How does norepinephrine, the sympathetic neurotransmitter, increase the heart rate?
a. It shortens the duration of the pace-maker action potentials.
b. It increases coupling among cardiomyocytes.
c. It increases transmission in the Bundle of His.
d. It makes the atrioventricular node the primary pace-maker of the heart.
e. It accelerates the recovery phase of the spontaneously pace-maker activity