Locomotion and Movement · Lesson 2 of 10
Skeletal Muscle Architecture and the Sarcomere
“Trace skeletal-muscle organisation from whole muscle to sarcomere and explain the A, I and H regions.”
• Identify fascicles, muscle fibres, sarcolemma, sarcoplasm and sarcoplasmic reticulum. • Trace myofibrils and sarcomere boundaries. • Label Z line, M line, A band, I band and H zone. • Explain how filament placement produces striations.
A skeletal muscle contains many nested units
A muscle used to bend the elbow may appear to be one fleshy structure, but it contains many bundles arranged side by side. The organised skeletal muscle includes groups called fascicles or muscle bundles, held together by collagen-containing connective tissues. Each fascicle contains many long muscle fibres, and each muscle fibre is a single specialised muscle cell. These fibres extend within the muscle and are connected mechanically to connective tissue that helps transmit generated force to tendons and the skeleton.
The membrane of a skeletal muscle fibre is called the sarcolemma, and the intracellular fluid and contents are called sarcoplasm. Mature skeletal fibres are multinucleate because precursor cells fuse during development, producing a syncytium. Their many nuclei support the needs of a very long cell containing extensive contractile structures. The sarcoplasmic reticulum, a specialised intracellular membrane network related to endoplasmic reticulum, stores Ca²⁺ and releases it when a fibre receives an appropriate activating signal.
Myofibrils create the striped pattern
Each fibre contains many parallel myofibrils. At the microscopic level, a myofibril displays alternating dark and light stripes. These repeat because thick myosin filaments and thin actin filaments are arranged in an orderly overlapping pattern. Dark A bands correspond to the entire length of thick filaments, including zones of overlap with thin filaments; light I bands contain thin filaments without thick myosin. The word isotropic is associated with the I band and anisotropic with the A band in the textbook's terminology.
A common simplification says that A bands contain only myosin and I bands contain actin. A more precise statement is that the A band spans the full length of thick myosin filaments and may also include overlapping actin at its outer portions; only the central H zone of a relaxed sarcomere has thick filaments without overlapping thin filaments. In the I band, only thin filaments occur. This distinction becomes essential for correctly predicting changes during contraction.
The sarcomere is the repeating contractile unit
A sarcomere extends from one Z line to the next Z line along a myofibril. Each Z line anchors thin actin filaments extending toward the centre. Thick myosin filaments lie approximately at the centre and are aligned with the M line. The A band corresponds to the thick-filament length. Each I band lies across a Z line and contains the thin-only region. The H zone is the central section of the A band where thin filaments do not overlap thick ones in a resting sarcomere.
What happens to the bands when muscle shortens?
During a contraction, actin filaments slide farther between the thick filaments, drawing the Z lines closer together. The overlap of actin and myosin increases, so the I bands and central H zone become narrower. The actual lengths of the thin and thick filaments remain largely unchanged, so the length of the A band stays the same. The key observation is shortening of the distance between Z lines, not shortening of individual actin and myosin protein filaments.
If a sarcomere were shown with a narrower A band after contraction, the drawing would wrongly imply that the thick filaments shorten. If the H zone were shown at maximal overlap, it might become extremely small or disappear in a simplified diagram, while the A band would still correspond to the thick-filament span. By comparing rest and contracted states against fixed Z, A and M landmarks, students can infer filament sliding instead of memorising the phrase without understanding the geometry.
How fibre organisation contributes to force
Myofibrils in a skeletal fibre are arranged mostly in parallel, enabling force contributions from many contractile elements. Sarcomeres are arranged end-to-end along each myofibril, so many small shortening events together produce meaningful movement. Connective tissues surrounding fibres and fascicles transmit tension beyond the individual cells. The architecture connects molecular actin–myosin interactions to a whole arm or leg moving through a joint.
| Structure | Location or boundary | Role |
|---|---|---|
| Sarcolemma | Around each muscle fibre | Electrical excitation and cell boundary |
| Sarcoplasmic reticulum | Within muscle fibre | Ca²⁺ storage and release |
| Z line | Sarcomere ends | Thin-filament anchorage |
| M line | Sarcomere centre | Thick-filament organisation |
| A band | Full myosin filament span | Thick filaments and overlap |
| I band | Thin-only region | Shortens as overlap increases |
| H zone | Central thick-only region | Narrows on contraction |
Label two Z lines as the boundaries of one sarcomere. Draw thick myosin in the middle and thin actin entering from both ends; then mark H, A and I regions by the presence or absence of overlap.
Quick Quiz
Quiz
What spans one complete sarcomere?
Which band remains constant in length during ordinary sliding-filament shortening?
Which region is thick myosin without actin overlap in a resting sarcomere?
Where does a skeletal fibre store much of the Ca²⁺ used for contraction?
Why is a skeletal muscle fibre called a syncytium?
Practice Problems
- Draw and label a resting sarcomere with A band, I band, H zone, M line and two Z lines.
- Describe how muscle, fascicle, fibre, myofibril and sarcomere relate to each other.
- What changes during contraction and what remains unchanged in banding?
- Why does the A band contain both thick and thin filaments in its overlap regions?
- Differentiate sarcolemma from sarcoplasmic reticulum.
Model Answers
1. Draw thick myosin centred on M line, thin actin attached to Z lines and extending inward. A band spans myosin, I bands are thin-only, H zone is thick-only and Z-to-Z defines the sarcomere.
2. Whole muscles contain fascicles; fascicles contain muscle fibres; fibres contain myofibrils; myofibrils have serial sarcomeres that generate tension through filament interaction.
3. Z lines move closer, I band and H zone narrow while A band remains constant; actin and myosin filaments slide without individually shortening.
4. It is defined by the presence of the full thick-filament length. Thin filaments extend partly into that region from Z lines, producing overlap toward the edges.
5. Sarcolemma is the muscle-fibre plasma membrane that propagates electrical excitation. Sarcoplasmic reticulum is an internal membrane store controlling Ca²⁺ availability.
Key Takeaways
• Whole skeletal muscles contain fascicles, muscle fibres, myofibrils and repeating sarcomeres. • The sarcolemma surrounds fibres; the sarcoplasmic reticulum stores contraction-triggering Ca²⁺. • Each sarcomere extends from Z line to Z line; the M line lies centrally. • A band spans myosin filaments, I band is actin-only and H zone is central myosin-only. • Contraction increases overlap: I and H narrow, while A-band length stays constant.