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The ciliary muscle is an intrinsic muscle of the eye that allows us to focus on objects at different distances. It is a multi-unit smooth muscle, formed as a ring in the eye's middle layer, the uvea (vascular layer). The ciliary muscle is composed of muscle fibres oriented in three directions: longitudinal, radial and circular. When the ciliary muscle contracts, it pulls itself forward, moving the frontal region toward the axis of the eye. This releases the tension on the lens, allowing it to become more spherical and adapt to short-range focus. Conversely, when the ciliary muscle relaxes, the lens flattens out, increasing long-range focus.
| 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 | Adjusts the shape of the lens to enable focusing on objects at different distances |
| Blood Supply | Branches of the ophthalmic artery |
| Innervation | Parasympathetic innervation from the short ciliary nerves (postganglionic fibres) arising from the ciliary ganglion |
| Clinical Significance | Contraction of the ciliary muscle can be used to treat glaucoma by facilitating drainage of aqueous humour and reducing intraocular pressure |
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What You'll Learn
The ciliary muscle is a multi-unit smooth muscle
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 a multi-unit smooth muscle, made up of bundles of muscle cells surrounded by connective tissue cells. The muscle bundles form three distinct fibre types: longitudinal, radial and circular.
The ciliary muscle occupies the biggest portion of the anterior aspect of the ciliary body, with ciliary processes lying posterior to it. It is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial and circular. The outermost muscle layer is the longitudinal layer, followed by the middle layer, the radial layer, and finally, the innermost muscle layer, the annular or circular layer.
The ciliary muscle controls accommodation for viewing objects at varying distances. When the ciliary 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 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 also regulates the flow of aqueous humour into Schlemm's canal. It receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion.
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It adjusts the shape of the lens to enable focusing on objects at different distances
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 muscle fibres oriented in three different directions: longitudinal, radial, and circular.
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 pull the muscle forward and move 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. 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 flow of aqueous humour, a clear fluid that provides pressurisation to the eye and brings nutrients to the lens and cornea. By contracting and relaxing its longitudinal fibres, the ciliary muscle can increase and decrease the pore size in the trabecular meshwork, facilitating or impeding the flow of aqueous humour.
The ciliary muscle is the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. The ciliary body is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. It includes the ciliary muscle and finger-like ciliary processes, which secrete the aqueous humour. The ciliary muscle receives parasympathetic innervation from the short ciliary nerves that arise from the ciliary ganglion.
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It consists of longitudinal, radial, and circular fibres
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 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 outermost layer of the ciliary muscle is the longitudinal layer, which lies adjacent to the loose connective tissue of the ciliary body. The longitudinal fibres run parallel to the sclera from the scleral spur to the posterior visible limit of the ciliary muscle. During contraction, these fibres are responsible for the anterior shift in muscle mass. They also contract and relax to increase and decrease the size of the pore in the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye.
The middle layer is the radial layer of muscle. The radial fibres run perpendicular to the longitudinal fibres. The radial fibre cells contain the most mitochondria organelles, while the tips of the longitudinal fibre cells contain the fewest mitochondria and more myofibrils, possibly facilitating faster contraction and providing greater stiffness than the rest of the fibres.
The innermost muscle layer is the annular or circular layer (Müller's muscle), which 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 circular fibres encircle the ciliary muscle aperture and are the closest fibres to the crystalline lens. They act as a sphincter, causing an inward movement of muscle mass during contraction.
Together, the contraction of these fibres 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 and increasing the focal distance for long-range focus.
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The ciliary muscle is thicker temporally than nasally
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 responsible for adjusting the shape of the lens to enable focusing on objects at different distances. The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle's function is to adjust the shape of the lens, allowing the eye to focus 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 on the zonular fibres causes the lens to become more spherical, enabling short-range focus.
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 apex of the ciliary muscle is oriented inward in the eye toward the lens equator and is covered by the pars plicata region of the ciliary processes.
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It is the biggest portion of the ciliary body
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 the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. 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 smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle controls accommodation for viewing objects at varying distances by adjusting the shape of the lens. It does this by contracting and relaxing its longitudinal fibres to increase and decrease the size of the pore in the trabecular meshwork, which is responsible for aqueous humour drainage in the anterior portion of the eye. 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 release of tension on 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 and increasing the focal distance, which is necessary for long-range focus.
The ciliary muscle is thicker temporally than nasally. The longitudinal fibres are responsible for the anterior shift in muscle mass during contraction, while the radial and circular fibres are responsible for the inward movement of muscle mass during contraction, with the circular fibres acting as a sphincter. The radial fibre cells contain the most mitochondria organelles, while the tips of the longitudinal fibre cells contain the fewest mitochondria and more myofibrils, possibly facilitating faster contraction and providing greater stiffness than the rest of the fibres.
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.
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Frequently asked questions
The ciliary muscle is an intrinsic muscle of the eye that controls aspects of near and far vision.
The ciliary muscle changes the shape of the lens within the eye, allowing the eye to focus on objects at different distances.
The ciliary muscle is composed of smooth muscle fibres oriented in three different directions: longitudinal, radial, and circular.
The ciliary muscle is located in the middle layer of the eye, known as the uvea or vascular layer. It occupies the biggest portion of the anterior aspect of the ciliary body.
The ciliary muscle contracts and relaxes its longitudinal fibres, which alters the shape of the lens and adjusts the eye for near or far vision.
