CVS Practice Test

What is the resting membrane potential of the SA node?

1. -60 to -70 mV

2. No "true" RMP

3. Approximately -40 mV

4. Variable based on activity

The correct answer is: No "true" RMP

The resting membrane potential of the sinoatrial (SA) node is often described as "no true resting membrane potential" because of its unique characteristics compared to other cardiac cells. Unlike typical neurons or muscle fibers, the SA node cells exhibit a gradual depolarization during diastole, known as automaticity. This means that instead of a stable negative resting potential, the membrane potential of SA node cells slowly rises over time, leading to the generation of spontaneous action potentials that initiate the heartbeat. The cells of the SA node have a less negative membrane potential that is not fixed; rather, it continuously fluctuates due to ion channel activities, particularly the opening of sodium (Na+) and calcium (Ca2+) channels. This feature allows the SA node to be the primary pacemaker of the heart, setting the rhythm for cardiac cycles without maintaining a stable resting state like other excitable cells. Therefore, the assertion that the SA node has "no true resting membrane potential" is correct, as its membrane potential is always in a state of flux, reflecting its role in initiating the electrical activity in the heart.