You are watching: The plateau phase of the cardiac action potential is due to
Unlike pacemaker cells found in nodal organization within the heart, non-pacemaker cells have a true resting membrane potential (phase 4) that remains close to the equilibrium potential because that K+ (EK). The resting membrane potential is very an adverse during phase 4 (about -90 mV) due to the fact that potassium channels are open up (K+ conductance
When these cells are quickly depolarized to a threshold voltage of about -70 mV (e.g., by an action potential in an nearby cell), over there is a rapid depolarization (phase 0) the is resulted in by a transient rise in fast Na+-channel conductance (gNa+) through rapid sodium channels. This boosts the inward directed, depolarizing Na+ currents (INa) that are responsible because that the generation of this "fast-response" action potentials (see over figure). At the same time sodium networks open, gK+ and also outward directed K+ currents autumn as potassium channels close. These two conductance transforms move the membrane potential away from EK (which is negative) and also closer toward the equilibrium potential for salt (ENa), i m sorry is positive.
Therefore, the action potential in non-pacemaker cells is primarily established by relative transforms in rapid Na+, slow-moving Ca++ and K+ conductances and currents. As defined under the discussion on membrane potentials and summarized in the complying with relationship and in the figure to the right, the membrane potential (Em) is identified by the loved one conductances of the significant ions distributed throughout the cabinet membrane. As soon as g'K+ is high and also g'Na+ and also g'Ca++ are low (phases 3 and also 4), the membrane potential will certainly be much more negative (resting state in the figure). Once g'K+ is low and also g'Na+ and/or g'Ca++ are high, the membrane potential will certainly be more positive (phases 0, 1 and also 2) (depolarized state in the figure).
Em = g'K+ (−96 mV) + g'Na+ (+50 mV) + g'Ca++ (+134 mV)
These fast-response activity potentials in non-nodal organization are transformed by antiarrhythmic drugs that block particular ion channels. Sodium-channel blockers such together quinidine inactivate fast-sodium channels and reduce the rate of depolarization (decrease the slope of step 0). Calcium-channel blockers such together verapamil and also diltiazem impact the plateau step (phase 2) the the action potential. Potassium-channel blockers hold-up repolarization (phase 3) by impede the potassium channels that space responsible because that this phase.
Effective Refractory Period
Once an action potential is initiated, over there is a duration of time consisting of phases 0, 1, 2, 3 and early step 4 that a brand-new action potential cannot be initiated (see figure at optimal of page). This is termed the effective refractory period (ERP) or the absolute refractory period (ARP) of the cell. During the ERP, stimulation that the cabinet by an surrounding cell undergoing depolarization walk not develop new, propagated action potentials. This occurs because fast sodium channels remain inactivated adhering to channel closing during phase 1. They perform not change to your closed, resting (excitable) state until some time ~ the membrane potential has completely repolarized. The ERP acts together a protective mechanism in the love by avoiding multiple, compounded action potentials from occurring (i.e., it limits the frequency the depolarization and therefore heart rate). This is important since at very high love rates, the heart would certainly be can not to adequately fill with blood and also therefore ventricular ejection would be reduced.
Many antiarrhythmic drugs transform the ERP, thereby transforming cellular excitability. Because that example, drugs the block potassium channels (e.g., amiodarone, a course III antiarrhythmic) delay phase 3 repolarization and increases the ERP. Medicine that boost the ERP have the right to be specifically effective in abolishing reentry currents that result in tachyarrhythmias.
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Transformation of non-pacemaker right into pacemaker cells
It is essential to note that non-pacemaker activity potentials can adjust into pacemaker cell under details conditions. For example, if a cabinet becomes hypoxic, the membrane depolarizes, which closes quick Na+ channels. At a membrane potential of about –50 mV, every the quick Na+ channels are inactivated. When this occurs, activity potentials deserve to still it is in elicited; however, the inward present are carried by Ca++ (slow inward channels) exclusively. These activity potentials resemble those found in pacemaker cells located in the SA node, and also can sometimes display spontaneous depolarization and automaticity. This mechanism may serve as the electrophysiological system behind certain species of ectopic beats and arrhythmias, specifically in ischemic love disease and following myocardial infarction.