Excitable cells 2

• Neurotransmission


  Chemical signalling between cells
• Junctions between cells


  • Neuron - neuron
  • Neuron - muscle
  • Neuron - secretory cell
• Synapse


  Junction between a neuron and another cell
• Parts of a synapse

  • Cell body
  • Axon hillock
  • Dendrites
  • Axon from excitatory neuron
  • Axon from inhibitory neuron
  • Axon terminal
  • Presynaptic
  • Postsynaptic
  • Neurotransmitter
  • Synaptic cleft
• Synapses in the CNS

  • Neurons receive inputs from hundreds of other neurons
  • Excitatory input depolarises postsynaptic membrane
  • Inhibitory input hyperpolarises postsynaptic membrane or holds at resting potential
• Types of neurotransmitters


  • Amino acids (glutamate, GABA)
  • Amines (dopamine, noradrenaline, 5-HT)
  • Peptides (CRH, opioids, substance P)
  • ATP
  • Nitric Oxide
• Elements of neurotransmission


  1. Synthesis

  2. Storage

  3. Release

  4. Receptor interactions

  5. Termination
• Synthesis


  Neurotransmitters are synthesised in the cell body (peptides) or close to the site of release (small molecules)
• Storage


  Neurotransmitters are stored in synaptic vesicles, protecting them from enzymatic degradation and providing a ready package for release
• Release


  Exocytosis of synaptic vesicles in response to an action potential and calcium influx
• Receptor interactions


  Neurotransmitters bind to specialised receptor proteins, which are linked to ion channels or second messengers
• Termination


  Specialised transporters remove the neurotransmitter from the synaptic cleft, and enzymes break it down
• Classes of neurotransmitter receptors

  • Ionotropic (ligand-gated ion channels)
  • Metabotropic (G-protein coupled)
• Ionotropic receptors


  Binding of the transmitter causes the receptor to open, allowing ions to flow down their concentration gradient and depolarize the membrane
• Metabotropic receptors


  Binding of the transmitter causes G-protein activation, which then triggers intracellular signalling cascades
• Each neurotransmitter binds to several distinct receptor subtypes
• Many drugs interact with neurotransmission, either therapeutically or as drugs of abuse