What is phase 3 of cardiac action potential?
Repolarization (phase 3 of the action potential) occurs because of an increase in potassium permeability. At the SA node, potassium permeability can be further enhanced by vagal stimulation. This has the effect of hyperpolarizing the cell and reducing the rate of firing. Sympathetic stimulation has the opposite effect.
What is refractory period in cardiac action potential?
After an AP is triggered, the cardiac myocyte is unable to initiate another AP for some duration of time (which is slightly shorter than the duration of action potential itself). This period of time is referred to as the refractory period.
What are the phases of cardiac action potential?
Membrane currents that generate the a normal action potential. Resting (4), upstroke (0), early repolarization (1), plateau (2), and final repolarization are the 5 phases of the action potential.
What is the refractory period of the heart?
The absolute refractory period for cardiac contractile muscle lasts approximately 200 ms, and the relative refractory period lasts approximately 50 ms, for a total of 250 ms.
What happens in Phase 4 of action potential?
Phase 4 is the spontaneous depolarization (pacemaker potential) that triggers the action potential once the membrane potential reaches threshold between -40 and -30 mV). As ions flow through open channels, they generate electrical currents that change the membrane potential.
What is the refractory period of an action potential?
By definition, the refractory period is a period of time during which a cell is incapable of repeating an action potential. In terms of action potentials, it refers to the amount of time it takes for an excitable membrane to be ready to respond to a second stimulus once it returns to a resting state.
What is the relative refractory period of an action potential?
Refractory periods As voltage-gated Na+ channels transition from the inactivated to the closed state (i.e., they become capable of being activated), the membrane becomes capable of supporting a 2nd action potential – this period is called the RELATIVE REFRACTORY PERIOD.
Which drug increases duration of the action potential by prolonging repolarization in phase 3?
Potassium-channel blockers delay phase 3 repolarization, thereby lengthening the action potential duration and ERP.
What are the refractory periods of the action potential?
Absolute and relative refractory periods during the action potential. As seen in Figure 2 the relative refractory period coincides with the T-wave apex.
When does a cardiac cell have a refractory period?
Cardiac cells have two refractory periods, the first from the beginning of phase 0 until part way through phase 3; this is known as the absolute refractory period during which it is impossible for the cell to produce another action potential.
How does the relative refractory period affect potassium conductance?
The relative refractory period immediately follows the absolute. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases dramatically. K+ ions moving out of the cell bring the membrane potential closer to the equilibrium potential for potassium.
What is Phase 3 of cardiac myocyte action potential?
Plummet = Potassium = Phase 3 After myocyte contraction has occurred in phase 2, the action potential will then “plummet” again as the voltage across the cell membrane becomes negative. This is referred to as repolarization and is phase 3 of the cardiac myocyte action potential.