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The ciliary muscle is an intrinsic muscle of the eye that controls near and far vision 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 enable the eye to change power and focus at different distances, a process called accommodation. The ciliary muscle also regulates the flow of aqueous humour, a fluid that nourishes the lens and cornea.
| Characteristics | Values |
|---|---|
| Definition | An intrinsic muscle of the eye |
| Function | Controls near and far vision by changing the shape of the lens |
| Composition | Smooth muscle fibers oriented in longitudinal, radial, and circular directions |
| Control | Controlled by the oculomotor nerve (CN III) and the parasympathetic nerve fibers |
| Accommodation | Enables the eye to change power and focus at different distances |
| Aqueous Humor | Regulates the flow of aqueous humor into Schlemm's canal |
| Trabecular Meshwork | Controls the pore size of the trabecular meshwork, facilitating or impeding aqueous humor drainage |
| Glaucoma Treatment | Muscarinic receptor agonists cause contraction of the ciliary muscles, facilitating drainage of aqueous humor and treating glaucoma |
| Size | Occupies the biggest portion of the anterior aspect of the ciliary body |
| Structure | Forms a semi-transparent ring on the outer surface of the choroid |
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What You'll Learn
Controls near and far vision
The ciliary muscle is an intrinsic muscle of the eye that controls near and far vision 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 is an important part of the eye that controls aspects of near and far vision.
The ciliary muscle controls the shape of the lens by contracting and relaxing. When the ciliary muscle contracts, it loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision. This is because when you are looking at something closer, your lens becomes more rounded. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens, increasing the focal distance, and improving long-range focus. This process is called "accommodation" and allows the eye to change power and focus at different distances.
The ciliary muscle fibres have a greater abundance of mitochondria than any other muscle in the body, indicating their extraordinarily high energy demands and requirements. The ciliary muscle also regulates the flow of aqueous humour into Schlemm's canal. A clear fluid, known as aqueous humour, constantly flows out of the anterior chamber of the eye to maintain intraocular pressure.
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Maintains the shape of the lens
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 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 the iris. The ciliary body is a 5–6 mm wide ring of tissue extending from the scleral spur anteriorly to the ora serrata posteriorly. It is the anterior continuation of the choroid.
The ciliary muscle maintains the shape of the lens by contracting and relaxing its longitudinal fibres. This action increases and decreases the size of the pore in the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye. The contraction of the ciliary muscle loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision. 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 fibres have a greater abundance of mitochondria than any other muscle in the body, indicating their extraordinarily high energy demands and requirements. The ciliary muscle controls the shape of the lens, while the ciliary epithelium regulates aqueous humour production. The ciliary body also makes aqueous fluid, which nourishes the lens and cornea.
The ciliary muscle is an important part of the eye that controls aspects of near and far vision. 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.
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Produces aqueous fluid
The ciliary muscle is an intrinsic muscle of the eye that controls near and far vision. It is 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 and is responsible for the production of aqueous fluid, also known as aqueous humour.
The ciliary body, a component of the uvea, makes aqueous fluid, which nourishes the lens and cornea. The ciliary epithelium regulates aqueous humour production, which may commence as early as 20 weeks into development. The ciliary muscle controls the flow of aqueous humour into Schlemm's canal.
The ciliary muscle fibres have a greater abundance of mitochondria than any other muscle in the body, indicating their extraordinarily high energy demands and requirements. The ciliary muscle receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion.
The ciliary muscle alters the shape of the lens by contracting and relaxing, which changes the thickness of the lens and simultaneously alters its curvature. This enables the eye to change power and focus at different distances, a process called "accommodation".
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Regulates the flow of aqueous humour
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 formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle alters 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 regulates the flow of aqueous humour by controlling the tightness of the zonular fibres through contraction and relaxation. The zonular fibres are suspensory ligaments that attach to the lens. When the ciliary muscle contracts, the zonular fibres relax, and the lens becomes 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 also regulates the pore size of the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye. By changing the pore size, the ciliary muscle can impede or facilitate aqueous humour flow in the canal of Schlemm. This regulation of aqueous humour flow is particularly important in the context of glaucoma, an eye disease that can damage the optic nerve due to increased ocular pressure.
The ciliary body, of which the ciliary muscle is a part, produces the aqueous humour. The ciliary epithelium, specifically, regulates aqueous humour production. The aqueous humour is a clear fluid that flows out of the anterior chamber of the eye to maintain intraocular pressure and provide nourishment to the lens and cornea.
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Has high energy demands
The ciliary muscle is an intrinsic muscle of the eye that controls the movements of the lens and pupil, thus accommodating vision. It is a circular band of smooth muscle that controls the tightness of the zonular fibres by contracting and relaxing, thereby altering the shape of the lens and enabling the eye to focus on objects at varying distances.
The ciliary muscle fibres have a greater abundance of mitochondria than any other muscle in the body, indicating their extraordinarily high energy demands and requirements. The pars plicata region of the ciliary body surrounding the ciliary muscle contains a dense vascular supply that provides oxygen and nutrients to the muscle.
The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The contraction of the longitudinal fibres increases and decreases the size of the pore in the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye. This process facilitates and impedes the flow of aqueous humour into the canal of Schlemm.
The contraction of the circular fibres decreases the diameter of the ring of ciliary muscle, causing relaxation of the zonular fibres. This increases the convexity of the lens, making it more spherical and increasing its power to refract light for near vision.
The ciliary muscle also plays a role in maintaining eye health by regulating the flow of aqueous humour, a clear fluid that flows out of the anterior chamber of the eye to maintain intraocular pressure and drain into the canal of Schlemm.
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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.
The ciliary muscle controls the shape of the lens, allowing the eye to focus on objects at varying distances.
The ciliary muscle contracts and relaxes its longitudinal fibres to change the shape of the lens. When the muscle contracts, the lens becomes rounded, increasing its power to refract light for near vision. When the muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range focus.
Conditions that affect the ciliary muscle can cause vision problems. For example, glaucoma is a disease that damages the optic nerve, often due to increased ocular pressure, and can lead to blindness.
