Action potential in cardiac muscle

Action potential in cardiac muscle

The resting potential of a cardiac muscle cell is about –90 mV: the inside of the cell is negative with respect to the outside (phase 4). The threshold for the generation of an action potential is –60 mV. Once the threshold is reached, the following events occur: Rapid depolarization occurs as fast sodium channels open and Na+ enters the cell (phase 0). The membrane potential thus becomes +30 mV and the voltage-dependent sodium channels close and are inactivated. Partial repolarization occurs as potassium channels open

and transiently let K+ exit the cell (phase 1). Slow, voltage-dependent calcium channels open and Ca2+ enters the cell, causing the membrane potential to fall more slowly (phase 2). This phase is known as the plateau. When the membrane potential further decreases, the calcium channels close and the slow potassium channels open. The outflow of K+ causes a rapid repolarisation (phase 3). The entire action potential lasts 200–400 msec. Compare this to action potentials in skeletal muscle, which merely last 2–4 msec.

1

During the absolute refractory period, a cell membrane cannot respond because

2

What brings a refractory cell back below threshold?