Muscle and Tendon Physiology

Muscle Fundamentals - Mighty Machines

• Muscle Fiber (Cell): Basic structural unit; multinucleated. Contains myofibrils.
• Myofibril: Composed of repeating sarcomeres, the functional contractile units.
  • Sarcomere: Z-disc to Z-disc.
    • A-band: Anisotropic (dArk); contains Myosin ± Actin. Length constant during contraction.
    • I-band: Isotropic (lIght); Actin only. Shortens during contraction.
    • H-zone: Myosin only. Shortens during contraction.
    • M-line: Midpoint of sarcomere; anchors Myosin. and thin (actin) filaments, titin, and nebulin)
• Motor Unit: A single α-motor neuron and all the muscle fibers it innervates. Obeys "all-or-none" law.
• Muscle Fiber Types: , Type IIA (fast oxidative glycolytic), Type IIX/B (fast glycolytic) detailing contraction speed, fatigue resistance, color, mitochondria, myoglobin, capillary density, and primary metabolic pathway)

⭐ Titin is a giant protein responsible for passive elasticity of muscle and stabilizing myosin filaments within the sarcomere. It's the largest known protein in the human body!

Contraction Action - Sliding Story

• NMJ Activation: AP → Ca²⁺ influx → ACh release → ACh binds receptors → EPP → Muscle AP.

• Excitation-Contraction Coupling:

• Sliding Filament Mechanism:
  • Ca²⁺ binds Troponin C → Tropomyosin shifts, exposing actin sites.
  • Myosin head (energized by ATP hydrolysis: $ATP \rightarrow ADP + P_i$) binds actin (cross-bridge).
  • Power Stroke: Myosin pulls actin; ADP and $P_i$ are released.
  • New ATP binds myosin → detachment.
  • Cycle repeats if Ca²⁺ & ATP are present.
  • Sarcomere shortens: H & I bands ↓; A band constant. 📌 Mnemonic: "HI" disappears (H and I bands shorten).
• Relaxation: Ca²⁺ pumped back to SR (SERCA pump, ATP-dependent) → Tropomyosin covers actin sites.

⭐ Rigor mortis occurs due to ATP depletion, preventing myosin head detachment from actin.

Tendon Tidbits - Tough Connectors

• Function: Transmit muscle force to bone; enable movement & stability.

• Composition:

  • Cells: Tenocytes & tenoblasts (synthesize/maintain ECM).
  • ECM: Primarily Type I collagen (~95%), elastin, proteoglycans (e.g., decorin).

• Structure: Hierarchical: Collagen molecule → microfibril → fibril → fascicle → tendon.

  • Endotenon: Connective tissue around fascicles.
  • Epitenon: Connective tissue sheath covering entire tendon.
  • Paratenon: Outer loose connective tissue layer (if present, aids gliding).

• Vascularity: Limited; supplied by musculotendinous junction (MTJ), osteotendinous junction (OTJ) & paratenon/synovial sheath.

• Innervation: Golgi Tendon Organs (GTOs - proprioception), mechanoreceptors (Ruffini, Pacinian), nociceptors (free nerve endings).

• Biomechanics: Viscoelastic properties (creep, stress relaxation).

  • Stress-Strain Curve: Toe region (crimp unfolds), linear region (elastic), plastic region (microfailure), ultimate failure point.

• Healing: Slow due to ↓ vascularity; often results in mechanically inferior scar tissue.

⭐ Type I collagen is the predominant collagen type in tendons, providing their characteristic high tensile strength.

Healing & Hurt - Repair & Rebuild

• Muscle Injury:
  • Strain (indirect), Contusion (direct).
  • Grades: 1 (mild, <5% fibers, min. loss), 2 (moderate, partial tear, pain, ↓strength), 3 (severe, complete tear, defect).
• Tendon Injury:
  • Tendinopathy, Rupture.
• Healing Phases (Muscle & Tendon):
  1. Inflammation (0-7d): Hematoma, necrosis, phagocytosis. Cells: neutrophils, macrophages.
  2. Proliferation (1-6w): Fibroblasts, myoblasts active. Collagen III. Angiogenesis.
  3. Remodeling (>3w-yrs): Collagen III→I. Cross-linking, alignment, scar matures. Strength regains.

  ⭐ Tendon healing primarily forms scar tissue. Early Type III collagen is replaced by stronger Type I, but final tensile strength rarely matches the original, uninjured tendon.

• Key Points: RICE/PRICE acutely. Controlled mobilization is vital for optimal healing. Scar tissue is biomechanically inferior.

High‑Yield Points - ⚡ Biggest Takeaways

• Sarcomere: Basic contractile unit; actin-myosin interaction drives contraction.
• Calcium (Ca²⁺) binding to troponin C initiates muscle contraction.
• ATP is vital for cross-bridge cycling (contraction) and detachment (relaxation).
• Motor unit: A single motor neuron and all muscle fibers it innervates.
• Tendons: Primarily Type I collagen, ensuring high tensile strength.
• Golgi Tendon Organs sense muscle tension; Muscle Spindles sense muscle length/stretch.
• Henneman's size principle: Smaller motor units are recruited before larger ones during muscle activation.