A skeletal muscle fiber is a long, multinucleated muscle cell specialized for producing force. Most of its interior is filled with parallel cylindrical bundles called myofibrils.

Each myofibril is built from repeating contractile units called sarcomeres. A sarcomere extends from one Z disc to the next.

Two filament systems interdigitate inside each sarcomere:

• thin filaments, built mainly from actin and anchored at the Z discs;
• thick filaments, built from myosin and centered around the M line.

The two filament types overlap through part of the sarcomere but are not attached end-to-end.

Sarcomere banding reflects filament geometry

Striated muscle has alternating light and dark bands because thick and thin filaments occupy different regions.

• The A band spans the full length of the thick filaments.
• The I band contains thin filaments but no thick filaments.
• The H zone, when visible, is the central part of the A band containing thick filaments but no thin-filament overlap.

A useful schematic is

Z disc        M line        Z disc
  |              |              |
  |---- thin ----|---- thin ----|
       ===== thick =====

The real arrangement is three-dimensional: many thin filaments surround each thick filament so that numerous myosin heads can interact with actin.

Regulatory proteins are part of the thin filament

Thin filaments also contain tropomyosin, a long regulatory protein lying along actin, and the troponin complex, which can change tropomyosin's position in response to Ca$^{2+}$.

Their regulatory mechanism is a separate concept; structurally, they form part of the contractile apparatus.

Hierarchical organization

The mechanical hierarchy is

skeletal muscle
   ↓
muscle fibers
   ↓
myofibrils
   ↓
repeating sarcomeres
   ↓
thick and thin filaments

This organization lets microscopic actin–myosin forces from many sarcomeres combine into macroscopic muscle tension.