Skeletal muscle contraction • Contraction, force and tension • Sliding Filament Theory of Contraction • Contraction cycle • Regulation of the contraction cycle
Muscle contraction • Movement or resist a load (force) • Load is the weight or force that opposes the contraction of a muscle • Tension is the force created by a muscle • Need ATP to generate tension
Observations during muscle contraction Muscle shortens when it moves a load. (When muscle contracts, it does not always shorten)
Observations during muscle contraction: A band does not shorten during contraction.
Sliding filament theory of contraction: movement and force Resting length How about force without movement?
During contraction • Z discs move closer together • Sarcomere shortens • A band same length • I band reduced • H band reduced
What pushes the actin filaments into the myosin? • Cross-bridges link myosin to actin • Power stroke: myosin head binds to actin myosin head release actin. Repeated many times. • Myosin molecules are flexible • ATP causes movement of myosin molecules
Myosin • A motor protein • Converts chemical bond energy of ATP to mechanical energy of motion • Each myosin as ATPase • Energy from ATP hydrolysis is stored as potential energy in the myosin molecule, and is used to create the power stroke.
Why don’t actin and myosin continuously bind together? • ATP is usually available • Actin’s binding site for myosin is revealed only during cross-bridge (binding). • During relaxation, actin’s binding site for myosin is concealed
Energy for skeletal muscle contraction • ATP sources • The many causes of muscle fatigue • Classification of skeletal muscle fiber types
ATP and muscle contraction • Need ATP for • Cross-bridge formation, power stroke (myosin ATPase) • Ca++ transport to SR (Ca++ ATPase) • Na+/K+ transport across sarcolemma (Na+/K+ ATPase)
Sources of ATP • ATP pool • Phosphocreatine. • At rest, ATP phosphorylates creatine. • During exercise, creatine kinase (creatine phosphokinase) moves phosphate from phosphocreatine to ATP
Sources of ATP • Glucose (glycolysis) to pyruvate citric acid cycle oxidative phosphorylation (about 30 ATP per glucose molecules) • Anaerobic glycolysis: glucose lactic acid (2 ATP per glucose molecule)
Sources of ATP • Beta oxidation of fatty acids. Fatty acids are converted to acetyl CoA citric acid cycle in the mitochondria, need oxygen • Slow • During light exercise
Sources of ATP • glucose catabolism during heavy exercise • carbo loading builds up glycogen stores • Protein catabolism during starvation
Fatigue • Muscle is no longer able to generate sustained expected power output • A variety of contributing factors • depends on the degree of muscle activity
Fatigue: contributing factors • Intensity of muscle activity • Duration of muscle activity • Aerobic/anaerobic metabolism • Muscle composition • Fitness level • Ions • Nutrients • Neurotransmitter
Fatigue during extended submaximal exertion • Not ATP shortage • Glycogen depletion may affect Ca++ release from SR
Fatigue during short duration maximal exertion • Lots of inorganic phosphate from ATP hydrolysis • may slow P release from myosin:ADP:Pi • slows power stroke • Acidosis may inhibit some enzymes
More factors for muscle fatigue • K+ : intracellular K+ lowered during repetitive action potentials affects Ca++ release channels on SR membrane • Acetyl choline depletion at the myoneural junction low end-plate potential (disease)
More factors for muscle fatigue • CNS: • Subjective feelings preceding physiological fatigue • Acidosis may influence perception of fatigue