Muscles

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Types of muscle fibres

Slow oxidative

  • slow in generating force -> suited to endurance events/aerobic exercise e.g. marathon
  • adapted for slow rhythmical contractions 
  • smaller in size and produce less overall force than fast-twitch fibres
  • high aerobic capacity
  • high number of mitochondria
  • high level of myoglobin
  • high capillary density
  • high levels of oxidative enzymes 

Fast oxidative glycolytic

  • used mainly during short high-intensity endurance events e.g. 400m
  • fast contraction
  • large force of contraction
  • fatigues easily
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Types of muscle fibres

Fast glycolytic

  • used during explosive events e.g. 100m
  • very rapid contractions
  • very large forces - calcium ions released quicker
  • fatigues very easily
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Motor units and spatial summation

  • Motor unit -> a motor neurone and its mucle fibres
  • Need fast twitch fibres rather than slow twitch fibres
  • Spatial summation -> for a greater force of contraction, more and/or bigger motor unit are recruited by the brain
  • All or none law -> each muscle fibre within a motor unit contracts maximally or not at all
  • Motor unit unable to relax/increase the force
  • Muscle spindles detect changes in muscle length/speed of contraction
  • Send information to brain/CNS
  • Compares information to long term memory to ensure correct force applied
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Motor unit

  • Receptors/senses detect stimuli and a signal is sent from the medulla oblongata
  • Nerve impulse travels down the axon (i.e. spinal cord) to muscle fibres
  • Axon is surrounded by a fatty covering (myelin sheath)
  • This insulates and protects the nerves
  • Nodes of ranvier are the breaks in myelin sheath and allow the nerve to conduct more rapidly
  • When the nerve reaches the motor end plate, acetylcholine is released over the nueromuscular junctions 
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Sliding filament hypothesis

  • Filaments unable to bind due to tropomyosin -> prevents myosin heads attaching
  • Nerve impulse -> Releases calcium ions from sarcoplasmic reticulum 
  • Calcium ions attach to troponin (on actin filaments) -> causes change of shape of troponin
  • Exposes myosin binding site (on actin filament)
  • Allows myosin to bind to actin/cross-bridges formed 

 

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