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Sagot :
Answer:
Explanation:
In cardiac muscle, after the plateau phase and at the beginning of repolarization, the correct answer is:
**b. Voltage-gated potassium channels open**
Here's a brief explanation of the cardiac action potential phases:
1. **Phase 0 (Depolarization)**: This phase is characterized by the rapid influx of sodium ions (Na\(^+\)) through voltage-gated sodium channels, causing a sharp increase in membrane potential.
2. **Phase 1 (Initial Repolarization)**: This phase involves the transient outward flow of potassium ions (K\(^+\)) and the closure of sodium channels, leading to a slight decrease in membrane potential.
3. **Phase 2 (Plateau Phase)**: This phase is sustained by the influx of calcium ions (Ca\(^{2+}\)) through voltage-gated calcium channels, which balances the outward flow of potassium ions. This results in a prolonged depolarized state (the plateau).
4. **Phase 3 (Repolarization)**: This phase begins at the end of the plateau phase. It is characterized by the closure of voltage-gated calcium channels and the opening of voltage-gated potassium channels, leading to an increased outflow of potassium ions, which brings the membrane potential back to its resting state.
5. **Phase 4 (Resting Membrane Potential)**: The membrane potential is maintained by the activity of the sodium-potassium pump and leak channels, keeping the cell ready for the next action potential.
So, during the transition from the plateau phase to the beginning of repolarization in cardiac muscle cells, voltage-gated potassium channels open to facilitate the repolarization process.
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