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The ciliary muscle is an intrinsic muscle of the eye that adjusts the shape of the lens to enable focusing on objects at different distances. It is located in the middle layer of the eye, known as the uvea or vascular layer, and forms a ring of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The ciliary muscle functions by contracting and relaxing its fibres, which alters the tightness of the zonular fibres that support the lens. This, in turn, changes the shape of the lens, allowing the eye to adapt to near or far vision. The ciliary muscle also regulates the flow of aqueous humour, a clear fluid that provides nourishment and pressurization to the eye.
| 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 | Beneath the anterior sclera, just posterior to the limbus. |
| Shape | Elongated and triangular. |
| Composition | Smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. |
| Function | Controls accommodation for viewing objects at varying distances by adjusting the shape of the lens. Regulates the flow of aqueous humour into Schlemm's canal and controls the pore size of the trabecular meshwork. |
| Blood Supply | Branches of the ophthalmic artery, including the central artery of the retina, short and long posterior ciliary arteries, and anterior ciliary arteries. |
| Innervation | Parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. |
| Clinical Significance | Contraction of the ciliary muscle is associated with the treatment of open-angle glaucoma and closed-angle glaucoma, facilitating drainage of aqueous humour and reducing intraocular pressure. |
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What You'll Learn
Ciliary muscle layers
The ciliary muscle is an intrinsic muscle of the eye that controls accommodation for viewing objects at varying distances. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The muscle is made up of bundles of muscle cells surrounded by connective tissue cells.
The ciliary muscle occupies the biggest portion of the anterior aspect of the ciliary body, with ciliary processes lying posterior to it. The outer surface of the muscle lies parallel to the sclera, while the innermost layer of the muscle is located near the major circle of the iris. The innermost layer, or annular/circular layer (also known as Müller’s muscle), 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 outermost layer is the longitudinal layer, which is situated adjacent to the loose connective tissue of the ciliary body. The middle layer is the radial layer, which represents the transition from the longitudinal to the circular muscle layer.
The ciliary muscle is responsible for adjusting the shape of the lens to enable focusing on objects at different distances. When the muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye, releasing tension on the lens caused by the zonular fibres. This release of tension 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 long-range focus.
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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 accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. 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 composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular. The longitudinal fibres are the outermost muscle layer, situated adjacent to the loose connective tissue of the ciliary body. The radial layer of muscle is the middle layer and represents the transition from the longitudinal to the circular muscle layer. The annular or circular layer (Müller's muscle) is the innermost muscle layer that functions as a sphincter of the eye.
The ciliary muscle functions are mainly instructed by the parasympathetic nerve fibres of the oculomotor nerve (CN III). 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, which 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 for near vision.
The ciliary muscle's contractile response is thought to be greater temporally than nasally, possibly to align the lenticular axes during convergence. The ciliary muscle connective tissue is mainly made up of collagen fibrils and fibroblasts.
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Ciliary muscle function
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 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 in the middle, and the innermost layer is the circular layer.
The ciliary muscle functions to adjust the shape of the lens to enable focusing on objects at different distances. It does this by contracting and relaxing its longitudinal fibres to change the shape of the lens, making it more spherical for near vision and flatter for distant vision. This change in the shape of the lens is caused by the relaxation and tightening of the zonular fibres, which are attached to the ciliary body and support the lens.
The ciliary muscle also regulates the flow of aqueous humour, a clear fluid that fills the space between the cornea and iris, providing nourishment and pressurization to the eye. The muscle controls the drainage of aqueous humour into the canal of Schlemm, which is important in maintaining intraocular pressure.
Additionally, the ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The activation of M3 muscarinic receptors causes ciliary muscle contraction, which is important for accommodating near vision.
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Ciliary muscle shape
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 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 annular or circular layer (Müller’s muscle), which functions as a sphincter of the eye.
The ciliary muscle is responsible for adjusting the shape of the lens to enable focusing on objects at different distances. It does this by contracting and relaxing its longitudinal fibres, which control the tightness of the zonular fibres that support the lens. When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye, 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.
The ciliary muscle is thicker temporally than nasally, and its connective tissue is mainly made up of collagen fibrils and fibroblasts. The contractile response is thought to be greater temporally than nasally, possibly to align the lenticular axes during convergence. The ciliary muscle is elongated and triangular in shape, located beneath the anterior sclera just posterior to the limbus. The shortest side of the triangular region faces anterior-inward, and it is to this region that the base of the iris inserts.
The ciliary muscle is an important part of the eye that controls aspects of near and far vision. It receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion is a parasympathetic ganglion located behind the eye, containing about 2,500 neurons.
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Ciliary muscle blood supply
The ciliary muscle is an intrinsic muscle of the eye that is responsible for adjusting the shape of the lens to enable focusing on objects at different distances. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The blood supply to the ciliary muscle comes from the branches of the ophthalmic artery, which form the major arterial circle located near the root of the iris. These arteries supply blood to the ciliary body, the iris, and the anterior aspect of the choroid. The ciliary body is a ring-shaped thickening that separates the posterior chamber from the vitreous chamber. It is composed of the ciliary muscle, ciliary stroma, and both pigmented and non-pigmented epithelial cells. The ciliary muscle occupies the biggest portion of the ciliary body and receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion.
The blood supply to the ciliary muscle is essential for its function in changing the shape of the lens during the accommodation reflex. 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 increasing its power to refract light for near vision. 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.
The parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction. The contraction of the longitudinal fibres of the ciliary muscle also widens the iridocorneal space and venous sinus of the sclera (canal of Schlemm), facilitating the draining of eye fluid. The ciliary muscle also receives innervation from the sympathetic fibres of the autonomic nervous system (ANS), which allegedly provide inhibitory impulses that inhibit the accommodation reflex.
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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 accommodation for viewing objects at varying distances. It does this by changing the shape of the lens within the eye, allowing the eye to focus on near objects.
The ciliary muscle consists of longitudinal, radial, and circular fibres that contract during accommodation. The contraction of the longitudinal fibres causes an anterior shift in muscle mass, while the radial and circular fibres cause an inward movement. This changes the shape of the lens, making it more spherical for near vision and flatter for far vision.
The ciliary muscle occupies the biggest portion of the anterior aspect of the ciliary body, with the outer surface of the muscle lying parallel to the sclera. The innermost layer of the muscle is located near the major circle of the iris. The ciliary muscle is connected to the zonular fibres, which act as suspensory ligaments that attach to the lens.
The blood supply to the ciliary muscle comes from the branches of the ophthalmic artery, which form the major arterial circle located near the root of the iris.
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