MUSCLES

Characteristically, all living cells, including protozoa and slime molds, contain the contractile proteins actin and myosin. Thus, actin and myosin are present in all the cells of the human body—from the most highly differentiated nerve cells to the shed fragments of megakaryocyte cytoplasm, the platelets, which are important in the formation of blood clots. Actin and myosin are arranged in the cytoplasm of a cell to interact and slide in relationship to one another to produce contraction of the cell when driven by the energy supplied by the hydrolysis of adenosine triphosphate.

During the evolution of single-celled protozoa into metazoa, or multicellular organisms, cells became specialized to perform specific functions. Certain cells accumulated larger than usual amounts of actin and myosin in their cytoplasm to become muscle cells scattered throughout the body of the primitive metazoan. As the higher forms developed distinct organ systems, the muscle cells grouped together to become the smooth (involuntary, visceral, nonsegmental) muscles of the viscera and blood vessels.

SMOOTH AND CARDIAC MUSCLE

All the smooth and cardiac muscle cells in the human embryo arise from mesoderm, except the sphincter and dilator smooth muscles of the iris of the eye and the myoepithelial cells of the sweat and mammary glands, which arise from ectoderm. Both smooth and cardiac muscle cells have a centrally placed nucleus. During development, numerous smooth muscle cells become elongated in the same direction and form layers, such as the circular and longitudinal smooth muscle layers of the small intestine.

For a time during evolution, a simple layer of smooth muscle surrounding the vessels of the circulatory system was also sufficient for the demands of function. However, as organisms became larger and increasingly complex, the need arose for the system to have a strong pump—the heart. In the human embryo, two endothelial tubes fuse to become one vessel, which then becomes surrounded with mesenchyme that differentiates into cardiac. The muscle cells surrounding the developing heart accumulated a larger amount of more compactly and more orderly arranged actin and myosin molecules than did simple smooth muscle cells. Despite undergoing repeated mitotic divisions, they remained attached to one another in such a manner that they formed long tubes of cells known as fibers.