Muscles L.2

• Muscular System


  Sliding Filament Hypothesis
• Skeletal Muscle Structure


  • Muscles - largest unit; attached to bone by tendons
  • Muscle fibres - organized into larger bundles; up to 20 cm long
  • Myofibrils - thousands of cylindrical subunits that contain myofilaments
  • Myofilaments - protein structures responsible for muscle contraction; two types
• Actin


  Thin myofilaments (diameter = 5 nm) - composed of globular actin proteins and ion receptor proteins called troponin and tropomyosin
• Myosin


  Thick myofilaments (diameter = 11 nm) - composed of myosin proteins two polypeptide chains wrapped around each other; ends have globular heads
• Muscle Fibre (Muscle Cell)

  • Myoglobin - oxygen-binding pigment (similar to hemoglobin); stores oxygen for muscle contractions
  • Sarcolemma - membrane surrounding muscle fibre; regulates entry and exit of materials
  • Sarcoplasm - cytoplasm of muscle fibre; site of metabolic processes; contains myoglobin and glycogen (which stores energy for muscle contractions)
  • Sarcoplasmic reticulum - smooth endoplasmic reticulum (ER) in muscle fibre; stores calcium ions for muscle contractions
• Sarcomere


  Contractile unit of muscle cell
• Sliding Filament Model


  1. Sliding of actin past myosin during muscle contraction

  2. Presence of calcium ions allow myosin head to attach to actin

  3. Myosin head flexes, pulling on the actin filament

  4. Myosin head releases and unflexes via ATP

  5. Myosin reattaches to actin further down fibre
• The cycle of attachment, power stroke, and release continues as long as calcium ions and ATP remain available
• Typically half the myosin molecules at any time are bound to the actin while the other half are preparing to bind again
• Sliding Filament Model analogy

  Climbing a rope hand over hand
• Role of Calcium


  1. Muscle relaxed - tropomyosin blocks myosin binding sites along the actin molecule

  2. Muscle contracted - calcium ions bind to troponin on actin causing tropomyosin to reposition and expose myosin binding sites