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  • Synapses
    • Transmission
      • A junction between a neurone and the next neurone or effector cell
      • The tiny gap between the cells of a synapse is called the synaptic cleft
      • The presynaptic neurone has a swelling called the synaptic knob
        • This contains synaptic vesicles filled with chemicals called neurotransmitters
      • When an action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft
        • They diffuse across to the postsynaptic membrane and bind to specific receptors
      • Because the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional- they only travel in one direction
      • Neurotransmitters are removed from the cleft so the response doesn't keep happening
    • Electrical impulses
      • Electrical synapses in vertebrates contain many gap junctions
      • Membrane proteins form tiny pores linking the two cells allowing movement of ions between them
      • The potential difference and action potential can be transferred directly from one cell to another
      • Electrical synapses allow for the very fast transmission of impulses from one neurone to anoher
      • They are used for rapid signalling within the CNS
      • They have many limitations and the chemical synapse is still regarded as the main method of communication between neurons
    • Drugs
      • Stimulation
        • Nicotine: mimics the action of the acetylcholine on post synaptic membranes
          • Stimulates the nervous system
        • Strychnine + serine: inactivates cholinesterase at the post-synaptic membrane, preventing the breakdown of acetylcholine
          • Enhances and prolongs the effects of acetylcholine as it is no longer destroyed, meaning nerves fire continuously
      • Inhibition
        • Curare: Interferes with the action of acetylcholine and stops the depolarisation of the post-synaptic membrane
          • Causes paralysis as the muscles can no longer be stimulated by the nerevs
        • Botulinus toxin: affects the presynaptic membrane and stops the release of acetylcholine
          • Prevents transmission of impulses across synapses, preventing the nervous system from working
    • Electron micrographs
      • Showed that the synaptic cleft is too wide for an impulse the size of an action potential to 'jump' across it (transmission chemical)
      • Shows that after a nerve has been strongly stimulated, the neurone has a lack of vesicles (it has become accommodated)
      • Normal transmission across the synapse results from the release of transmitter from a small number of vesicles
      • Drugs and poisons can interfere with the working of synapses
      • Substances that interfere with the formation of acetylcholine, stops it being released, prevents it reacting with synaptic receptors or reduces the rate its broken down will have a major impact on the nervous communication system of the body
    • Sequence of events
      • 1. An action potential arrives
        • 2. The membrane depolarises. Calcium ion channels open. Calcium ions enter the neurone
          • 3. Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane
            • 4. Neurotransmitter is released into the synaptic cleft
              • 5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Sodium ions flow through the channels
                • 6. The membrane depolarises and initiates an action potential
                  • 7. When released the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down), or it can diffuse away and be broken down


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