Synaptic transmission

• Synapse


  Gaps between the end of an axon on one neurone and the dendrite of another one
• What happens in the synapse?

  Action potential is transmitted as neurotransmitters which diffuse across the synapse
• Synaptic cleft


  Gap between neurones at synapse
• Synaptic knob


  • Swelling at end of pre-synaptic neurone
  • Contains synaptic vesicles containing neurotransmitters
• What happens when an action potential reaches the end of a neuron?

  1. Causes neurotransmitters to be released from synaptic vesicles

  2. into synaptic cleft

  3. Diffuse across to postsynaptic membrane and bind to complementarily receptors
• When neurotransmitters bind to receptors


  • Trigger action potential in next neurone
  • Cause muscle contraction - in muscles cell
  • Cause a hormone to be secreted from gland
• Neurotransmitters


  • Chemical messengers
  • Allow communication between neurones, muscles and glands
• Sequence of events at a cholinergic synapse

  1. An action potential arrives at synaptic knob

  2. Depolarisation of synaptic knob leads to the opening of calcium ion channels

  3. Calcium ions diffuse into synaptic knob

  4. Vesicles contain acetylcholine move towards and fuse with presynaptic membrane

  5. Acetylcholine released into synaptic cleft

  6. Acetylcholine diffuses down concentration gradients across synaptic cleft

  7. to post-synaptic membrane

  8. Acetylcholine binds to complementary chlorogenic receptors on post-synaptic membrane

  9. Sodium ion channels on post-synaptic membrane open and sodium ions diffuse in

  10. If enough acetylcholine bonds and enough solidum ions diffuse into raise membrane potential above -55 threshold

  11. then post synaptic neurone becomes depolarised

  12. Acetylcholine degraded and released from receptor

  13. Sodium ion channels close

  14. Post-synaptic neurone can re-establish resting potential
• Recycling acetylcholine


  1. Enzyme which degrades acetylcholine known as acetylcholinesterase

  2. Breaks acetylcholine into choline and acetate

  3. Reabsorbed and recycled in pre-synaptic neurone
• Unidirectionality


  • Impulse can only travel in one direction
  • Neurotransmitter only released from pre-synaptic neurone
  • Only diffuses down concentration gradient to post-synaptic neurone
  • Post-synaptic receptors only present on post-synaptic membrane
• Summation


  Rapid build-up of neurotransmitters in the synapse to help generate an action potential
• Spatial summation


  • Many different neurones collectively trigger a new action potential
  • By combining the neurotransmitters they release to exceed the -55 threshold
• Temporal summation


  • One neurone releases a neurotransmitter repeatedly
  • Over a short period of time to exceed -55 threshold
• Inhibitory synapse


  • Negatively charged ions to move into post synaptic neurone
  • Potassium ions move out
  • The combined effect of negative ions moving in and positive ions moving out makes membrane potential increase to -80 (hyperpolarised)
  • Action potential unlikely to occur
• Neuromuscular junction


  • Synapse which occurs between motor neurone and muscle
  • Similar to synaptic junction
• Cholinergic synapse and neuromuscular junction similarities

  Unidirectional as neurotransmitter receptors only present on post-synaptic neurone
• Cholinergic synapse excitatory or inhibitory whereas neuromuscular junction only excitatory
• The presynaptic terminal contains synaptic vesicles filled with neurotransmitters.
• Summation importance

  • Some action potentials do not result in sufficient concentrations of neurotransmitters being released
  • Unable to exceed -55 threshold and trigger action potential
• Neuromuscular junction connects muscle and neurone whereas…


  cholinergic synapse connects two neurone which could be sensory, relay or motor
• Neuromuscular junction is the end point for an action potential whereas…


  at a cholinergic synapse a new action potential is generated in the next neurone
• At a cholinergic synapse acetylcholine binds to receptors on post-synaptic membrane of neurone whereas…


  at a neuromuscular junction acetylcholine binds to receptors on muscle fibre membranes