How Do Skeletal Muscles Contract? Sliding Filament Theory

Skeletal muscles contract when myosin filaments pull actin filaments past them, shortening the sarcomeres inside muscle fibres. This is called the sliding filament theory. The filaments slide; they do not get shorter.

How contraction happens

1. A nerve impulse reaches the neuromuscular junction, causing an impulse in the muscle fibre. This leads to calcium ions being released from the sarcoplasmic reticulum, a store of calcium inside the fibre.
2. Calcium ions cause tropomyosin to move away from binding sites on the actin filaments. Myosin heads can now attach to actin, forming actinomyosin bridges.
3. The myosin heads change angle, pulling the actin filaments towards the centre of the sarcomere. This movement is the power stroke.
4. ATP binds to the myosin heads, causing them to detach from actin. ATP is then hydrolysed, providing energy to return the heads to a position where they can attach again. Repeated attachment, pulling and detachment makes the sarcomere shorter.

A sarcomere is the repeating contractile unit of a myofibril, bounded by two Z lines. As it contracts, the Z lines move closer together. The region containing only actin gets narrower, but the length of the myosin filaments stays the same.

A myofibril is a long bundle of these repeating units. When many sarcomeres shorten together, the muscle fibre shortens and produces a pulling force. Muscles work in antagonistic pairs: one contracts while the other relaxes, allowing movement around a joint.

When stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum. Tropomyosin covers the binding sites on actin again, so new actinomyosin bridges cannot form and the muscle relaxes.

Exam mistake to avoid: Do not say that actin or myosin filaments shrink. Explain that myosin pulls actin past it, and that ATP is needed both to detach myosin from actin and to reset the myosin head.