6 Steps of Information Transfer Across Chemical Synapses

Question 1

1

Answer 1

1. AP arrives at axon terminal of presynaptic neuron

Question 2

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Answer 2

2. Voltage-gated Ca2+ channels open, and Ca2+ enters the axon terminal
   - Ca2+ flows down its electrochemical gradient from ECF to the inside of the axon terminal

Question 3

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Answer 3

3. Entry of Ca2+ causes synaptic vesicles to release neurotransmitter
   - Synaptotagmin - a Ca2+ sensing protein on the vesicle – binds Ca2+ and reacts with SNARE proteins to control fusion of the synaptic vesicles with the axon membrane
   - Fusion results in exocytosis of neurotransmitter into the synaptic cleft – higher impulse frequency = more vesicles exocytose

Question 4

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Answer 4

4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane – often chemically gated ion channels

Question 5

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Answer 5

5. Binding of neurotransmitter opens ion channels –creating graded potentials in the postsynaptic cell
   - Graded potentials can be excitatory or inhibitory

Question 6

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Answer 6

6. Neurotransmitter effects are terminated when neurotransmitter is no longer binding to receptors for 1 of 3 reasons:
   - Reuptake by astrocytes or axon terminal
   - Degradation by enzymes
   - Diffusion away from the synaptic cleft