Muscle Physiology L2

• Excitation-contraction coupling


  The process by which an action potential in a muscle fibre leads to muscle contraction

  
• Skeletal muscle is neurogenic
• Motor unit


  • A group of muscle fibres that receive signals from the same motor neuron
  • Each muscle contains several intermingled motor units

  
• Innervation ratio


  The average number of muscle fibres per motor neuron in a muscle

  
• Excitation-contraction coupling


  1. Nerves communicate with muscle fibres at neuromuscular junctions, using acetylcholine (ACh) as a transmitter (depolarization)

  2. The action potential propagates down the transverse tubules and triggers the release of calcium ions from the sarcoplasmic reticulum (SR)

  3. The calcium ions bind to troponin and tropomyosin slides out of its blocking position

  4. Actin can now bind to myosin (forming cross-bridges), activating ATP-ase. Contraction occurs!

  5. Once action potentials cease, Ca2+ is actively pumped into the SR

  6. Tropomyosin blockage is restored and the muscle relaxes

  
• Removal of calcium is key to muscle relaxation. ATP required for muscle relaxation- explains the phenomenon of rigor mortis
• Rigor mortis


  Stiffness of death, obvious 2-4 hrs post-mortem, lasts 15-25 hrs depending on conditions, disappears once proteins start degrading
• Contraction-Relaxation Cycle


  1. Depolarisation of the sarcolemma

  2. Depolarisation of T-tubules

  3. Release of calcium from SR

  4. Increased levels of calcium in the cytoplasm

  5. Calcium binds to troponin, tropomyosin moves out of the blocking position

  6. Actin binds to myosin, and contraction of muscle occurs

  7. Calcium is actively pumped back into SR. Tropomyosin blocks bindings site on actin, and muscle relaxes
• Latent period


  The time between excitation and contraction
• Single twitch


  • A single action potential produces a single twitch
• Wave summation


  • The frequency of stimulation influences tension development
• Tetanus


  • Contractile activity outlasts the electrical activity that started it
• Motor unit recruitment


  For stronger contractions, more motor units are recruited
• Asynchronous recruitment of motor units

  To delay fatigue during a sustained contraction, only possible for sub-maximal contractions
• Muscle fatigue


  • Depletion of glycogen stores
  • Accumulation of lactate (acidosis)
  • Accumulation of extracellular K+
  • Local increase in ADP and inorganic phosphate
• Central fatigue


  CNS no longer adequately activates the motor neurons supplying the muscle
• Excess post-exercise oxygen consumption (EPOC)


  A period of increased oxygen consumption required to recover following fatigue
• During a contraction, if all the ATP is consumed, only 25% is realised as external work, the other 75% is converted to heat
• Environmental temperature


  Body temperature is a function of environmental temperature
• Ectotherms


  • Most fish are ectothermic but a handful (<0.1%) of species are special!
• Endotherms


  • Majority (~98%) of all species are ectothermic
• Regional endothermy


  The ability to maintain elevated temperatures in certain regions of the body (brain, eyes, swimming muscles)
• Fishes with warm muscles swim faster and perform longer migrations than similar-sized ectothermic fishes
• Exercise training & muscle plasticity

  The ability of muscle to adapt to increased use