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Excitation is Conducted from Cell to Cell Through Gap Junctions

Sarcolemma T tubule

Excitation is Conducted from Cell to Cell Through Gap Junctions

Sarcolemma T tubule

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FIGURE 3 Ultrastructure of a contractile cell. A contractile cell in the heart is very similar to skeletal muscle in its basic cellular organization. (Modified from Katz AM. Physiology of the heart. New York: Raven Press, 1992.)

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FIGURE 3 Ultrastructure of a contractile cell. A contractile cell in the heart is very similar to skeletal muscle in its basic cellular organization. (Modified from Katz AM. Physiology of the heart. New York: Raven Press, 1992.)

transverse aspect of the intercalated disc is filled with structures called desmosomes. The desmosomes make strong mechanical attachments between the cells and transmit the force of contraction. The transverse aspect as well as the longitudinally oriented region of the intercalated disc are rich in low-resistance connections between the cells called gap junctions. Small pores in the center of each gap junction allow ions and even small peptides to flow from one cell to another along their electrochemical gradients. Cardiac muscle gap junctions serve an identical function as the gap junctions at electrical synapses between nerve cells (Chapter 6) and nexuses in smooth muscle cells (Chapter 8). An action potential in one muscle cell is propagated to adjacent muscle cells via direct electrotonic propagation across the gap junctions. The gap junctions cause every cell in the heart to be electrically coupled to its neighboring cells and that is what causes the heart to behave like a single motor unit. Theoretically, an ion inside an SA nodal cell could travel through every cell in the heart without ever having to enter the extracellular space.

The Purkinje cells are still muscle cells but contain fewer contractile proteins than contractile cells. They are specialized not for contraction but, rather, for fast electrical propagation. The large diameter of the Purkinje cells gives them a high conduction velocity.

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