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, known as the uvea (vascular layer). It is responsible for facilitating lens accommodation for near vision and receives parasympathetic innervation from short ciliary nerves, which arise from the oculomotor nerve via the ciliary ganglion. The ciliary muscle plays a crucial role in changing the shape of the lens to enable clear vision, while the muscles that move the eyeballs help with focusing. When the ciliary muscle contracts, it stretches the choroid, releasing tension on the lens, making it thicker and more spherical, thus increasing its power to refract light for near vision.
| Characteristics | Values |
|---|---|
| Definition | One of the muscles around the lens of the eye that can change the shape of the lens |
| Location | Within the ciliary body of the eye |
| Shape | Triangle-shaped region of smooth muscle fibers |
| Composition | Thin processes attaching to a complex network of zonular fibers |
| Innervation | Parasympathetic innervation from short ciliary nerves, arising from the oculomotor nerve via the ciliary ganglion |
| Function | Controls accommodation for viewing objects at varying distances, changes the shape of the lens, regulates the flow of aqueous humor into Schlemm's canal |
| Contraction | Decreases the diameter of the ring of ciliary muscle, relaxes zonule fibers, increases the lens's power to refract light for near vision |
| Relaxation | Causes zonular fibers to become taut, flattening the lens, increasing the focal distance, enhancing long-range focus |
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What You'll Learn
Ciliary muscle contraction
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 considered a cranial neural crest derivative, developing from mesenchyme within the choroid. The main function of the ciliary muscle is to control the eye's accommodation for viewing objects at varying distances. It does this by changing the shape of the lens within the eye, making it thicker and more spherical, and thus increasing its power to refract light for near vision. This is particularly useful when a higher degree of accommodation of the lens is required, such as when reading a book.
The ciliary muscle is also involved in the regulation of the flow of aqueous humour, a fluid produced by the ciliary processes, into Schlemm's canal. This regulation is particularly important in the treatment of open-angle glaucoma (OAG) and closed-angle glaucoma (CAG). Muscarinic receptor agonists, such as pilocarpine, can be used to treat these conditions by causing rapid miosis and contraction of the ciliary muscles. This opens the trabecular meshwork, facilitating drainage of the aqueous humour into Schlemm's canal and ultimately decreasing intraocular pressure.
The ciliary muscle, along with the pupillary sphincter muscle and pupillary dilator muscle, is sometimes referred to as intrinsic ocular muscles or intraocular muscles. These muscles receive parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion.
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Ciliary muscle fibres
The ciliary muscle is an intrinsic muscle of the eye that controls accommodation for viewing objects at varying distances. 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 differentiates during fetal development from the mesenchyme between the neuroectoderm and scleral condensation external to the early ciliary folds.
The ciliary muscle fibres are oriented in three different directions: longitudinal, radial, and circular. These fibres work together to control the shape and accommodation of the lens within the eye. When the ciliary muscle contracts, it pulls itself forward, moving the frontal region toward the axis of the eye. This action stretches the choroid and releases tension on the lens, allowing it to become thicker and more spherical. The circular ciliary muscle fibres, in particular, are believed to affect zonular fibres in the eye, enabling changes in lens shape for light focusing.
The longitudinal fibres are responsible for the anterior shift in muscle mass during contraction. In contrast, the radial and circular fibres contribute to the inward movement of muscle mass during contraction, with the circular fibres acting as a sphincter. The contractile response is thought to be more pronounced temporally than nasally, possibly to align the lenticular axes during convergence.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that originate in the ciliary ganglion. Additionally, there is evidence suggesting that the ciliary muscle may also receive innervation from the sympathetic fibres of the autonomic nervous system (ANS). These fibres are believed to provide inhibitory impulses that suppress the accommodation reflex.
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Ciliary muscle innervation
The ciliary muscle is an intrinsic muscle of the eye. 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 is responsible for controlling the eye's accommodation for viewing objects at varying distances. It also regulates the flow of aqueous humour into Schlemm's canal and changes the shape of the lens within the eye.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres). These nerves arise from the ciliary ganglion, which receives preganglionic fibres via the oculomotor nerve (cranial nerve III). The oculomotor nerve originates from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. The ciliary muscle is supplied by branches of the ophthalmic artery, which form a major arterial circle near the root of the iris.
