6.2.2 Synaptic Transmission

• Transmission across a cholinergic synapse

  Cholinergic synapses use the neurotransmitter acetylcholine (ACh)

  At pre- synaptic neurone

  1. Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels → Ca2+ diffuse into pre- synaptic neurone

  2. Causing vesicles containing ACh to move and fuse with pre-synaptic membrane → releasing ACh into synaptic cleft (exocytosis)

  
• Transmission across a cholinergic synapse

  Cholinergic synapses use the neurotransmitter acetylcholine (ACh)

  At post- synaptic neurone

  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

  
• ACh hydrolysed by acetylcholinesterase → products reabsorbed by presynaptic neurone to stop overstimulation
• Features of synapses: unidirectionality

  Synapses result in unidirectional nerve impulses because:

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

  ● Receptors only on post-synaptic membrane
• Features of synapses: summation

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

  ● Causing rapid buildup of neurotransmitter

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

  Importance: low frequency action potentials release insufficient neurotransmitter to exceed threshold
• 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

  
• Temporal summation

  One pre-synaptic neurone releases neurotransmitter many times over a short time

  ● Sufficient neurotransmitter to reach threshold to trigger an action potential

  
• Features of synapses: inhibition by inhibitory synapses

  ● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane because:

  ○ Cl- channels open → Cl- diffuse in

  ○ K+ channels open → K+ diffuse out

  ● More Na+ required for depolarisation

  ● So reduces likelihood of threshold being met

  and action potential formation at post-synaptic membranes

  Example: acetylcholine is an inhibitory neurotransmitter at cholinergic synapses in the heart

  
• inhibition by inhibitory synapses- Importance: both excitatory and inhibitory neurones

  forming synapses with same post-synaptic membrane gives control of whether it ‘fires’ an action potential
• Structure of a neuromuscular junction

  Very similar to a synapse except:

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

  ● Muscle fibre forms clefts to store enzyme eg. acetycholinesterase to break down neurotransmitter
• Students should be able to use information provided to predict and explain the effects of specific drugs on a synapse.

  Stimulate nervous system - more action potentials

  • Similar shape to neurotransmitter
  • Stimulate release of more neurotransmitter
  • Inhibit enzyme that breaks down neurotransmitter → Na* continues to enter
• Students should be able to use information provided to predict and explain the effects of specific drugs on a synapse

  Inhibit nervous system → fewer action potentials

  • Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels
  • Block receptors by mimicking shape of neurotransmitter
• 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