Ciliary Muscle Function

The ciliary muscle of the eye is a ring of striated smooth muscle that is present in the middle layer of the eye. This is an important involuntary muscle as it helps to control the amount of light entering the eye. The ciliary muscle innervation is both sympathetic and parasympathetic. It receives these fibers from the ciliary ganglion, which gives off short ciliary nerves. These fibers are a part of the ophthalmic branch of the trigeminal nerve. This dual innervation helps the ciliary muscle functioning to proceed smoothly. However, the adrenergic tone, when compared to the parasympathetic tone is more dominant. Given below are details regarding ciliary muscle functions and how it carries them out.

Accommodation of the Eye
The most important function is accommodation. Accommodation is the process by which the eye manages to change its optical power so as to maintain a clear image on an object. In simple words, this is the process by which the eye manages to focus on objects that are at a distance or are near. Thus, it helps the eye to adjust to changes in the vision field within a matter of less than a second. Although accommodation is a reflex, it can also be a controlled movement. This is the reason why the human eye can change focus from a considerable distance to a distance less than 7 cm from the eye in approximately 350 milliseconds. This is brought about due to a reduction in the zonular tension that is induced by ciliary muscle contraction. There is an age related decline in the amount of accommodation that can be done by the human eye.

There are many theories as to how the ciliary muscle functions to bring about accommodation of the eye. The most widely believed theory is that of Hermann von Helmholtz. This theory claims that when the eye tries to focus at an object that is near to the eye, the circular muscle fibers of the ciliary muscle contract, thus, reducing the zonular tension and allowing the lens to round up and increase its optical power. When the eye views a distant object, the ciliary muscle fibers relax, causing an increase in the zonular tension. This increase in the zonular tension causes the surfaces of the lens to flatten and thus, decreases the optical power of the lens. So, when there is contraction of ciliary muscle, lens rounds up and vice versa.

Regulation of Aqueous Humor
Another important function of this muscle is to regulate the amount of aqueous humor. Aqueous humor is the liquid that is present in the eye. The amount of aqueous humor is controlled by contraction and relaxation of the longitudinal fibers of the ciliary muscle. These fibers insert into the trabecular meshwork that is present in the anterior chamber of the eye. This causes an increase and decrease in the meshwork pore size, thus, facilitating and impeding aqueous humor flow into the canal of Schlemm, depending on the requirement of the eye.

There are many pathologies that can affect the ciliary muscle. One such pathology is ciliary muscle spasm, due to which accommodation of the eye is hindered. This can be temporary or permanent. It is claimed that there are certain ciliary muscle exercises which can help prevent this condition. The pathological significance of ciliary muscle function lies in the fact that it is useful when treating glaucoma. This is an eye disease where there is an increase in the amount of aqueous humor in the eye. This leads to an increase in the intra-ocular pressure, which can even lead to blindness if not treated in time. One way of treating a type of glaucoma, known as closed angle glaucoma, is by causing rapid miosis, that is, constriction of the lens by contraction of the ciliary muscles. This leads to opening of the tubular meshwork thus, facilitating drainage of the aqueous humor into the canal of Schlemm leading to a decrease in the intra-ocular pressure.