6.2.2 Synaptic transmission

• structure of a synapse

  A) axon

  B) voltage gated calcium ion channel

  C) synaptic cleft

  D) vesicle containing neurotransmitter

  E) axon terminal

  F) receptor and sodium ion channel

  
• What are cholinergic synapses?

  Synapses that use the neurotransmitter acetylcholine (ACh)
• Describe transmission across a cholinergic synapse

  1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels

  ○ Ca2+ diffuse into pre-synaptic neurone / knob

  2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane

  ○ Releasing ACh into the synaptic cleft (by exocytosis)

  3. ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane

  4. Causing Na+ channels to open

  ○ Na+ diffuse into post-synaptic knob causing depolarisation

  ○ If threshold is met, an action potential is initiated
• Explain what happens to acetylcholine after synaptic transmission

  ● It is hydrolysed by acetylcholinesterase

  ● Products are reabsorbed by the presynaptic neurone

  ● To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation
• Explain how synapses result in unidirectional nerve impulses

  ● Neurotransmitter only made in / released from pre-synaptic neurone

  ● Receptors only on post-synaptic membrane
• Explain summation by synapses

  ● Addition of a number of impulses converging on a single post-synaptic neurone

  ● Causing rapid buildup of neurotransmitter (NT)

  ● So threshold more likely to be reached to generate an action potential

  Importance - low frequency action potentials release insufficient neurotransmitter to exceed threshold
• Describe spatial summation

  ● Many pre-synaptic neurones share

  one synaptic cleft / post-synaptic

  neurone

  ● Collectively release sufficient

  neurotransmitter to reach threshold

  to trigger an action potential
• Describe temporal summation


  ● One pre-synaptic neurone releases

  neurotransmitter many times over

  a short time

  ● Sufficient neurotransmitter to reach

  threshold to trigger an action

  potential
• Describe inhibition by inhibitory synapses

  ● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:

  ○ Cl- channels open → Cl- diffuse in

  ○ K+ channels open → K+ diffuse out

  ● This means inside of axon has a more negative charge relative to outside / below resting potential

  ● So more Na+ required to enter for depolarisation

  ● Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes
• Describe the structure of a neuromuscular junction

  Very similar to a synapse except:

  ● Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more

  ● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter
• Compare transmission across cholinergic synapses and neuromuscular

  junctions


  In both: transmission is unidirectional

  Cholinergic synapse

  Neurone to neurone (or effectors, glands)

  Neurotransmitters can be excitatory or inhibitory

  Action potential may be initiated in postsynaptic neurone

  Neuromuscular junction

  (Motor) neurone to muscle

  Always excitatory

  Action potential propagates along sarcolemma down T tubules
• Use examples to explain the effect of drugs on a synapse

  ● Some drugs stimulate the nervous system, leading to more action potentials, eg.:

  ○ Similar shape to neurotransmitter

  ○ Stimulate release of more neurotransmitter

  ○ Inhibit enzyme that breaks down neurotransmitter → Na+ continues to enter

  ● Some drugs inhibit the nervous system, leading to fewer action potentials, eg.:

  ○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels

  ○ Block receptors by mimicking shape of neurotransmitter