Which cells are primarily responsible for contraction of atria and ventricles in the heart?

• A. Sinoatrial and atrioventricular nodes
• B. Atrial and ventricular contracting myocytes; Purkinje fibers
• C. Purkinje fibers exclusively
• D. Pacemaker cells

Answer: (B)

The key idea is which cells actually generate the force of contraction in the heart chambers. The contraction of atria and ventricles is produced by the atrial and ventricular contracting myocytes—the cardiac muscle cells that shorten to push blood. The rhythm and timing come from pacemaker tissues like the sinoatrial and atrioventricular nodes, which set the pace, and Purkinje fibers, which rapidly spread the impulse to coordinate ventricular contraction. These conduction components organize when and how the heart contracts, but they do not themselves provide the muscle force. Pacemaker cells initiate impulses, while the contracting myocytes do the work.