Skeletal Muscle Metabolism and Fiber Types Dr.Mohammed Sharique Ahmed Quadri Assistant Professor, Physiology

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

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 Ca 2+ back into sarcoplasmic reticulum during relaxation depends on energy derived from breakdown of ATP

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

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

Oxidative phosphorylation In muscle mitochondria if sufficient O 2 is present Relatively slow – many steps involved Supports aerobic ( with O 2 ) or endurance-type exercise Adequate O 2 & nutrients required Fueled by glucose or fatty acids depending upon intensity and duration of activity Rest / light exercise – Fatty acids High intensity exercise – Glucose

Glycolysis When O 2 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

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

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

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

The Oxygen Debt Mechanism After a period of exertion is over, extra O 2 is consumed to Remove the excess lactate Replenish the ATP and phosphorylcreatine stores Replace the small amounts of O 2 that have come from myoglobin.

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

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