Logan Steele
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 has several functions, including controlling the eye's accommodation for viewing objects at varying distances, changing the shape of the lens within the eye, and regulating the flow of aqueous humour into Schlemm's canal. The ciliary muscle is also involved in the treatment of open-angle glaucoma (OAG) and closed-angle glaucoma (CAG). While its function is preserved in older humans, it is unknown if it remains operational at very advanced ages.
| Characteristics | Values |
|---|---|
| Type of muscle | Intrinsic muscle of the eye |
| Muscle composition | Smooth muscle fibres oriented in three directions: longitudinal, radial and circular |
| Muscle function | Controls the movements of the lens and pupil, participating in the accommodation of vision |
| Accommodation | When the ciliary muscle contracts, it loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision |
| Regulation of flow | Regulates the flow of aqueous humour into Schlemm's canal |
| Shape of the lens | Controls the shape of the lens, making it more or less spherical depending on the distance of the object being viewed |
| Glaucoma treatment | Muscarinic receptor agonists that cause contraction of the ciliary muscles can help treat open-angle and closed-angle glaucoma |
| Nutrients | Receives blood vessels from the long posterior ciliary arteries and the major iris circle |
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What You'll Learn
Ciliary muscle contraction
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, while the ciliary epithelium regulates aqueous humour production.
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, which are fibres that hold or flatten the lens. This release of tension of the zonular fibres causes the lens to become more spherical, adapting to short-range focus.
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). 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 then becomes more spherical, increasing its power to refract light for near vision.
The contraction of the ciliary muscle can also lead to relief in both open-angle and closed-angle glaucoma, making it a target for drugs like pilocarpine. Glaucoma is an eye disease that damages the optic nerve and can eventually lead to blindness.
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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, known as the uvea or vascular layer. It is responsible for controlling the accommodation of vision for objects at varying distances. The ciliary muscle achieves this by changing the shape of the lens within the eye, allowing for the necessary adjustments in focus.
When the ciliary muscle contracts, it moves the frontal region towards the axis of the eye, releasing the tension on the lens caused by the zonular fibres. This release of tension allows the lens to become more spherical, enabling short-range focus.
Conversely, ciliary muscle relaxation causes the zonular fibres to tighten, resulting in a flattening of the lens. This flattening increases the focal distance, facilitating long-range focus.
The ciliary muscle is also involved in the production of aqueous fluid by the ciliary body, which nourishes the lens and cornea, further contributing to the maintenance of clear vision.
Additionally, the ciliary muscle regulates the flow of aqueous humour into Schlemm's canal. This regulation is significant in maintaining eye health and intraocular pressure. Conditions such as open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) can be treated by targeting the ciliary muscles to facilitate drainage of the aqueous humour and reduce intraocular pressure.
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Glaucoma treatment
Glaucoma refers to a group of eye diseases that cause a build-up of pressure inside the eyeball, leading to permanent vision loss and blindness. While glaucoma cannot be prevented, early detection and treatment can help manage intraocular pressure and prevent or delay vision loss.
The ciliary muscle is an intrinsic muscle of the eye that participates in the accommodation reflex, allowing us to view objects at varying distances. It controls the accommodation of vision by changing the shape of the lens within the eye, making it rounded for close-up objects and flattening it for objects that are farther away.
Open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists, which cause rapid miosis and contraction of the ciliary muscles. This opens the trabecular meshwork, facilitating the drainage of aqueous humour into the canal of Schlemm and ultimately decreasing intraocular pressure.
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Ciliary muscle and age
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, known as the uvea or vascular layer. The ciliary muscle controls the accommodation of vision at varying distances by changing the shape of the lens within the eye. This muscle also regulates the flow of aqueous humour, a fluid that nourishes the lens and cornea, into Schlemm's canal.
With age, the ciliary muscle undergoes certain changes. While it retains its ability to contract throughout an individual's lifespan, the muscle displays age-related variations. It has been observed that the ciliary muscle of older individuals is shorter, wider, and has a greater amount of connective tissue. Additionally, there is a decrease in the unaccommodated ciliary muscle diameter with advancing age. This reduction in diameter is highly correlated with age and supports the theory of presbyopia, which is the difficulty older individuals experience when focusing on nearby objects.
The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The outermost layer is the longitudinal layer, followed by the radial layer, and the innermost layer is the circular layer (Müller's muscle). The circular layer functions as a sphincter of the eye and is typically well-developed in hypermetropic (long-sighted) individuals but may be rudimentary or absent in myopic (short-sighted) eyes.
The ciliary muscle plays a crucial role in vision by facilitating the accommodation process. When the muscle contracts, it moves the frontal region toward the axis of the eye, releasing the tension on the lens caused by the zonular fibres. This release of tension allows the lens to become more spherical, enabling short-range focus. Conversely, when the ciliary muscle relaxes, the zonular fibres flatten the lens, increasing the focal distance and enhancing long-range focus.
Maintaining eye health is important at any age. Adequate lighting, a healthy diet rich in vitamins and nutrients, regular exercise, staying hydrated, and protecting the eyes from strain and UV light are all recommended practices to keep the eyes in optimal condition.
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Ciliary muscle and zonular fibres
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 controls the shape of the lens, while the ciliary epithelium regulates aqueous humour production. The ciliary body, a component of the uvea, makes aqueous fluid, which nourishes the lens and cornea.
The ciliary muscle is the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. 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, thereby facilitating the accommodation of vision. 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 (fibres that hold or flatten the lens). This 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 improving long-range focus.
The ciliary muscle is also involved in the treatment of glaucoma, an eye disease that damages the optic nerve and can lead to blindness. Glaucoma is characterised by increased ocular pressure, which can be relieved by the contraction of the ciliary muscles, opening the trabecular meshwork and facilitating drainage of the aqueous humour.
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Frequently asked questions
Ciliary muscles are intrinsic muscles of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer).
Ciliary muscles control accommodation for viewing objects at varying distances. They also change the shape of the lens within the eye, increasing its power to refract light for near vision.
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, allowing the lens to become more spherical and adapt to short-range focus.
Studies have shown that ciliary muscle function is preserved in older humans, but it is unknown whether the muscle still operates at very advanced ages.
