Sliding Filament Theory

Question 1

Process

Answer 1

1. Impulse arrives at neuromuscular junction
2. Calcium ions are released from sacroplasmic reticulum
3. Calcium ions attach to troponin on actin filament causing it to move.
4. As a result tropomyosin on actin filament shifts exposing myosin binding sites.
5. Myosin head on myosin filament attaches to myosin binding site on actin filament forming cross bridges.
6. ADP and Pi are released from myosin head
7. Causes myosin head to change shape and nod forward and causing attached actin filament to slide over myosin.
8. ATP attaches to myosin head causing myosin head to detach from actin filament.
9. ATPase in myosin head hydrolysed ATP to ADP and Pi causing myosin head to change shape and return to its upright position.

Question 2

What happens when a muscle relaxes?

Answer 2

The muscle is no longer stimulated by a nerve impulse.

1. Calcium ions are actively pumped out of the muscle sacroplasm back into the sacroplasmic reticulum using ATP (form of active transport)
2. Troponin and Tropomysoin move back to their original positions where Tropomysoin blocks the myosin binding sites.
3. Actin filament slides over to their relaxed position and this lengthens the sacromere.

Question 3

What happens in the absence of ATP?

Answer 3

Cross bridges remain attached
Muscle is remains contracted
Calcium ions can not leave sacroplasm of muscle