Synapses

• Synapse

  = The junction between two neurones.

  • Impulses are transmitted across the synapse using chemicals called neurotransmitters.
• Excitatory neurotransmitters

  = Result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered.

  Eg: acetylcholine
• Inhibitory neurotransmitters

  = Result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being triggered.
• Synaptic transmission

  1. Action potential reaches the end of the presynaptic neurone.
  2. Depolariation of the presynaptic membrane causes Ca+ ion channels to open.
  3. Calcium ions diffuse into the presynaptic knob.
  4. This cause synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
  5. Neurotransmitter diffuses across the synaptic cleft and binds with specific receptor molecules on the postsynaptic membrane.
  6. This causes sodium ion channels to open.
• continued synaptic transmission:

  7.  Sodium ions diffuse into postsynaptic neurone. 8. Triggering an action potential along the postsynaptic neurone.

  9. Any neurotransmitter left in the synaptic cleft is removed so the stimulus is not maintained.
• Cholinergic synapses

  Use the neurotransmitter acetylcholine.

  • Common in the CNS of vertebrates and neurotransmitter junctions.
  • If the neurotransmitter reaches the receptors on a muscle cell it will cause the muscle to contract.
• Transmission across cholinergic synapses

  Once an action potential has been triggered in the postsynaptic neurone.

  • The enzyme acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone. This prevents a action potential from continuously being generated.
  • ATP is used to recombine choline and ethanoic acid into acetycholine.
  • Sodium ion channels close.
• Role of synapses

  -They ensure impulses are unidirectional. As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from pre to the postsynaptic.

  -They can allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses. This results in a single stimulus creating a number of simultaneous responses.

  -Alternatively a number of neurones may feed in to the same synapse with a single postsynaptic neurone. This results in stimuli from different receptors interacting to produce a single response.
• Summation and control

  Each stimulus from the presynaptic neurone causes the release of the same amount of neurotransmitter into the synapse.

  However the amount of neurotransmitter from a single impulse is not enough to trigger an action potential in the postsynaptic neurone, as the threshold level is not reached.

  • However if the amount of neurotransmitter build up sufficiently to reach the threshold then this will trigger an action potential= summation
• Spatial summation

  = Occurs when a number of presynaptic neurones connect to one postsynaptic. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone.
• Temporal summation

  = Occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient trigger an action potential.
• Inhibitory synapse

  = Prevents an action potential occuring in the postsynaptic neurone.

  • Chlorine ion channels open
  • Membrane potential decreases
  • Threshold is not reached
  • No action potential produced in postsynaptic neurone.
• Stimulating drugs

  • Mimic neurotransmitter
  • Open calcium ion channels
  • Inhibit enzymes so they don't break down neurotransmitter
  • Block reuptake of neurotransmitter
• Inhibitory drugs

  • Close calcium ion channels
  • Block receptor sites
  • Reuptake faster
  • Bind to neurotransmitter so cant bind to receptor sites.