Multiple choice

The action potential in contractile muscle fibers of the heart is similar to that in skeletal muscles of the body. Among the following statements, select the incorrect statement regarding the action potential of human heart?

  1. Within the cell, Ca 2+ binds to troponin.

  2. Rapid depolarization occurs when fast‐opening Na + channels in the sarcolemma open.

  3. A plateau phase occurs during which K+ enters the cytosol of the muscle cell.

  4. Repolarization occurs as K + channels open and K + diffuses out of the cell.

  5. The refractory period of cardiac muscle is dramatically longer than that of the skeletal muscle.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A plateau phase occurs, during which Ca2+ enters the cytosol of the muscle cell. Ca2+ enters from the sarcoplasmic reticulum within the cell and also from outside the cell through slow‐opening Ca2+ channels in the sarcolemma. 

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