What feature of cardiac muscle supports its autorhythmicity?

• A. Pacemaker cells in the SA node with gap junctions enabling coordinated automatic electrical activity.
• B. A high density of troponin complexes in sarcomeres.
• C. Absence of intercalated discs between cardiomyocytes.
• D. A lack of mitochondria in most fibers.

Answer: (A)

Autorhythmicity in the heart comes from specialized pacemaker cells that initiate electrical impulses on their own. In the sinoatrial node, these cells depolarize spontaneously and set the heartbeat pace. Gap junctions between cardiac cells then allow the impulse to spread quickly and uniformly across the atria and into the ventricles, producing a coordinated rhythm. The other features described don’t drive rhythm: a high troponin density mainly affects contraction regulation, not automatic pacing; absence of intercalated discs would disrupt electrical coupling rather than support automaticity; and cardiac muscle cells rely on abundant mitochondria to meet their energy needs, not a lack of them.