Microscopic Structure of Skeletal Muscle

Muscle Fiber or Muscle Cell

• Definition: Long, cylindrical, multinucleated cells made up of myofibrils.
• Structure:
  • Sarcolemma: Plasma membrane of a muscle cell.
  • Endomysium: Connective tissue surrounding each muscle fiber.
  • T-tubules (Transverse Tubules): Infoldings of the sarcolemma filled with interstitial fluid.
    • Function: Allows action potentials to spread rapidly throughout the muscle fiber.

Cellular Components

• Sarcoplasm: Cytoplasm of a muscle fiber.
  • Contains glycogen for ATP production.
  • Includes myoglobin, a protein that binds oxygen.

Myofibrils

• Contractile organelles of skeletal muscle.
• Diameter: Very narrow but extend the entire length of the muscle.
• Sarcoplasmic Reticulum: Fluid-filled membranous sacs surrounding myofibrils.
  • Terminal Cisterna: Expanded ends of the sarcoplasmic reticulum.
  • Function: Stores calcium ions, releases them to trigger muscle contraction.
  • Triad: Formed by a T-tubule flanked by two terminal cisternae.

Myofilaments

• Types:
  • Thin Filaments: Composed primarily of actin.
  • Thick Filaments: Composed primarily of myosin.
• Sarcomere: Basic functional unit of a myofibril.
  • Z-discs: Define the boundaries of each sarcomere.
  • I-band: Light area containing only thin filaments.
  • A-band: Dark area where thick and thin filaments overlap.
  • H-zone: Central region with only thick filaments.
  • M-line: Center of the sarcomere, anchors thick filaments.

Muscle Proteins

• Contractile Proteins:
  • Myosin: Forms thick filaments, head binds to actin for contraction.
  • Actin: Forms thin filaments, has binding sites for myosin.
• Regulatory Proteins:
  • Tropomyosin: Blocks myosin binding sites on actin when muscle is relaxed.
  • Troponin: Holds tropomyosin in place; binding of calcium ions changes its shape to allow muscle contraction.

Muscle Contraction Overview

• Calcium ions released from the sarcoplasmic reticulum bind to troponin.
• Troponin changes shape, moving tropomyosin to expose myosin binding sites on actin.
• Myosin heads bind to actin, pulling thin filaments towards the center of the sarcomere, resulting in muscle contraction.

This summary presents a detailed view of the microscopic structure and function of skeletal muscle, focusing on the interactions between different muscle fibers, cellular components, and proteins involved in muscle contraction.