Myocardial action potential

Phase         Description                     Mechanism

0           Rapid depolarisation                    Rapid sodium influx

These channels automatically deactivate after a few ms

1           Early repolarisation                     Efflux of potassium

2           Plateau                                    Slow influx of calcium

3           Final repolarisation                     Efflux of potassium

4           Restoration of ionic

concentrations

Resting potential is restored by Na+/K+ ATPase

There is slow entry of Na+ into the cell decreasing the potential difference until

the threshold potential is reached, triggering a new action potential

NB cardiac muscle remains contracted 10‐15 ties longer than skeletal muscle

Conduction velocity

Atrial conduction Spreads along ordinary atrial myocardial fibres at 1 m/sec

AV node conduction   = 0.05 m/sec

Ventricular conduction

Purkinje fibres are of large diameter and achieve velocitis of 2‐4 m/sec (this allows a rapid