Ciliary muscle

Structure

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. 

Relations 

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. 

Innervation

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 accessory/Edinger-Westphal nucleus of oculomotor 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

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. 

Function

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 venous sinus of sclera (canal of Schlemm) which facilitates the draining of eye fluid.