Organization of Myofibrils
In skeletal and cardiac muscle tissues, contraction is driven by long organelles called myofibrils. They run parallel to the long axis of the fiber or cell and extend along its entire length.
- In cardiac muscle, myofibrils occupy about 40% of the cytoplasm (sarcoplasm) volume, with hundreds present in a single cardiomyocyte.
- In skeletal muscle, they occupy up to 70% of the volume, and a single muscle fiber (myosymplast) can contain up to 1,400 myofibrils.
Striations and Sarcomere Boundaries
The characteristic optical cross-striation pattern results from the highly ordered organization of myofibrils. Alternating regions of differing density run along the organelle:
- Light bands — I bands.
- Dark bands — A bands.
Because these zones align across adjacent parallel myofibrils, the entire muscle fiber appears striated. Right in the middle of the light I band lies a dark boundary called the Z-line (telophragma). The segment of the myofibril situated between two adjacent Z-lines is the sarcomere.
Ultrastructure and Contraction Mechanism
Each sarcomere contains thousands of longitudinally oriented protein filaments—myofilaments. They are divided into two types:
- Thin (actin).
- Thick (myosin).
At rest, myofilaments are distributed unevenly, creating the visible bands. At the edges of the sarcomere, near the Z-lines, only thin filaments are present. In the center, only thick filaments are found. Between these areas lies the zone of overlap, where both types of filaments coexist.
The sliding filament mechanism involves the mutual sliding of actin and myosin filaments past one another. Their overlap zone increases, shortening the sarcomere. The shortening of thousands of sarcomeres along a myofibril leads to the contraction of the entire muscle.