Synapse

• First part

  • Action potential arrives at the axon terminal (synaptic knob)
  • There is a change in axon membrane permeability to Ca2+. Ca2+ channel opens
  • Ca2+ are pumped out of the cell by active transport so Ca2+ is more concentrated outside the membrane of the synaptic knob
  • Ca2+ rapidly diffuse into the cell
  • Ca2+ cause synaptic vesicles to migrate towards the presynaptic membrane
  • Synaptic vesicles fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) into the synaptic cleft via exocytosis
• second part

  • Synaptic vesicles fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) into the synaptic cleft via exocytosis
  • Acetylcholine molecules diffuse across the cleft to the postsynaptic membrane
  • Acetylcholine molecules bind to the Na+ gated channels on the postsynaptic membrane
  • Na+ chemically gated channels open
  • Na+ diffuses into the dendrite of the postsynaptic neurone
  • Action potential generated in postsynaptic membrane (depolarisation)
  • Acetylcholinesterase breaks down acetylcholine in the protein receptor to give choline and acetate.
• Third part

  • Acetylcholinesterase breaks down acetylcholine in the protein receptor to give choline and acetate.
  • Na+ channels in the postsynaptic membrane close. No more action potentials.
  • Choline and acetate diffuse back to the presynaptic membrane
  • Acetyl co-enzyme A is used to resynthesise acetylcholine in the synaptic knob from acetate and choline
  • Energy from mitochondria (ATP) used to repackage acetylcholine into vesicles
• Where is ATP used:

  • synthesis of neurotransmitter
  • exocytosis
  • active transport of Ca2+ out the cell
  • synthesis of neurotransmitter receptors - Na+ channels
  • package of neurotransmitter into synaptic vesicles
• Enzymes:

  • Acetylcholinesterase
  • Acetyl co-enzyme A
• What is the purpose of the synapse?

  They make sure that the flow of impulses is in one direction only. This is because the vesicles containing transmitter are only in the presynaptic neurone and the receptor molecules are only on the postsynaptic membrane