Muscle Tissue Anatomy and Function

Muscle Tissue

• Muscle tissue is composed of cells specialized for contraction
• Muscle cells are called myocytes

Muscle Cell Characteristics

• Myocyte cytoplasm is called sarcoplasm
• Myocyte plasma membrane is called sarcolemma
• Sarcoplasmic reticulum (SR) is a modified endoplasmic reticulum
  • Forms a web-like network surrounding myofibrils
  • Varies in structure in different types of muscle tissue

Skeletal Muscle Organization

• Composed of muscle fibers that are long, cylindrical cells

The Skeletal Muscle Fiber

• Contains multiple nuclei
• Contains myofibrils, which are bundles of protein filaments

Sarcoplasmic Reticulum

• Network of interconnected tubules that run parallel to myofibrils
• Stores and releases calcium ions, crucial for muscle contraction

Structure of the Myofibril

• Contains three types of myofilaments
  • Thick filaments: composed of myosin protein
  • Thin filaments: composed of actin, tropomyosin, and troponin proteins
  • Elastic filaments: composed of the protein titin; stabilizes myofibril structure and resists excessive stretching

Myofilament Arrangement and the Sarcomere

• Myofilaments are arranged into repeating units called sarcomeres
• Sarcomere: functional contractile unit of a muscle fiber.
  • Bounded by two Z-discs
  • Contains alternating light and dark bands
    • Light band (I band) - only contains thin filaments
    • Dark band (A band) - contains both thick and thin filaments
    • Z discs: located in the center of the I band
      • Anchors thin filaments
      • Attaches myofibrils to each other

Protein Myofilament Arrangement

• Sarcomere: The region from one Z-disc to the next
• Myosin: located in the center of the sarcomere
• Actin: extends toward the edges of the sarcomere

Sliding Filament Mechanism of Contraction

• Actin and myosin filaments slide past each other, shortening the sarcomere
• The length of the filaments themselves do not change
• The I band and H zone shorten
• The A band remains the same length

Membrane Potential

• Muscle cells like all cells have a resting membrane potential
• The difference in electrical charge across the plasma membrane
• Maintained by sodium potassium pumps
• Pumps three sodium ions out of the cell for every two potassium ions pumped into the cell