Question

1. what specific steps of muscle contraction cycle cost energy? list relevant steps

2. how are the processes leading to muscle cramps associated with intense activity in rigor mortis related?

Answer 1

1)

solu :

• The propagation of action potential along the sarcolemma and down the T tubules,Calcium released from the SR and calcium concentration rises in the sarcoplasm.

1. Calcium binds to troponin C and induces a conformational change in the troponin molecule.
2. This result in shifting of troponin I from the actin filament,allowing tropomysoin to move deeper into the actin groove ,unmasking the myosin binding sites. Moreover,the conformational change alters the position of troponin T that drags tropomyosin away from the myosin-binding sites.
3. The exposure of the binding sites allows myosin heads to interact with actin and engage in cross - bridge cycling.
4. Thus,the troponin -tropomysoin complex regulates the contraction of the muscles.
5. As the regulation of the contractile process by calcium is implemented through the actin filaments ,it is also called Actin-linked regulation.

​​​​​​​​​​​​​​2)

solu :

• During the cross bridge cycle ,binding of ATP to the myosin heads break the actin-myosin interaction and brings in relaxation of the muscle.If the cellular energy stores are depleted,as happens after death ,the cross-bridge detachment cannot occur due to lack of ATP.Therefore the myosin head remain in the attached state.Following death ,the cytoplasmic calcium concentration remains elevated.
• The raised cytoplasmic calcium concentration initiates contraction by exposing the myosin heads on actin and permitting formation of cross-bridges that do not detach due to lack of ATP.
• This leads to stiffness in the muscle,which is known as rigor mortis.