Action at synapses

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  • Synapses
    • The junction between two neurones
      • an action potential cannot cross this gap between the neurones (synaptic cleft)
        • Instead is carried by chemicals called neurotransmitters
          • Made by the cell sending the impulse (pre-synaptic neurone)
            • Stored in synaptic vesicles at the end of the axon
              • The cell that is receiving the impulse (post-synaptic neurone) has chemical -gated ion channels in its membrane, called nueroreceptors
    • 1. At the end of the pre-synaptic neurone there are voltage-gated calcium channels
      • When an action potential reaches the synapse these channels open
        • Causing calcium ions to diffuse into the cell down their concentration gradient
    • 2. these calcium ions cause the synaptic vesicles to fuse with the membrane
      • Releasing their contents by exocytosis
    • 3. The neurotransmitters diffuse across the synaptic cleft
    • 4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane
      • Causing the ion channels to open
    • 5. This causes  a depolarisation of the post-synaptic cell membrane
      • called the post-synaptic potential (PSP)
        • which may initiate an action potential
    • 6. The neurotransmitter must be removed from the synaptic cleft to stop the synapse being permanently on
      • This can be achieved by breaking down the neurotransmitter by a specific enzyme in the synaptic cleft
        • he breakdown products are absorbed by the pre-synaptic neurone by endocytosis
          • used to re-synthesise more neurotransmitter; using energy from the mitochondria
      • Alternatively the neurotransmitter may be absorbed intact b the pre-syanptic neurone


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