A sarcomere (Greek σάρξ sarx "flesh", μέρος meros "part") is the smallest functional unit of striated muscle tissue.

Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as alternating dark and light bands.

Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes.

A change in the receptor conformation allows an influx of sodium ions and initiation of a post-synaptic action potential.

The L-type calcium channels are in close association with ryanodine receptors present on the sarcoplasmic reticulum.

The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction.

At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross-bridge binding sites on the actin.

A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration.

Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy.

Arthropods, however, show tremendous variation (over seven-fold) in sarcomere length, both between species and between muscles in a single individual.