Final answer:
Epinephrine and norepinephrine increase heart rate by binding to β-adrenergic receptors on myocardial pacemaker cells, thereby increasing the rate of depolarization and decreasing the time to reach the threshold for action potentials.
Step-by-step explanation:
Epinephrine and norepinephrine are catecholamines released by the adrenal medulla that play a central role in the body's 'fight-or-flight' response. When these substances bind to β-adrenergic receptors on myocardial pacemaker cells, a cascade of events occurs leading to an increased heart rate. These hormones bind to β1-receptors, which results in the opening of sodium and calcium ion channels. The influx of these positively charged ions into the myocardial pacemaker cells causes a faster rate of depolarization and, consequently, a reduced threshold for action potential generation.
The increased depolarization rate means the cardiac cells reach their threshold and fire action potentials more quickly. This increase in the rate of action potential generation translates to a higher heart rate. Moreover, epinephrine and norepinephrine also shorten the repolarization period of cardiac cells, allowing them to recover and prepare for the next action potential faster. It's crucial to note that while these effects enhance the fight-or-flight response, excessive release of these hormones, along with extensive sympathetic stimulation, may lead to arrhythmias. The adrenal medulla does not receive parasympathetic stimulation, so the balance between the sympathetic and parasympathetic systems is vitally important for maintaining normal heart function.