What happens to heart rate during over stimulation of the parasympathetic nervous system?

When the parasympathetic nervous system is overstimulated, it predominantly exerts an inhibitory influence on heart rate through the release of acetylcholine. This neurotransmitter acts on the heart’s pacemaker cells, specifically in the sinoatrial (SA) node, leading to a decrease in the heart rate. The mechanism of action involves increasing the permeability of the heart cells to potassium ions, resulting in hyperpolarization of the cells and slowing down depolarization. Consequently, when the parasympathetic nervous system is overly active, it can cause bradycardia, which is a significant reduction in heart rate.

This physiological response is essential for maintaining the body's homeostasis, particularly in resting states or during relaxation. It contrasts with the sympathetic nervous system, which raises heart rate during stress or physical activity. Understanding this balance between the two branches of the autonomic nervous system is crucial in the field of cardiac sonography and can help in interpreting patient conditions and their implications on heart function.