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 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 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 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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