Iris sphincter muscle Iris sphincter muscle The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil. It is found in vertebrates and some cephalopods. Initially, all the myocytes are of the smooth muscle type but, later in life, most cells are of the striated muscle type. Its dimensions are about 0.75 mm wide by 0.15 mm thick. In humans, it functions to constrict the pupil in bright light (pupillary light
Smooth muscle chains are phosphorylated, they become active and will allow contraction to occur. The enzyme that phosphorylates the light chains is called myosin light-chain kinase (MLCK), also called MLC kinase. In order to control contraction, MLCK will work only when the muscle is stimulated to contract. Stimulation will increase the intracellular concentration of calcium ions. These bind to a molecule called calmodulin, and form a calcium-calmodulin complex. It is this complex that will bind to MLCK to activate it, allowing the chain of reactions for contraction to occur. Activation consists of phosphorylation of a serine on position 19 (Ser19) on the