SYNAPSE

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SYNAPSE
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• Synapse
•  
• A junction that mediates the transfer of
  information from
• - neurone to neurone
• - neurone to effector cell (NMJ)
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•   Pre-synaptic neurone
• neurone conducting impulses towards
  synapse
•  
• Post-synaptic neurone
• neurone conducting impulse away from
  synapse

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The green neurone is postsynaptic to the yellow
neurone and presynaptic to the purple neurone
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Presynaptic and postsynaptic neurons

• The first neuron, which transmit signals to the
  second neuron is defined as the presynaptic
  neurone
• The second neuron, which receives signals from
  the first neuron, is defined as the postsynaptic
  neurone.

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• There are 2 types of synapse
• - Electrical synapse
• - Chemical synapse
• Electrical Synapses
• - contain protein channels which interconnect
  cytoplasm of adjacent neurones
• - specialised as allow ions to flow directly
  from one neurone to next
• - provide low resistance electrical pathways
• - not common

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• In adults found in
• - Brain areas of brain responsible for movement
  (eye)
• - Cardiac and Smooth muscle allow rhythmic
  excitation
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• Embryonic tissue very prominent,
• aid nervous system development
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• Most electrical synapses replaced by chemical
  synapses as development continues.

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• Chemical Synapse
• consist of
• Presynaptic axonal terminal
• Postsynaptic receptor region
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• The Pre and Postsynaptic neurones are separated
  by a fluid filled Synaptic Cleft.
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• Electrical impulse from a pre-synaptic neurone
  is not directly transmitted to the postsynaptic
  neurone.

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• Transmission of electrical signal across chemical
  
• synapse is a chemical event.
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• This depends upon
• - release of neurotransmitter
• - diffusion of neurotransmitter
• - binding of neurotransmitter molecule to
  receptors.

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1. The membrane is depolarised by the arrival of
an action potential 2. Calcium channels open 3.
Calcium enters the cell and triggers the release
of neurotransmitter (by exocytosis 4.
Neurotransmitter diffuses across the synaptic
cleft and some binds to receptors on the
post-synaptic cell membrane 5. A response is
produced in the post-synaptic cell
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• Depolarisation of axonal terminal
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• Arrival of nerve impulse at axonal terminal
• Influx of Ca2 into terminal
• Ca2 promotes fusion of synaptic vesicles with
  presynaptic membrane
• Exocytosis of vesicles
• Release of neurotransmitter which diffuses across
  synaptic cleft
• Molecules attach to postsynaptic membrane
• Binding causes ion channels to open on
  postsynaptic membrane
• Ion changes occur
• Results in a response

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