Synaptic transmission

• Synapse 


  A junction between 2 neurones or a neurone and a effector
• Transmission across a synapse 1

  • Action potential arrives at presynaptic membrane
  • Calcium channels open and calcium ions diffuse into the knob
  • These are actively transported out after
  • Synaptic vesicle fuses with presynaptic membrane
  • Acetycholine is released and moves into the synaptic cleft via exocytosis
• Transmission across a synapse 2

  • Acetycholine diffuses down a concentration gradient to the post synaptic membrane
  • Acetycholine binds to specific protein receptors of post synaptic membrane
  • Sodium ion channels open at post synaptic membrane and sodium ions diffuse into and depolarise the membrane
  • starts a new action potential
  • Acetycholinesterase breaks down acetycholine and it diffuses back into the presynaptic knob where it is resynthesised (requires ATP)
• Excitatory synapses 


  The binding of neurotransmitters to the receptor proteins opens up sodium channels and starts an action potential
• Inhibitory synapses


  The binding of the neurotransmitters makes the resting potential of the post synaptic membrane more negative

  So it is less likely for the threshold potential to be exceeded and an action potential not initiated
• Summation


  Several action potentials are needed to produce enough transmitter so depolarisation can occur
• Spatial summation 


  When several action potentials from different neurones arrive at the post synaptic membrane at the same time

  each releasing small quantities of neurotransmitter which combine to produce an action potential
• Temporal Summation


  When several action potentials arrive very closely after one another and each release a set volume of neurotransmitter to produce an action potential