Lecture Notes: Skeletal Muscle Unit

Introduction to Muscle Tissue

• Three Types of Muscle Tissue:
  • Cardiac Muscle: Located in the heart.
  • Smooth Muscle: Found in hollow organs, circulatory vessels, digestive tract.
  • Skeletal Muscle: Attaches to bones, facilitates skeleton movement.

Muscle Tissue Characteristics

• Striations:
  • Skeletal and cardiac muscles are striated.
  • Smooth muscle lacks striations.
• Nuclei:
  • Skeletal Muscle: Multi-nucleated, nuclei peripherally located.
  • Smooth Muscle: Uninucleated, centrally located nuclei.
  • Cardiac Muscle: Uni- or bi-nucleated, centrally located nuclei.
• Voluntary/Involuntary:
  • Skeletal muscle is primarily voluntary, some muscles have involuntary components (e.g., diaphragm).
  • Smooth and cardiac muscles are involuntary.

Skeletal Muscle Structure

• Attachment:
  • Muscles attach to bones via tendons (muscle to bone), ligaments attach bone to bone.
• Connective Tissue Layers:
  • Epimysium: Outermost layer, surrounds whole muscle.
  • Perimysium: Surrounds bundles of muscle fibers (fascicles).
  • Endomysium: Around individual muscle fibers.
• Muscle Fiber: A muscle fiber is a muscle cell.

Histology of Skeletal Muscle

• Cross Section: Reveals arrangement of fascicles and connective tissue layers.
• Longitudinal Section: Highlights nuclei and striations in skeletal muscle.
• Striations: Result from the arrangement of protein filaments (actin, myosin) within a muscle cell.

Sarcomere and Muscle Contraction

• Structure: Functional unit of muscle contraction; extends from Z line to Z line.
  • Actin (thin filament), Myosin (thick filament), Tropomyosin, Troponin.
• Bands and Zones:
  • Z Line: Boundary of sarcomere.
  • M Line: Middle of sarcomere.
  • I Band: Actin only.
  • A Band: Myosin plus overlapping actin.
  • H Band: Myosin only.

Muscle Contraction Mechanism

• Excitation-Contraction Coupling:
  • Motor Neuron: Initiates action potential at neuromuscular junction.
  • Acetylcholine Release: Binds to sarcolemma, propagates impulse.
  • Calcium Release: Triggers muscle contraction.
  • Sliding Filament Theory: Myosin heads pull actin filaments to contract muscle, requires ATP for detachment and reattachment.

Motor Units and Recruitment

• Motor Unit: One motor neuron plus all muscle fibers it innervates.
  • Smaller motor units for precise movements; larger motor units for gross movements.
• Recruitment: Activating smaller motor units before larger ones for efficiency.

Types of Muscle Contractions

• Isometric: Muscle contracts but doesn’t change length.
• Isotonic: Muscle tension constant, muscle length changes.
  • Concentric: Muscle shortens.
  • Eccentric: Muscle lengthens.

Muscle Fiber Types

• Slow Twitch (Type I): Aerobic, fatigue-resistant, smaller force, smaller cross-sectional area.
• Fast Twitch (Type IIa and IIx): Anaerobic, high power output, fatigue quickly.

Exercise Physiology

• Hypertrophy: Increase in muscle fiber diameter with strength training.
• Delayed Onset Muscle Soreness: Result of microscopic tears, not lactic acid.
• Sex Differences: Men typically have greater muscle strength; women often more fatigue-resistant.

Clinical Applications

• Rigor Mortis: Post-mortem muscle stiffening due to calcium influx, lack of ATP.
• Sarcopenia: Age-related muscle mass decline, can be mitigated with strength training.
• Muscular Dystrophy: Genetic muscle diseases, most common is Duchenne's, no current cure.

Conclusion

• Next Exam: Covers cellular and muscle physiology.
• Support Available: Instructor reachable via email for help.