The intrinsic muscles of the eye are muscles that control the movements of the lens and pupil and thus participate in the accommodation of vision. There are three smooth muscles that comprise this group; ciliary, dilatator pupillae and sphincter pupillae muscles.
The ciliary muscle occupies the biggest portion of the ciliary body, which lies between the anterior border of the choroid and iris. It is composed of smooth muscle fibers oriented in three different directions; longitudinal, radial and circular. Together with the sphincter pupillae, the ciliary muscle functions are mainly instructed by the parasympathetic nerve fibers of oculomotor nerve (CN III).
The contraction of the ciliary muscle loosens the zonular fibers increasing the convexity of the lens, which induces accommodation for near vision. It also regulates the pore size of the trabecular meshwork.
|Insertion||Longitudinal part: anterior one-third of the choroid;
Radial part: connective tissue near the base of the ciliary processe
|Action||Accommodation; regulation of trabecular meshwork pore size|
|Innervation||Parasympathetic component of oculomotor nerve (CN III)|
|Blood supply||Long posterior ciliary arteries; vorticose veins|
In this article, we will discuss the anatomy and function of ciliary muscle.
- Origin and insertion
- Blood supply
- Clinical relations
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 the fingerlike ciliary processes. The ciliary processes are attached to the lens via zonular fibers. Through this indirect attachment, the ciliary muscle acts on the lens facilitating the accommodation.
The layers of ciliary muscle are described differently by several authors in the literature, but the most used classification divides this muscle into three separate layers;
- The longitudinal layer is the outermost muscle layer situated adjacent to the loose connective tissue of the ciliary body.
- The middle layer is the radial layer of muscle. It 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. This layer is typically well-developed in hypermetropic (being longsighted), but rudimentary or absent in myopic eyes (shortsighted).
Origin and insertion
The attachments of ciliary muscle are yet to be determined. According to some authors, the ciliary muscle originates from a protrusion of the sclera into the anterior chamber of the eye, also known as the scleral spur. The outermost longitudinal layer of ciliary muscle inserts onto the anterior one-third of the choroid, while the radial fibers insert onto the connective tissue of the base of ciliary processes.
The ciliary muscle occupies the biggest portion of the anterior aspect of the ciliary body, with ciliary processes lying posterior to it. It comprises the lateral wall of the posterior chamber of the eye. The outer surface of the muscle lays parallel to the sclera, while the innermost layer of the muscle is located near the major circle of the iris.
The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibers) that arise from the ciliary ganglion. The ciliary ganglion receives preganglionic fibers via the oculomotor nerve (cranial nerve III) that originate from the Edinger-Westphal nucleus in the midbrain.
There is a shred of existing evidence in the literature that ciliary muscle also receives innervation from the sympathetic fibers of the autonomic nervous system (ANS). Allegedly these fibers provide the inhibitory impulses and thus inhibit the accommodation reflex.
Blood supply for the ciliary body, and with it the ciliary muscle, comes from the branches of the ophthalmic artery;
- Anterior ciliary arteries
- Long posterior ciliary arteries
These arteries form the major arterial circle located near the root of the iris. Besides the ciliary body, they also supply the iris and the anterior aspect of the choroid. The blood from the ciliary body is drained by vorticose veins (vortex veins). These veins drain into superior orbital and inferior orbital veins which are tributaries to the cavernous sinus and pterygoid plexus respectively.
The main action of ciliary muscle is changing the shape of the lens which occurs during the accommodation reflex. In addition, when contracting, the longitudinal fibers of ciliary muscle widen the iridocorneal space and canal of Schlemm which facilitates the draining of eye fluid.
The state of the ciliary muscle changes depending if we observe distant or close objects. When looking at the distant object, the ciliary muscle is relaxed, the zonular fibers are tightened and the lens is flattened. In this state the refractive power of the lens is enough to form a clear image of the focused object on the retina. However, in order to focus on a close object, the inner structures of the eye must adapt, which is possible through the process of accommodation.
The accommodation reflex consists of three responses;
- Increase of the curvature of the lens
- Constriction of the pupil
- Convergence of the eyes
The reflex pathway includes optic nerve (CN II), visual and frontal cortex, oculomotor and accessory oculomotor nuclei and oculomotor nerve (CN III). In need of accommodation, the optic nerve sends initial impulses to the primary visual cortex through the lateral geniculate body and the optic radiation. From here the impulse travels to the Edinger-Westphal nucleus in the midbrain through the visual association cortex. Efferent parasympathetic fibers carried by the oculomotor nerve synapse with the ciliary ganglion, which sends postganglionic fibers to the ciliary muscle.
The action of ciliary muscle is instructed by the parasympathetic fibers originating from the Edinger-Westphal nucleus in the midbrain. The contraction of this muscle loosens the zonular fibers allowing the lens to relax. When the lens relaxes, its degree of curvature increases, making it rounder. This way the refractive power of the lens is increased allowing the creation of sharply focused images of the near objects on the retina.
The aqueous humor is a transparent fluid located within the eye that supplies the inner structures with nutrients and maintains the intraocular pressure. It is secreted by the cells of the ciliary body into the posterior chamber of the eye. It circulates through the pupil entering to the anterior chamber. From here, the fluid exits the eye through the small canas located on the limbus of the eye (the canals of Schlemm). The secretion process of aqueous humor must be in synergy with the drainage. If this is not the case, the accumulation of this fluid can cause an increase in intraocular pressure, which is the main risk factor for glaucoma.
Glaucoma is a group of conditions that results in damage to the optic nerve. It is the most common cause of irreversible vision loss affecting more than 70 million people worldwide. This condition can be detected through ophthalmoscopy and tonometry. Glaucomas are classified into two categories;
- Open-angle glaucoma - the most common type which involves sclerosis of the canals of Schlemm
- Closed-angle glaucoma - less common type that represents a medical emergency in which the resorption of aqueous humor is blocked
Both types of glaucoma are treated with muscarinic receptor agonists that cause the contraction of ciliary muscle leading to the opening of the trabecular meshwork. Once this happens, the aqueous humor can flow through the canal of Schlemm and eventually lead to a decrease in intraocular pressure.