The flattening of the action potentials of myocardial contractile cells, called the plateau phase, is due to a combination of efflux and influx occurring at the same time.

A) K Ca
B) Ca K
C) Na K
D) K Na
E) none of the above

The flattening of the action potentials of myocardial contractile cells, called the plateau phase, is due to a combination of K+ efflux and Ca2+ influx occurring at the same time.

Explanation:

Plateau phase (maintained depolarization) is due to Ca2+ inflow due to opening of voltage-gated slow Ca2+ channels and K+ outflow with some K+ channels opening. It is maintained for 200-300 ms.

As the voltage of the cardiac muscle cell changes-it causes slower later opening L-Calcium channels and also the opening of some special slow delayed rectifier potassium channels (not the usual leak K channels or usual Voltage dependant K channels seen in neurons and skeletal muscle cells). The plateau phase is sustained by a balance between inward movement of Ca2+ through L-type Ca2 channels and outward movement of K+ thrugh slow delayed rectifier K+ channels.

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