Skeletal Muscle Force Generation, Energy Usage and Fibre Types Flashcards
What is meant by excitation contraction coupling ?
Linkage between excitation of the muscle fibre membrane and the onset of contraction
What occurs as a result of contraction in the Sliding-Filament Mechanism ?
Explain what specifically happens on a microscopic level.
Shortening of a skeletal muscle fiber
The overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges.
Which one(s) of A band, I band H zone change upon shortening of the the muscle fiber and which one(s) stay unchanged ?
A band- unchanged
I band- reduced
H zone- reduced
What is the energy source for contraction ?
ATP being converted to ADP + Pi
How much ATP is contained in muscles ?
Around 4 mM/kg of ATP
What is rigor mortis ?
Stiffening of skeletal muscles after death
When does rigor mortis begin ?
Begins 3-4 hours after death
How long after death is rigor mortis maximised ?
Maximum after 12 hours
What is the molecular explanation of rigor mortis ?
Cells accumulate calcium
In the absence of ATP, cross bridges can bind to actin but the cross-linkage is irreversible
What are the 4 functions of ATP in skeletal muscle contraction ?
- Hydrolysis of ATP by Na+/K+ -ATPase in the plasma membrane maintains Na+ and K+ gradients, which allows the membrane to produce and propagate actions potentials
- Hydrolysis of ATP by Ca2+ ATPase in the sarcoplasmic reticulum provides the energy for the active transport of Calcium ions into the reticulum, lowering cytosolic Ca2+ concentrations to prerelease concentrations, ending the contraction, and allowing the muscle fiber to relax
- Hydrolysis of ATP by myosin-ATPase energizes the cross-bridge, providing the energy for force generation.
- Binding of ATP to myosin dissociates cross-bridges bound to actin, allowing the bridges to repeat their cycle of activity.
Are contraction and shortening always equivalent ? If not, explain what contraction refers to, and give a situation in which one occurs but not the other.
No
Refers to the activation of the force-generating sites within muscle fibers (the cross-bridges)
Holding a dumbbell at a constant position requires muscle contraction, but not muscle shortening.
What are the three ways a muscle fibre can form ATP ? Which is used in different intensity levels/lengths of times ?
– Phosphorylation of ADP by creatine phosphate (MOST RAPID LEVEL OF ACTIVITY)
– Oxidative phosphorylation of ADP in the mitochondria (MODERATE LEVEL OF ACTIVITY)
– Phosphorylation of ADP by the glycolytic pathway in the cytosol (AT HIGHER INTENSITY LEVEL OF ACTIVITY, e.g. distance running)
What are the two main bases upon which skeletal muscle fibers can be classified ?
Maximal velocities of shortening (fast or slow)
Major pathway they use to form ATP
Which microscopic property of skeletal muscle causes different muscle fibers to have different maximal velocities of shortening ?
Fast and slow fibers contain forms of myosin that differ in the maximal rates at which they use ATP.
This determines the maximal rate of cross-bridge cycling
What are the two types of skeletal muscle fibers, classified according to the type of enzymatic machinery available for synthesizing ATP ?
Oxidative or Glycolytic
What are some of the cellular features of oxidative fibers ?
Contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation
What does ATP production from oxidative fibers depend on ?
Blood flow to deliver oxygen and fuel molecules to the muscle
Which molecule keeps oxygen in muscle ?
Myoglobin
Are oxidative fibres darker or lighter than their glycolytic counterparts ? What is another name for them ?
Darker
Dark Meat
What are some of the cellular features of glycolytic fibers ?
Few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen
Are glycolytic fibres darker or lighter than their oxidative counterparts ? What is another name for them ?
Lighter
White Meat
What is a major function of muscles containing many oxidative fibres ?
many glycolytic fibers ?
Long term contraction (e.g. standing)
Quick burst of activity
On the basis of these two characteristics (maximal shortening velocity and major pathway to form ATP), what are the three principal types of skeletal muscle fibers which can be distinguished ?
- Slow-oxidative fibers (Type I): combine low myosin- ATPase activity with high oxidative capacity.
- Fast-oxidative-glycolytic fibers (Type IIa): combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.
- Fast-glycolytic fibers (Type IIb): combine high myosin- ATPase activity with high glycolytic capacity.
What are the two factors upon which the tension a muscle can develop depend on ?
AMOUNT OF TENSION DEVELOPED BY EACH FIBER:
- AP frequency
- Fiber length
- Fiber diameter
- Fatigue
NUMBER OF FIBERS CONTRACTING AT ANY TIME:
- Number of fibers per motor unit
- Number of active motor units
What does the shortening velocity of a whole muscle depend on ?
- LOAD ON THE MUSCLE
- TYPES OF MOTOR UNITS IN THE MUSCLE
- NUMBER OF MOTOR UNITS RECRUITED TO WORK AGAINST THE LOAD
How does muscle adapt to exercise ?
Increase in the amount of contractile activity increases the size of muscle fibers and increases their capacity for ATP production.
What is muscle atrophy ?
“decrease in the mass of the muscle”
What are the types of atrophy ?
Disuse atrophy (like an arm in a cast) Denervation atrophy (nerve damage = loss of function)
