Neurons and synapses

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  • Neurons and synapses
    • neurons transmit electrical impulses
      • dendrites - short branched nerve fibres
      • axons - elongated nerve fibres
    • the myelination of nerve fibres allows for saltatory conduction
      • schwann cells are covered in myelin.
      • node of Ranvier
        • impulse can jump from one node to the other
          • saltatory conduction
    • Neurons pump sodium and potassium ions across their membranes to generate a resting potential
      • sodium (3) out for every 2 potassium in.
        • potassiums leak back in the membrane - charge imbalance
          • resting membrane potential of - 70 mV
            • outside more positive than inside
    • An action potential consists of depolarisation and repolarisation of the neuron
      • depolarisation
        • sodium ions get back in its concentration gradient
          • reversing of balance - inside more positive than outside (30 mV)
      • repolarisation
        • potassium channels make potassium ions go out from the membrane
          • inside more negative than outside
    • Nerve impulses are action potentials propagated along the axons of neurons
      • ion movements that depolarize one part of the neuron trigger depolarisation in the next part of neuron
        • refractive period
          • prevents propagation backwards
    • propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential
      • due to movement of sodium ions
      • local currents reduce concentration gradient
    • synapses are junctions between neurons and between neurons and receptor or effector cell
      • effector - the thing that carries out a response to a stimulus
      • neurotransmitters
      • pre synaptic and post synaptic cells
    • when pre-synaptic neurons are depolarised they release a neurotransmitter into the synapse
      • nerve impulse is propagated along the pre synaptic neuron
        • depolarisation of the pre-synaptic cell causes calcium ions to diffuse in the neuron
          • calcium makes vesicles containing neurotransmitters to fuse with the membrane of the cell
            • neurotransmitter released in the synaptic cleft by exocytosis
              • nt binds to receptors in the post synaptic cell membrane
                • sodium channels  open and sodium ions diffuse in through their concentration gradient
                  • action potential
    • secretion and reabsorption of acetylcholine by neurons at synapses
    • a nerve impulse is only initiated if the threshold potential is reached


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