Dominic Stone
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 and receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion. The ciliary muscle is responsible for controlling the eye's accommodation for viewing objects at varying distances, changing the shape of the lens to enable near or far vision.
| Characteristics | Values |
|---|---|
| Definition | An intrinsic muscle of the eye |
| Shape | Ring of smooth muscle |
| Location | Eye's middle layer, the uvea (vascular layer) |
| Control | Accommodation for viewing objects at varying distances |
| Regulation | Flow of aqueous humour into Schlemm's canal |
| Development | From mesenchyme within the choroid |
| Nerve Supply | Short ciliary nerves from the ciliary ganglion |
| Innervation | Parasympathetic and Sympathetic |
| Function | Changes the shape of the lens |
| Glaucoma Treatment | Muscarinic receptor agonists cause contraction of ciliary muscles, facilitating drainage |
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What You'll Learn
The ciliary muscle is an intrinsic muscle of the eye
The ciliary muscle plays a crucial role in vision by controlling the shape of the lens within the eye. When the ciliary muscle contracts, it relaxes the zonular fibres, allowing the lens to become more spherical and adapt to near or short-range vision. Conversely, when the ciliary muscle is relaxed, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range vision. This process is known as the accommodation reflex, and it allows us to focus on objects at varying distances.
The ciliary muscle is innervated by the parasympathetic and sympathetic subdivisions of the autonomic nervous system. Parasympathetic activation causes the ciliary muscle to contract, while sympathetic activation relaxes the muscle. The ciliary muscle receives parasympathetic input 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 sympathetic input to the ciliary muscle comes from the superior cervical ganglion.
The ciliary muscle also plays a role in maintaining intraocular pressure by regulating the flow of aqueous humour, also known as aqueous fluid. The ciliary body produces this fluid, which nourishes the lens and cornea. The aqueous humour drains into the canal of Schlemm, and the ciliary muscle helps control the drainage process by contracting to open the trabecular meshwork. This function is particularly important in the treatment of glaucoma, where increased intraocular pressure can cause damage to the optic nerve.
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It controls accommodation for viewing objects at varying distances
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 accommodation, allowing us to view objects at varying distances.
The ciliary muscle achieves this by altering the shape of the lens within the eye. When focusing on a distant object, the ciliary muscle is relaxed, and the zonular fibres—which act as suspensory ligaments that attach to the lens—are tightened, flattening the lens. Conversely, when focusing on a close object, the ciliary muscle contracts, releasing tension on the lens caused by the zonular fibres. This release of tension allows 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). The ciliary muscle is also innervated by sympathetic subdivisions of the autonomic nervous system.
The ciliary muscle is composed of three layers of muscle fibres with different orientations: longitudinal, radial, and circular. These muscle fibres work together during contraction to facilitate the accommodation reflex. The longitudinal fibres are responsible for the anterior shift in muscle mass, while the radial and circular fibres are responsible for the inward movement of muscle mass.
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Glaucoma treatments target ciliary muscles
Glaucoma is a group of eye conditions that result in damage to the optic nerve and can eventually lead to blindness. The most common cause of irreversible vision loss, it affects over 70 million people worldwide. 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 the eye's ability to view objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It also changes the shape of the lens within the eye but not the size of the pupil.
The ciliary muscle, along with the pupillary sphincter muscle and pupillary dilator muscle, is sometimes called an intrinsic ocular muscle or intraocular muscle. 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).
Both types of glaucoma—open-angle glaucoma (OAG) and closed-angle glaucoma (CAG)—can be treated with muscarinic receptor agonists, such as pilocarpine, that cause the ciliary muscle to contract and lead to the opening of the trabecular meshwork. This facilitates the drainage of the aqueous humour into the canal of Schlemm and ultimately decreases intraocular pressure.
Gene therapy for glaucoma has also been explored by targeting the ciliary body. In one study, the ShH10 serotype was used to deliver a single vector CRISPR-Cas9 system disrupting Aquaporin 1, resulting in reduced IOP in treated eyes compared to control and non-injected eyes. This approach could benefit patients with glaucoma by providing a new treatment that could lower IOP following a single intravitreal injection.
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Ciliary muscles are innervated by sympathetic and parasympathetic nerves
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 the eye's accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It also changes 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 is dually and reciprocally innervated by sympathetic and parasympathetic subdivisions of the autonomic nervous system. The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion is a parasympathetic ganglion that is located behind the eye and contains about 2500 neurons. These parasympathetic fibres arise from cranial nerve V, also known as the nasociliary nerve of the trigeminal. 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.
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. This allows the lens to become more spherical, increasing its power to refract light for near vision. The parasympathetic tone is dominant when a higher degree of accommodation of the lens is required, such as when reading a book.
There is some evidence that the ciliary muscle also receives innervation from the sympathetic fibres of the autonomic nervous system (ANS). These fibres are believed to provide the inhibitory impulses that inhibit the accommodation reflex.
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The ciliary body is an inner eye structure
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). The ciliary muscle also receives sympathetic innervation from the long ciliary nerves. The muscle fibres have their fixed origin at the scleral spur and their movable posterior insertion attached posteriorly to the choroid at the ora serrata of the retina. The outermost longitudinal layer of ciliary muscle inserts into the anterior one-third of the choroid, while the radial fibres insert into the connective tissue of the base of the ciliary processes.
The ciliary muscle controls the process of accommodation, which allows us to view objects at varying distances. When looking at a distant object, the ciliary muscle is relaxed, the zonular fibres are tightened, and the lens is flattened. Conversely, when looking at a close object, the ciliary muscle contracts, the zonular fibres relax, and the lens becomes more spherical. This change in the shape of the lens is necessary for focusing on objects at different distances.
The ciliary body also makes aqueous fluid, which nourishes the lens and cornea. The ciliary processes, which are protrusions on the internal surface of the ciliary body, secrete this aqueous fluid. The ciliary body receives its blood supply from the branches of the ophthalmic artery, including the long posterior ciliary arteries and the anterior ciliary arteries. The blood flow through the ciliary body is about 7% of the total ocular flow.
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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 the eye's ability to view objects at varying distances, changing the shape of the lens to focus on objects.
Ciliary muscles work with zonular fibres, which act as suspensory ligaments that attach to the lens. When the ciliary muscle contracts, it relaxes the zonular fibres, allowing the lens to become more spherical, adapting to short-range focus.
The word 'ciliary' originated in 1685-1695, with the term 'cilia' (the Neo-Latin plural of cilium, meaning 'eyelash') coming into use in 1705-1715. Together, cili(a)-ary pertains to various anatomical structures around the eye.
The ciliary body can be affected by inflammation, infections, tumours, masses, and other diseases. Severe infections can cause inflammation and then cause the ciliary body to shut down.
