Final answer:
Norepinephrine and acetylcholine both influence pacemaker cells by altering their depolarization rate; however, they produce opposite effects on the heart rate with norepinephrine increasing it and acetylcholine generally decreasing it.
Step-by-step explanation:
The actions of norepinephrine and acetylcholine on pacemaker cells in the heart are similar in that both neurotransmitters can affect the heart rate by influencing the rate of depolarization in these cells. Norepinephrine binds to beta-1 adrenergic receptors, initiating a signaling cascade that results in an increased rate of depolarization and therefore an increased heart rate. Similarly, acetylcholine affects pacemaker cells by binding to muscarinic receptors, which leads to a decrease in heart rate; however, the mechanism by which they create these effects differs substantially. While acetylcholine generally acts to decrease heart rate by increasing potassium efflux and inhibiting cyclic AMP production, norepinephrine increases heart rate by promoting calcium influx and cyclic AMP production. Despite their different outcomes, the similarity lies in their capacity to modulate pacemaker activity, albeit in opposite directions.