learning and memory (anton) Flashcards

1
Q

define
1. learning
2. memory
3. recall
4.the engram

A

Learning = acquisition of information
Memory = storage of learned information
Recall = reacquisition of stored information
The engram = physical embodiment of a memory

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2
Q

what is procedural memory?

A

Skills and associations we do unconsciously - like riding a bike, playing an instrument i.e. you don’t have to think about it

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3
Q

what is declarative memory?

A

Can be encoded in symbols and language - available to conscious mind

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4
Q

what is explicit memory?

A

memory that can be consciously recalled (e.g. recalling riding a shiny new bike on the Christmas day when you were 5)

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5
Q

what is implicit memory?

A

Implicit memory
Memory that cannot consciously recalled (e.g. learning to ride a bike)

Can be different types:
Includes procedural memory
Classical conditioning (associating a stimulus with a behaviour)
Priming (when one stimulus influences the response to subsequent stimuli)

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6
Q

what animal is used to show habituation and sensitisation?

A

the aplysia snail
has a siphon (tail? idk)
and delicate gills it hides if hurt/sensing danger etc…

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7
Q

what neurons are involved in the gill withdrawal reflex and how is habituation shown?

A

Sensory neuron (presynaptic) of the syphon skin synapses with a motor neuron (L7) resulting in gill withdrawal

researchers poke the syphon, and eventually the aplysia stops withdrawing its gills when it realises the pokes aren’t harmful

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8
Q

what did recordings from the presynaptic sensory neuron and the L7 motor neuron show?

what can this be concluded about where the memory is learnt/stored?

A

Response at the postsynaptic neuron is reduced as number of pokes (stimuli) increases (while the presynaptic neuron maintains a steady response)

This means the ‘memory’ is learnt somewhere after the body of the presynaptic and before/at the body of the postsynaptic

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9
Q

in habituation, we know that the memory (of ‘no harm’ in this example) is learnt after the presynaptic neuron.

so what is the mechanism of habituation?

A

when looking at vesicles of NT released at a synapse, you have three populations:

  1. Readily releasable pool (attached to active zone)
  2. Proximal pool (very close but not yet docked)
  3. Reserve/resting pool (a bit further away, takes longer - mins - to be released)

The proximal pool takes some time to dock (reserve pool even longer), so…
Increasing repeated stimulation will result in reduced response as the readily releasable pool has been depleted and the reserves need a little bit more time

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10
Q

what is the procedure to investigate sensitisation (in aplysia)?

A

Application of a painful/noxious stimulus alone (e.g. shock to the tail) then results in a benign stimulus (e.g. poke of the syphon) eliciting a stronger response (faster/longer withdrawal of the gills) when this benign stimulus would have previously caused habituation

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11
Q
A
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11
Q

what neurons are involved in sensitisation in aplysia?

A

got the sensory neuron at the syphon synapsing with the motor neuron L7 of the gills, but the sensory neuron ISN’T being activated (at first)

Additional ‘interneuron’ involved, activated by the painful stimulus being applied - the L29 neuron

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12
Q

what are the mechanisms occurring in sensitisation?

A

the painful stimulus activates the L29 interneuron

Its presynaptic terminal releases serotonin to act on the sensory neuron just before said sensory neuron synapses with the motor neuron L7

Activation of receptor on sensory neuron causes it to produce adenylate cyclase → cAMP → PKA

More PKA in the presynaptic sensory neuron causes phosphorylation and therefore inactivation of K+ channels, slightly depolarising the membrane…

When a gentle poke is then applied to the syphon, more NT vesicles are released from sensory neuron at the same time because of the slower repolarisation, so there is more activation of motor neuron = larger withdrawal of the gill muscle (protecting it’s sensitive parts)

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13
Q

what is the mechanism/s occurring during this process of associative learning?

A

You’ve got the ^^sensitisation mechanism occurring due to the pain (US): L29 releasing serotonin causing adenylate cyclase activation → more cAMP → PKA phosphorylating and inactivating some K+ channels etc…

AT THE SAME TIME - sensory neuron is being activated
Increases intracellular Ca2+ in the sensory neuron terminal, the Ca2+ provides +ve feedback to adenylate cyclase that’s being activated due to L29 neuron, produces even more cAMP, causing a larger response

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14
Q

is it that simple (sensitisation and associative learning mechanisms)?

A

There are more things going on -
Complex models of sensitization and learning:
Multiple intracellular signalling pathways are needed, pre and postsynaptic

Long term phase involves the nucleus and altered gene expression, uses proteins like MAPK. early phase uses other pathways like PKC, and AMPA and NMDA receptors etc…

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15
Q

Hebbian synapse?

A

Coordinated activity of a presynaptic terminal and a postsynaptic neuron strengthen the synaptic connections between them

suggesting that when two neurons are repeatedly and persistently activated together, the strength of the synaptic connection between them increases. Summarised as “cells that fire together wire together,” basis of associative learning and memory

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16
Q

hippocampus and memory?

