What happens in each of the five phases cardiac muscle depolarization?
Cardiac Muscle Depolarization Phases:
Phase 0 - Rapid Depolarization:
1. Action potential begins triggered by a pacemaker signal or an external stimulus.
2. Sodium-potassium pumps open, allowing a rapid influx of sodium ions (Na+) into the myocardial cells.
3. The intracellular sodium concentration increases, leading to a rapid depolarization of the membrane.
4. The cell's membrane potential becomes positive, reversing its polarity.
Phase 1 - Early Repolarization:
1. Some potassium channels open, allowing a slight efflux of potassium ions (K+) from the cell.
2. The outflow of potassium ions partially repolarizes the membrane, causing a small decrease in the membrane potential.
3. This短暂 repolarization is quickly followed by phase 2.
Phase 2 - Plateau Phase:
1. Calcium channels open, allowing an influx of calcium ions (Ca++) into the myocardial cells.
2. Calcium ions play a crucial role in sustaining the depolarization and maintaining the action potential.
3. The calcium influx counteracts the repolarizing forces of potassium efflux, leading to a prolonged plateau in the membrane potential.
4. The sustained depolarization is essential for ensuring a strong and prolonged contraction of the heart muscle.
Phase 3 - Rapid Repolarization:
1. Potassium channels open more extensively, allowing a significant efflux of potassium ions from the myocardial cells.
2. The potassium efflux rapidly repolarizes the membrane, bringing the membrane potential back to the resting negative state (more negative).
3. The calcium channels close, and the sodium-potassium pumps resume their normal activity, restoring the ionic balance across the membrane.
Phase 4 - Resting Phase (Diastole):
1. The membrane potential remains negative during this phase as there is no active depolarization.
2. The sodium-potassium pumps continue to maintain the ionic gradients across the membrane, ensuring the cell is ready for the next depolarization cycle.
3. The heart muscle relaxes during phase 4, preparing for the next contraction cycle when the action potential initiates again.
These five phases of cardiac muscle depolarization are essential for the coordinated and rhythmic contraction of the heart. The proper functioning of ion channels and pumps ensures that the electrical impulses are generated, propagated, and terminated appropriately, allowing the heart to maintain a regular heartbeat and pump blood effectively throughout the body.