SarcomereThe functional unit of a skeletal muscle fiber: the stretch of myofibril from one Z disc to the next, about 2 micrometers long. One myofibril holds thousands of them end to end.
Z disc (Z line)The border of the sarcomere. Thin filaments are anchored to it through the protein alpha-actinin, and titin is fixed there too. Z is from the German zwischen, between.
I band (light band)The light band: thin filaments with no thick filaments over them. A Z disc runs through its middle, so each I band is shared by two sarcomeres. I is for isotropic.
A band (dark band)The dark band. It covers the whole length of the thick filaments, including the ends where thin filaments overlap them. Its width does not change when the muscle contracts. A is for anisotropic.
H zoneThe paler middle of the A band. Thin filaments do not reach it, so it holds thick filaments only. It narrows as the sarcomere contracts. H is from the German heller, brighter.
M lineThe center of the sarcomere, where accessory proteins such as myomesin hold the thick filaments together. M is from the German mittel, middle.
Thick filament (myosin)Built from about 300 myosin molecules, with the tails bundled into a shaft and the heads pointing outward. It is anchored at the M line and stops short of the Z discs.
Thin filament (actin)Built mainly of actin, with tropomyosin and troponin along it. One end is anchored in a Z disc. The free end points toward the M line and, in a relaxed sarcomere, stops short of it.
Relaxed sarcomere 2
Zone of overlapThe part of the A band where thick and thin filaments lie side by side. Cross-bridges can form only here, so the pull that slides the filaments begins in this zone.
Titin (elastic filament)A springy protein that runs from the Z disc to the M line, so one molecule spans half a sarcomere. It keeps the thick filament centered and recoils after a stretch. Only myosin and actin are more abundant in muscle.
Contracted sarcomere 5
Shortened sarcomereAs the thin filaments slide toward the M line they pull the Z discs closer together. The filaments themselves do not shorten. They slide past one another.
Shortened I bandThe I band gets narrower as the Z discs are pulled toward the ends of the thick filaments. Less of each thin filament is left uncovered.
Unchanged A bandThe A band stays the same width, because it is the length of the thick filaments, and they do not shorten.
Shortened H zoneThe H zone narrows as the thin filaments slide inward. At full contraction the thin and thick filaments overlap completely, and the H zone is filled in.
Longer zone of overlapMore of each thin filament now lies beside a thick filament than in the relaxed figure. Cross-bridges can form only where the two overlap.
Cross-bridge close-up 5
Myosin tailThe long tail of one myosin molecule. Tails of neighboring molecules join to form the shaft of the thick filament, and the stretch next to the M line is tails only, with no heads.
Myosin head (cross-bridge)The head has one binding site for actin and one for ATP. Bound to actin it forms a cross-bridge, and each power stroke pulls the thin filament about 10 nanometers toward the M line.
Actin molecule (G-actin)A globular protein. Chains of it form the backbone of the thin filament, and each unit carries a binding site for a myosin head.
TroponinThree globular subunits: one holds tropomyosin, one holds actin and one binds calcium. When calcium binds, troponin changes shape and lets tropomyosin move off the binding sites.
TropomyosinA strand-like protein that lies along the actin. In a resting muscle it covers the myosin-binding sites, so no cross-bridge can form.
How this list was compiled: Part names and detail lines were checked on 2026-09-19 and 2026-09-20, for facts only and written in our own words, against OpenStax Anatomy and Physiology 2e, sections 10.2 Skeletal Muscle and 10.3 Muscle Fiber Contraction and Relaxation (https://openstax.org/books/anatomy-and-physiology-2e/pages/10-2-skeletal-muscle, https://openstax.org/books/anatomy-and-physiology-2e/pages/10-3-muscle-fiber-contraction-and-relaxation), and OpenStax Biology 2e, section 38.4 Muscle Contraction and Locomotion (https://openstax.org/books/biology-2e/pages/38-4-muscle-contraction-and-locomotion). What those sections do not state was checked on Wikipedia: https://en.wikipedia.org/wiki/Sarcomere, https://en.wikipedia.org/wiki/Titin and https://en.wikipedia.org/wiki/Actin. Licence: own work
The drawing: Drawn for this site, 2026-09-20. Original line drawing built in code from written proportions; nothing was traced or copied from an existing illustration. Read for accuracy on 2026-09-20 (text only): https://en.wikipedia.org/wiki/Sarcomere, https://en.wikipedia.org/wiki/Myofilament, https://en.wikipedia.org/wiki/Actin, https://en.wikipedia.org/wiki/Tropomyosin, https://en.wikipedia.org/wiki/Troponin, https://en.wikipedia.org/wiki/Muscle_contraction Licence: own work
How it works
This sarcomere diagram labeled with 20 parts opens with every label on: a relaxed sarcomere with its bands, zones, lines and filaments, the same sarcomere contracted, and a close-up of one myosin head gripping actin. Name it turns the sarcomere labeled diagram into a quiz.
Read the full guide
Reading the bands from Z disc to Z disc
One sarcomere runs from one Z disc (2) to the next. The light I band (3) around each disc has no thick filaments. The dark A band (4) is as long as the thick filaments. Its paler middle, which thin filaments do not reach, is the H zone (5), around the M line (6). On either side lies a zone of overlap (7).
The same sarcomere, contracted
Parts 11 to 15 make this a sliding filament theory diagram labeled in both states. On the middle figure the Z discs are closer, the I band and H zone narrower and the zone of overlap longer. Dashed guides show the A band as wide as before. No filament shortens: thin ones slide inward over thick ones.
Dark has an a, so the dark band is the A band. Light has an i, so the light band is the I band.
The Z disc is a zigzag and the M line is straight. Z is from the German zwischen, between, and M from mittel, middle.
Under Start from, take Simple: 8 core parts first, then Contracted sarcomere. Older books say Z line for Z disc; either counts when typed.
Questions and answers
Which names do you need when asked to label the structures of a sarcomere?
Begin with Simple, under Start from: sarcomere, Z disc, M line, A band, I band, H zone, thick filament and thin filament. Titin and the zone of overlap come next, then the close-up: myosin head and tail, actin, tropomyosin and troponin.
Can I print the sarcomere unlabeled for a worksheet?
Yes. Print blank gives the sarcomere unlabeled: three figures, rings numbered 1 to 20 and an answer line for each. Print labelled is the answer key.
Why does the contracted figure still have an H zone?
The middle figure is only partly contracted, so a sliver of H zone is left to find. At full contraction the thin and thick filaments overlap completely and the H zone is filled in.