How do the sympathetic and parasympathetic nervous systems interact to regulate heart rate?

The interaction between the sympathetic and parasympathetic nervous systems is crucial for the regulation of heart rate, functioning through a balanced mechanism known as autonomic control. The sympathetic nervous system is responsible for the body's 'fight or flight' response and helps to prepare the heart for increased activity. When activated, it releases neurotransmitters such as norepinephrine that bind to beta-adrenergic receptors in the heart, leading to an increase in heart rate and the force of contraction.

On the other hand, the parasympathetic nervous system counters this effect. It primarily uses the vagus nerve to release acetylcholine, which acts on muscarinic receptors, ultimately decreasing the heart rate. This action is predominant when the body is at rest or in a state of relaxation.

Therefore, the correct choice accurately reflects the distinct roles each system plays in heart rate regulation: the sympathetic system increases heart rate and contractility while the parasympathetic system decreases heart rate, ensuring a dynamic and responsive balance according to the body’s needs.