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7/27/2019 PS1009 BioPsych 12-13-04 Synapse
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LECTURE 4
Transmission of informationbetween neurones
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http://www.youtube.com/watch?NR=1&v=dSkxlpNs3tU
http://www.youtube.com/watch?NR=1&v=dSkxlpNs3tUhttp://www.youtube.com/watch?NR=1&v=dSkxlpNs3tU7/27/2019 PS1009 BioPsych 12-13-04 Synapse
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Synaptic transmission
Information transmission betweenneurons.
Allows integration and processing ofinformation.
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Thousands of connections per neurone
Post-synaptic sites on dendrites (green)
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What kind of signal?
Electrical, mechanical or chemical?
Loewi, 1921 discovery of acetyl
choline.
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Gleitman, 8th Edition, P98
The heart in Jar B slowed when the f lu id from Jar A
was added
... therefo re, the signal mus t bechemical.
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Synaptic transmission
Chemical neurotransmitters cross thesynapse
From the presynaptic to postsynaptic cell.
The synapse is very narrow, so transmission isfast.
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dendritic spine
presynaptic membrane
postsynaptic membrane
extracellular fluid
Structure of the synapse
axon of presynaptic cell
axon terminal
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The processes involved
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Release
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An action potential causes neurotransmitterrelease from the presynaptic membrane.
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Neurotransmitter release
The action potential causes voltage-gated channelsto open; calcium (Ca2+) ions flood in.
Calcium ions affect synaptic vesicles containing
neurotransmitter.
vesicles
Ca2+Ca2+ Ca2+Ca
2+Ca2+
Ca2+Ca2+
Ca2+
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Neurotransmitter release
Ca2+ causes vesicle membrane to fuse withpresynaptic membrane and empty contents.
Ca2+
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Neurotransmitters diffuse across the
synapse.
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Diffusion
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Diffusion
Random movement of particles within a
fluid, e.g. a drop of ink in water. The will eventually become evenly distributed
within the water.
N.B. There is no force acting to push/pull theink; no energy is used. By chance, some willmove away; no reason for them to form adrop again.
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Diffusion
Click to watch on YouTube
N.B. Ignore movement caused by ink landing;diffusion occurs in still liquids because the
molecules are always moving watch after 1 minute
http://www.youtube.com/watch?v=9ghYur0vqgE&feature=fvwrelhttp://www.youtube.com/watch?v=9ghYur0vqgE&feature=fvwrelhttp://www.youtube.com/watch?v=9ghYur0vqgE&feature=fvwrel7/27/2019 PS1009 BioPsych 12-13-04 Synapse
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Transmission of the chemical signal ispassive (takes no energy) but because the
gap is small, it is nevertheless fast.
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Binding
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The processes involved
Neurotransmitters bind to receptors withinthe postsynaptic membrane, altering the
membrane potential.
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22
The effects of binding
Neurotransmitter bindingchanges the receptorsshape.
It will open a channel for ions to pass through.
Ligand-gated ion channels.
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+
nt
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nt
+
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nt
+
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+
nt
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Postsynaptic potentials
The effect of the ion channel opening isto either depolarise or hyperpolarise
the postsynaptic membrane.
This will make the postsynaptic cell
either more or less likely to fire.
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Excitatory postsynaptic potentials
(EPSPs)
If the inside of the cell becomes less negative, theneuron is more likely to fire (remember the actionpotential).
Channels allowing in positively charged ions (e.g.sodium and calcium) will do this.
They produce excitatory PSPs: EPSPs.
Na+
++
- -
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Inhibitory postsynaptic potentials
(IPSPs)
If the inside of the cell becomes more negative, theneuron is less likely to fire.
Channels allowing in negatively charged ions (e.g.chloride) will do this.
They produce inhibitory PSPs: IPSPs.
++
- -Cl-
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How is the signal terminated?
Fast, clear signalling requires a rapidoff mechanism.
This can occur by reuptake ordeactivation.
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1: Reuptake
Transporter molecules in the presynapticmembrane take neurotransmitter back into
the terminal.
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2: Deactivation
An enzyme can deactivate the transmitter.
E.g. Acetyl cholinesterase splits acetylcholine into acetate and choline, disabling it.
ACh AChE Acetate Choline
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Some NeurotransmittersFast type
Excitatory: glutamate, acetyl choline (atneuromuscular junctions)
Inhibitory: gamma-amino butyric acid (GABA)
Modulatory type
Dopamine, serotonin These have slower effects, not directly
opening ion channels, but altering theirsusceptibility to fast neurotransmitters.
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How does this allowinformation processing?
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Combining PSPs
PSPs are small.
An individual EPSP will not produce enoughdepolarization to trigger an action potential.
If there are enough coincident EPSPs, thecell will fire.
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Combining PSPs
E.g. Picking up a plate.
A fairly warm plate will
trigger a small proportionof pain inputs; no action.
A hot plate will trigger alarge proportion of pain
inputs -> drop the plate!
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Combining PSPs
IPSPs will counteract the effect of EPSPs atthe same neuron.
E.g., cat is standing beneath the plate -> do
not drop it yet!
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Integration of information
EPSPs and IPSPs are integratedwithinthe postsynaptic cell.
This allows for flexibility: the meaningof a signal is changed according to
other signals.
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What about psychology??
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e.g. Executive function
(See Lecture 8, Frontal Lobes)
Inhibition of prepotent/automatic
responses
Stroop task name the colour:
REDGREEN
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e.g. Vision
(See Lecture 7, Sensory Pathways)
Centre-surround receptive fields
detection of contrast, e.g. edges
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This system allows forINFORMATION PROCESSING
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http://www.youtube.com/watch?NR=1&v=dSkxlpNs3tU
http://www.youtube.com/watch?NR=1&v=dSkxlpNs3tUhttp://www.youtube.com/watch?NR=1&v=dSkxlpNs3tU7/27/2019 PS1009 BioPsych 12-13-04 Synapse
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LECTURE 5
Brain anatomy
Next week...