Transcript Observations during muscle contraction

Skeletal muscle contraction
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Contraction, force and tension
Sliding Filament Theory of Contraction
Contraction cycle
Regulation of the contraction cycle
Muscle contraction
• Movement or resist a load (force)
• Load is the weight or force that opposes
the contraction of a muscle
• Tension is the force created by a muscle
• Need ATP to generate tension
Observations during muscle
contraction
Muscle shortens when
it moves a load.
(When muscle contracts,
it does not always
shorten)
Observations during muscle contraction:
A band does not shorten during contraction.
Sliding filament theory of contraction: movement and force
Resting length
How about force without movement?
During contraction
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Z discs move closer together
Sarcomere shortens
A band same length
I band reduced
H band reduced
What pushes the actin filaments
into the myosin?
• Cross-bridges link myosin to actin
• Power stroke: myosin head binds to actin
 myosin head release actin. Repeated
many times.
• Myosin molecules are flexible
• ATP causes movement of myosin
molecules
Myosin
• A motor protein
• Converts chemical bond energy of ATP to
mechanical energy of motion
• Each myosin as ATPase
• Energy from ATP hydrolysis is stored as
potential energy in the myosin molecule,
and is used to create the power stroke.
Why don’t actin and myosin
continuously bind together?
• ATP is usually available
• Actin’s binding site for myosin is revealed
only during cross-bridge (binding).
• During relaxation, actin’s binding site for
myosin is concealed
Energy for skeletal muscle
contraction
• ATP sources
• The many causes of muscle fatigue
• Classification of skeletal muscle fiber
types
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)
Sources of ATP
• ATP pool
• Phosphocreatine.
– At rest, ATP phosphorylates creatine.
– During exercise, creatine kinase (creatine
phosphokinase) moves phosphate from
phosphocreatine to ATP
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)
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
Sources of ATP
• glucose catabolism during heavy exercise
• carbo loading builds up glycogen stores
• Protein catabolism during starvation
Fatigue
• Muscle is no longer able to generate
sustained expected power output
• A variety of contributing factors
• depends on the degree of muscle activity
Fatigue: contributing factors
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Intensity of muscle activity
Duration of muscle activity
Aerobic/anaerobic metabolism
Muscle composition
Fitness level
Ions
Nutrients
Neurotransmitter
Fatigue during extended
submaximal exertion
• Not ATP shortage
• Glycogen depletion may affect Ca++
release from SR
Fatigue during short duration
maximal exertion
• Lots of inorganic phosphate from ATP
hydrolysis
–  may slow P release from myosin:ADP:Pi
–  slows power stroke
• Acidosis may inhibit some enzymes
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)
More factors for muscle fatigue
• CNS:
– Subjective feelings preceding physiological
fatigue
– Acidosis may influence perception of fatigue