The Neuron & Synaptic Transmission

• Presynaptic neuron


  • Axon terminal with presynaptic membrane
  • Vesicles, filled with neurotransmitter (NT) molecules
  • Ca++ ion channels
• Synaptic cleft


  • Between pre- & post-synaptic neuron
• Postsynaptic neuron


  • Postsynaptic membrane (e.g., at dendritic spine)
  • Various ion channels
• Neurotransmitter release


  1. Action Potential arrives

  2. Ca++ channel open

  3. Ca++ ions enter axon terminal

  4. Vesicles fuse with presynaptic membrane

  5. NT molecules released into synaptic cleft
• Neurotransmitter binding


  1. NT diffuse across synaptic cleft

  2. NT bind with ion channels at receptor site

  3. Ion channels open

  4. Ions enter postsynaptic neuron

  5. Generating postsynaptic potential (PSP)

  6. NT molecules removed from receptor site

  7. Channels close
• Voltage-gated ion channels


  Open in response to a change in membrane potential
• Voltage-gated ion channels


  • K+-channel and Na+-channels in axon hillock & axon
  • Ca++ channels in membrane of axon terminal
• Transmitter-gated ion channels


  Open in response to a NT molecule binding with the channel's receptor site
• Transmitter-gated ion channels


  • Ionotropic: open directly
  • Metabotropic: open indirectly
• All ion channels in the post-synaptic membrane are transmitter-gated
• Different channels respond to different neurotransmitters because they have differently-shaped receptor sites
• Excitatory synapse


  Positive ions enter (depolarisation), new AP becomes more likely
• Inhibitory synapse


  Negative ions enter (hyperpolarisation), new AP becomes less likely
• For an AP to be triggered, the membrane potential at the axon hillock must depolarise beyond -50 mV
• Post-synaptic summation


  1. Temporal summation: Combines PSPs occurring in rapid succession

  2. Spatial summation: Combines PSPs from different synapses (of one post-synaptic neuron)
• Integration and transformation processes in neurons are the basis of ALL behaviour
• Neurotransmitter removal


  1. Degradation: Special enzymes in the synaptic cleft break down (inactivate) NTs, components partly recycled

  2. Re-uptake: Receptor molecules at pre-synaptic axon terminal take up NTs, return them into pre-synaptic cell