Chapter 9 skeletal muscle microanatomy

Microanatomy of Skeletal Muscle

• Overview

  • Focus on Chapter 9: Microanatomy of Skeletal Muscle

  • Concepts applicable to cardiac muscle as well

Organization of Muscle

• Muscle Fiber

  • Defined by the sarcolemma (membrane of the muscle fiber)

  • Contains essential organelles and myofibrils

• Myofibril Structure

  • Comprised of sarcomeres arranged end to end

  • Z discs delimit each sarcomere

Sarcomere Structure

• Distinct Bands

  • I Band (Light Band): Isotropic; light can pass through

  • A Band (Dark Band): Anisotropic; light does not pass through

• Striated Appearance

  • Alternating I and A bands create striated pattern in skeletal and cardiac muscle

  • Cross-section Views: Show differences in arrangement when viewed at various angles

Myofibrils and Contractile Proteins

• Contractile Proteins

  • Organized to form sarcomeres

  • Thin filaments interdigitate with thick filaments

• Overview of Sarcomere Components

  • A Band: Area with thick filaments, may also include thin filaments

  • M Line: Central line of the A Band, anchors thick filaments

  • H Zone: Region at rest with thick filaments, no thin filaments

  • I Band: Contains only thin filaments and Z discs

Cross-section of Sarcomeres

• Cross-section Observations

  • A Band: Shows both thick and thin filaments interacting

  • I Band: Comprised solely of thin filaments, maintains specific organization (6 thin filaments for every 1 thick filament)

Thick Filaments

• Myosin Structure

  • Myosin is a dimer, composed of two protein strands

  • Regions of Myosin:

    • Tail: Anchors myosin in thick filament

    • Hinge: Allows flexibility; critical for contraction

    • ATP Binding Site: Vital for energy during muscle contraction

    • Actin Binding Site: Myosin's site of interaction with actin

Thin Filaments

• Composition of Thin Filaments

  • Primarily made of actin (small globular protein)

  • Actin molecules form a helical structure

• Regulatory Proteins:

  • Tropomyosin: Covers the myosin binding site on actin; prevents binding

  • Troponin Complex: Moves tropomyosin to allow myosin to bind during contraction

Sarcolemma and Sarcoplasmic Reticulum

• Muscle Cell Structure

  • Sarcolemma is the plasma membrane; has organelles including myofibrils and mitochondria

• Sarcoplasmic Reticulum (SR)

  • Specialized endoplasmic reticulum in muscle cells; crucial for calcium storage

  • Functions:

    • Stores calcium

    • Releases calcium into the cytosol during contraction

    • Sequesters calcium back into the SR via primary active transport

T Tubules and Triads

• T Tubules

  • Extensions of the sarcolemma, associated with myofibrils

  • Facilitate communication of signals through the muscle fiber

• Triad Formation

  • Comprised of a T tubule flanked by two terminal cisternae of the SR

  • Structure is essential for effective calcium release and muscle contraction

Neuromuscular Junction (NMJ)

• Definition

  • Synapse between a motor neuron and skeletal muscle fiber

• Function

  • Mediates communication; muscle contraction is initiated here

  • Existence of NMJs is unique to skeletal muscle; not found in involuntary muscles

• Importance

  • Critical for muscle contraction regulation

  • Involved in maintaining muscle health and functionality

Conclusion

• Next slides will cover cardiac muscle anatomy after this overview of skeletal muscle microanatomy.