Final answer:
Norepinephrine accelerates depolarization of pacemaker cells by opening sodium and calcium ion channels, which increases the influx of positively charged ions and speeds up the heart rate.
Step-by-step explanation:
Norepinephrine has a significant impact on pacemaker cells in the heart. When it binds to the beta-1 receptors on the pacemaker cells, it opens sodium and calcium ion chemical- or ligand-gated channels. This action results in an increased influx of positively charged ions, which leads to the acceleration of depolarization. This process decreases the time to reach threshold potential and subsequently shortens the period of repolarization. Therefore, norepinephrine accelerates depolarization by increasing sodium influx, aligning with option b of the initial question.