Muscle Tissue and Skeletal Muscle Structure

Muscle Types and Control

  • Skeletal Muscle:
    • Voluntary - we can control it.
  • Cardiac Muscle:
    • Specialized skeletal muscle.
    • Involuntary - we don't control it.
    • Organized similarly to skeletal muscle.
  • Smooth Muscle:
    • Associated with organs (viscera).
    • Involuntary - we cannot control it.

Contraction and Relaxation

  • Contraction can occur outside of muscle tissue with actin-myosin interaction.
  • Sponges and Nigerians (likely referring to cnidarians like Hydra) can contract without muscle tissue.
    • They can change body shape by contracting and elongating.
  • Actin: Thin filament.
  • Myosin: Thick filament.
  • Contraction: Shortening of muscle.
  • Relaxation: Elongation of muscle.
  • Contraction: Stimulus required.
  • Relaxation: Stimulus removed.

Muscle Groups: Agonists and Antagonists

  • Agonists: Muscles that work together for a particular function (flexion or extension).
  • Antagonists: Muscles that oppose a particular function.
  • Example: Biceps and Triceps
    • Act as antagonists to each other.
    • Act as Agonists depending on whether flexing or extending.

Skeletal Muscle Structure

  • Individual cells, groups of cells, and the entire muscle are surrounded by connective tissue.
  • Fiber = Cell (in skeletal muscle).

Connective Tissue Layers

  • Individual fiber (cell): Surrounded by connective tissue.
  • Group of fibers (cells): Bundled together in the paramesiae, surrounded by connective tissue.
  • Groups of paramesiae: Form the whole muscle, surrounded by the peptomethane.

Tendons

  • Connective tissue extends beyond the muscle proper to become the tendon.
  • The tendon connects the muscle to the bone.

Skeletal Muscle Cell Components

  • Plasma Membrane:
    • Referred to as the Sarcolemma.
  • Cytoplasm:
    • Referred to as the Sarcoplasm.
  • Organelles:
    • Including the Sarcoplasmic Reticulum (modified endoplasmic reticulum).

Transverse Tubules (T-Tubules)

  • Plasma membrane invaginates to form T-tubules.
  • T-tubules run through the cell.
  • Critical for transmitting electrical impulses through the cell.

Triad Structure

  • Organization:
    • Sarcoplasmic Reticulum - T-tubule - Sarcoplasmic Reticulum.
  • Importance: Electrical impulses travel down the T-tubule during muscle contraction.

Myofibrils and Myofilaments

  • Myofiber: Skeletal muscle cell (fiber).
  • Myofibril: Unit inside the cell that contains myofilaments.
  • Myofilaments: Actin (thin) and myosin (thick).
  • Arrangement: Myofilaments are grouped together into myofibrils.

Muscle Contraction Mechanism

  • Actin and myosin interact to create tension.
  • Tension builds during contraction, leading to muscle movement.
  • Regulatory proteins (e.g., troponin, tropomyosin) regulate the interaction between actin and myosin, often involving calcium binding.
  • Structural proteins (e.g., elastin) provide elasticity to sarcomeres, allowing them to return to their original shape after contraction.

Sarcomere Structure

  • Sarcomere: Structural unit of skeletal muscle. Repeating units make skeletal muscle segmented.
  • Defined by Z lines (or Z discs).
  • One sarcomere exists between two Z lines.
  • Z line: Anchors actin filaments for adjacent sarcomeres.

Bands and Zones within the Sarcomere

  • Z line (Z disc): At the end of each sarcomere.
  • A band: Entire length of the myosin filament, includes overlapping actin.
  • I band: is only actin.
  • H zone: Middle of the A band, contains only myosin.
  • M line: Structural line running down the center of the sarcomere; anchors proteins to maintain integrity.

Cross-Sectional View of Myofilaments

  • Actin filaments surround a single myosin fiber.
  • Allows myosin to interact with multiple actin fibers.
  • Ensures smooth contraction and relaxation.