3.6.2.2 Synaptic Transmission

• Transmission across a synapse:

  1. AP arrives at synaptic knob - depolarisation of presynaptic membrane occurs. Gated Ca2+ ion channel proteins open
  2. Diffusion of Ca2+ ions into presynaptic neurone
  3. Ca2+ ions cause neurotransmitter vesicles to move and fuse with presynaptic membrane - Acetylcholine diffuses across synaptic gap
  4. Acetylcholine binds to receptor proteins on postsynaptic membrane causing depolarisation (Na+ channel proteins open and Na+ diffuses in) and AP to be generated to travel along postsynaptic membrane
• Restoring Synapse:

  • Acetylcholinesterase enzyme hydrolyses acetylcholine to release it from the receptor - products returned to synaptic knob reforming acetylcholine back into vesicles.
  • Ca2+ actively transported back out of synaptic knob
• Unidirectional transmission:

  • Ca2+ ion channel proteins only in presynaptic membrane
  • Neurotransmitter in vesicles in presynaptic neurone
  • Neurotransmitter receptors only on postsynaptic membrane
• Excitatory synapse: Neurotransmitters make it more likely for depolarisation in postsynaptic membrane by opening Na+ channel proteins.

  Inhibitory synapse: Neurotransmitters make it less likely for depolarisation in postsynaptic neurone by the inside of the neurone becoming more negative from the opening of cl- channel proteins and cl- diffusing in.
• Temporal summation: One presynaptic neuron sending high frequency of AP's to the postsynaptic neurone. Enough neurotransmitter is released to bind to receptor proteins on sodium ion channels leading to depolarisation.

  Spatial summation: More than one presynaptic neurone sending AP's at the same time to the postsynaptic neurone. Enough neurotransmitter is released binding to receptor proteins on sodium ion channels leading to depolarisation.
• Effect of drugs on synaptic transmission:

  • Many places of action - increase AP's or prevent AP's in postsynaptic neurone.
  • Mimic a neurotransmitter (similar shape) causing an AP
  • Block receptor proteins by binding to binding site
  • Cause other ion channel proteins to open/close on postsynaptic membrane
  • Prevent acetylcholinesterase action, preventing reuptake of neurotransmitters into presynaptic neurone.