The ciliary muscle of the eye is a ring of striated smooth muscle, which 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 muscle's innervation are 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 functioning of this muscle to proceed smoothly. However, the adrenergic tone is more dominant as compared to the parasympathetic tone.
Accommodation of the Eye
The most important function of this muscle is accommodation. It 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, the eye manages to focus on objects that are at a distance or are nearby. 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, which is induced by ciliary muscle contraction. There is an age related decline in the amount of accommodation, which 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 any nearby object, the circular muscle fibers contract, reducing the zonular tension, and allowing the lens to round up and increase its optical power. When the eye views a distant object, the fibers relax, causing an increase in the zonular tension. This causes the surfaces of the lens to flatten, and thus, decreases its optical power. Simply said, when there is a contraction of the ciliary muscle, the lens rounds up and vice versa.
Regulation of Aqueous Humor
Aqueous humor is the liquid that is present in the eye. The amount of this liquid is controlled by contraction and relaxation of the longitudinal fibers of the ciliary muscle. These fibers insert into the trabecular mesh-work, which is present in the anterior eye chamber. This causes an increase and decrease in the meshwork pore size, 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 the accommodation of the eye is hindered. This can be temporary or permanent. It is claimed that there are certain muscle exercises, which can help to prevent this condition. The pathological significance of this muscle lies in the fact that it is useful when treating glaucoma. This is an eye disease, wherein there is an increase in the amount of aqueous humor in the eye, leading to an increase in the intra-ocular pressure, which can even lead to blindness if not treated in time. One way of treating closed angle glaucoma (a type of this disease), is by causing constriction of the lens by contraction of the ciliary muscles. This leads to opening of the tubular meshwork, facilitating drainage of the aqueous humor into the canal of Schlemm, and leading to a decrease in the intra-ocular pressure.