Final answer:
The autonomic nervous system influences heart rate via the sympathetic and parasympathetic branches, which release norepinephrine and acetylcholine, respectively. These neurotransmitters affect pacemaker cells by altering the rate of depolarization and hyperpolarization. Drugs can also affect heart rate by mimicking or interfering with these neurotransmitters.
Step-by-step explanation:
Autonomic Nervous System and Heart Rate
The autonomic nervous system regulates heart rate through its sympathetic and parasympathetic fibers. When norepinephrine (NE) is released onto cardiac cells, it binds to adrenergic receptors causing depolarization to occur more quickly, thereby increasing heart rate.
Conversely, when acetylcholine (ACh) is released, it binds to muscarinic receptors and induces hyperpolarization, making it harder for the cells to reach the depolarization threshold and slowing down the heart rate. In the absence of parasympathetic input, the heart would naturally beat at approximately 100 bpm. However, sympathetic stimulation during activities such as exercise can increase the rate to 120-140 bpm, whereas parasympathetic stimulation slows it to the normal resting rate of 60-80 bpm.
Additionally, drugs affecting autonomic function, such as sympathomimetic or sympatholytic drugs, can significantly alter the heart rate by mimicking or interfering with these neurotransmitters and their receptors.