Excitation-contraction coupling is the process linking the generation of an action potential with muscle contraction ( KaD.dM®i.,.a,l 1991). Sodium influx triggers an action potential which propagates over the surface of the muscle fiber and into its interior via the T-tubule system. This spreading depolarization acts at voltage-sensitive channels located in terminal cisterns of the T tubules adjacent to the sarcoplasmic reticulum. As a result, Ca 2+ is released from the sarcoplasmic reticulum into the intracellular space of the muscle fiber. Intracellular Ca 2+ binds to troponin molecules located on actin polymers of the thin filaments. This induces a conformational change in actin, exposing a binding site for the myosin heads of thick filaments ( CD F.ig.u.r.e.Z). The subsequent rotation of the myosin heads pulls the thin filaments over the thick filaments, shortening the muscle sarcomere, the basic unit of contraction in muscle. In a process dependent on ATP and Ca 2+, the myosin heads detach from actin binding sites and rotate back to the resting position, ready for reattachment to actin. When muscle depolarization is complete, Ca 2+ is pumped back into the sarcoplasmic reticulum and new myosin-actin cross-bridges can no longer form, resulting in cessation of contraction. If ATP is present, the existing myosin-actin contacts detach and the muscle relaxes.

CD Figure 7. (a) The basic units involved in excitation-contraction coupling including the myofibrils within a muscle fiber. (b) Structure of an individual muscle sarcomere. (b) Actin and myosin filaments.

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