Synapses

• 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

  
• 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
• Summation

  • Unlikely 1 impulse along presynaptic neurone will cause generator potential above threshold required to trigger action potential
  • Summation is when several impulses arrive over short period + raise above threshold
  • Temporal Summation: Impulse sequence arrival along 1 neurone over short period
  • Spatial Summation: Impulse number arrival along several presynaptic neurones at once (convergence)
• 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
  • Neurotransmitter hyperpolarises by binding to chloride ion channels
  • Potassium channels open (K+ out)
  • Makes it more negative + prevent new impulse
  • Key Features:
  • Unidirectional: Impulse travel 1 direction
  • Filter unwanted signals: Low level stimuli make AP in presynaptic but may not stimulate enough acetylcholine for 1 in postsynaptic
  • Amplification: Low level signals from weak stimuli amplified; summation
  • Memory: Function of interconnections between neurones in brain
• Neurone Types

  • Pre-synaptic neurone
  • Many mitochondria; active process, need ATP
  • Large SER amounts
  • Many secretory vesicles w/ acetylcholine
  • Voltage-gated Ca^2+ ion channels
  • Post-synaptic neurone
  • Specialised Na+ ion channels w/ acetylcholine-specific receptor sites
  • When acetylcholine binds; Na+ ion channel opens