Explain the sliding filament theory

The sliding filament theory describes the process by which muscles contract.

Muscle fibres are made up of myofibrils. Myofibrils comprise of sarcomeres, containing actin and myosin.

1.) A nerve impulse arrives at the neuromuscular junction, releasing acetylcholine.Depolarisation continues down the t-tubules, causing Ca2+ release.

2.) Ca2+ binds to troponin, altering the shape of troponin, causing tropomyosin to move off actin binding sites. Myosin heads are now able to bind to the exposed actin binding sites, forming a cross-bridge.

3.) Hydrolysis of ATP releases energy, allowing myosin to 'cock' its head,pulling actin to create an overlap. This is muscular contraction (shortening).

4.) ATP binds to myosin heads, causing the cross-bridge to be broken.Once the ATP is hydrolysed it can bind to another actin binding site, further down actin. Allowing contraction to continue. 

5.) When ATP and Ca2+ are depleted in the muscle contraction terminates. Actin binding sites are covered again with tropomyosin.