Synapses

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• Termination of nerve axons
• Best understood is neuromuscular junction (nicotinic receptor)
  1. Action potential arrives at junction
  2. Voltage gated calcium channels open
  3. Flood of calcium causes docking of synaptic vesicles to plasma membrane
  4. vesicles membrane and plasma membranes fuse
  5. contents released (ACh) across synaptic cleft
  6. encounter nicotinic receptor on post-synaptic membrane
  7. binding of 2 acetycholine molecules causes opening of Na⁺ channel and discharge of action potential in muscle fibre
  • 500+ vesicles released at any one AP

• Adrenergic synapes are less well defines
  • synaptic boutons found along axon rather than at termination, spread over wide area of target tissue
  • at AP 0,1 or maybe 2 synaptic vesicles may be release at each bouton
  • hence less precise effect, more cumulative

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