Muscle System Summary

Types of Muscle Tissue

  • Skeletal Muscle: Attached to skeleton, voluntary, over 600 muscles, composed of muscle, nervous, blood, and connective tissues.

  • Cardiac Muscle: Involuntary, forms heart wall, responsible for pumping blood.

  • Smooth Muscle: Involuntary, found in internal organs (e.g., digestive tract).

Structure of Skeletal Muscles

  • Connective tissue coverings:

    • Fascia: Covers muscle.

    • Tendon: Connects muscle to bone.

    • Aponeurosis: Connects muscle to bone, skin, or muscle.

  • Types of connective tissue:

    • Epimysium: Surrounds whole muscle.

    • Perimysium: Surrounds fascicles.

    • Endomysium: Surrounds muscle fibers.

Skeletal Muscle Fiber Structure

  • Multinucleated with:

    • Sarcolemma: Cell membrane.

    • Sarcoplasm: Cytoplasm.

    • Myofibrils: Contain actin (thin) and myosin (thick) filaments; organized into sarcomeres.

    • Sarcoplasmic Reticulum (SR): Stores calcium; connects with T tubules.

Muscle Contraction

  • Excitation-Contraction Coupling: Neural stimuli trigger calcium release from SR, enabling contraction.

  • Sliding Filament Model: Thick and thin filaments slide past each other, sarcomeres shorten without changing filament length.

  • Cross-Bridge Cycling: Myosin heads attach, pull actin, release ADP; requires ATP for myosin reattachment.

Muscle Relaxation

  • Ends with cessation of neural stimulation. ACh is decomposed by acetylcholinesterase, calcium is pumped back into SR, preventing contraction.

Energy Sources for Muscle Contraction

  • ATP Reserves: Initial energy source, depleted quickly.

  • Creatine Phosphate: Regenerates ATP for a short duration.

  • Cellular Respiration: Glucose breakdown for long-term energy.

Muscle Fatigue and Oxygen Debt

  • Fatigue caused by blood flow deficiency, ion imbalances, lactic acid build-up.

  • Oxygen Debt: Oxygen needed to convert lactic acid back to glucose after intense activity.

Types of Muscle Contractions

  • Isotonic: Changes length (concentric - shortening; eccentric - lengthening).

  • Isometric: No change in length, develops tension.

Muscle Fiber Types

  • Type I (Slow-twitch): Fatigue-resistant, oxidative.

  • Type IIa (Fast-twitch): Intermediate, fatigue-resistant.

  • Type IIb (Fast-twitch): Glycolytic, fatigable, anaerobic.

Muscular System Disorders**

  • Myasthenia Gravis: Autoimmune disorder affecting ACh receptors.

  • Muscular Dystrophy: Genetic disorder affecting dystrophin.

Smooth and Cardiac Muscles

  • Smooth Muscle: Lack striations, involuntary, slower to contract, can stretch without tension change.

  • Cardiac Muscle: Striated, involuntary, intercalated discs, rhythmic contractions.

Aging Effects on Muscles

  • Decline in key muscle proteins after 40s, leading to sarcopenia (muscle loss) and reduced strength and reflexes.