D. Slow influx of calcium – Explanation
Slow influx of calcium is responsible for the plateau phase of the action potential
Electrical activity of the heart
Myocardial action potential
| Phase | Description | Mechanism |
| 0 | Rapid depolarisation | Rapid sodium influxThese 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 isreached, triggering a new action potential |
NB cardiac muscle remains contracted 10-15 times 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 velocities of 2-4 m/sec (this allows a rapid and coordinated contraction of the ventricles |
