The sliding filament model

When a sarcomere shortens, neither actin nor myosin gets any shorter. They slide past each other.

Two kinds of filament

Look at a skeletal muscle fibre under a strong microscope and you see alternating dark and light bands. The pattern comes from two kinds of protein filament inside each sarcomere. Thin filaments are made mostly of actin, twisted into a strand like two strings of beads, with the regulatory proteins tropomyosin and troponin lying along it. They are anchored at the Z discs at each end of the sarcomere and point toward the middle. Thick filaments are made of myosin, roughly 300 molecules in each. They sit in the middle, held together at the M line, and their heads stick out toward the thin filaments everywhere except a bare stretch in the centre.

Naming the bands

The bands were named from how they look. The A band is the dark band, and its width is the length of the thick filaments, with thin filaments overlapping its outer parts. The I band is the light zone on either side of a Z disc, where there are only thin filaments. The H zone is the paler middle of the A band, where there are only thick filaments, and the M line runs down its centre. A sarcomere runs from one Z disc to the next, so it holds one whole A band and half an I band at each end.

What changes and what stays the same

Compare a relaxed sarcomere with a contracted one and three things change: the Z discs move closer together, the I band narrows and the H zone narrows. The A band keeps exactly the same width. If the filaments themselves were getting shorter, the A band would shrink too. Since it does not, the filaments must keep their length and slide past each other, so that the zone where they overlap grows. This explanation is called the sliding filament model. At full contraction the thin and thick filaments overlap completely, and the I band all but disappears.

Drag the Contraction slider in the model and read the numbers in the corner. The sarcomere, the I band and the H zone all get smaller as you go. The A band stays at the same percentage the whole way.

Who pulls whom

The thick filaments stay put and the thin filaments move. Myosin heads grab the thin filaments and pull them toward the M line, and because the thin filaments are fixed to the Z discs, the Z discs are dragged inward. The heads on the left half of a thick filament pull one way and the heads on the right half pull the other way, so both ends of the sarcomere close in on the middle. Neighbouring sarcomeres share their Z discs, so when every sarcomere in a myofibril shortens a little, the myofibril and then the whole fibre shortens a lot.

Too long or too short

Myosin heads can only grab actin where the two kinds of filament overlap, so the length of a sarcomere affects how much tension it can make. This is the length-tension relationship. Sarcomeres produce the most tension between about 80 and 120 percent of their resting length. Stretch a sarcomere far beyond that and the overlap shrinks, leaving fewer heads within reach of actin. Squeeze it much shorter and the thin filaments run past the last heads and crowd the middle, so tension falls again. A sarcomere stretched so far that the filaments did not overlap at all could make no tension, but connective tissue and other proteins in the muscle normally stop that from happening.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.