Physiology of the Iris and Ciliary Body
The iris and ciliary body are the anterior parts of the uvea (also called uveal tract), which is continuous with the choroid posteriorly. The iris is a highly pigmented tissue that functions as a movable diaphragm between the anterior and posterior chambers of the eye to regulate the amount of light that reaches the retina. It is a delicate, dynamic structure that can make precise and rapid changes in pupillary diameter in response to light and specific pharmacologic stimuli. The ciliary body produces the aqueous humor, regulates its composition, and contributes to uveoscleral outflow, thereby directly influencing the ionic environment and metabolism of the cornea, lens, and trabecular meshwork.
The ciliary body is the main pharmacologic target in the treatment of glaucoma. Many of the agents used to lower intraocular pressure (IOP) in glaucoma, such as adrenergic and cholinergic drugs and prostaglandin analogues, work through receptors and their respective signal transduction pathways. The iris–ciliary body is rich in many types of receptors that bind to various ligands. Chapter 16 discusses these receptors and pharmacologic agents relevant to the treatment of glaucoma.
The ciliary body is a major contributor to the defense against oxidative stress, via molecules secreted into the aqueous humor, as discussed later in this chapter. It has the highest concentration of redox (oxidation-reduction) enzymes in the anterior segment. The ciliary body also contains proteins of the cytochrome P450 family, though only a small number compared with the liver. These enzymes are involved in detoxification, whereby they convert hydrophobic compounds to hydrophilic ones via hydroxylation. CYP2D6 is one such enzyme that metabolizes timolol. It is expressed in low levels in ocular tissues but is abundant in the liver.
See Chapter 2 of this volume for further discussion of the structures mentioned in this section.
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.