neuromuscular junction mam saima

• Neuromuscular junction


  Junction between terminal branch of nerve fiber and skeletal muscle fiber

  
• Neuromuscular junction


  • Converts electrical impulses generated by motor neurons into chemical and then again into electrical activity in the muscle fibers

  
• Components of neuromuscular junction

  • Axon terminal
  • Presynaptic membrane
  • Calcium channels
  • Synaptic vesicles
  • Synaptic cleft or space
  • Acetylcholinesterase enzyme
  • Postsynaptic membrane or motor end plate
  • Postsynaptic receptors

  
• Axon terminal


  Terminal branch of motor nerve fiber, enlarged into a knob-like structure that fits into a shallow depression in the underlying muscle fiber

  
• Axon terminal


  • Contains mitochondria that supply ATP for acetylcholine synthesis
  • Contains synaptic vesicles that contain acetylcholine
• Presynaptic membrane


  Membrane of nerve ending that contains voltage-gated calcium channels
• Synaptic vesicles


  Made by the Golgi apparatus in the nerve soma and carried by axoplasmic transport to the axon terminal, each containing around 10,000 molecules of acetylcholine

  
• Synaptic cleft


  20-30 nm wide space between the axon terminal and the muscle cell membrane, containing the enzyme acetylcholinesterase
• Postsynaptic membrane or motor end plate

  Part of the sarcolemma in close contact with the motor nerve ending, containing multiple folds called subneural folds to accommodate large numbers of acetylcholine receptors
• Postsynaptic (acetylcholine) receptors


  Nicotinic type of receptors that contain ligand-gated channels allowing the passage of Na+ and K+

  
• Sequence of events at neuromuscular junction

  1. Action potential arrives at axon terminal and depolarizes presynaptic membrane

  2. Opening of voltage-gated calcium channels, calcium influx

  3. Calcium influx causes migration of synaptic vesicles and release of acetylcholine by exocytosis

  4. Binding of acetylcholine to postsynaptic receptors, forming acetylcholine receptor complex

  5. Acetylcholine receptor complex opens ligand-gated sodium channels, sodium influx

  6. Sodium influx depolarizes membrane, producing end-plate potential

  7. End-plate potential is not a true action potential, but a local depolarization

  8. Sodium influx spreads to neighboring areas, opening voltage-gated sodium channels and producing a true action potential in the muscle fiber

  9. Acetylcholinesterase enzyme breaks down acetylcholine, preventing constant muscle contraction

  
• Acetylcholinesterase enzyme is responsible for acetylcholine synthesis and breakdown

  
• End-plate potential


  A local, non-propagated depolarization of the specialized motor end plate, not a true action potential