Muscle Contraction Mechanisms

• Types of Muscles


  • Skeletal (muscle attached to the skeleton)
  • Cardiac (muscle in the heart)
  • Smooth (muscle involved in many involuntary processes in blood vessels, airways and gut)

  
• Skeletal and Cardiac Muscles

  • Morphologically striated or striped, often called striated muscles
• Skeletal Muscle


  Muscle that functions together with the skeleton as the musculoskeletal system, under conscious control

  
• What is skeletal musclecomposed of?

  • Composed of numerous parallel, elongated, multinucleated muscle fibres or myofibres
  • Grouped together to form fasciculi
  • Each fasciculus surrounded by perimysium, each myofibre encased by endomysium
  • Sarcolemma (excitable plasma membrane) beneath the endomysium

  
• Myofibrils


  • 1 μm in diameter, separated by cytoplasm and arranged in parallel along the long axis of the cell
  • Further sub-divided into thick and thin myofilaments, responsible for the cross striations
• Thin Filament Proteins


  • Actin
  • Tropomyosin
  • Troponin
• Thick Filament Proteins


  • Myosin
• 1. tubules

  • Regular invaginations that project from outside the cell and wrap around sarcomeres
  • Contain extracellular fluid and go deep into the cell
• Sarcoplasmic Reticulum

  • Massive calcium ion store
• Motor Nerve Activation

  Body sends motor nerves from motor centres down to spinal cord and out through nerve root, moving towards muscle fibres
• Muscle Contraction

  Interaction of actin and myosin, two types of proteins, brings about contraction of the muscle

  
• Muscle Contraction


  1. Actin and myosin slide past each other

  2. Myosin heads bind to actin chain and tilt

  3. Constant process of binding, tilting, releasing and rebinding of cross-bridges

  4. Rotation of myosin filaments as they interact with actin filaments and bind with alternate myofibril

  
• Neuromuscular Junction (NMJ)


  Structure where nerves in muscle fibres make contact with the muscle fibres

  
• Neuromuscular Junction

  • Motor axon splits into branches that make contact with individual muscle fibres
  • Allows one-to-one transmission of excitatory impulses from motor neurone to muscle fibre

  
• Neuromuscular Transmission


  1. Motor neurone releases acetylcholine (ACh) into synaptic cleft

  2. ACh binds to nicotinic ACh receptors on muscle fibre

  3. Receptor opens ion channel allowing sodium influx, exciting the muscle fibre

  4. ACh is broken down by acetylcholinesterase

  
• Dihydropyridine Receptor (DR)


  Component that links excitation to calcium release, responds to voltage

  
• What causes calcium release in skeletal muscle contraction?

  Physical interaction between DR and calcium release channel (RyR) causes calcium release

  
• What causes cardiac muscle contraction?

  Opening of DR calcium channels promotes calcium-induced calcium release (CICR)

  
• Contractile Mechanism


  • Myosin has stiff tail and flexible head, can burn ATP
  • Actin has myosin binding site, tropomyosin prevents actin-myosin interaction at rest
  • Troponin has calcium binding domain and tropomyosin binding site

  
• Muscle Contraction Cycle


  1. Increased calcium binds to troponin-C

  2. Removal of suppressive tropomyosin

  3. Allows actin to interact with primed myosin

  4. ATP-dependent cross-bridge cycling

  
• Smooth Muscle Contraction


  • Slower mechanism than skeletal/cardiac
  • No troponin, tropomyosin doesn't interact with myosin binding sites
  • Myosin has lower ATPase activity and affinity for ATP

  
• Smooth Muscle Contraction


  1. Calcium binds to calmodulin, activating myosin light chain kinase

  2. Myosin light chain phosphorylation increases myosin ATPase activity and affinity for actin

  3. Myosin forms cross-bridges with actin, using ATP energy

  4. Dephosphorylation by myosin light chain phosphatase

  
• Comparison of Skeletal, Cardiac and Smooth Muscle

  • Skeletal: T-tubules present, myofilaments arranged, calcium sensor is troponin
  • Cardiac: T-tubules present, myofilaments arranged, calcium sensor is troponin
  • Smooth: No T-tubules, myofilaments not regularly arranged, calcium sensor is calmodulin