Physiologic Anatomy of Cardiac Muscle Figure 9-2 shows the histology of cardiac muscle, demon- strating cardiac muscle fibers arranged in a latticework, with the fibers dividing, recombining, and then spreading again. One also notes immediately from this figure that cardiac muscle is striated in the same manner as in skel- etal muscle. Fuher, cardiac muscle has typical myofibrils that contain actin and myosin filaments almost identical to those found in skeletal muscle; these filaments lie side by side and slide along one another during contraction in the same manner as occurs in skeletal muscle (see Chapter 6). But in other ways, cardiac muscle is quite different from skeletal muscle, as we shall see. Cardiac Muscle as a Syncytium. The dark areas crossing the cardiac muscle fibers in Figure 9-2 are called intercalated discs; they are actually cell membranes that separate individual cardiac muscle cells from one another. That is, cardiac muscle fibers are made up of many indi- vidual cells connected in series and in parallel with one another. At each intercalated disc the cell membranes fuse with one another in such a way that they form permeable "com- municating" junctions (gap junctions) that allow rapid diffusion of ions. Therefore, from a functional point of view, ions move with ease in the intracellular fluid along the longitudinal axes of the cardiac muscle fibers so that Aoa Pulmonary aery Inferior vena cava Superior vena cava Right ventricle Tricuspid valve Pulmonary valve Right atrium Pulmonary veins Left atrium Mitral valve Aoic valve Left ventricle Lungs HEAD AND UPPER EXTREMITY TRUNK AND LOWER EXTREMITY Figure 9-1 Structure of the hea, and course of blood flow through action potentials travel easily from one cardiac muscle cell to the next, past the intercalated discs. Thus, cardiac muscle is a syncytium of many hea muscle cells in which the cardiac cells are so interconnected that when one of these cells becomes excited, the action potential spreads to all of them, from cell to cell throughout the latticework interconnections. The hea actually is composed of two syncytiums: the atrial syncytium, which constitutes the walls of the two atria, and the ventricular syncytium, which constitutes the walls of the two ventricles. The atria are separated from the ventricles by fibrous tissue that surrounds the atrio- ventricular (A-V) valvular openings between the atria and ventricles. Normally, potentials are not conducted from the atrial syncytium into the ventricular syncytium directly through this fibrous tissue. Instead, they are con- ducted only by way of a specialized conductive system called the A-V bundle, a bundle of conductive fibers sev- eral millimeters in diameter that is discussed in detail in Chapter 10. This division of the muscle of the hea into two func- tional syncytiums allows the atria to contract a sho time ahead of ventricular contraction, which is impoant for effectiveness of hea pumping. Ref guyton and hall textbook of medical physiology 12/e pg 101