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are more richly innervated by sympathetic fibers than parasympathetic fibers.

Correct answer: True

Purkinje fibers are specialized conduction fibers located in the subendocardial layer of the ventricles, extending from the bundle branches to distribute impulses throughout the ventricular myocardium. Under normal conditions, they rapidly conduct impulses initiated by the SA node via the AV node to coordinate ventricular contraction. If the AV node is blocked, the Purkinje fibers can act as a backup pacemaker, generating spontaneous impulses at an intrinsic rate of approximately 30–40 beats per minute. This slower rate maintains minimal ventricular contraction, but it is insufficient to sustain adequate cardiac output for normal physiologic demands over extended periods.

A. SA node: The sinoatrial node is the primary pacemaker of the heart. It is richly innervated by both sympathetic and parasympathetic fibers, allowing autonomic modulation of heart rate. Its sympathetic and parasympathetic balance is critical for initiating and regulating cardiac rhythm rather than forceful contraction.

B. Atria: The atrial myocardium receives autonomic innervation from both sympathetic and parasympathetic fibers. Parasympathetic input via the vagus nerve predominates in controlling heart rate and atrial conduction, whereas sympathetic input modulates contractility modestly. Atrial innervation is less dominated by sympathetic fibers compared with the ventricles.

C. Ventricles: Ventricular myocardium is more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation increases ventricular contractility (positive inotropy) and conduction velocity, which enhances stroke volume and cardiac output during stress or exercise. Parasympathetic influence on ventricles is minimal.

D. AV node: The atrioventricular node is innervated by both sympathetic and parasympathetic fibers. Parasympathetic input slows conduction through the AV node, prolonging the PR interval, while sympathetic stimulation increases conduction velocity. However, its innervation is not predominantly sympathetic compared with ventricular myocardium.