What is the sliding filament theory?

At a neuromuscular junction, an impulse arrives, causing Ca²⁺ to be released from the sarcoplasmic reticulum of your muscle cell (a system of membrane bound organelles around myofibril).

1. Ca²⁺ diffuses through sarcoplasm 

2. Ca²⁺ attaches to troponin, which causes tropomysoin to shift position, exposing the myosin binding site 

3. Myosin head can now bind to the actin, forming a cross bridge 

4. ADP + Pi released from myosin head 

5. Myosin changes shape and the head nods forward - this causes actin to slide over myosin

6. ATP then binds to myosin to detach it 

7. ATP then breaks down into ADP and Pi to return the myosin head back to its original position 

This is the sliding filament theory. It is much easier to learn it in a form of a flow chart.

N.B. When your muscles are relaxed, Ca²⁺ is actively pumped out of sarcoplasm using ATP, and so myosin binding site is blocked again by tropomyosin.