How do muscle fibres contract?

Contraction occurs when an impulse from a motor neurone reaches the synapse at the junction with the muscle, the neuromuscular junction (NMJ). 

At the presynaptic membrane of the NMJ, depolarisation results in the entry of calcium ions. This causes vesicles in the membrane containing the neurotransmitter acetylcholine to fuse with the membrane and release the neurotransmitter into the synapse, this is known as exocytosis. The neurotransmitter molecules diffuse across the synaptic cleft and bind to complementary receptors on the post-synaptic membrane. This causes sodium ions to enter the post-synaptic membrane and the signal has been transmitted across the synapse. 

Synaptic transmission then affects the muscle fibres through the release of calcium ions from within the muscle. Muscle is made up broadly of actin and myosin fibres. Normally, actin is bound to a glycoprotein called tropomyosin - the release of calcium ions displaces tropomyosin from actin. This exposes binding sites on actin fibres and allows myosin to bind, forming cross-bridges.

ATP, attached to the head of the myosin molecule is hydrolysed to ADP. The energy released from this causes the myosin heads to alter their conformational shape. This pulls the actin filament past the myosin filament.

The cross-bridges then detach and reattach, in the same fashion, this time further along the actin filament. In this way, the formation of fibres gradually shortens, in contraction.