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Learning Objectives. What is a synapse? Explain the role of calcium ions Explain the role of the neurotransmitter Explain the role of Acetylcholinesterase. The Basic Idea. Synapses are gaps between neurones Information is sent between neurones by chemical transmission
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Learning Objectives • What is a synapse? • Explain the role of calcium ions • Explain the role of the neurotransmitter • Explain the role of Acetylcholinesterase
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
Presynaptic Neurone Smooth Endoplasmic Reticulum Incoming Action Potential Calcium ion channel Mitochondrion Synaptic Knob Synaptic vesicle containing neurotransmitter Membrane of postsynaptic neurone Synaptic Cleft Sodium ion channels Postsynaptic Neurone
Incoming Action Potential Neurotransmitter New action Potential
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
Incoming Action Potential Ca2+ Ca2+ Ca2+ Ca2+
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
Incoming Action Potential Ca2+ Ca2+ Ca2+ Ca2+
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
Ca2+ Ca2+ Ca2+ Ca2+ Neurotransmitter (acetlycholine) is released into the synaptic cleft. Acetlycholine binds to the receptor site on the sodium ion channels.
Sodium Channels Na+ • 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.
Empty Synaptic Vesicles Sodium ions diffuse into the postsynaptic neurone Depolarised
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
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
Incoming Action Potential Neurotransmitter New action Potential
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.
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
Step 5 Acetylcholinesterase • A hydrolytic enzyme • Breaks up acetylcholine (the neurotransmitter) into acetyl (ethanoic acid) and choline.
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.
Acetylcholine binds and opens Sodium channels Depolarised Acetylcholinesterase breaks up acetylcholine. Sodium channels close
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
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?
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.
Incoming Action Potential Ca2+ Ca2+ Ca2+ Ca2+