Question

What happens when the calcium ion level increases in the sarcoplasm?

• A. Filament sliding occurs
• B. Filament sliding stops
• C. Muscle relaxes
• D. No change in muscle state

Answer 1

The correct option is A Filament sliding occurs

The calcium ion level determines muscle contraction. The increase in calcium ion levels in the sarcoplasm initiates the sliding of filaments. Calcium ions are stored inside the sarcoplasmic reticulum. When an action potential arrives, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm around the thick and thin filaments. The calcium ions then bind with troponin, which causes a change in the shape of the troponin molecule. Since troponin is bound to tropomyosin, this causes a shift in the position of tropomyosin. So, the myosin-binding sites on actin, which are normally covered by tropomyosin, are now exposed. Now, the myosin heads can bind actin, initiating the sliding of the filaments, i.e. muscle contraction.