Understanding Skeletal Muscle Structure

Feb 15, 2025

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.