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Action Potential in SA Node
SA Node
is also called
sinoatrial node
sinus node
is a small & flat ellipsoid strip of specialized cardiac muscle.
is the fastest among all pacemaker tissues in the heart โ normally the SA node sets the pace of the heart.
Action Potential in SA Node
Slow Pacemaker Potential (Phase 4)
Resting membrane potential is about - 60 mV
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1. Slow Na channels allow slow entry of Na even without any external stimulus โ produces funny current (If).
2. Slow entry of Ca.
โ Slow rise in membrane potential
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Potential reach the threshold, which is about -40 mV
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This phase is called slow pacemaker potential (phase 4)
Upstroke (Phase 0)
At threshold
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More Ca channel opens
โ Rapid influx of Ca
โ Upstroke in the action potential
Repolarization (Phase 3)
Ca channels close & K channels open
โ Ca influx stops & K starts leaving the cell
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Net loss of positive ions
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Brings the potential to electronegativity
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Eventually, the potential reaches to initial point i.e. - 60 mV
Next Action Potential
K channels start closing
โ K efflux decreases
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Slow entry of Na and Ca overbalances K efflux
โ Slow rise in membrane potential
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โ
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Next action potential
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The cycle is repeated
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Impulses thus generate spread all over the heart and trigger heartbeats.
Note: Phase 1 (initial repolarization) and phase 2 (plateau) are seen in ventricular myocytes. They are not seen in SA node.