The sliding filament model of contraction

The big idea: A muscle shortens, but the protein filaments inside it do not.

Inside each muscle fibre, repeating units called sarcomeres contain two kinds of filament:

Thin filaments made of actin, anchored to plates called Z-discs.

Thick filaments made of myosin, sitting in the centre.

During contraction the actin and myosin filaments slide past each other so that they overlap more. The filaments themselves keep the same length — only the amount of overlap changes. This is the sliding filament model.

Sarcomere

The repeating contractile unit of a muscle fibre, running from one Z-disc to the next.

Z-disc

The plate at each end of a sarcomere; the thin actin filaments are anchored to it.

Actin (thin filament)

The thin protein filament anchored to the Z-discs and projecting inward toward the centre.

Myosin (thick filament)

The thick protein filament in the centre of the sarcomere; it pulls on the actin.

Sliding filament model

The model that explains contraction: actin and myosin slide past each other so they overlap more, and the sarcomere shortens — the filaments do not shorten.

Slide, not shrink: The single most-tested point: the filaments slide, they do not shorten.

A sarcomere gets shorter because the overlap increases, just like two combs pushed into each other take up less space without either comb getting shorter.

A sarcomere has a striped banding pattern you can name. Because contraction only increases overlap, each band changes in a fixed, predictable way — and you can work each one out from where actin and myosin sit.

Read it as structure → observation: think about what overlaps with what, and the band change follows.

The A band is your control: The A band equals the length of the thick myosin filaments.

Since myosin never shortens, the A band never changes length — not when relaxed, not when fully contracted.

This is the proof that the filaments slide rather than shrink: if the filaments themselves got shorter, the A band would shrink too. It doesn't, so they must be sliding.

One sarcomere → the whole muscle: Each muscle fibre is a long chain of sarcomeres joined end to end (in series).

When every sarcomere shortens by a small amount at the same time, the shortenings add up along the fibre, so the whole fibre — and the whole muscle — shortens noticeably.

So a big movement comes from many tiny sliding events happening together in series.

How this is tested: The classic HL question gives you the banding pattern of a relaxed and a contracted sarcomere (a micrograph or diagram) and asks you to explain the changes, or to deduce which one is contracted.

To score well: say the filaments slide and do not shorten; that overlap increases; that the Z-discs come closer so the sarcomere shortens; and give the band changes — I band shorter, H zone shorter, A band unchanged. Always justify the A band staying the same by the unchanged myosin length.

✓ Why this scores full marks: It names the mechanism (filaments slide, don't shorten), links more overlap to the Z-discs closing and the sarcomere shortening, and explains each band change with its reason — ending with the A band unchanged because myosin keeps its length.

A common way to lose marks is to write 'the filaments contract/shorten' — that's the wrong mechanism and usually costs the first mark and the A-band mark.