Muscle Physiology PDF

MUSCLE PHYSIOLOGY 4. Muscle contraction Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Describe how the excitation-contractio...

MUSCLE PHYSIOLOGY 4. Muscle contraction Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Describe how the excitation-contraction coupling occurs Describe the role of T tubules, sarcoplasmic reticulum, and calcium in the initiation of the muscle contraction Describe the “Walk Along Theory” of muscle contraction Neuromuscular transmission (review) Action potential on muscle cell (review)  Excitation-contraction coupling Is the overall process that transforms the nerve impulse into muscle contraction Action potentials on the sarcolemma spread from the motor end plate along the length of the fiber but also to the interior of the cell along the transverse tubules (T Tubules) The fast depolarization of the membrane penetrates the cell  Excitation-contraction coupling T tubules are associated to the sarcoplasmic reticulum Sarcoplasmic Reticulum (SR) is composed by Large chambers called terminal cisternae Long longitudinal tubules that surround all surfaces of the actual contracting myofibrils The action potential causes a conformational change in voltage-sensing proteins (dihydropyridine receptors – DHPRs) on the T tubule These receptors are mechanically coupled to the ryanodine receptors (RyRs) calcium channels on the SR membrane Calcium channels open  calcium rapidly diffuses out of the sarcoplasm and initiates muscle contraction  Excitation-contraction coupling thisworkscontinuously A calcium pump (SERCA 1a) removes calcium ions from the myofibrillar fluid after contraction occurs SERCA1a = sarcoendoplasmic reticulum calcium ATPase Continually active calcium pump Located in the walls of the SR Pumps Ca back into the SR Muscle contraction continues as long as Ca ion concentration remains high Excitation-contraction coupling video https://www.youtube.com/watch?v=IOkn1ldFO60  Walk along theory of muscle contraction Before muscle contraction begins, myosin heads bind with ATP (low energy configuration) The ATPase activity of the myosin head immediately cleaves the ATP ADP and phosphate ion Cleavage products are kept bound to the head Head becomes energized in a “cocked position”  Walk along theory of muscle contraction Forces are generated by the interaction of the cross-bridges from myosin and actin filaments In the resting muscle, myosin-binding sites on the actin filament are inhibited (physically covered by the troponin- tropomyosin complex) The sites cannot attach to the heads of the myosin filaments Calcium (released from SR) binds to troponin, exposing these sites, allowing myosin heads to bind The cross-bridge is formed  Walk along theory of muscle contraction Immediately after the cross-bridge is formed, the phosphate is released The release of Pi triggers the POWER STROKE – a conformational change in the myosin head, which moves from a high-energy to a low-energy state. Myosin heads bend toward the center of the sarcomere (“micro-contraction”), causing the actin to slide toward the M line ADP is released from the myosin head The myosin head remains tightly bound to actin in a state known as the rigor state The binding of new ATP causes detachment of the myosin head from the actin filament The new molecule of ATP is cleaved to begin a new cycle Walk along theory of muscle contraction RELAXED CONTRACTED Muscle contraction overview Muscle contraction video https://www.youtube.com/watch?v=sIH8uOg8ddw