Synapses

  • is a physical and functional gap which occur between one neuron and the other and muscle or neuron and gland which serves the purpose of electrical or chemical transmission of impulses
  • contains the end of presynaptic neuron + synaptic cleft + end of postsynaptic neuron

 

Mechanism of synaptic transmission

  • action potential arrives at presynaptic neuron
  • stimulates opening of voltage-gated channels for Ca2+
  • Ca2+ diffuse into cytoplasm of presynaptic membrane
  • Ca2+ cause vesicles containing acetylcholine (ACh – a type of neurotransmitter) move towards the presynaptic membrane
  • Vesicle fuses with the membrane, Ach is released and diffuses across the synaptic cleft
  • Ach temporarily binds to receptor proteins on the postsynaptic membrane; causes chemically-gated ion channels for Na+ to open
  • Na+ diffuse through postsynaptic membrane à depolarizes membrane à generates action potential
  • Recycling of Ach, catalyzed by acetylcholinsterase (stops continuous production of action potentials)
    • acetylcholine → acetate + choline
  • choline moves back into the presynaptic neuron.
    • choline + acetyl coenzymeA → acetylcholine
  • ACh transported back to presynaptic vesicles

Q/A Explain the mechanism of transmission of a nerve impulse across a cholinegic synapse

Logical sequence of events is very important when explaining a mechanism

  • arrival of an impulse
  • Depolarization of presynaptic membrane
  • opening if calcium channels
  • increase in membrane permeability to Ca2+
  • rushing of Ca2+ into presynaptic knob
  • synaptic vesicle fuse with presynaptic cleft(by exocytosis)
  • acetylcholine released into synaptic cleft
  • attaches to a receptor on the post synaptic membrane
  • entry of Na+
  • depolarization of membrane

 

Functions of synapses

  • ensures one-way transmission: neurotransmitters are only released on one side of synapse;
    receptors on the other side of synapse
  • allow connection of nerve pathways
  • integration of impulses
  • involved in memory making and learning
  • Decreases the overload of information in the brain

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