Ciliary muscle

The ciliary muscle /ˈsɪli.ɛəri/ is a ring of smooth muscle in the eye's middle layer (vascular layer) that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye, not the size of the pupil which is carried out by the sphincter pupillae muscle and dilator pupillae.

Structure

Development

The ciliary muscle develops from the mesenchyme within the choroid and is considered a cranial neural crest derivative.

Innervation

The ciliary muscle receives only parasympathetic fibers from the short ciliary nerves that arise from the ciliary ganglion. These postganglionic fibers are part of cranial nerve V (Nasociliary nerve of the trigeminal).

Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III (the oculomotor nerve) and travel through the ciliary ganglion. 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. The zonule fibers relax and the lens becomes more spherical, increasing its power to refract light for near vision.