Structure of skeletal muscle

• Skeletal muscles act in antagonistic pairs against an incompressible skeleton
• Skeletal muscles are stimulated to contract by impulses from motor neurones
• Tendons are made of bundles of muscle fibres, which are made of muscle fibres, which are made of myofibrils
• Muscle fibres are multinucleated so they can code for protein synthesis
• Myofibrils appear striped because of the way that actin and myosin filaments are arranged
• Actin and myosin filaments are arranged in units known as sarcomeres
• Actin is a thin filament
• Myosin is a thick filament
• Neuromuscular junctions are evenly spread across the muscle

  • ensures simultaneous contraction of the muscle fibres
  • produces a smooth coordinated contraction
  • maximises efficiency and force
• Depolarisation of the sarcolemma at the neuromuscular junction

  1. arrival of action potential
  2. release of acetylcholine
  3. binding of acetylcholine to receptors
  4. opening of sodium ion channels
  5. depolarisation of sarcolemma
• Similarities between neuromuscular junctions and cholinergic synapses

  • have neurotransmitters that are transported by diffusion
  • have receptors that cause an influx of sodium ions
  • use sodium-potassium pump to repolarise the axon
  • use enzymes to break down the neurotransmitter
• Key differences for neuromuscular junctions

  • only excitatory
  • only links neurones to muscles
  • only involves motor neurones
  • action potential ends here
  • acetylcholine binds to receptors on membrane of muscle fibre
• Slow-twitch muscle fibres structure

  • high mitochondria density
  • rich blood supply
  • high myoglobin content
• Slow-twitch muscle fibres are located in muscles used for endurance
• Slow-twitch muscle fibres use aerobic respiration as their source of energy
• Fast-twitch muscle fibres structure

  • few mitochondria
  • low myoglobin content
  • high glycogen content
• Fast-twitch muscle fibres are located in muscles used for short bursts of power
• Fast-twitch muscle fibres use anaerobic respiration to generate ATP quickly, and have high glycogen stores to provide an immediate energy source
• The H-band is the section that only contains myosin
• The A-band spans the whole length of the myosin, which includes some actin
• The Z-line is the end of the sarcomere
• The I-band only contains actin