Synapses

• Synase is junction between 2 or more neurones and enables them to communicate and signal; synaptic cleft (~20nm wide) and neurones close to it
• Action potential cannot move over it, instead neurotransmitter chemical molecules are released from one; they diffuse over cleft; causing action potentials in next neurone
• Pre-synaptic neurone: Many mitochondria (active process, needs ATP), large SER amounts, many secretory vesicles with acetylcholine and voltage-gated Ca^2+ ion channels
• Transmission across Synaptic cleft: Generator potential or post-synaptic potential is created in postsynaptic membrane, if enough potentials combine threshold potential is reached and new action potential
• Acetylcholine (cholinergic) and noradrenaline (adrenergic) are the main neurotransmitters outside the brain, more than 50 natural neurotransmitters e.g. endorphins, dopamine, serotonin
• There are many drugs which mimic natural neurotransmitters or block action (antagonistic) e.g. heroin, LSD, cocaine, nicotine, caffeine
• Many organophosphorous compounds such as malathion (insecticide) and sarin gas (nerve gas) inhibit acetylcholinesterase causing Ach to bind continuously
• Many organophosphorous compounds such as malathion (insecticide) and sarin gas (nerve gas) inhibit acetylcholinesterase causing Ach to bind continuously
• If threshold is reached, an action potential is generated in postsynaptic neurone; excitatory synapse, they can also be inhibitory; depend on neurotransmitter or synapse location
• At inhibitory synapses, neurotransmitter hyperpolarises by binding to chloride ion channels moving Cl^- and causes potassium channels to open (K^+ out); makes it more negative and prevents new impulse
• Synaptic Gap/Cleft

  A) pre-synaptic

  B) synaptic

  C) mitochondrion

  D) synaptic vesicle

  E) transmitter

  F) post-synaptic

  G) synaptic cleft

  
• Pre-synaptic Neurone

  A) SER

  B) Mitochondrion

  C) Synaptic vesicle

  D) acetylcholine

  E) Ca^2+

  F) Ca^2+

  G) Calcium ion

  H) Myelin Sheath

  I) Axon

  J) Membrane

  
• Post-synaptic neurone: Specialised Na+ ion channels with acetylcholine-specific receptor sites, when acetylcholine binds; Na+ ion channel opens

  A) 5

  B) sodium ion

  C) Acetylcholine

  D) receptor

  
• Synapses

  • Junction between 2 or more neurones
  • Enables them to communicate + signal
  • Synaptic cleft (~20nm wide)
  • Action potential cannot move over it
  • So neurotransmitter chemical molecules diffuse over cleft; causing action potential in next neurone
  • If threshold reached, action potential generated in postsynaptic neurone; excitatory synapse
  • Can also be inhibitory; depend on neurotransmitter or synapse location
  • At inhibitory synapses
  • Neurotransmitter hyperpolarises by binding to chloride ion channels
  • Potassium channels to open (K+ out)
  • Makes it more negative + prevents new impulse
• Cholinergic synapses

  1. Action potential transmitted to presynaptic neurone end
  2. Depolarisation: Ca2+ voltage gated channels open + diffuse in synaptic bulb
  3. Ca2+ stimulate synaptic vesicles to move to presynaptic membrane
  4. Vesicles fuse with membrane + release Ach to synaptic cleft
  5. Diffuse over cleft + bind to receptor sites on postsynaptic Na+ channels
  6. Na+ channels open
  7. Na+ influx depolarises, generator potential
  8. Ach leaves ion channels for acetylcholinesterase enzyme active sites
  9. Ehanoic acid + choline; brief stimulation
  10. Choline molecules re-enter presynaptic + re-synthesised by ATP