Synapses
Bill Indge
Synapses
What happens at a synapse?
Impulse travels down the
presynaptic neurone
Neurotransmitter diffuses
across synaptic cleft
Action potential produced in
postsynaptic neurone
Synapses
Presynaptic
neurone
Vesicle containing
neurotransmitter
Voltage-gated
calcium channel
Synaptic cleft
Postsynaptic
neurone
Synapses
1
Action potential passes down
presynaptic neurone
2
Calcium channels
open and allow
calcium ions to
diffuse into the
neurone
Synapses
3
Some of the
vesicles fuse with
the presynaptic
membrane and
release the
neurotransmitter,
acetylcholine, into
the synaptic cleft
Synapses
4
Acetylcholine diffuses
across synaptic cleft
and binds to a
ligand-gated
sodium channel on
the postsynaptic
membrane
Synapses
5
Action potential
initiated in
postsynaptic
neurone
Synapses
6
Neurotransmitter is
broken down by
enzymes. Inactive
products are taken
up by reuptake
transporters into
the presynaptic
neurone
Synapses
Summary
1 An action potential passes down the presynaptic neurone.
2 Calcium channels open and allow calcium ions to diffuse into the
neurone.
3 Some of the vesicles fuse with the presynaptic membrane and
release the neurotransmitter, acetylcholine, into the synaptic
cleft.
4 Acetylcholine diffuses across the synaptic cleft and binds to a
ligand-gated sodium channel on the postsynaptic membrane.
5 An action potential is initiated in the postsynaptic neurone.
6 The neurotransmitter is broken down by enzymes. Inactive
products are taken up by reuptake transporters into the
presynaptic neurone.
Red back spider
Synapses
A bite from a red-back spider results in the
uncontrolled release of acetylcholine.
One of the symptoms of being bitten by this spider is
severe sweating. Explain why.
Synapses
Remember what happens at a synapse:
Impulse travels down the
presynaptic neurone
Neurotransmitter diffuses
across synaptic cleft
Action potential produced in
postsynaptic neurone
Synapses
A sting from a cone shell prevents calcium ions
crossing the presynaptic membrane. One of the
symptoms of being stung by a cone shell is paralysis.
Explain why.
Synapses
Remember what happens at a synapse:
Impulse travels down the
presynaptic neurone
Calcium channels open
and allow calcium ions to
diffuse into the neurone
Neurotransmitter diffuses
across synaptic cleft
Action potential produced in
postsynaptic neurone
Synapses
The following slides refer to the worksheet on
synapses.
• Look at the instructions:
Read the following passage.
Use the information in the passage and
your own biological knowledge to answer
the questions.
• Start by reading the passage through quickly.
• Pick out the key ideas in each paragraph. This will
provide you with some signposts so that you can
find what you want quickly.
Synapses
Here is a section from the passage. Pick out the key
ideas in each paragraph:
Benzodiazepines are drugs that are widely used to
treat anxiety and insomnia. They work by affecting the
activity of a neurotransmitter called GABA. GABA is
the most common inhibitory neurotransmitter in the
brain.
Synapses
Signpost
You now have a set of signposts. In this
passage, the three paragraphs cover the
following topics:
• Benzodiazepines, drugs that affect inhibitory
synapses
• Ethanol and its effect on GABA
• Ethanol and its effect on glutamate
This should enable you to find your way round
the passage quickly.
Synapses
• Unless there is a good reason to do something
different, the order of the questions reflects the
order of the topics in the passage.
• Line references are there to help you. Make sure
that you use them.
1 GABA receptors are examples of
transmembrane ligand-gated ion
channels (lines 5–6). Explain why. (3
marks)
• This question is testing your understanding of
technical terms.
Synapses
1 GABA receptors are examples of
transmembrane ligand-gated ion
channels (lines 5–6). Explain why. (3
marks)
• (Transmembrane means that) the receptor
spans the cell surface membrane;
• (Ligand-gated means that) the channel is
opened by the binding of a specific substance, in
this case GABA;
• (Ion channel means that) there is a pore running
through the molecule through which ions such
as chloride can pass;
Synapses
2 (a) Explain the part played by sodium ions
and potassium ions in establishing the resting
potential in a neurone. (4 marks)
This is the first part of a three-part question. It is
a straightforward recall question.
The main points that you need to make in your
answer are shown on the next slide.
Synapses
• A lot of positive potassium ions diffuse out;
• Few positive sodium ions diffuse in;
• Therefore there are fewer positive ions inside the
neurone;
K+
Na+
Resting potential of –60 mV
• In other words, the inside of the neurone is now
negative with respect to the outside;
This is the resting potential.
Synapses
2 (b) An increased influx of chloride ions makes
the resting potential of the neurone more
negative (lines 7–8). Explain how. (2 marks)
To be really useful, recall must be based on
understanding. This question is designed to test
understanding of the information required in part
(a) of this question.
• Chloride ions have a negative charge;
• This makes the resting potential more negative;
Synapses
2 (c) A larger difference between the resting
potential and the threshold results in a
neurone being less likely to generate an
action potential. Explain why. (3 marks)
The final part of Question 2 completes the story
linking benzodiazepines with the behaviour patterns
associated with worry, stress and anxiety.
• The same number of sodium ions entering;
• Will not reach threshold;
• Must reach threshold to generate action
potential;
Synapses
3 Explain how the effects of ethanol on the
presynaptic side of the synapse (lines 15–16)
may affect GABA release. (2 marks)
You need to refer to lines 15–16 here to find out the
effects of ethanol on the postsynaptic side of a synapse.
• GABA-containing vesicles are more likely to fuse
the presynaptic membrane;
• More GABA will be released;
The first step in this question is to locate the relevant
material in the passage. It is better to express this idea
in your own words rather than simply copying it out.
Synapses
4 Memory loss can occur after drinking large
amounts of alcohol. Use information in the
passage to suggest an explanation for this
memory loss. (2 marks)
This question wants you to use information from
the passage. You will have to search for the
answer here as there are no line references. This
is where your signposts will come in useful.
• (The passage says that) ethanol prevents
glutamate binding to receptors;
• No excitation of neurones associated with
memory;