Figure 32.8: The Process of Activation of the Mammalian Heart during Each Heartbeat The muscle cells of the heart, like virtually all muscle cells, are activated to contract by electrical depolarization of their cell membrane. (A) Structures composed of modified muscle cells—the S-A node, A-V node, and conducting system—initiate and coordinate the spread of depolarization through the heart during each heartbeat. (B) Muscle cells in pink areas are depolarized during the successive stages shown. Cells in white areas are not depolarized; they are either awaiting depolarization or have repolarized following depolarization.