Muscle Physiology L3

• School of Biological Sciences | Te Kura Pūtaiao Koiora

  Rm 239 Julius von haast
• Learning objectives


  • Explain how the three muscle fibre types differ to one another
  • How do muscle phenotypes change following exercise training?
  • Describe how muscle phenotypes change following prolonged periods of inactivity
• Type I: slow-oxidative, aerobic fibres (red) - walking, standing, jogging


  • Slow contraction rates
  • High fatigue resistance
  • High mitochondrial density
  • High myoglobin levels
  • Small diameter (so small diffusion distances)
  • Good blood supply (for O2 delivery)
  • Use lipids (and protein) as fuel stores
  • Low capacity for anaerobic metabolism
• Type IIa: fast-oxidative, aerobic fibres (pink)- intermediate activity


  • Fast contraction rates (fast myosin ATPase)
  • Medium-high fatigue resistance
  • Moderate levels of mitochondria
  • Moderate levels of myoglobin
  • Moderate to poor blood supply (depending on species and level of training)
  • Medium to large fibres
  • Will use lipid, but also have the capacity to use glucose anaerobically
• Type IIb: fast-glycolytic, anaerobic fibres (white) - "sprint muscles"


  • Fast contraction rates (fast myosin ATPase)
  • Low resistance to fatigue
  • Low numbers of mitochondria
  • Low myoglobin levels
  • High glycolytic capacity (anaerobic metabolism)
  • Poor blood supply
  • Large fibres
  • Use glucose as an energy substrate
• Muscle fibre types


  • Type I: Red (slow oxidative)
  • Type IIa: Pink (fast-oxidative)
  • Type IIb: White (fast-glycolytic)

  
• Aerobic reliance


  • High
  • Low
• Mitochondrial density


  • High
  • Low
• Contraction speed


  • Slow
  • Fast
• Fatigue resistance


  • High
  • Low
• At birth, amphibian, reptile, bird and mammalian muscles have their allocated maximum number of muscle fibres

  
• Biological exception: Fishes
• Muscle growth


  Via hypertrophy, or hyperplasia
• Exercise & training


  What happens when you contract your skeletal muscles on a regular basis?
• Human (and non-human) skeletal muscle is very plastic
• Changes that can occur

  • ATP-synthesising capacity (e.g. mitochondrial density)
  • Diameter (size)
  • Contractile speed and force (interconversion between fast muscle fibre-types)
• Exercise & training: ATP-synthesising capacity

  1. Regular endurance training induces metabolic changes in oxidative muscle fibres

  2. Increase in the number of capillaries supplying blood to the fibres

  3. Increase in mitochondrial density

  4. Muscles do not enlarge but they are better able to withstand prolonged activity without fatiguing
• Exercise and training: Fibre diameter

  1. Muscle fibre size (not number) can be increased by regular bouts of high-intensity, anaerobic exercise

  2. Muscle enlargement results from an increase in diameter of white muscle fibres

  3. Comes from increased synthesis of myosin and actin filaments, which create more opportunity for cross-bridge interactions and results in greater contractile strength

  
• Exercise & training: Contractile speed and force

  1. Regular endurance training can convert white into pink fibres

  2. Alternatively, weight training can convert pink fibres into white fibres

  3. Muscle fibres cannot change between red and pink, because the nerves are different
• Exercise & training: Interconversion between fast muscle fibre-types


  • Type I: Red (slow oxidative)
  • Type IIa: Pink (fast-oxidative)
  • Type IIb: White (fast-glycolytic)
• Some amphibious fish locomote on land
• Amphibious fish


  • Hawaiian goby (Sicyopterus stimpsoni)
  • Blennies (Alticus kirkii)
  • Mudskipper (Boleophthalmus pectinirostris)
• Movement on land can be important for

  Predator avoidance, dispersal and prey capture
• Terrestrial jumping important for

  • Prey capture
  • Avoiding predators
  • Dispersing to new habitats
  • Seek moist habitats during the dry season
  • Deposit embryos out of water
• Mangrove killifish (Kryptolebias marmoratus)


  Move across the mangrove forest floor by jumping
• Experimental design


  • Control fish (no training, n = 25)
  • Exercise-trained fish (trained every 2nd day for 14 days, n = 26)
  • Training: 3 minutes of consecutive jumping
• Hypothesis: Terrestrial exercise improves locomotion in amphibious fishes out of water as a result of skeletal muscle remodelling
• Response variables measured


  • Skeletal muscle phenotype: Number of red (slow-oxidative) and white (fast-glycolytic) muscle fibres, Cross-sectional area of red and white muscle fibres, Aerobic capacity of muscle fibres: succinate dehydrogenase (SDH) stain was used as a proxy for the aerobic capacity of red muscle
  • Jumping performance (total distance jumped and number of jumps)

  
• Fish are a bit weird! The majority is white muscle >90% and a small band of red fibres run along the lateral line
• Muscle phenotype- muscle fibres


  Hypertrophy
• Muscle phenotype- aerobic capacity

  Increased
• Jumping performance


  Improved
• Hypothesis: Terrestrial exercise improves locomotion in amphibious fishes out of water as a result of skeletal muscle remodelling - Accept
• Muscle atrophy


  What happens when muscles are not use for prolonged periods of time?
• Muscle disuse atrophy


  Muscles decrease in mass and become weaker
• Types of muscle atrophy

  • Disuse atrophy- occurs when muscle is not used for a long period
  • Denervation atrophy- occurs when the nerve supply to a muscle is lost
• Physiological mechanisms of muscle atrophy

  1. Decline in protein synthesis: Progressive loss of muscle protein, Impairment of sarcolemma ion transport, Loss of Ca2+ homeostasis

  2. Oxidative damage: Reactive oxygen species (ROS) leak out of mitochondrial membranes and damage muscle fibres, ROS are a natural by-product of the normal aerobic metabolism, Oxidative damage occurs only when the rate of formation of ROS exceeds the rate of removal
• Case study: Aestivating frogs

  • Green-striped burrowing frog (Cyclorana alboguttata)
  • Endemic to semi-arid environments in eastern Queensland
  • When conditions are dry and resources are scarce, they burrow underground
  • Remain underground, inactive for periods up to 9 months!
• Aestivation


  Prolonged period of dormancy. Similar to hibernation but it often occurs in Summer in response to high temperatures and/or drought conditions
• A person forced into similar inactivity would lose more than 90% of their muscle strength!