Sliding filament model of muscle contraction (Biology HL)

The big idea: Striated (skeletal) muscle looks striped under the microscope because it is built from millions of identical repeating units lined up end to end.

Sarcomere

The functional contractile unit of striated muscle — the region between two adjacent Z-discs.

Z-disc

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

Thin filament (actin)

A thin protein filament anchored to the Z-discs and projecting inward toward the centre of the sarcomere.

Thick filament (myosin)

A thick protein filament that carries protruding heads (cross-bridges); it sits in the centre of the sarcomere.

M-line

The line down the middle of the sarcomere that links the myosin filaments and holds them in register.

Titin

A giant elastic protein that anchors myosin to the Z-disc, keeps it centred, and provides passive recoil (elasticity).

Thin = actin, thick = myosin: Thin filaments are actin — anchored to the Z-discs at the ends.

Thick filaments are myosin — in the centre, with heads that stick out.

An easy way to remember: myosin = the middle, massive (thick) filament with the moving heads.

A sarcomere is built from just a few proteins, each in a fixed place.

Once you know where each one sits, the striped banding you see under the electron microscope (EM) follows automatically. The bands are simply named after which filaments lie there and whether they overlap.

Titin — the molecular spring: Titin is the largest protein in the body, and it does three jobs at once.

Now the banding. The bands are just labels for regions of the sarcomere, based on what lies there:

The logic in one line: Light = actin only (I band), dark = myosin present (A band), and the myosin-only middle = H zone.

Every band name is just a statement about which filament is there — that's all you need to remember to label a sarcomere diagram correctly.

The big idea: A muscle shortens, but the protein filaments inside it do not. This is the sliding filament model.

What happens in contraction: Inside each muscle fibre, repeating units called sarcomeres contain both kinds of filament. During contraction the actin and myosin filaments slide past each other so that they overlap more. The filaments keep the same length; only the amount of overlap changes.

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.

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.

Draw it like this: the same sarcomere relaxed and contracted, with the bands marked.

Read the mechanism as a chain of cause and effect. It starts with the sites blocked, and the Ca²⁺ switch flips them open.

Then the myosin head runs a repeating four-step cross-bridge cycle as long as Ca²⁺ and ATP are both present.

The two jobs: Ca²⁺ is the switch, ATP is the energy: Keep the two molecules' roles separate: questions often test this. Ca²⁺ unlocks the door; ATP does the work of letting go and resetting.

How the muscle relaxes: When the nerve impulse stops, the Ca²⁺ is actively pumped back into the sarcoplasmic reticulum. With Ca²⁺ gone, troponin returns to its original shape, tropomyosin slides back over the binding sites, and no new cross-bridges form. The muscle relaxes.

Rigor mortis: This is why a dead body goes stiff (rigor mortis): with no ATP left, the heads cannot detach from actin, so the cross-bridges stay locked.

The cycle in four pictures: