Cardiac myocytes (contractile cells) have a prolonged action potential including a plateau phase which lasts up...

Cardiac myocytes (contractile cells) have a prolonged action potential including a plateau phase which lasts up to ~250ms. This is to enable a longer, sustained contraction so that the ventricles have time to pump blood out. In order to enable this ‘plateau’ phase (Phase 2):

Group of answer choices

Slow Na⁺ channels enable a sustained inward Na⁺ current.

Slow K⁺ channels enable a sustained inward K⁺ current, balance with Na⁺ efflux.

L-type Ca²⁺ channels enable Ca²⁺ influx which is balanced by a slight K⁺ efflux.

Only Na⁺ and K⁺ are involved in this action potential, just like a neuron, only it takes longer for the voltage-gated K⁺ channels to open and repolarise the cell.

Expert Solution

Option C-L-type Ca2+ channels enable Ca2+ influx which is balanced by a slight K+ efflux.

Phase 2 is the plateau phase of the cardiac action potential. A plateau phase occurs during which Ca 2+ enters the cytosol of the muscle cell. Ca 2+ enters from the sarcoplasmic reticulum (endoplasmic reticulum) within the cell and also from outside the cell through slow‐opening Ca 2+channels in the sarcolemma. Within the cell, Ca 2+ binds to troponin, which in turn triggers the cross‐bridge binding that leads to the sliding of actin filaments past myosin filaments. The sliding of the filaments produces cell contraction. At the same time that the Ca 2+ channels open, K + channels, which normally leak small amounts of K + out of the cell, become more impermeable to K + leakage. The combined effects of the prolonged release of Ca 2+ and the restricted leakage of K + lead to an extended depolarization that appears as a plateau when membrane potential is plotted against time.

Phase 0-Depolarization due to operating fast sodium channels

Phase 1-Partial repolarisation Due to Rapid decrease in Sodium ion passage as fast sodium channels close

Phase 2-Plateau Phase in which movement of Ca2+ ions maintain depolarisation.

Phase 3-Repolarisation-sodium and calcium channels all close and membrane potential returns to base line.

Phase 4-Resting membrane potential (-90 mV) resulting from activity of Na+/K+ pump

Sydney Mullin answered 1 year ago

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