Muscular System: Anatomy

MUSCULAR SYSTEM: ANATOMY

Functions:

  • Posture Maintenance: Helps maintain standing position.

  • Support Soft Tissues: E.g., abdominal wall protects intestines.

  • Guard Entrances & Exits: Voluntary control of swallowing and waste release.

  • Thermoregulation: Muscle energy use increases body temperature.

  • Protein Storage: Muscles break down when protein-deficient.

    • Proteins involved: Actin, Myosin, Titin, Troponin, Tropomyosin, etc.

  • Movement: Enabled by muscle contraction.

LEVELS OF ORGANIZATION

  1. Skeletal Muscle (Organ Level)

  2. Muscle Fascicle (Bundle of Cells; Tissue Level)

  3. Muscle Fiber (Cell Level)

CONNECTIVE TISSUES

  • Endomysium: Surrounds and stabilizes each muscle fiber.

  • Perimysium: Surrounds and stabilizes each fascicle.

  • Epimysium: Surrounds and stabilizes the entire muscle.

  • Tendon: Fusion of all three layers to attach muscle to bone.

    • Aponeurosis: Tendon for flat muscles.

OTHER TISSUES

  • Nervous Tissue: Facilitates communication between brain and muscle.

  • Vascular Tissue: Delivers oxygen, nutrients, and removes waste.

STEM CELLS

  • Myoblasts: Fuse to create muscle fibers.

  • Myosatellite Cells: Assist in repairing damaged mature muscle fibers.

MUSCLE CELL ORGANELLES

  • Nuclei: Control centers containing DNA (blueprint for proteins).

  • Mitochondria: Release energy.

  • Sarcolemma: Cell membrane receiving electric signals.

  • Myofibril: Organelle responsible for muscle contraction.

  • Sarcoplasm: Cytoplasm within the muscle cell.

  • Sarcoplasmic Reticulum (SR): Transport system within the cell.

  • Terminal Cisternae: Store calcium (Ca²⁺).

  • Transverse Tubules: Relay signals from sarcolemma to SR.

  • Triad: Combination of 2 terminal cisternae and 1 transverse tubule.

PARTS OF THE SARCOMERE

  • Sarcomere: Basic unit of muscle contraction; multiple along each myofibril.

  • Thick Myofilament: Made of myosin protein.

  • Thin Myofilament: Made of actin protein.

    • Cross-bridges: Formed between thick and thin filaments for contraction.

SARCOMERE BANDS & LINES

  • A Band: Dark band; length remains constant during contraction.

  • H Band: Contains only thick filaments; length changes during contraction.

  • I Band: Light band; length changes during contraction.

  • M Line: Middle of the sarcomere.

  • Z Line: Start and end of a sarcomere.

PROTEINS IN SARCOMERE

  • Tropomyosin: Blocks bonding sites on thin filaments to prevent constant contraction.

  • Troponin: Moves tropomyosin to allow muscle contraction.

SLIDING FILAMENT THEORY

  • Explains how muscles contract:

    • Before Contraction:

      • H & I bands are longer.

    • During Contraction:

      • Cross-bridges form.

      • H & I bands shorten.

      • A band length remains unchanged.

      • Z lines move inward, shortening the sarcomere.

      • Important: Filaments slide past each other; they do not shorten.

LEVELS OF ORGANIZATION (PHYSIOLOGY OF CONTRACTION)

  • From Smallest to Largest:

    1. Myofilament

    2. Sarcomere

    3. Myofibril

    4. Muscle Fiber

    5. Muscle Fascicle

    6. Skeletal Muscle

    7. Muscular System

    8. Human Body