Glossary
sarcomere
A sarcomere is one repeating contractile unit of a muscle fiber, running from one Z line to the next, and it is the part that shortens in a contraction.
A sarcomere is the repeating unit of a muscle fiber, and it is where contraction actually happens. A myofibril is a chain of sarcomeres laid end to end, each one bounded by a Z line. Inside a sarcomere are two sets of filaments: thin filaments of actin and thick filaments of myosin. In a contraction the myosin heads bind the actin and pull the thin filaments toward the center of the sarcomere. The filaments themselves do not shorten — they slide past one another, which is why the mechanism is called the sliding-filament model. What shortens is the sarcomere, and a whole muscle shortens because every sarcomere in every fiber shortens a little at the same time. The signal for it starts at the neuromuscular junction, where a motor neuron releases acetylcholine; calcium released inside the fiber then binds troponin and moves tropomyosin off the actin binding sites, and ATP powers the stroke and the release.
Taught in
Related terms
- muscle tissue — Muscle tissue shortens to produce movement, and its three kinds — skeletal, smooth and cardiac — differ in where they are and whether they obey you.
- flexion — Flexion is a movement that decreases the angle at a joint, as bending the elbow brings the hand toward the shoulder.
Source: The anatomy and physiology guide in this course, whose organ and tissue descriptions follow the US National Cancer Institute's SEER Training Modules and MedlinePlus consumer health pages. Both are US federal government material, not subject to copyright in the United States. The definition above is written for this course in our own words. (SEER Training Modules accessed 2026-10-26; definition written 2026-10-27)