CVS Practice Test 2025 – Complete Exam Preparation

The vagus nerve is responsible for innervating the SA node through the parasympathetic nervous system (PNS). It plays a crucial role in regulating heart rate by releasing acetylcholine, which decreases the heart rate when stimulated. The PNS generally serves to promote a "rest and digest" state within the body, and its action on the SA node is vital for maintaining appropriate heart rhythms and preventing excessive heart rate, especially during times of rest or relaxation.

The other options relate to different aspects of cardiac innervation. The T1-T4 cardioaccelerators refer to the sympathetic nervous system, which increases heart rate and contractility, contrasting the effects of the vagus nerve. Thoracic nerves are involved in the sympathetic nervous system's influence on the heart but do not pertain to the parasympathetic innervation of the SA node. “Phi nerves” do not correspond to established anatomical terminology related to cardiac innervation. Thus, the vagus nerve is distinctly identified as the primary nerve influencing the SA node through PNS activity.