7.8 Structure of Skeletal Muscles

The main biological concepts cover the hierarchical organization of skeletal muscle, from the whole muscle down to the molecular proteins responsible for its function. This includes the macroscopic and microscopic anatomy, with a focus on the sarcomere as the fundamental contractile unit.

Macroscopic and Microscopic Structure of Skeletal Muscle

Skeletal muscles, also known as striated muscles, have a complex, bundled structure.

• Connective Tissue Coverings:
  • Epimysium: A dense connective tissue layer that wraps the entire muscle.
  • Perimysium: Surrounds bundles of muscle cells called fascicles.
  • Endomysium: A delicate connective tissue layer that covers each individual muscle fibre (muscle cell).
• Tendon: At the ends of the muscle, the connective tissue layers merge to form a strong tendon, which attaches the muscle to bone.
• Muscle Fibre (Cell):
  • A single, cylindrical, multinucleated cell.
  • Sarcolemma: The plasma membrane of the muscle fibre.
  • Transverse Tubules (T-tubules): Deep invaginations of the sarcolemma that penetrate into the cell, allowing nerve impulses to travel deep into the muscle fibre.
  • Sarcoplasm: The cytoplasm of the muscle fibre.
  • Sarcoplasmic Reticulum: The specialized endoplasmic reticulum of a muscle cell, which stores and releases calcium ions.
  • Myofibrils: Numerous thin, rod-like structures that run parallel to the length of the muscle fibre. They contain the contractile elements.

The Sarcomere: The Contractile Unit

Myofibrils exhibit a repeating pattern of light and dark bands, giving skeletal muscle its striated appearance. This pattern is created by the arrangement of myofilaments within a unit called the sarcomere.

• Sarcomere: The fundamental contractile unit of a myofibril, defined as the region between two successive Z-lines.
• The arrangement of thick and thin myofilaments creates distinct bands and zones within the sarcomere.

Myofilaments: The Contractile Proteins

Myofibrils are composed of two types of protein filaments: thick and thin myofilaments.

• Thick Myofilaments:
  • Primarily composed of the protein myosin.
  • Each myosin molecule has a long tail and two globular heads.
  • The heads contain binding sites for actin and ATP.
  • About 300 myosin molecules are bundled together, with their tails forming the core and their heads pointing outwards.

• Thin Myofilaments:
  • Composed of three proteins: actin, tropomyosin, and troponin.
  • Actin: The primary structural protein. Globular actin (G-actin) subunits polymerize to form two intertwined strands of fibrous actin (F-actin). Each G-actin has a myosin-binding site.
  • Tropomyosin: A rod-shaped protein that spirals around the actin core. In a relaxed muscle, it blocks the myosin-binding sites on actin.
  • Troponin: A complex of three regulatory proteins bound to tropomyosin.
    • TnI (inhibitory): Binds to actin.
    • TnT (tropomyosin-binding): Binds to tropomyosin and helps position it on actin.
    • TnC (calcium-binding): Binds to calcium ions (Ca²⁺), which triggers the contraction process.

Possible Questions/Answers

• Q: What gives skeletal muscle its striated (striped) appearance? A: The regular, repeating arrangement of dark A-bands and light I-bands within the myofibrils.
• Q: What is a sarcomere? A: It is the basic contractile unit of a muscle fibre, extending from one Z-line to the next.
• Q: What is the role of tropomyosin in a relaxed muscle? A: Tropomyosin physically blocks the myosin-binding sites on the actin filaments, preventing the myosin heads from attaching and thus preventing contraction.
• Q: How are thick and thin filaments arranged relative to each other? A: In the regions of overlap, each thick filament is surrounded by six thin filaments in a hexagonal pattern.

Summary

• Skeletal muscle is organized hierarchically: Muscle → Fascicles → Muscle Fibres → Myofibrils → Myofilaments.
• Connective tissue layers (epimysium, perimysium, endomysium) provide structure and support.
• The sarcomere is the functional unit of contraction, located between two Z-lines. Its structure includes A-bands, I-bands, H-zones, and an M-line.
• Myofilaments are the proteins responsible for contraction.

• Biological Significance: This highly organized, intricate structure of proteins and membranes allows for the precise control and powerful force generation required for voluntary movement, powered by the sliding of myofilaments past one another.