synaptic transmission

• electrical synapse

  neurones joined by gap junction channels - allows ions to move directly from pre to post synaptic neurone

  
• gap junctions

  made up of connexon channels made by connexin proteins = fast neurotransmission

  
• examples of electrical synapses

  found in inferior olive

  • brain stem nucleus
  • movement
  • Neurones form electrical synapses with each other to synchronise activity of networks

  ASTROCYTES

  • inter-connected via gap junctions to form syncytia
• chemical synapses

  1. neurotransmitter molecules synthesized and packaged into vesicles
  2. action potential arrives
  3. voltage gates Ca2+ channels open and ca2+ enters
  4. triggers fusion of vesicles with presynaptic membrane
  5. neurotransmitter diffuse across cleft and binds to specific receptors on postsynaptic membrane
  6. bound receptors activate postsynaptic cell
  7. neurotransmitter broken down and taken up by presynaptic terminal or other cells or diffuses away.
• vesicle docking and release

  calcium dependent proteins - SNARE - activated by increased calcium -causes the movement and fusion of vesicles

  • SNARE proteins form a complex of synaptobrevin, SNAP 25 and syntaxin

  
• motor end plate (neuromuscular junctions)

  terminal of axon of motor neurone contacts a muscle fibre

  Motor unit – axon has multiple terminal fibres so can stimulate many muscle fibres
• acetylcholine used on neuromuscular junctions

  acetylcholinesterase - breaks down ACh into choline and acetic acid
• End plate potentials

  EPP- brief surge of current into muscle fibre locally at the end plate that passively spreads in both directions becoming smaller

  -          Long time course

  -          decay with distance
• Nicotinic ACh receptors

  ionotropic

  • channels opens in response to binding of ACh on extracellular side
  • allows Na+ to pass across membrane = depolarisation
  • rapid
• botox

  Botulinum toxin

  • causes paralysis of muscle fibres
  • interferes with SNARE proteins preventing ACh release
• synaptic potentials

  Excitatory postsynaptic potentials (EPSPs)

  • principle neurotransmitter - glutamate
  • Result of neurotransmitter opening channels which allow the movement of ions with positive reversal potentials e.g. Na+ and Ca2+
  • Small response (subthreshold)

  Inhibitory postsynaptic potentials (IPSPs)

  • Principle neurotransmitters = GABA and glycine
  • Result of neurotransmitter opening channels which allow the movement of ions with negative reversal potentials positive e.g. K+ and Cl-

  ·       Small response
• synaptic arrangement

  Axon terminal to dendrite:

  • Likely excitatory

  Axon terminal to soma:

  • Likely inhibitory
  • Greater effect - closer to hillock

  Axon terminal to axon terminal:

  • Presynaptic inhibition