What effect does the parasympathetic nervous system have on the heart?

The parasympathetic nervous system primarily reduces the heart rate and contractile force through the action of acetylcholine, a neurotransmitter it releases. The vagus nerve, one of the major components of the parasympathetic nervous system, plays a critical role in this process. When stimulated, the vagus nerve acts on the heart's pacemaker, the sinoatrial (SA) node, leading to a decrease in heart rate. Additionally, through its effects on the atrioventricular (AV) node and myocardium, the parasympathetic system can also reduce the strength of contractility in the heart's ventricles. This is part of the body's mechanism to promote rest and digestion during states of relaxation or lower activity, which is essential for maintaining homeostasis after periods of stress or activity.

In contrast, options involving increases in heart rate or blood volume pertain to the effects of the sympathetic nervous system, which is responsible for the body's 'fight or flight' response, while respiratory rate alterations relate more directly to the respiratory system rather than the heart specifically.