Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

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Prepare for the COMLEX-USA with quizzes featuring flashcards and multiple-choice questions, each with hints and explanations. Gear up for your osteopathic medicine exam!

Each practice test/flash card set has 50 randomly selected questions from a bank of over 500. You'll get a new set of questions each time!

Practice this question and more.

How does the PNS affect the heart compared to the SNS?

Increases heart rate
Decreases heart rate
Has no effect
Increases contractility

The correct answer is: Decreases heart rate

The correct answer is that the PNS, or parasympathetic nervous system, decreases heart rate. The parasympathetic fibers, primarily through the vagus nerve, release acetylcholine, which acts on muscarinic receptors in the heart. This action slows down the firing rate of the sinoatrial (SA) node, which is the heart's natural pacemaker, leading to a decrease in heart rate. In contrast, the sympathetic nervous system (SNS) increases heart rate through the release of norepinephrine, which acts on beta-1 adrenergic receptors in the heart. This stimulation enhances the speed of conduction through the heart and increases contractility (the strength of heart muscle contractions). The PNS's role in heart function is primarily restorative and calming, counterbalancing the excitatory and stimulating effects of the SNS, leading to a net decrease in heart rate during rest or relaxation. This understanding of the autonomic regulation of the heart is crucial for grasping cardiovascular physiology and the interplay between these two branches of the autonomic nervous system.