Following a stimulus explain how the opening of sodium ion channels affects the potential difference across a neurone cell membrane Describe and explain the movement of sodium ions if the potential difference across a neurone cell membrane reaches the threshold level ID: 261323
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Slide1
Starter Questions
Following a stimulus, explain how the opening of sodium ion channels affects the potential difference across a neurone cell membrane.
Describe and explain the movement of sodium ions if the potential difference across a neurone cell membrane reaches the threshold level.
Describe the structure of a militated neurone. Slide2
Answers
Sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative and so decreases the potential difference across the membrane.
More sodium ions diffuse into the neurone because more sodium ion channels open.
A
myelinated
neurone has a myelin sheath. The myelin sheath is made of a type of cell called a
schwann
cell. Between the
schwann
cells are tiny patches of bare membrane called the nodes of
ranvier
. Sodium ion channels are concentrated at the nodes of
ranvier
. Slide3
http://www.youtube.com/watch?v=LT3VKAr4rooSlide4
Synapses
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 Slide5
The synapse
Where two neurones meet, there is a gap, usually about 20nm wide. This is the
synaptic cleft
The end of the neurone immediately before the synaptic cleft, ends in a
presynaptic
bulb.After the synaptic cleft is the postsynaptic bulb, i.e. the start of the next neurone Together they make up the synapseSlide6
Structure of the synapseSlide7
Neurotransmitters
There are more than 40 different neurotransmitter substances.
Noradrenalin
and
acetylecholine
(ACh) are found throughout the nervous systemDopamine and glutamate are found only in the brain.Synapses releasing achetylcholine and known as cholinergic synapses Slide8
Postsynaptic Neurone
Presynaptic Neurone
Synaptic
Cleft
Synaptic
Knob
Calcium ion channel
Sodium ion channels
Membrane of postsynaptic neurone
Synaptic vesicle containing neurotransmitter
Smooth Endoplasmic Reticulum
Mitochondrion
Incoming
Action PotentialSlide9
Incoming
Action Potential
New action Potential
NeurotransmitterSlide10Slide11
Step 1 – Calcium Channels Open
The incoming action potential causes depolarisation in the synaptic knob
This causes calcium channels to open
Calcium ions (Ca
2+
) flood into the synaptic knobSlide12
Incoming
Action Potential
Ca
2+
Ca
2+
Ca
2+
Ca
2+Slide13
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 neurotransmitterSlide14
Incoming
Action Potential
Ca
2+
Ca
2+
Ca
2+
Ca
2+Slide15
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 openSlide16
Ca
2+
Ca
2+
Ca
2+
Ca
2+
Neurotransmitter (acetlycholine) is released into the synaptic cleft. Acetlycholine binds to the receptor site on the sodium ion channels.Slide17
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.
Neurotransmitter binds and opens the channel.
Na
+Slide18
Sodium ions diffuse into the postsynaptic neurone
Depolarised
Empty Synaptic VesiclesSlide19
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 depolarisesSlide20
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 neuroneSlide21
Incoming
Action Potential
New action Potential
NeurotransmitterSlide22
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.Slide23
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 AcetylcholinesteraseStep 6 Remaking acetylcholineSlide24
Step 5 Acetylcholinesterase
A hydrolytic enzyme
Breaks up acetylcholine (the neurotransmitter) into acetyl (ethanoic acid) and choline.Slide25Slide26
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.Slide27
Acetylcholine binds and opens Sodium channels
Acetylcholinesterase breaks up acetylcholine. Sodium channels close
DepolarisedSlide28
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 thisSlide29
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?Slide30
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.Slide31
The Whole ProcessSlide32
Incoming
Action Potential
Ca
2+
Ca
2+
Ca
2+
Ca
2+Slide33Slide34Slide35
Neuromuscular Junctions
A neuromuscular junction is a specialised cholinergic between a motor neurone and a muscle cell. Neuromuscular junctions use the neurotransmitter acetylcholine (Ach), which binds to cholinergic receptors called nicotinic cholinergic receptors. Slide36
Very similar to Cholinergic synapses with a few differences
Postsynaptic membrane has lots of folds that form clefts. These clefts store the enzyme that breaks down Ach (
acetylcholinesterase
)
Postsynaptic membrane has more receptors than other synapses
Motor neurone fires an action potential, it always triggers a response in a muscle cell. This isn't always the case for a synapse between two neurones. Slide37
Excitatory and Inhibitory Neurotransmitters
Excitable neurotransmitters depolarise the postsynaptic membrane.
