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Characteristics of Cardiac Muscle Cells (CMC)

1.  Forms a functional syncitium.
    a.  Cellular bridges interconnect CMC's to each other.
    b.  Intercalated discs form junctions between CMC's.  These porous junctions
         permit ions of sodium and potassium to diffuse
         from cell to cell.  This facilitates the spread of depolarization and
         repolarization throughout the myocardium.
    c.  The entire myocardium behaves as a single coordinated unit.

2.  Mitochondria are large and very abundant.  CMC's are capable of producing high quantities of ATP.

3.  The cisternae of the sarcoplasmic reticulum of CMC's are smaller than in skeletal muscle cells.  As a result, CMC's must take Calcium ions from adjacent cells through the intercalated discs and from the extracellular environment.

4.  CMC's have a very long absolute refractory period.  This period of time corresponds to the time it takes a CMC to depolarize and repolarize.  The CMC's refractory period is about as long as its period of contraction and relaxation.  As a result, CMC's cannot enter tetanus or fatigue like skeletal muscle cells.  The significance of this is that the heart cannot begin a new cardiac cycle until it has finished with the previous one.

5.  Myocardial cells possess the property of automaticity or spontaneous depolarization.  This is the direct result of a cleverly designed "leaky membrane" which allows sodium ions to slowly enter the cell until the threshold is reached for depolarization.  Calcium ions follow and extend the depolarization even further.   Once calcium stops moving inward,  potassium ions move out slowly to produce repolarization.  The very slow repolarization of the CMC membrane is responsible for the long refractory period.