Muscle Fiber Coloring Worksheet: Sarcomere Structure & Skeletal Muscle Anatomy

*Updated 2026*

Structures of the Sarcomere

• Sarcomere – The repeating contractile unit of a muscle fiber. It shortens during contraction, causing the entire muscle to contract.

• Thin (Actin) Filaments – Protein filaments attached to the Z discs. They slide toward the center of the sarcomere during muscle contraction.

• Thick (Myosin) Filaments – Protein filaments located in the center of the sarcomere. Myosin heads bind to actin and pull the thin filaments inward using ATP.

• Z Disc (Z Line) – The boundary of each sarcomere. It anchors the thin filaments and moves closer to adjacent Z discs during contraction.

• A Band – The dark region containing the entire length of the thick filaments. Its length remains the same during contraction.

• I Band – The light region containing only thin filaments. It becomes narrower as the muscle contracts.

• H Zone – The central area of the A band where only thick filaments are present. It decreases in width or disappears during contraction.

• M Line – The center of the sarcomere. It contains proteins that hold the thick filaments together and keep them aligned.

• Myofibril – A long bundle of repeating sarcomeres inside a muscle cell. Myofibrils are responsible for generating the force of muscle contraction.

• Sarcoplasmic Reticulum (SR) – A specialized membrane network that surrounds each myofibril. It stores and releases calcium ions needed for muscle contraction.

• Terminal Cisternae – Enlarged sacs of the sarcoplasmic reticulum that store large amounts of calcium. They release calcium when the muscle receives a nerve signal.

• Transverse (T) Tubules – Deep invaginations of the sarcolemma that carry electrical impulses into the interior of the muscle fiber, ensuring all sarcomeres contract at the same time.

• Triad – A structure formed by one T tubule and two terminal cisternae. It coordinates the release of calcium in response to a muscle impulse.

• Sarcolemma – The plasma membrane surrounding a muscle fiber. It conducts electrical signals that trigger muscle contraction.

• Mitochondria – Organelles that produce ATP through cellular respiration. Muscle cells contain many mitochondria to provide the energy required for repeated contractions.