Synaptic Transmission

• What are Cholinergic synapses?

  Synapses that use the neurotransmitter acetylcholine (ACh)
• Describe the transmission across a cholinergic synapse at the pre-synaptic neuron:

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

  • Ca2+ diffuse into pre-synaptic neuron / knob

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

  • Releasing ACh into the synaptic cleft via exocytosis
• Describe the transmission across a cholinergic synapse at the post-synaptic neuron:

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

  4)Causing Na+ channels to open

  • Na+ diffuses into post-synaptic knob causing depolarisation
  • If threshold value is reached and action potential is initiated
• Explain what happens to Acetylcholine after synaptic transmission:

  • It is Hydrolysed by acetylcholinesterase
  • Products are reabsorbed by the presynaptic neuron
  • 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 neuron
  • Receptors only on post-synaptic membrane
• Explain summation by synapses:

  • Addition of a number of impulses converging on a single post-synaptic neuron
  • Causing a rapid build up of neurotransmitters
  • So threshold value is more likely to be reached to generate an action potential
• Describe spatial summation:

  • Many pre-synaptic neurons share one synaptic cleft / post-synaptic neuron
  • Collectively release sufficient Neurotransmitter to reach threshold value to trigger an action potential
• Describe temporal summation:

  • One pre-synaptic neuron releases neurotransmitter many times over a short time
  • Sufficient neurotransmitter to reach threshold value to trigger an action potential
• Describe inhibition by inhibitory synapses:

  • Inhibitory neurotransmitters hyperpolarise post-synaptic membrane as:

  -(Cl−) channels open -> Cl− diffuses in

  -(K+) channels open -> K+ diffuses 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 value being met / action potential formation at post-synaptic membranes
• Describe the structure of a neuromuscular junciton:

  Very similar to synapse except:

  • Receptors are on muscle fibre sarcolemma instead of post-synaptic membrane and there are more
  • Muscle fibre forms clefts to store enzyme e.g. acetylcholinesterase to break down neurotransmitter
• Compare transmission across cholinergic synapses and neuromuscular junctions:

  In both: Transmission is unidirectional

  Cholinergic Synapse:

  • Neuron to neuron (or effectors, glands)
  • Neurotransmitter can be excitatory and inhibitory
  • Action potential may be initiated in post-synaptic neuron

  Neuromuscular Junction:

  • (Motor) neuron to muscle
  • Always excitatory
  • Action potential propagates along sarcolemma down T tubules
• Use examples to explain the effect of drugs on a synapse:

  1)Some drugs stimulate the nervous system, leading to more action potentials e.g.

  • Similar shape to neurotransmitter
  • Stimulate release of more neurotransmitter
  • Inhibit enzyme that break down NT -> Na+ continues to enter

  2)Some drugs inhibit the nervous system, leading to fewer action potentials e.g.

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