Skeletal Muscle Metabolismand Fiber Types

Dr.Mohammed Sharique Ahmed QuadriAssistant Professor, Physiology

بسم الله الرحمن الرحيم

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 Ca2+ 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 kinaseCreatine 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 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

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

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 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.

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, 7th edition

• Text book physiology by Guyton &Hall,12th edition

• Text book of physiology by Linda .s contanzo,third edition

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