Which of the following serves as an actuating signal, or as part of an actuating signal, in a negative feedback system?
A. Action potentials in sympathetic neurons that release acetylcholine (ACh) near the SA node of the heart.
B. Action potentials in parasympathetic neurons that release norepinephrine (NE) near the SA node of the heart.
C. Action potentials in parasympathetic neurons that release norepinephrine (NE) near the smooth muscles that surround the arterioles.