Which cardiac structures receive dual autonomic innervation enabling modulation by autonomic tone?

• A. SA and AV Nodes
• B. Purkinje Fibers
• C. Atrial Appendage
• D. Ventricular Myocardium

Answer: (A)

Autonomic tone can finely tune heart rate and the timing of electrical conduction by targeting structures that receive both sympathetic and parasympathetic input. The sinoatrial node, the heart’s natural pacemaker, speeds up with sympathetic stimulation and slows with parasympathetic (vagal) input. The atrioventricular node likewise receives both branches of autonomic innervation, adjusting conduction between atria and ventricles according to the balance of input. In contrast, Purkinje fibers and the ventricular myocardium are driven mainly by sympathetic input for contractility and conduction, while parasympathetic influence there is minimal, so they don’t exhibit the same dual innervation pattern. The atrial tissues do receive autonomic input as well, but the clearest dual innervation enabling modulation by autonomic tone is found in the SA and AV nodes.