Excitation Contraction Coupling

As described in Chapter 11, the mechanism that couples excitation—an action potential in the plasma membrane of the muscle cell—and contraction is an increase in the cell's cytosolic calcium concentration. As is true for skeletal muscle, the increase in cytosolic calcium concentration in cardiac muscle is due mainly to release of calcium from the sarcoplasmic reticulum. This calcium combines with the regulator protein tro-ponin, and cross-bridge formation between actin and myosin is initiated.

But there is a difference between skeletal and cardiac muscle in the sequence of events by which the action potential leads to increased release of calcium from the sarcoplasmic reticulum. In both muscle types, the plasma-membrane action potential spreads into the interior of muscle cells via the T tubules (the lumen of each tubule is continuous with the extracellular fluid). In skeletal muscle, as we saw in Chapter 11, the action potential in the T tubules then causes the direct opening of calcium channels in the sarcoplasmic reticulum adjacent to the T tubules. In cardiac muscle (Figure 14-22): (1) The action potential in the T tubule opens voltage-sensitive calcium channels in the T tubule membrane itself; calcium diffuses from the extracellular fluid through these channels into the cells, causing a small increase in cytosolic calcium concentration in

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

"Excitation" (Depolarization of plasma membrane)

Opening of voltage-sensitive plasma membrane Ca2+ channels in T tubules

Flow of Ca2+ into cytosol

Ca2+ binds to Ca2+ receptors on the external surface of the sarcoplasmic reticulum

Opening of Ca2+ channels intrinsic to these receptors

Flow of Ca2+ into cytosol t Cytosolic Ca2+ concentration

Multiple iple ^^

steps

Contraction

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