Action potentials in cardiac muscle cells

Step 1

Voltage-gated Na+ channels open.

Steps 2

Na- inflow depolarizes the membrane and triggers the opening of still more Na+ channels, creating a positive feedback cycle and a rapidly rising membrane voltage.

Steps 3

Na- channels close when the cell depolarizes and the voltage peaks at nearly +30 mV.

Step 4

Ca +2 entering through slow Ca -2 channels prolongs depolarization of membrane, creating a plateau.

Plateau falls slightly because of some +K leakage, but most +K channels remain closed until end of plateau.

Step 5

Ca +2 channels close and Ca +2 is transported out of cell.

K+ channels open, and rapid K+ outflow returns membrane to its resting potential.