2. Learning Objectives What is a synapse?
Explain the role of calcium ions
Explain the role of the neurotransmitter
Explain the role of Acetylcholinesterase

3. The Basic Idea Synapses are gaps between neurones
Information is sent between neurones by chemical transmission
Neurotransmitters pass across the synaptic cleft
A new action potential will be triggered in the post synaptic neurone

4. Postsynaptic Neurone Presynaptic Neurone Synaptic Cleft Synaptic Knob
Smooth Endoplasmic Reticulum
Incoming
Action Potential
Calcium ion channel
Mitochondrion
Synaptic
Knob
Membrane of postsynaptic neurone
Synaptic vesicle containing neurotransmitter
Synaptic
Cleft
Sodium ion channels
Postsynaptic Neurone

5. Incoming Action Potential Neurotransmitter New action Potential
The basic process (GCSE level)
Neurotransmitter
New action Potential

6. Give students 2 minutes to add as many labels as they can

7. Step 1 – Calcium Channels Open
The incoming action potential causes depolarisation in the synaptic knob
This causes calcium channels to open
Calcium ions (Ca2+) flood into the synaptic knob

8. Incoming
Action Potential
Ca2+
Ca2+
Ca2+
Ca2+

9. Step 2 – Neurotransmitter Release
The influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane
This releases neurotransmitter in to the cleft
 So calcium ions cause the release of neurotransmitter

10. Incoming
Action Potential
Ca2+
Ca2+
Ca2+
Ca2+

11. Step 3 – Sodium Channels
Neurotransmitter (acetylcholine) is released into the synaptic cleft.
Acetylcholine binds to the receptor site on the sodium ion channels.
Sodium ion channels open

12. Ca2+
Ca2+
Ca2+
Ca2+
Neurotransmitter (acetlycholine) is released into the synaptic cleft. Acetlycholine binds to the receptor site on the sodium ion channels.

13. Sodium Channels
The sodium channels on the postsynaptic membrane are normally closed.
When the neurotransmitter binds there is a conformational change opening the channel.
This allows sodium ions to flood in and causes depolarisation.
Na+
Neurotransmitter binds and opens the channel.

14. Depolarised Empty Synaptic Vesicles
Sodium ions diffuse into the postsynaptic neurone
Depolarised

15. Step 3 – Sodium Channels
Neurotransmitter (acetylcholine) is released into the synaptic cleft.
Acetylcholine binds to the receptor site on the sodium ion channels.
Sodium ion channels open
Sodium ions diffuse in (down steep concentration gradient)
Postsynaptic neurone depolarises

16. Step 4 – New Action Potential
Depolarisation inside the postsynaptic neurone must be above a threshold value
If the threshold is reached a new action potential is sent along the axon of the post synaptic neurone

17. Incoming
Action Potential
Neurotransmitter
New action Potential

18. Questions When do the calcium channels open and close?
Why are the calcium ions important?
What is the name of the neurotransmitter?
Explain how the neurotransmitter causes a new action potential to be generated.

19. Questions When do the calcium channels open and close?
Incoming AP causes depolarisation in the synaptic knob which causes Ca+ ions to open
Why are the calcium ions important?
Calcium ions causes the release of Ach by binding to pre-synaptic membrane
What is the name of the neurotransmitter?
Acetylcholine
Explain how the neurotransmitter causes a new action potential to be generated.
Causes depolarisation of post synaptic neurone

20. The rest of the process Step 1 Calcium channels open
Step 2 Neurotransmitter release
Step 3 Sodium Channels
Step 4 New action potential
Step 5 Acetylcholinesterase
Step 6 Remaking acetylcholine

21. Step 5 Acetylcholinesterase
A hydrolytic enzyme
Breaks up acetylcholine (the neurotransmitter) into acetyl (ethanoic acid) and choline.

23. Acetylcholinesterase
Acetylcholinesterase is an enzyme that hydrolyses acetylcholine in to separate acetyl (ethanoic acid) and choline.
Sodium ion channels close.
The two bits diffuse back across the cleft into the presynaptic neurone.
This allows the neurotransmitter to be recycled.

24. Depolarised Acetylcholine binds and opens Sodium channels
Acetylcholinesterase breaks up acetylcholine. Sodium channels close

25. Why break down acetylcholine?
If the neurotransmitter is not broken down this could allow it to continuously generate new action potentials
Breaking down acetylcholine prevents this

26. Questions Name the hydrolytic enzyme and the products of the reaction.
Why must the neurotransmitter be broken down?
What happens to the remnants of the neurotransmitter?

27. Questions Name the hydrolytic enzyme and the products of the reaction.
Acetylcholinesterase
Why must the neurotransmitter be broken down?
Prevent continual stimulation/depolarization of the post synaptic neurone (tetanus)
What happens to the remnants of the neurotransmitter?
Acetyl (ethanooic acid) and choline can be recycled – made back into ACh

28. Step 6 Remaking AcetylcholineHL
Step 6 Remaking Acetylcholine
ATP released by mitochondria is used to recombine acetyl (ethanoic acid) and choline thus recycling the acetylcholine.
This is stored in synaptic vesicles for future use.
More acetylcholine can be made at the SER.
Sodium ion channels close in the absence of acetylcholine at their receptor sites.
The synapse is now ready to be used again.

29. The Whole Process

30. Incoming
Action Potential
Ca2+
Ca2+
Ca2+
Ca2+
Steps 1 + 2

31. Step 3

32. Step 4 + 5