Force generation, energy usage and fibre types Flashcards
Describe the sliding filament mechanism?
Force generation produces shortening of skeletal muscle fibre, the overlapping of thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross bridges
What does the ability of a muscle fibre to generate a force and movement dependent of?
It is dependent on the interaction of the contractile proteins actin and myosin
What happens in the sliding filament mechanism from relaxation to contraction?
Z-lines move closer to one another and towards the M line in the middle of the Sarcomere
Where does the sliding filament mechanism occur?
In A band where there is overlap of Mysoin and actin filaments
What are the events involved in the cross-bridge cycle?
- Calcium levels rise and Troponin pulls Tropomyosin to allow Myosin binding sites to be free:
- When calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell, they bind to the troponin complex. This binding causes a conformational change in troponin, which then pulls tropomyosin away from the myosin-binding sites on the actin filament, allowing cross-bridge formation.
2. Cross-bridge binds to Actin, ADP + Pi is stored in cross bridges: - The myosin head, already energized with ADP and inorganic phosphate (Pi) bound to it from the previous cycle, attaches to the exposed binding site on actin to form a cross-bridge.
3. Cross-bridge moves and ADP and Pi are released as a byproduct, filaments are moved: - During the power stroke, the release of ADP and Pi from the myosin head triggers the myosin to pivot and pull the actin filament toward the center of the sarcomere, resulting in muscle contraction.
4. ATP binds to Myosin, causing cross-bridge to detach: - After the power stroke, a new molecule of ATP binds to the myosin head, causing it to detach from the actin filament.
5. ATP is hydrolyzed to energize the cross-bridge, giving ADP + Pi in the cross-bridge: - The myosin head acts as an ATPase and hydrolyzes ATP into ADP and Pi. This hydrolysis reaction provides the energy necessary to return the myosin head to its energized, cocked position, ready for the next cycle of attachment and movement.
6. The muscle is now at rest (if Ca²⁺ levels decrease): - If calcium ion levels decrease, Ca²⁺ is pumped back into the sarcoplasmic reticulum, and tropomyosin moves back to cover the myosin-binding sites on actin, preventing further cross-bridge formation and leading to muscle relaxation. If Ca²⁺ levels remain high, the cycle will continue, and the muscle will remain contracted.
- When calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell, they bind to the troponin complex. This binding causes a conformational change in troponin, which then pulls tropomyosin away from the myosin-binding sites on the actin filament, allowing cross-bridge formation.
What converts ATP into ADP + Pi
Calcium ATPase and Myosin ATPase
What event occurs in Rigor Mortis?
In Rigor mortis after there is no ATP to bind to the cross-bridge after contraction to detach the cross-bridge from the Myosin filament
Hence all muscles are still contracted and stiff after death
What is Rigor Mortis?
Stiffening of skeletal muscles after death
- Begins 3-4 hours after death and is at maximum after 12 hours
- Cells accumulate in Calcium
- In the absence of ATP, cross bridges can bind to actin but cross-linkage is irreversible
What does hydrolysis of ATP in the plasma membrane maintain?
Hydrolysis of ATP by the Sodium Potassium pump in the plasma membrane maintains the sodium and potassium gradients which allows the membrane to produce and propagate action potentials
What does hydrolysis of ATP by the Calcium ATPase pump in the Sarcoplasmic reticulum provide?
Hydrolysis of ATP by the Calcium ATPase pump in the Sarcoplasmic reticulum provides the energy for the active transport of Calcium ions into the reticulum, lowering cytosolic Calcium ion concentration to pre-release concentrations, ending the contraction and allowing the muscle fibre to relax
What does hydrolysis of ATP by Myosin-ATPase do?
- Energizes the cross-bridges: The energy released from ATP hydrolysis is what “cocks” the myosin head into its high-energy conformation, making it ready for the next cycle of attachment to actin and force generation.
- Provides myosin with ADP and Pi: After ATP is hydrolyzed, the products (ADP and inorganic phosphate, Pi) remain bound to the myosin head. This state is the “cocked” position, ready to form a new cross-bridge with actin once the myosin-binding sites are exposed.
Hydrolysis of ATP by Myosin-ATPase energises the cross-bridges providing the energy for force generation
What does the binding of ATP by Myosin do?
The binding of ATP by Myosin dissociates cross-bridges bound to Actin, allowing the bridges to repeat their cycle of activity
What does contraction mean?
It refers to the activation of the force-generating sites within muscle fibres - the cross bridges
(Not necessarily ‘shortening’ of muscle)
e.g - holding a dumbbell at a constant position requires muscle contraction but not muscle shortening
What are the 3 ways that a muscle fibre can form ATP?
1) . Phosphorylation of ADP by creatine phosphate (rapid response)
2) . Oxidative phosphorylation of ADP in the mitochondria (most ATP made this way)
3) . Phosphorylation of ADP by the glycolytic pathway in the cytosol
What resources do the 3 different ways to form ATP in a muscle fibre require ?
1) . Phosphorylation of ADP by creatine phosphate - requires: Creatine Phosphate
2) . Oxidative phosphorylation of ADP in the mitochondria - requires; Proteins to be broken into amino acids or fatty acids from blood
3). Phosphorylation of ADP by the glycolytic pathway in the cytosol - requires; Glycogen to be converted to Glucose or Glucose from the blood to undergo Glycolysis
(this makes Lactic acid as a bi-product)
