ABSTRACT
Contractility is an intrinsic property of all cells that is necessary for the creation of basic functions involving movements such as cell division and motility. These cells may function as individual contractile units, or may be aggregated to form special tissue – muscle. The contractile apparatus consists of fibrillar proteins set in a structured manner in the cytoplasm, and it is connected by intermolecular bonds. The contractions result from the reorganization of the intermolecular bonds with the utilization of chemical energy. Three different types of muscle tissues may be distinguished: 1) Striated muscle is composed of long multinucleated cells named muscle fibers comprising sets of fine myofibrils, each composed of discrete series of contractile units termed sarcomeres. 2) Smooth muscle forms muscular components of several structures such as the intestinal tract and blood vessels. They are composed of single spindle shape cells, fibers that run parallel to each other. The cytoplasm of smooth muscle fibers is homogenous and lacks the alternating dark and light bands characteristic of striated muscles. The contractile proteins of the smooth muscle fibres are composed of filaments that run as parallel fine threads along the long axis of the fibers. They represent myosin and actin units, respectively, that are similar to those of striated muscle. 3) Cardiac muscle is composed of a population of single elongated cells named cardiomyocytes that are linked together by specialized junctions to form physiological units. Cardiac muscle has several structural and functional characteristics of smooth and striated muscle. Particular interest in this chapter is devoted to the molecular mechanisms of contraction.