Parasympathetic activation of the M3 muscarinic receptors causes the ciliary muscle to contract. This contraction decreases the diameter of the ciliary muscle ring, leading to the relaxation of the zonule fibres. As a result, the lens becomes more spherical, increasing its power to refract light for near vision. The parasympathetic tone is dominant when a higher degree of accommodation is required, such as when reading a book.
In addition to the parasympathetic innervation, there is some evidence suggesting that the ciliary muscle also receives sympathetic innervation from the autonomic nervous system (ANS). These sympathetic fibres are believed to provide inhibitory impulses that suppress the accommodation reflex.
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Ciliary muscle and glaucoma
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 eye's accommodation for viewing objects at varying distances and regulating the flow of aqueous humour into Schlemm's canal. The ciliary muscle also changes the shape of the lens within the eye, allowing for the adaptation to near or far vision.
The ciliary muscle is one of three intrinsic ocular muscles, along with the pupillary sphincter and dilator muscles. It develops from mesenchyme within the choroid and is considered a cranial neural crest derivative. The muscle receives parasympathetic fibres from the short ciliary nerves, which arise from the ciliary ganglion. The ciliary ganglion is a parasympathetic ganglion located behind the eye, containing approximately 2500 neurons.
The ciliary muscle plays a crucial role in the accommodation reflex, which is the process of adjusting the eye's focus to clearly see objects at different distances. When the muscle contracts, it pulls itself forward, moving the frontal region toward the axis of the eye. This contraction loosens the zonular fibres, increasing the convexity of the lens and enabling better refraction of light for near vision. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and improving long-range focus.
The ciliary muscle's function is closely related to the maintenance of healthy eye pressure. It regulates the flow of aqueous humour, a fluid produced by the eye, into Schlemm's canal. Aqueous humour drains through the trabecular meshwork, a porous structure in the anterior chamber of the eye, into Schlemm's canal. The ciliary muscle's contraction and relaxation directly impact the pore size of the trabecular meshwork, influencing the rate of aqueous humour drainage.
Glaucoma is an eye condition associated with impaired drainage of aqueous humour and a rapid increase in intraocular pressure. In angle-closure glaucoma, the iris presses against the trabecular meshwork, obstructing the outflow of aqueous humour. Both open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) can be treated with muscarinic receptor agonists, such as pilocarpine. These drugs induce rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork and facilitating the drainage of aqueous humour into Schlemm's canal. This, in turn, leads to a decrease in intraocular pressure, providing relief in both types of glaucoma.
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Ciliary muscle and astigmatism
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 adjusting the eye's lens shape to focus on objects at varying distances. The ciliary muscle, along with the pupillary sphincter and dilator muscles, are referred to as intrinsic ocular or intraocular muscles.
When the ciliary muscle contracts, it pulls itself forward, moving the frontal region toward the axis of the eye. This contraction decreases the diameter of the ciliary muscle ring, causing the zonule fibres to relax and the lens to become more spherical. As a result, the lens's power to refract light for near vision increases. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance, improving long-range focus.
Now, let's discuss the connection between the ciliary muscle and astigmatism. Astigmatism is a refractive error in the eye, and studies have explored the potential link between astigmatism and ciliary muscle function. Some sources suggest that certain forms of astigmatism may be corrected by reinstating the correct neuromuscular message to the ciliary muscle. This message stimulates the ciliary muscle to offset the astigmatic condition by modifying the shape of the crystalline lens.
Additionally, there are indications that asymmetric contraction of the ciliary muscle may contribute to astigmatism of the lens. The oblique muscles' spasms can lead to astigmatism as the visual cortex interprets this tension as an eye focusing on nearby objects. This interpretation overrides depth perception information, disrupting the ability to bring distant images into focus. However, the treatment for simple myopic astigmatism involves using a plain contact lens to relax the oblique and ciliary muscles, allowing for the proper focusing of distant images.
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Frequently asked questions
Ciliary muscles are the muscles around the lens of the eye that can change the shape of the lens, but not the size of the pupil.
The ciliary muscle is formed of smooth muscle fibres, which are divided into longitudinal, radial, and circular groups.
Ciliary muscles control accommodation for viewing objects at varying distances. They also regulate the flow of aqueous humour into Schlemm's canal.
The ciliary muscle contracts and relaxes to change the shape of the lens. When contracted, it pulls itself forward and moves the frontal region toward the axis of the eye. This reduces zonular tension, increasing the convexity and refractive power of the lens for near vision.
