Question

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

Answer 1

Answer:

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.

Answer 2

Answer:

D) K Na

Explanation:

Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions. The resulting movement of sodium ions creates spontaneous depolarization. At this point, calcium ion channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate. At this point, the calcium ion channels close and K+ channels open, allowing outflux of K+ and resulting in repolarization, the K+ channels close and Na+ channels open, and the prepotential phase begins again.