What is the cardiac cycle and the action potional for 3rd year medical student

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Welcome to our study of the cardiac cycle. The cardiac cycle is the complete sequence of mechanical and electrical events that occur during one heartbeat. It consists of two main phases: systole, which is the contraction phase, and diastole, which is the relaxation phase. This coordinated pumping action ensures continuous blood circulation throughout the body. The cardiac cycle consists of two main phases. During diastole, the heart relaxes and fills with blood. This includes isovolumetric relaxation when all valves are closed, followed by ventricular filling through rapid filling, reduced filling, and atrial contraction. During systole, the heart contracts and ejects blood. This includes isovolumetric contraction when all valves are closed, followed by ventricular ejection with rapid and then reduced ejection phases. The cardiac action potential is the rapid change in membrane potential that triggers cardiac contraction. In contractile myocytes, it has five distinct phases. Phase 4 is the resting potential at minus 90 millivolts. Phase 0 shows rapid depolarization due to sodium influx. Phase 1 is initial repolarization from potassium efflux. Phase 2 is the plateau phase maintained by calcium influx. Phase 3 is repolarization back to resting potential through potassium efflux. Pacemaker cells in the SA and AV nodes have a different action potential pattern. Unlike contractile myocytes, they have no stable resting potential. Instead, they show spontaneous depolarization during phase 4, called the pacemaker potential. This is driven by the funny current and increasing calcium influx. Phase 0 shows slower depolarization through calcium channels, not sodium. Phase 3 is repolarization through potassium efflux. This automatic rhythmicity sets the heart rate. To summarize what we have learned about the cardiac cycle and action potentials: The cardiac cycle is a coordinated sequence of systole and diastole that pumps blood efficiently. Action potentials are electrical signals that trigger mechanical contraction through excitation-contraction coupling. Contractile myocytes have a distinctive five-phase action potential with a plateau phase, while pacemaker cells show spontaneous depolarization that sets the heart rate. Understanding these fundamental mechanisms is essential for comprehending cardiac pathophysiology in clinical practice.