Ciliary muscle

# Overview
The ciliary muscle is a smooth muscle of the head that is responsible for accommodation of the eye

# 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 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.

# Innervation
Contraction of the lens happens when there is parasympathetic activation of the M3 muscarinic receptors on the ciliary muscles. 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.

# Function

## Accommodation
The ciliary fibers have circular (Ivanoff), longitudinal (meridional) and radial orientations.
According to Hermann von Helmholtz's theory, the circular ciliary muscle fibers affect 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 the focal distance, increasing long range focus.  Although Helmholtz's theory has been widely accepted since 1855, its mechanism still remains controversial.  Alternative theories of accommodation have been proposed by others, including L. Johnson, M. Tscherning, and Ronald A. Schachar.

## Trabecular meshwork pore size
Contraction and relaxation of the longitudinal fibers, which insert into the trabecular meshwork in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impeding aqueous humour flow into the canal of Schlemm.

# Clinical significance

## Glaucoma
Open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists (e.g., pilocarpine), which cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork, facilitating drainage of the aqueous humour into the canal of Schlemm and ultimately decreasing intraocular pressure.

# History

## Etymology
The word ciliary had its origins around 16851695.  The term cilia originated a few years later in 17051715, and is the Neo-Latin plural of cilium meaning eyelash.  In Latin, cilia means upper eyelid and is perhaps a back formation from supercilium, meaning eyebrow.  The suffix -ary originally occurred in loanwords from Middle English (-arie), Old French (-er, -eer, -ier, -aire, -er), and Latin (-arius); it can generally mean "pertaining to, connected with," "contributing to" and "for the purpose of."  Taken together, cili(a)-ary pertains to various anatomical structures in and around the eye, namely the ciliary body and annular suspension of the lens of the eye.