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.