshwork and decreasing intraocular pressure |
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What You'll Learn
- The ciliary muscle is an intrinsic muscle of the eye
- It receives parasympathetic innervation from the short ciliary nerves
- The ciliary ganglion receives preganglionic fibres via the oculomotor nerve
- The ciliary muscle controls the shape of the lens
- It is dually innervated by the autonomic nervous system
The ciliary muscle is an intrinsic muscle of the eye
The ciliary muscle plays a crucial role in controlling the accommodation of vision for objects at varying distances. When the ciliary muscle contracts, it pulls itself forward, moving the frontal region towards the axis of the eye. This action releases the tension on the lens caused by the zonular fibres, which are fibres that suspend the lens in position. The release of tension on the zonular fibres causes the lens to become more spherical, increasing its power to refract light and enabling clear vision for nearby objects. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance, which is essential for focusing on distant objects.
The ciliary muscle is dually innervated by the autonomic nervous system. It 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) from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. The parasympathetic activation of the M3 muscarinic receptors causes the contraction of the ciliary muscle. Additionally, there is evidence suggesting that the ciliary muscle also receives innervation from the sympathetic fibres of the autonomic nervous system, which likely have an inhibitory effect.
The ciliary muscle is an essential component of the eye's accommodation reflex, allowing for clear vision at different distances. Its contraction facilitates the accommodation of the lens, enabling the eye to focus on nearby objects, while its relaxation aids in focusing on distant objects.
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It receives parasympathetic innervation from the short ciliary nerves
The ciliary muscle is an intrinsic muscle of the eye that controls the movements of the lens and pupil, thus participating in the accommodation of vision. It is a smooth muscle that occupies the largest part of the ciliary body, which lies between the anterior border of the choroid and iris.
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 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 via the postganglionic parasympathetic fibres, which 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. This, in turn, makes the lens more spherical, increasing its power to refract light. 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 that is a function of the level of parasympathetic activity.
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The ciliary ganglion receives preganglionic fibres via the oculomotor nerve
The ciliary muscle is an intrinsic muscle of the eye that controls the accommodation of vision. It is part of the ciliary body, which is a ring-shaped thickening of tissue inside the eye that divides the posterior chamber from the vitreous body. The ciliary muscle itself forms a triangle-shaped region of smooth muscle fibres within the anterior aspect of the ciliary body, located anterior to the choroid and posterior to the iris.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion, in turn, 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 Edinger-Westphal nucleus lies posterior to the main oculomotor nucleus in the rostral midbrain at the level of the superior colliculus.
The oculomotor nerve carries presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus. These signals travel through the ciliary ganglion via the postganglionic parasympathetic fibres, which are part of the short ciliary nerves. The short ciliary nerves supply the ciliary body and the iris. Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction, which decreases the diameter of the ring of ciliary muscle. This contraction leads to the relaxation of the zonule fibres, causing the lens to become more spherical and increasing its power to refract light.
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. The ciliary muscle also receives sympathetic innervation from the autonomic nervous system, which allegedly provides inhibitory impulses that inhibit the accommodation reflex.
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The ciliary muscle controls the shape of the lens
The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens. It is a part of the ciliary body, which is a ring-shaped thickening of tissue inside the eye. The ciliary muscle itself forms a triangle-shaped region of smooth muscle fibres within the anterior aspect of the ciliary body, located anterior to the choroid and posterior to the iris. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle is dually innervated by the autonomic nervous system. It 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 parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as when reading a book.
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). This release of tension of the zonular fibres causes the lens to become more spherical, increasing its power to refract light and adapting to short-range focus. The ciliary muscle also regulates the pore size of the trabecular meshwork, aiding the drainage of aqueous humour into the canal of Schlemm.
Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens, increasing the focal distance, and improving long-range focus. The ciliary muscle works in synergy with the other intrinsic muscles of the eye: dilatator pupillae and sphincter pupillae muscles, which control the size of the pupil.
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It is dually innervated by the autonomic nervous system
The ciliary muscle is an intrinsic muscle of the eye, which is responsible for controlling the shape of the lens and, thus, the accommodation of vision. The ciliary muscle is dually innervated by the autonomic nervous system. This means that it receives signals from both the parasympathetic and sympathetic nervous systems.
The parasympathetic innervation of the ciliary muscle is well understood. Parasympathetic fibres arise from the ciliary ganglion, which receives preganglionic fibres via the oculomotor nerve (cranial nerve III) from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. The ciliary ganglion then gives off short ciliary nerves (postganglionic fibres) that innervate the ciliary muscle. Activation of the M3 muscarinic receptors by the parasympathetic system causes the ciliary muscle to contract. This contraction decreases the diameter of the ciliary muscle ring, relaxing the zonule fibres and increasing the convexity of the lens. This, in turn, increases the lens's power to refract light, improving focus for closer objects.
The ciliary muscle also receives innervation from the sympathetic fibres of the autonomic nervous system. These fibres are believed to provide inhibitory impulses, reducing the accommodation reflex. The sympathetic fibres may pass directly to the retrobulbar plexus behind the eye or through the ciliary ganglion uninterrupted. From the ciliary ganglion, fibres are distributed to the blood vessels of the eye via the short ciliary nerves. The sympathetic action is mediated by the release of norepinephrine, which acts on the α1 and β2 adrenoceptors.
The ciliary muscle, therefore, receives dual innervation from the parasympathetic and sympathetic nervous systems, which work synergistically to control the accommodation of vision.
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Frequently asked questions
The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibers) that arise from the ciliary ganglion.
The ciliary muscle controls the accommodation of vision by changing the shape of the lens within the eye. It also regulates the flow of aqueous humor into Schlemm's canal.
Glaucoma is characterised by high intraocular pressure. The ciliary body produces aqueous humor, so it is the main target of many medications against glaucoma. Muscarinic receptor agonists cause the contraction of the ciliary muscle, leading to the opening of the trabecular meshwork and a decrease in intraocular pressure.
