What are innervated by both the parasympathetic and sympathetic nervous system?

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

Answer: (D)

The key idea is how the autonomic nervous system regulates heart rate and conduction by targeting specific cardiac regions. The SA node (the heart’s natural pacemaker) and the AV node (the connection between atrial and ventricular conduction) receive dual innervation from both parasympathetic and sympathetic systems. Parasympathetic input via the vagus nerve releases acetylcholine onto M2 receptors in these nodes, slowing the rate of spontaneous depolarization in the SA node and increasing the delay through the AV node. This lowers heart rate and slows conduction. Sympathetic input via cardiac nerves releases norepinephrine onto beta-1 receptors, increasing cAMP, which speeds up depolarization in the SA node and increases conduction velocity through the AV node, raising heart rate and speeding conduction. Having both inputs allows precise, rapid adjustments to heart rhythm and rate in response to changing needs.

In contrast, Purkinje fibers and the ventricular myocardium are predominantly influenced by sympathetic input, with relatively limited parasympathetic innervation, so they are not innervated by both divisions in the same way. This makes the SA and AV nodes the primary regions that are innervated by both systems.