Energy for skeletal muscle contraction

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Energy for skeletal muscle contraction

• ATP sources
• The many causes of muscle fatigue
• Classification of skeletal muscle fiber types

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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)

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Sources of ATP

• ATP pool
• Phosphocreatine.
• At rest, ATP phosphorylates creatine.
• During exercise, creatine kinase (creatine
  phosphokinase) moves phosphate from
  phosphocreatine to ATP

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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)

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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

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Sources of ATP

• glucose catabolism during heavy exercise
• carbo loading builds up glycogen stores
• Protein catabolism during starvation

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Fatigue

• Muscle is no longer able to generate sustained
  expected power output
• A variety of contributing factors
• depends on the degree of muscle activity

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Fatigue contributing factors

• Intensity of muscle activity
• Duration of muscle activity
• Aerobic/anaerobic metabolism
• Muscle composition
• Fitness level
• Ions
• Nutrients
• Neurotransmitter

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Fatigue during extended submaximal exertion

• Not ATP shortage
• Glycogen depletion may affect Ca release from
  SR

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Fatigue during short duration maximal exertion

• Lots of inorganic phosphate from ATP hydrolysis
• ? may slow P release from myosinADPPi
• ? slows power stroke
• Acidosis may inhibit some enzymes

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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)

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More factors for muscle fatigue

• CNS
• Subjective feelings preceding physiological
  fatigue
• Acidosis may influence perception of fatigue