Which of the following best describes the slowing of the heart rate caused by the activation of the parasympathetic nervous system?

The activation of the parasympathetic nervous system primarily affects heart rate by hyperpolarizing the resting potential of cardiac pacemaker cells, particularly in the sinoatrial (SA) node, which is responsible for initiating heartbeats. When the parasympathetic nervous system is activated, primarily through the release of acetylcholine, it binds to muscarinic receptors on the cardiac cells. This binding leads to increased potassium ion permeability, resulting in hyperpolarization of the cell membrane.

Hyperpolarization means that the cell's membrane potential becomes more negative, moving it further away from the threshold that triggers depolarization and the subsequent action potential responsible for heart contractions. As the resting potential is hyperpolarized, it decreases the spontaneous depolarization rate of the pacemaker cells, thereby slowing down the heart rate.

The other options focus on different mechanisms that do not align with how the parasympathetic nervous system operates in this context. Increasing the depolarization rate, enhancing sodium ion influx, or stimulating beta-adrenergic receptors would instead lead to an increase in heart rate or excitability, which is contrary to the effects of parasympathetic activation. Thus, the best description of the slowing heart rate due to parasympat