PS1009 BioPsych 12-13-04 Synapse

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=dSkxlpNs3tU


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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=fvwrel


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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=dSkxlpNs3tU


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LECTURE 5

Brain anatomy

Next week...

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PS1009 BioPsych 12-13-04 Synapse

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