Mechanism of Muscle Contraction Flashcards
(28 cards)
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
