Question

which of the following statements regarding the sinoatrial (sa) node is true? when norepinephrine is released by sympathetic neurons, the rate of hyperpolarization increases, and the heart rate accelerates. any factor that changes either the resting potential or the rate of spontaneous depolarization at the sa node will alter the heart rate. nodal fibers require stimulation from the nervous system to cause depolarization. under normal resting conditions, sympathetic activity reduces the heart rate from the inherent nodal rate of 80 - 100 impulses per minute to 70 - 80 beats per minute.

Explanation:

• For the first option: When norepinephrine is released by sympathetic neurons, it causes depolarization (not hyperpolarization) to increase and heart rate to accelerate. So this option is wrong.
• For the second option: The SA node's ability to set the heart rate is based on its resting potential and rate of spontaneous depolarization. Any change in these factors will indeed alter the heart rate. This is a correct physiological principle.
• For the third option: Nodal fibers (SA node cells) are autorhythmic. They can depolarize spontaneously without external nervous system stimulation. So this option is wrong.
• For the fourth option: Under normal resting conditions, parasympathetic (not sympathetic) activity reduces the heart rate from the inherent nodal rate. So this option is wrong.

Answer:

Any factor that changes either the resting potential or the rate of spontaneous depolarization at the SA node will alter the heart rate.