Lecture Notes on Skeletal Muscle Structure and Function

Overview of Skeletal Muscles

• Total number of skeletal muscles: 75 pairs
• Functions:
  • Responsible for posture
  • Movements of limbs
  • Other functions include swallowing and eye movement

Key Topics Covered

1. Muscle Fibers
2. Motor Units
3. Types of Fiber
4. Fiber Architecture

Muscle Fibers

• Definition: Muscle fibers are the cells that make up skeletal muscle.
• Characteristics:
  • Thread-like appearance
  • Each fiber has a membrane called sacrolemma.
  • Contains sarcoplasm (similar to cytoplasm) that includes nutrients and mitochondria.

Myofibrils

• Definition: Myofibrils are structures within muscle fibers.
• Contain proteins responsible for muscle striations.
• Types of proteins:
  • Actin (thin filaments)
  • Myosin (thick filaments)

Organization of Muscle Fibers

• Muscle fibers are surrounded by membranes:
  • Endomysium: Covers individual muscle fibers.
  • Perimysium: Covers bundles of muscle fibers called fascicles.
  • Epimysium: Covers the entire muscle.

Sarcomeres

• Definition: The functional units of muscle contraction within myofibrils.
• Structure:
  • Composed of Z-lines, A-band, I-band, M-band, and H-zone.
  • I-band:
    • Isotropic, contains actin (thin filaments).
  • A-band:
    • Anisotropic, contains myosin (thick filaments).
  • M-band: Intermediate disc located between I-band and A-band.
  • H-zone: Hollow space in the center of the A-band.

Muscle Contraction Mechanism

• During contraction:
  • Z-lines move toward the H-zone.
  • Actin filaments slide over myosin filaments.
  • Muscle shortens, leading to contraction.

Additional Structures

• Sarcoplasmic Reticulum: Stores calcium ions needed for contraction.
• Transverse Tubules: Provide channels for the movement of electrolytes, generating action potential for muscle activation.

Muscle Fiber Count

• Determined at birth and remains consistent throughout life.
• Muscle growth primarily involves increasing the diameter of fibers, not the number of fibers.

Summary

• Covered the structure and function of skeletal muscle fibers, myofibrils, sarcomeres, and their role in muscle contraction.
• Emphasized the importance of certain membranes and additional structures in muscle function.