Skeletal Muscle Metabolism and Fiber Types

1. بسم الله الرحمن الرحيم Skeletal Muscle Metabolismand Fiber Types Dr.Mohammed Sharique Ahmed Quadri Assistant Professor, Physiology

2. Contraction-Relaxation Steps Requires ATPmuscle has been called "a machine for converting chemical energy into mechanical work."

3. Contraction-Relaxation Steps Requiring ATP • Splitting of ATP by myosin ATPase provides energy for power stroke of cross bridge • Binding of fresh molecule of ATP to myosin lets bridge detach from actin filament at end of power stroke so cycle can be repeated • Active transport of Ca2+ back into sarcoplasmic reticulum during relaxation depends on energy derived from breakdown of ATP

4. Energy Sources for Contraction • Transfer of high-energy phosphate from creatine phosphate to ADP • Oxidative phosphorylation (citric acid cycle and electron transport system • Glycolysis

5. CREATINE PHOSPHATE creatine kinase Creatine phosphate + ADP creatine + ATP • First source for supplying additional ATP when exercise begins . • First few minute or less of exercise • Supports short burst of high intensity contractile efforts • source of creatine in diet , MEAT

6. Oxidative phosphorylation • In muscle mitochondria if sufficient O2 is present • Relatively slow – many steps involved • Supports aerobic ( with O2) or endurance-type exercise • Adequate O2 & nutrients required • Fueled by glucose or fatty acids depending upon intensity and duration of activity • Rest / light exercise – Fatty acids • High intensity exercise – Glucose

8. Glycolysis • When O2 delivery or Oxidative phosphorylation can not keep pace with the demand for ATP • Advantage • Produce ATP in absence of O2 • Much faster than oxidative phosphorylation • Supports anaerobic or high-intensity exercise • Disadvantage • Depletion of nutrient /energy reserve • Production of lactate

10. FATIGUE • Contractile activity in a muscle can not be maintained at given level indefinitely • Fatigue • Muscle fatigue • Central fatigue

11. Muscle Fatigue • Exercising muscle can no longer respond to stimulation with same degree of contraction • Defense mechanism that protects muscle from reaching - (rigor mortis) • Underlying causes of muscle fatigue are unclear • Local increase in ADP & inorganic phosphate • Accumulation of lactate • Accumulation of extracellular K+ • Depletion of glycogen energy reserve

12. Central Fatigue • Occurs when CNS no longer adequately activates motor neurons supplying working muscles • Often psychologically based • Mechanisms involved in central fatigue are poorly understood Neuromuscular fatigue: inability of motor neuron to synthesize acetylcholine- possible only experimentally

13. The Oxygen Debt Mechanism • After a period of exertion is over, extra O2 is consumed to • Remove the excess lactate • Replenish the ATP and phosphorylcreatine stores • Replace the small amounts of O2 that have come from myoglobin.

14. Major Types of Muscle Fibers • Classified based on differences in ATP hydrolysis and synthesis • 3 major types • Slow-oxidative (type I) fibers • Fast-oxidative (type IIa) fibers • Fast-glycolytic (type IIx) fibers

16. References • Human physiology by Lauralee Sherwood, 7th edition • Text book physiology by Guyton &Hall,12th edition • Text book of physiology by Linda .s contanzo,third edition