During which phase of action potential does the membrane potential drop to -40mV?

The correct answer identifies Phase 2 of the action potential, which is characterized by a plateau phase. During this phase, the membrane potential is maintained around -40 mV due to a balance between calcium influx and potassium efflux. This influx of calcium ions helps to sustain depolarization, preventing the membrane potential from falling to more negative values.

Phase 1 actually represents the initial repolarization, where the membrane potential begins to decrease from its peak due to the closure of sodium channels and the opening of potassium channels. In Phase 3, there is a significant repolarization as potassium ions flow out of the cell, leading the membrane potential to become more negative. Phase 4 is known as the resting potential, where the membrane is at its resting state, typically around -70 mV to -90 mV.

Therefore, recognizing the attributes of Phase 2 is essential in understanding how the action potential functions in cardiac muscle, especially regarding the role of calcium in maintaining depolarized states during contraction.