Learning Objective
By the end of this section, students should be able to describe the structural organization of skeletal muscle at the myofibril level, identify the components of a sarcomere, and explain how band changes occur during muscle contraction.
Organization of Skeletal Muscle
Skeletal muscle is composed of elongated, multinucleated cells called muscle fibers.
Within each muscle fiber are numerous parallel cylindrical structures known as myofibrils, which run longitudinally along the length of the cell. Myofibrils are responsible for the striated appearance of skeletal muscle and contain the contractile machinery of the cell.
The Sarcomere: Functional Unit of Contraction
Each myofibril is subdivided into repeating contractile units called sarcomeres.
- A sarcomere extends from one Z line (Z disc) to the next.
- It is the basic functional unit of muscle contraction.
Activity
Filament Organization Within a Sarcomere
Sarcomeres are composed of two major types of filaments:
- Thin filaments → primarily actin
- Thick filaments → primarily myosin
Their precise arrangement creates distinct bands:
- A band – length of the thick (myosin) filaments
- I band – region containing only thin (actin) filaments
- H zone – central portion of the A band containing only thick filaments
- Z line – boundary of each sarcomere, where thin filaments anchor
This organized structure produces the characteristic striated appearance of skeletal muscle.
Structural Changes During Contraction (Sliding Filament Mechanism)
During muscle contraction:
- ✅ A band length remains unchanged (myosin filament length does not change)
- 🔻 I band shortens
- 🔻 H zone shortens or disappears
This occurs because actin filaments slide inward over myosin filaments — filaments do not shorten; they slide past each other.
Role of Titin
Titin is a large elastic protein that:
- Anchors myosin filaments to the Z line
- Maintains structural alignment of thick filaments
- Contributes to the elastic recoil of striated muscle
Titin is essential for maintaining sarcomere integrity and passive muscle elasticity.
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