Question

Using the sliding flament theory, explain (or draw) the process of sarcomere shortening. Start from the point where calcium would interact with troponin. Make sure to discuss the roles of actin, myosin, and ATP.

Answer 1

Answer:

1. Calcium binds to troponin C
2. Troponin T moves tropomyosin and unblocks the binding sites
3. Myosin heads join to the actin forming cross-bridges
4. ATP turns into ADP and inorganic phosphate and releases energy
5. The energy is used to impulse myofilaments slide producing a power stroke
6. ADP is released and a new ATP joins the myosin heads and breaks the bindings to the actin filament
7. ATP splits into ADP and phosphate, and the energy produced is accumulated in the myosin heads, starting a new cycle
8. Z-bands are pulled toward each other, shortening the sarcomere and the I-band, producing muscle fiber contraction.

Explanation:  

In rest, the tropomyosin inhibits the attraction strengths between myosin and actin filaments. Contraction initiates when an action potential depolarizes the inner portion of the muscle fiber. Calcium channels activate in the T tubules membrane, releasing calcium into the sarcolemma. At this point, tropomyosin is obstructing binding sites for myosin on the thin filament. When calcium binds to troponin C, troponin T alters the tropomyosin position by moving it and unblocking the binding sites. Myosin heads join to the uncovered actin-binding points forming cross-bridges, and while doing so, ATP turns into ADP and inorganic phosphate, which is released. Myofilaments slide impulsed by chemical energy collected in myosin heads, producing a power stroke. The power stroke initiates when the myosin cross-bridge binds to actin. As they slide, ADP molecules are released. A new ATP links to myosin heads and breaks the bindings to the actin filament.  Then ATP splits into ADP and phosphate, and the energy produced is accumulated in the myosin heads, which starts a new binding cycle to actin. Finally, Z-bands are pulled toward each other, shortening the sarcomere and the I-band, producing muscle fiber contraction.