X. Smooth Muscle Tissue

A. Anatomy 1. Smooth muscle tissue is nonstriated and involuntary 2. Smooth muscle is considerably smaller than skeletal a. They are thick at the center and taper toward the ends b. Each contains a single, oval, centrally located nucleus

c. Myofilaments are not in orderly sarcomeres 3. Smooth muscle fibers contain intermediate filaments and dense bodies a. Function as Z- discs and anchoring fibers 4. Two types

a. Visceral (single unit) smooth. muscle is found in the walls of a. Visceral (single unit) smooth muscle is found in the walls of hollow viscera and in small blood vessels 1. Fibers are arranged in a network b. Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye 1. Fibers operate singly rather than as a unit

B. Physiology 1. The duration of contraction and relaxation of smooth muscle is longer than skeletal 2. The regulator protein that binds Ca is calmodulin instead of troponin a. Activates myosin light chain kinase, which facilitates myosin actin binding and allows contraction to occur at a slower rate

3. Contract in response to nerve impulses, hormones, and local factors 4. Can stretch considerably without developing tension