The Ciliary Muscle is a muscle in the eye that controls the eye's accommodation for viewing objects at varying distances.
Mode of action
The ciliary muscle affects zonular fibers
in the eye
(fibers that suspend the lens
in position during accommodation
), enabling changes in lens shape for light focusing. When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibers (fibers that hold or flatten the lens). This release of tension of the zonular fibers causes the lens to become more spherical, adapting to short range focus.
The other way around, relaxation of the ciliary muscle causes the zonular fibers to become taut, flattening the lens, increasing long range focus.
Contraction of the lens happens when there is parasympathetic
activation of the M3 muscarinic receptors
on the ciliary muscles. The parasympathetic signal is carried by cranial nerve III (the oculomotor nerve
) synapsing on the ciliary ganglion
. This leads to contraction of the ciliary muscles, a consequent reduction in the size of the ciliary body, and a lessening of the tension on the lens, hence allowing the lens to spring back into a more spherical shape to accommodate for close vision.
Unlike the muscles of the iris (which receives both types of autonomic innervation--the iris sphincter is exclusively innervated by parasympathetics and the iris dilator exclusively by sympathetics), the ciliary muscle receives only parasympathetic innervation.