14) Plasticity and Modulation in the Nervous System

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Why do we need it?
Short term adaptation Longer term structural changes Learning and memory
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How does learning change the structure of the brain?
e.g. Areas of brain enlargement in highly skilled musicians, can change as you get older too (wiring of brain not size changes)
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Describe the The circuit for the gill withdrawal reflex in aplysia
Sensory neurons activated  signals causing reflex by motor neurons  withdrawal When tail is shocked it activates a group of neurons and somehow 5HT enhances SNMN
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Locus of plasticity within the gill withdrawal circuit?
Somewhere in the circuit between the SN and the MN, there is plasticity Synapses either get stronger (sensitization) or weaker (habituation)
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Types of synaptic plasticity? Type A
a)homosynaptic (activity dependent) plastic change) - Plasticity depends on the activity of the presynaptic neuron (fires action potential at high frequency  synapse gets stronger/low frequency weaker)
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Types of synaptic plasticity? Type B
Heterosynaptic (modulatory inputt-dependent) plastic change:Aplysia is heterosynaptic – facilitatory neuron releases facilitatory neurotransmitters/inhibitory releases inhibitory neurotransmitters
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Where does plasticity occur?
at the sensory neuron - motoneuron synapse ->Behavioural sensitization associated with facilitation of synapse(enhancement of the withdrawal reflex) Behavioural habituation associated with depression of synapse(weakening of withdrawal reflex)
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Where and when does AP broadening occur?
in the sensory neuron during heterosynaptic facilitation - This occurs because 5HT inhibits a K+ current involved in repolarization of the action potential-Repolarization of the action potential controlled by K+ channels
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What effect does broadening have?
more Ca2+ entry, more transmitter release
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Describe how action potential broadening occurs
5HT causes production of cAMP which activates PKA. PKA phosphorylates the S-type K+ channel preventing it from opening This broadens the action potential, and more transmitter is released
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Long term changes in synaptic function require what?
gene expression and synthesis of new proteins(Changes in synaptic function = creation of new synapses or making existing ones bigger)
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Role of hippocampus?
a brain structure involved in learning and memory especially spatial memory - synaptic connectivity in the hippocampus is highly plastic and encodes memory
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Decribe Excitatory synaptic transmission in the hippocampus
Glutamate is the transmitter involved: It acts at AMPA receptors and NMDA receptors
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Explain Long term potentiation (LTP) in the hippocampus
Our brain are homosynaptic plasticity like this 1 stimulus  enhancement  short lasting 4 stimuli  long lasting enhancement
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Key requirements for LTP?
Postsynaptic depolarization (provided either by current injection or by the tetanus). A calcium influx into postsynaptic cell Activation of NMDA receptors.
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3 critical features of the NMDA receptor?
Slower kinetics -window for coincidence detection, allows summation of the EPSP during the tetanus.Voltage dependent Mg2+ blockade-detection of coincidence,cell must be depolarized to unblock the NMDA channel.Ca2+-permeability of NMDA receptor-conseq
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The sequence of events underlying LTP?
Ca influx  triggers events which  strengthening of synapse
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How does Ca2+ enter and Mg leave?
Unblocking of the NMDA receptor allows a Ca2+ influx. Tetanus triggers depolarisation to get Mg to leave
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What happens to make the EPSP bigger?
LTP has important postsynaptic mechanisms: Phosphorylation of AMPA receptors Insertion of more AMPA receptors
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Hebbian Plasticity, after Donald Hebb?
If activation of a synapse results in the postsynaptic neuron firing an action potential the synapse strengthens
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How does The NMDA receptor gives synapses in the hippocampus a quasi-Hebbian character?
Instead of the postsynaptic neuron having to fire an action potential, it needs to be depolarized sufficiently to unblock the NMDA receptor
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What effect does CaMKII mutation have?
prevents autophosphorylation and persistent activation -> deficient in LTP-> impairs spatial learning
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Long term synaptic modification in hippocampus also involves what?
changes in gene expression
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How does learning change the structure of the brain?

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e.g. Areas of brain enlargement in highly skilled musicians, can change as you get older too (wiring of brain not size changes)

Card 3

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Describe the The circuit for the gill withdrawal reflex in aplysia

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Card 4

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Card 5

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