Published by Patrick Mutisya · 14 days ago
Describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams.
Striated (skeletal) muscle fibres are long, multinucleated cells that contract rapidly and voluntarily. Their characteristic banding pattern arises from the highly ordered arrangement of contractile proteins within the sarcomere, the fundamental repeating unit of a myofibril.
Myofibrils are composed of alternating thick (myosin) and thin (actin) filaments that are anchored to specialised protein complexes at the sarcomere boundaries.
The sarcomere extends from one Z‑disc to the next (≈2.2 µm in relaxed skeletal muscle). Its internal zones are defined as follows:
| Region | Primary Components | Function |
|---|---|---|
| Z‑disc | α‑actinin, titin, nebulin | Anchors thin filaments; transmits tension laterally. |
| I‑band | Thin filaments only (actin, tropomyosin, troponin) | Region that shortens during contraction. |
| A‑band | Entire length of thick filaments (myosin) plus overlapping thin filaments | Length remains constant; contains H‑zone and M‑line. |
| H‑zone | Central part of A‑band containing only thick filaments | Decreases in width as thin filaments slide inward. |
| M‑line | Myosin‑binding proteins (myomesin, C‑protein) | Stabilises centre of thick filament lattice. |
Each thin filament is a double‑helical polymer of G‑actin (\overline{1}.0 µm long) with tropomyosin strands winding every 7 nm and troponin complexes at 38.5 nm intervals. The pointed (−) end points toward the M‑line, the barbed (+) end attaches to the Z‑disc.
Myosin molecules assemble into bipolar filaments \overline{1}.6 µm in length. Each molecule has two globular heads (\overline{10} nm) that bind ATP and actin, and a long α‑helical tail that forms the filament backbone. The heads project outward from the centre, creating the cross‑bridge sites.
Titin spans from the Z‑disc to the M‑line, acting as a molecular spring that maintains sarcomere alignment and contributes to passive elasticity. Nebulin runs along the length of the thin filament, stabilising its length.
Transmission electron microscopy (TEM) reveals the ordered lattice of filaments and the distinct banding pattern:
The sliding filament theory explains how sarcomere shortening produces muscle contraction:
\$\text{Sarcomere length change} = \Delta L = (n_{\text{cross‑bridges}}) \times d\$
where \$d \approx 10\ \text{nm}\$ is the displacement per power stroke of a myosin head. As cross‑bridges cycle, the I‑band and H‑zone diminish while the A‑band length remains constant.