Synaptic transmission

• Describe synaptic transmission

  1. Action potentials arrive at presynaptic membrane which causes voltage-gated Ca2+ channels to open and it facilitatedly diffuses into the pre SN
  2. This causes vesicles containing neurotransmitters to move and fuse with the presynaptic membrane
  3. Neurotransmitters are released and diffuse across the synaptic cleft to bind to complementary receptors on the post SN
  4. The receptors open Na+ channels, allowing Na+ to diffuse into the postsynaptic neuron
  5. If enough Na+ enters to reach the depolarisation threshold, action potentials are produced in the post SN
• How is synaptic transmission unidirectional?

  • Neurotransmitters are only made and stored in the presynaptic neuron
  • Neurotransmitters receptors are only on the postsynaptic membrane
• Describe the role of acetylcholinesterase in a cholinergic synapse

  1. Acetylcholinesterase is an enzyme that hydrolyses acetylcholine into acetate and choline
  2. This removes acetylcholine from the synaptic cleft to prevent constant depolarisation of the postsynaptic neuron
  3. Acetate and choline are then reabsorbed by the presynaotic neuron and recombined
• Why does summation occur?

  One action potential at the presynaptic neuron does not release enough neurotransmitter to reach the threshold of depolarisation in the postsynaptic neuron
• Describe temporal summation

  1. One presynaptic neuron synapsing with one postsynaptic neuron with a high frequency of action potentials
  2. Releasing many neurotransmitters
  3. Neurotransmitters are added together to make it more likely that the depolarisation threshold is reached in the postsynaptic neuron
• Describe spatial summation

  1. More than one presynaptic neuron synapsing with one postsynaptic neuron
  2. Releasing many neurotransmitters
  3. Neurotransmitters are added together to make it more likely the depolarisation threshold is reached in the postsynaptic neuron
• What are the 2 benefits to summation?

  1. Weaker stimuli can be filtered out to prevent overwhelm
  2. Information from different stimuli can be combined to produce a suitable response
• What is the difference between excitatory and inhibitory synapses?

  Excitatory produces action potentials in the postsynaptic neuron whereas inhibitory makes it less likely action potentials are produced in the postsynaptic neuron
• Describe what happens at inhibitory synapses

  1. Inhibitory neurotransmitters bind to complementary receptors on the post synaptic membrane
  2. This opens K+ channels and Cl- channels
  3. K+ diffuses out and Cl- diffuses into the post synaptic neuron down electrochemical gradients
  4. This causes hyperpolarisation of the post synaptic membrane, so it is less likely to reach the threshold of depolarisation as more Na+ is needed
• Describe neuromuscular junctions

  • The point where neurons meet skeletal muscle fibres
  • There are thousands of neuromuscular junctions along muscle fibres to ensure rapid contraction
• Describe synaptic transmission at neuromuscular junctions

  1. Action potentials arrive at presynaptic membrane, causes voltage gated Ca2+ channels open
  2. Ca2+ facilitatedly diffuses into the pre SN
  3. Acetylcholine vesicles fuse with presynaptic membrane
  4. Acetylcholine is released by exocytosis and diffuses across the synaptic cleft
  5. Acetylcholine binds to complementary receptor on muscle fibre membrane
  6. Na+ channels open and Na+ diffuses into muscle fibre causing depolarisation
  7. Acetylcholinesterase hydrolyses acetylcholine to prevent constant contraction of the muscle
• Compare the cholinergic synapse with a neuromuscular junction

  • CS is neuron to neuron whereas NMJ is neuron to muscle
  • CS can be excitatory or inhibitory whereas NMJ is only excitatory
  • CS action potentials continue to the next neuron whereas NMJ action potentials end at the muscle