Final answer:
The vagus nerve acts as a 'brake' on the heart by exerting parasympathetic stimulation, which releases acetylcholine and slows the heart rate, demonstrating the heart's parasympathetic tone in its resting state.
Step-by-step explanation:
The Vagus Nerve as a 'Brake' on the Heart
The vagus nerve plays a crucial role in the regulation of heart rate (HR) by acting as a 'brake' through parasympathetic stimulation. The cardioinhibitor centers in the medulla oblongata decrease heart activity via the vagus nerve, which, upon stimulation, releases the neurotransmitter acetylcholine (ACh). ACh works to slow HR by opening potassium ion channels, thus prolonging depolarization and increasing the time needed for subsequent depolarizations. This process results in a lower resting heart rate than the intrinsic rate at which the SA node would fire if left unregulated. Hence, the vagus nerve's role can be compared to a person restraining a car's speed by applying the brake pedal; to increase HR, one would reduce the parasympathetic input, lowering ACh release and allowing heart rate to rise.
Autonomic tone, which refers to the balance between sympathetic and parasympathetic stimulation, is evidenced by a resting state that is generally dominated by parasympathetic activity, particularly in the heart's functioning, which is why the normal resting heart rate is around 72 beats per minute, significantly less than the intrinsic heart rate of about 100 bpm.