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๐Ÿ“š > Physiology > Cardiovascular System > Electrophysiology of Heart_>

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

โ†“

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

โ†“

Potential reach the threshold, which is about -40 mV

โ†“

This phase is called slow pacemaker potential (phase 4)


Upstroke (Phase 0)

At threshold

โ†“

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

โ†“

Net loss of positive ions

โ†“

Brings the potential to electronegativity

โ†“

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


Next Action Potential

K channels start closing

โ†“ K efflux decreases

โ†“

Slow entry of Na and Ca overbalances K efflux

โ†“ Slow rise in membrane potential

โ†“

โ†“

โ†“

Next action potential

โ†“

The cycle is repeated

โ†“

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.

ย 

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โ–ถ๏ธ Action Potential in Cardiac Muscle
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