Explain how skeletal muscle contracts.

There are several simple steps that occur each time a skeletal muscle contracts which follow the sliding filament model.
Firstly a nerve impulse travels down a motor neuron releasing neurotransmitter acetylcholine. This causes a depolarisation of the skeletal muscle fibre travelling down the T-tubules. This causes the release of calcium ions (Ca2+) from the sacroplasmic reticulum.
The muscle fibre is made up of sacromere units each containing thin actin filaments and thick myosin filaments that are blocked from binding by troponin and tropomyosin. When Ca2+ ions bind tropomyosin/ troponin they remove this block exposing myosin binding sites on the actin filaments. ADP from the last round of contraction allows myosin to bind the actin forming a cross bridge.
ATP then binds causing the myosin head to detach and swivel back away from the M line/ band, and its hydrolysis allows binding the next site. As myosin heads pull the actin filaments towards the centre of the sacromere it shortens and results in muscle contraction. This cycle is repeated as long as ATP and Ca2+ ions are available.