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Introduction to skeletal muscle organization

• Definition: Skeletal muscles are composed of muscle fibers that contract to generate movement.
• Hierarchy: Organized from the whole muscle to molecular components.

Gross Anatomy

• Muscle: An organ made up of muscle tissue, blood vessels, tendons, and nerves.
• Fascicles: Bundles of muscle fibers within the muscle.

Muscle Fibers

• Myofibers: Long, cylindrical cells that make up fascicles.
• Sarcolemma: The cell membrane of a muscle fiber.
• Sarcoplasm: The cytoplasm of a muscle fiber.

Myofibrils

• Definition: Rod-like units within muscle fibers.
• Sarcomeres: The basic contractile units of myofibrils.
• Myofilaments: Actin (thin) and myosin (thick) filaments that slide past each other to produce contraction.

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Detailed Sarcomere Structure

• Z-disc: Marks the lateral boundary of each sarcomere.
  • Function: Anchors the thin filaments and connects adjacent myofibrils.
• I-band: The lighter area containing only thin filaments.
  • Function: Changes width during muscle contraction and relaxation.
• A-band: The dark area containing the entire length of thick filaments.
  • Overlap Region: Where thick and thin filaments overlap.
  • Function: Remains constant in width during contraction.
• H-zone: The central part of the A-band with only thick filaments.
  • Function: Disappears during maximum muscle contraction.
• M-line: Located in the center of the H-zone; contains proteins that hold thick filaments in place.
  • Function: Maintains the alignment of the sarcomere during contraction.

Molecular Components of Muscle Contraction

• Actin Filaments (Thin Filaments):
  • G-actin (Globular Actin): Polymerizes to form F-actin strands.
  • F-actin (Filamentous Actin): Two F-actin strands twist together to form the thin filament.
  • Tropomyosin: Blocks the myosin-binding sites on actin in a relaxed muscle.
  • Troponin Complex: Binds to tropomyosin; moves it away from myosin-binding sites when calcium ions are present.
• Myosin Filaments (Thick Filaments):
  • Myosin Heavy Chains: Form the backbone of the filament.
  • Myosin Heads: Bind to actin to form cross-bridges; contain ATPase activity for energy transduction.
  • Titin: Connects myosin filaments to the Z-disc; provides elasticity and stability.
• Regulatory Proteins:
  • Troponin: Binds calcium ions and initiates the contraction process by moving tropomyosin.
  • Tropomyosin: Regulates the access of myosin to actin binding sites.

Contraction Mechanism

• Crossbridge Formation: Myosin heads bind to actin forming crossbridges.
• Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.
• ATP Binding and Myosin Detachment: A new ATP molecule binds to the myosin head, causing it to release from actin.
• ATP Hydrolysis and Cocking of Myosin Head: ATP is hydrolyzed, re-energizing the myosin head and returning it to the cocked position.

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These details provide a more comprehensive understanding of the sarcomere’s structure and the molecular components involved in muscle contraction. If you need further clarification or additional information, feel free to ask!

Neuromuscular Junction

• Motor End Plate: The site where the motor neuron communicates with the muscle fiber.
• Acetylcholine (ACh): The neurotransmitter that triggers muscle contraction.

Excitation-Contraction Coupling

• Action Potential: An electrical signal that triggers muscle contraction.
• Calcium Ions: Released from the sarcoplasmic reticulum, initiating the sliding filament mechanism.

Regeneration and Repair

• Satellite Cells: Stem cells that aid in muscle repair and growth.
• Hypertrophy: Increase in muscle size due to an increase in myofibril size.

Clinical Correlation

• Muscular Dystrophy: A group of diseases that cause progressive weakness and loss of muscle mass.
• Myopathies: Disorders characterized by muscle weakness.

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