Cardiovascular Physiology

Abstract

Structure. The heart is composed of three types of cardiac muscle; atrial muscle fibers, ventricular muscle fibers, and other fibers specialized for initiation and conduction of action potentials rather than for generating force. The morphology of cardiac muscle cells is similar to skeletal muscle cells, since they both contain the contractile proteins, actin and myosin, and the regulator proteins, tropomyosin and troponin. Large numbers of elongated mitochondria are located close to myofibrils. Transverse tubules aid in conducting electrical activation toward the interior regions of cells. There is an extensive sarcoplasmic reticulum, which serves as an intracellular store of calcium. The sarcolemmal membranes of adjacent cells are joined by intercalated disks to form cardiac muscle fibers. Intercalated disks provide a strong mechanical connection between cells, allowing axial mechanical forces to be transmitted from one cell to another. There is no protoplasmic continuity between cells. However, gap junctions provide low resistance pathways for spread of electrical excitation to adjacent cells. Thus, cardiac muscle behaves as a functional syncytium. Action potentials initiated in one cell will spread to all other cells through the latticework of cellular interconnections. By the all-or-nothing principle an effective stimulus anywhere in the heart always activates the entire heart.