The 3 layers of fibers in the ciliary muscle (Fig 2-24) are
Most of the ciliary muscle is made up of the outer layer of longitudinal fibers that attach to the scleral spur. The radial muscle fibers arise in the midportion of the ciliary body, and the circular fibers are located in the innermost portion. Clinically, the 3 groups of muscle fibers function as a unit. Presbyopia is associated with age-related changes in the lens (discussed in the section Lens, later in this chapter) rather than in the ciliary muscle. Even so, the muscle does change with age: the amount of connective tissue between the muscle bundles increases, and there is a loss of elastic recoil after contraction.
Figure 2-24 Schematic representation of the arrangement of the smooth muscle fibers in the ciliary body. Note the relationship of the ciliary body to the iris, the anterior chamber, the Schlemm canal, and the corneoscleral limbus.
(Modified with permission from Snell RS, Lemp MA. Clinical Anatomy of the Eye. Cambridge, MA: Blackwell Scientific Publications; 1989.)
The ciliary muscle fibers behave like other smooth, nonstriated muscle fibers. Ultrastructural studies reveal that they contain multiple myofibrils with characteristic electrondense attachment bodies, mitochondria, glycogen particles, and a prominent nucleus. The anterior elastic tendons insert into the scleral spur and around the tips of the oblique and circular muscle fibers as they insert into the trabecular meshwork. Both myelinated and unmyelinated nerve fibers are observed throughout the ciliary muscle.
Innervation is derived mainly from parasympathetic fibers of the third cranial nerve via the short ciliary nerves. Approximately 97% of these ciliary fibers are directed to the ciliary muscle, for accommodation, and about 3% are directed to the iris sphincter. Sympathetic fibers have also been observed and may play a role in relaxing the muscle. Cholinergic drugs contract the ciliary muscle. Because some of the muscle fibers form tendinous attachments to the scleral spur, their contraction increases aqueous flow by opening up the spaces of the trabecular meshwork.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.