Final answer:
Fewer open Na+ and Ca++ channels during the pacemaker potential in cardiac pacemaker cells will lead to a slower heart rate by lengthening the time it takes to reach the action potential threshold.
Step-by-step explanation:
If there are fewer open sodium (Na+) and calcium (Ca++) channels during the pacemaker potential of a cardiac pacemaker cell, it will result in a slower heart rate. This is because the pacemaker potential phase is characterized by the slow and progressive increase in membrane potential due to the gradual influx of Na+ ions. When Na+ and Ca++ channels are fewer in number or less active, the influx of these cations is reduced, leading to a slower rate of depolarization and thus a longer time to reach the threshold for an action potential. Since the pacemaker cells of the SA node set the pace of the heart based on how quickly they reach the action potential threshold, fewer active channels will slow down the heart rate.
During the prepotential or spontaneous depolarization, the membrane potential slowly rises from -60 mV toward threshold due to Na+ influx. Once the threshold is reached, rapid depolarization occurs due to a significant influx of Ca++. If the activity of these channels is reduced, the slope of the pacemaker potential won't be as steep, and the time to reach the threshold will be extended, resulting in a decelerated heartbeat.