CVS Practice Test 2025 – The All-In-One Guide to Exam Success!

Question: 1 / 400

What occurs during phase 0 of the pacemaker action potential?

Repolarization

Spontaneous depolarization

Upstroke of the action potential

During phase 0 of the pacemaker action potential, the upstroke of the action potential occurs. This phase is characterized by the rapid influx of sodium ions (Na+) through voltage-gated sodium channels, leading to a swift depolarization of the membrane potential. This rapid change in voltage is essential for the initiation of the action potential in pacemaker cells, which are responsible for setting the rhythm of the heart.

In contrast, the other phases, such as repolarization and resting state, occur later in the action potential cycle. Spontaneous depolarization happens during phase 4, where the cell gradually depolarizes due to the movement of sodium ions and calcium ions, but it is not the defining characteristic of phase 0. The clear distinction of phase 0 being the upstroke aligns it with the physiological processes that excite the heart muscle and regulate its rhythmic contractions.

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Resting state

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