E.g.Acetylcholine
.
Inhibitory neurotransmitters e.g. GABA is an inhibitory neurotransmitter, when it binds to its receptors it causes potassium ion channels to open on the postsynaptic membrane, hyperpolarising the neurone. Slide38
Summation
Low frequency action potentials often release insufficient amounts of neurotransmitter to exceed the threshold in the postsynaptic neurone
Summation allows action potentials to be generated
This enables a build up of neurotransmitter in the synapseSlide39
Spatial Summation
A number of different presynaptic neurones share the same synaptic cleft
Together they can release enough neurotransmitter to create an action potential
Multiple neuronesSlide40
Below the thresholdSlide41
Spatial Summation
Threshold reached action potential can be sentSlide42
Temporal Summation
A single presynaptic neurone releases neurotransmitter many times over a short period
If the total amount of neurotransmitter exceeds the threshold value an action potential is sent
1 neuroneSlide43
Temporal Summation
Low frequency action potentialsSlide44
Temporal Summation
High frequency action potentialsSlide45
Questions
What is summation?
What is the main difference between temporal summation and spatial summation?
Explain how temporal summation allows the postsynaptic membrane to reach the threshold value.
Suggest an advantage of responding to high-level stimuli but not low-level ones.Slide46
Inhibition
There are chloride ion channels on the postsynaptic membrane
If these are made to open chloride ions (
Cl
-
) flood into the postsynaptic neuroneThis hyperpolarises the neuroneThis make it harder to achieve a action potentialSlide47
Chloride ion (
Cl
-
)channel on the postsynaptic membrane
Neurotransmitter that opens calcium channelsSlide48
Chloride ions diffuse into the postsynaptic neurone
HyperpolarisedSlide49
Summation & Inhibition
Inhibitory and excitatory neurones will work antagonistically at the same synapse
Summation will occurSlide50
Exam Questions 1a
1.
(a) (
i
)
A Three marks for three of:Negatively charged proteins / large anions inside axon;Membrane more permeable to potassium ions than tosodium ions;Potassium ions diffuse* out faster than sodium ions diffuse in;Sodium / potassium pump;Sodium ions pumped* out faster than potassium ions pumpedin / 3 for 2; [3 max]* mechanism is necessary for mark Slide51
Exam Questions 1b
B
Sodium ion gates open / membrane more permeable to
sodium ions / sodium ions rush in; [1]
(ii) Two marks for two of:Membrane impermeable to sodium ions / sodium ion channels closed;Sodium ions cannot enter axon;Membrane becomes more negative than resting potential; [2 max] (b) (i) Two marks for two of:Unique shape of receptor protein / binding site; reject ‘active site’Due to (tertiary) structure of protein molecule;Concept of complementary shape / ref. to neurotransmitter ‘fitting’; [2 max]
(ii) Cause vesicles to move to
pres
ynaptic membrane /
fuse with membrane; [1]
Slide52
Exam Questions 1c
(c) (
i
)
Two marks for two of:
Impulses / action potentials from neurones A and B together /spatial summation;Cause sufficient depolarisation / open sufficient sodium ion channels;For threshold to be reached; [2 max] (ii) Two marks for two of:Impulses from A and B independent / no summation;Threshold not reached;Insufficient sodium ion channels opened; [2 max] (iii) Inhibitory;More IPSPs than EPSPs / reduces membrane potential / makes morenegative (allow hyperpolarisation) / cancels effect of action potentialfrom A; [2 max]Slide53
Exam Questions 2a
2.
(a) Initially membrane impermeable to Na+;
Sodium channels open;
allowing Na+ into axon;
reverses potential difference across membrane/ charge on either side/depolarised;membrane becomes more permeable to K+ ions/K+ leave the axon; [max. 4]Slide54
Exam Questions 2b
(
i
) All action potentials are the same size;
threshold value for action potential to occur [2]
(ii) frequency of action potentials [1] several (sub-threshold) impulses add to produce an action potential [1]Slide55
Drugs at Synapses
Drugs can affect the synaptic transmission. They can do this by various ways. E.g. Some drugs are the same shape of neurotransmitters so they mimic their action at receptors (Agonists)
Example: Nicotine mimics acetylcholine so binds to nicotinic cholinergic receptors in the brain.