Mechanism of Muscle Contraction Flashcards
Explain the 1st step of excitation-contraction coupling
- acetylcholine (ACh) released from axon terminal of motor neuron & binds to receptors of a motor end plate.
- binding elicits an end-plate potential, triggering an action potential in the muscle cell
Explain the 2nd step of excitation-contraction coupling
Action potential propagates along sarcolemma & down T tubules
Explain the 3rd step of excitation-contraction coupling
Action potential triggers Ca2+ release from sarcoplasmic reticulum
Explain the 4th step of excitation-contraction coupling
Ca2+ binds to troponin
- exposing myosin-binding sites
Explain the 5th step of excitation-contraction coupling
Cross-bridge cycle begins (muscle fiber contracts)
Explain the 6th step of excitation-contraction coupling
- Ca2+ is actively transported back into lumen of sarcoplasmic reticulum
- following the action potential
Explain the 7th step of excitation-contraction coupling
Tropomyosin blocks myosin-binding sites (muscle fiber relaxes)
What does the sliding filament theory involve?
- myosin
- actin
- tropomyosin
- troponin
- ATP
- calcium ions
What is the 1st step of the Contraction Cycle and Cross-Bridge Formation?
Arrival of Ca2+ within zone of overlap in sarcomere
What is the 2nd step of the Contraction Cycle and Cross-Bridge Formation?
- Ca2+ binds to troponin
- weakening bond between actin and troponin-tropomyosin complex
What happens to the troponin molecule in step 2 of the Contraction Cycle?
Changes position:
- rolls tropomyosin molecule away from active sites on actin
- allows interaction with the energised myosin heads
What is the 3rd step of the Contraction Cycle and Cross-Bridge Formation?
- active sites exposed
- energised myosin heads bind to them
- forming cross-bridges
What is the 4th step of the Contraction Cycle and Cross-Bridge Formation?
The power stroke action occurs after formation of bridge:
- energy stored in resting state is released
- as the myosin head pivots toward the M line
- bound ADP & phosphate group are released
What is the 5th step of the Contraction Cycle and Cross-Bridge Formation?
When ATP binds to myosin head:
- link between myosin head and active site (on actin molecule) is broken
- active site = exposed
- able to form another cross-bridge
What is the 6th step of the Contraction Cycle and Cross-Bridge Formation?
Myosin reactivation occurs when:
- free myosin head splits ATP into ADP and phosphate
- energy released used to recock myosin head
Summarise the Single Cross-Bridge Cycle in 6 steps
- Exposure of binding sites on actin
- Binding of myosin to actin
- Power stroke of cross-bridge
- disconnecting the myosin head from actin
- Re-energising & repositioning myosin head
- Removal of Ca ions
What happens to the length of the sarcomere during the contraction of a skeletal muscle fibre?
Contracts/shortens:
- A band = stays same width
- Z lines = move closer together
- I bands = get smaller
- H zone = changes
- myofilament = no change
Because the sarcomere units shorten …
the length of the muscle fiber shortens (contracts)
When one end of a myofibril is fixed in position …
the free end is pulled toward the fixed end
What is the role of ATP in the contraction of the skeletal muscle?
- energising the power stroke of the myosin cross-bridges
- disconnecting myosin head from binding site on actin at the end of a power stroke
- actively transports Ca ions into sarcoplasmic reticulum
What is the role of ATP in the contraction of the skeletal muscle?
- energising the power stroke of the myosin cross-bridges
- disconnecting myosin head from binding site on actin at the end of a power stroke
- actively transports Ca ions into sarcoplasmic reticulum
What is the 1st step of muscle relaxation?
ACh removed from receptors by AChE
What is the 2nd step of muscle relaxation?
ligand-gated Na+ channels close
What is the 3rd step of muscle relaxation?
Na/K pumps reestablish the RMP
What is the 4th step of muscle relaxation?
Ca2+ leave troponin & absorbed back into cisternae
What is the 5th step of muscle relaxation?
tropomyosin moves back over the actin active site
What is the 6th step of muscle relaxation?
myosin heads release their binding to actin
What 3 forces are needed for muscle relaxation?
- elastic forces
- opposing muscle contractions
- gravity
