Final answer:
The Vagus nerve decreases the heart rate through parasympathetic stimulation, which releases ACh and causes the pacemaker cells in the heart to slow down. The autonomic nervous system balances this effect with sympathetic stimulation when necessary, increasing the heart rate in response to the body's needs.
Step-by-step explanation:
The Vagus nerve plays a critical role in regulating the heart rate by exerting a parasympathetic influence over the cardiac function. Specifically, the Vagus nerve decreases the heart rate by releasing acetylcholine (ACh) at the sinoatrial (SA) node of the heart, which binds to muscarinic receptors causing hyperpolarization of the pacemaker cells. This results in a slower depolarization rate and, consequently, a reduced heart rate. In a healthy young adult, when heart rate increases above 160 bpm, cardiac output typically decreases due to a decrease in ventricular filling time.
On the other hand, sympathetic nerves, through the release of norepinephrine, bind to adrenergic receptors, which causes the heart to depolarize faster, leading to an increase in heart rate. The cardiovascular centers in the medulla of the brainstem regulate this balance of sympathetic and parasympathetic input to the heart. Therefore, the autonomic nervous system intricately controls the heart rate through both acceleratory and inhibitory stimuli, adapting to the body's needs at any given moment.