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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 → 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

↓

1. Slow Na channels allow slow entry of Na even without any external stimulus → produces funny current (If).

2. Slow entry of Ca.

↓ Slow rise in membrane potential

↓

Potential reach the threshold, which is about -40 mV

↓

This phase is called slow pacemaker potential (phase 4)


Upstroke (Phase 0)

At threshold

↓

More Ca channel opens

↓ Rapid influx of Ca

↓ Upstroke in the action potential


Repolarization (Phase 3)

Ca channels close & K channels open

↓ Ca influx stops & K starts leaving the cell

↓

Net loss of positive ions

↓

Brings the potential to electronegativity

↓

Eventually, the potential reaches to initial point i.e. - 60 mV


Next Action Potential

K channels start closing

↓ K efflux decreases

↓

Slow entry of Na and Ca overbalances K efflux

↓ Slow rise in membrane potential

↓

↓

↓

Next action potential

↓

The cycle is repeated

↓

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.

 

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