A

Involved in memory
First realised due to it being affected in Alzheimer’s
Taxi drivers have a larger hippocampus - spatial memory

17
Q

what simplified circuit was investigated in the hippocampus?

A

Entorhinal cortex → dentate gyrus = perforant path synapses

Dentate gyrus → CA3 = mossy fibre synapses

CA3 → CA1 = Schaffer collateral synapses

18
Q

investigating LTP -

which neurons were looked at, what was done to them and what was observed?

A

Stimulate enough presynaptic mossy fibres (CA3), measure effect on postsynaptic CA1 neuron (so we are looking at Schaffer collateral synapses)

A high frequency presynaptic stimulus produces long lasting-potentiation -
- Stimulus causes an EPSP
- A continuous high frequency stimulation (100 stimuli/sec) resulted in larger amplitude EPSPs being produced. I.e. the HFS (high frequency stimulation) has caused a modification of the stimulated synapses so that they are more effective
This effect can remain for days, weeks, or even years (tho ‘years’ is hard to prove)

19
Q

when looking at LTP what is meant by input specificity?

what does this suggest about where the LTP mechanisms occur?

A

Input specificity - e.g. if a neuron had two different inputs. You only stimulate one of them with high frequency
Only stimulation of this input showed increased amplitude EPSPs
Stimulation of the other input that wasn’t ‘trained’ did not show any difference in EPSP

Suggests the mechanisms involved don’t happen at level of CA1 (post) cell body, but most likely at synapse

20
Q

high frequency stimulation results in LTP when…?

A

High frequency stimulation results in LTP when it causes temporal summation of the EPSPs

21
Q

what is meant by cooperativity?

this demonstrates LTP can occur by ways other than temporal summation as seen with HFS)

A

Cooperativity = Two pathways converging on the same target can both be strengthened if they fire together (basis for associative learning)

Don’t need high frequency stimulation to cause LTP necessarily. If multiple synapses are active at the same time, they cause spatial summation of EPSPs, and EPSP amplitude was also seen to increase when the pre and postsynaptic neurons were stimulated at the same time

22
Q

what are the three kinds of glutamate receptors, including what they do?

A

AMPAR = slightly selective for Na+ ion channel, opens and depolarises neuron when Glu binds

NDMAR = Glu binds, ion channel selective for Ca2+ - doesn’t necessary depolarise the cell a lot, but causes downstream effects
there is a Mg2+ block, where despite Glu being bound, it prevents channel opening (it is removed by depolarisation)

mGluR = activated - G protein signalling cascade (not relevant here)

23
Q

CA3 - CA1 neurons -

what do we see in LTP (on a molecular level) and what is needed for this change to happen?

A

In LTP we see an inc. in number of AMPA receptors at postsynaptic membrane

To get this effect at the postsynaptic membrane…
Presynaptic needs to depolarise in order to release glutamate
Postsynaptic also needs to be depolarised at the same time, in order to remove the magnesium block on NMDA receptors
Opening of NMDARs = Ca2+ in, downstream effects causing increased gene expression of AMPARs

24
Q

early vs late LTP?

A

Early changes occur that don’t require protein synthesis
Later changes do need it

25
Q

early LTP - explain in detail how we get increased amplitude EPSPs and the effects on AMPARs

A

(covered previously)
1. Activation of presynaptic neuron with high frequency
2. Release glutamate
3. Activates AMPARs on postsynaptic, if great enough (high amplitude EPSPs)
4. Postsynaptic depolarises massively
5. Depolarisation of the postsynaptic membrane removes Mg block, allows Ca2+ in via NMDARs

(new bit)
Ca2+ activates calmodulin kinase II (CaMKII):
Ca and calmodulin bind to the regulatory domain of CaMKII, exposing its catalytic site allowing it to phosphorylate itself…
And then other proteins, first being the AMPA receptors
This increases the size of AMPA receptor response

This phosphorylation also leads to an increase in no. of AMPA receptors at postsynaptic membranes - recruiting AMPARs stored in vesicles close to the membrane

26
Q

late LTP changes - what happens and how?

A

Aside form early LTP effects, the incoming Ca2+ also causes: Adenylyl cyclase - cAMP increases -activates PKA which goes to the nucleus and… →

  1. CRE (cAMP response element) is a sequence present in promoter regions of certain genes
  2. CREB-2 is a protein that is bound to this CRE element
  3. When CREB-2 is substituted for (phosphorylated?) CREB-1, the genes in question are expressed

Translation of genes → proteins (‘LTP effectors’) produced travel to synapse and alter it - changing the structure/scaffolding for example

27
Q

in terms of late LTP effects, what is yet to be found out?

A

how these protein effectors know which synapse to go to. How come they don’t act on all synapses the neuron has, moving simply by diffusion?

28
Q

what is a behavioural experiment you can do when investigating memory (anton)?

A

Water maze - mouse is swimming, tries to find a platform hidden under the water

You can then do some experiments with knockout mice/genes inhibited in specific neurons (can’t entirely knock it out or the mouse would die) - lacking/affecting CaMKII, NMDARs etc…
Or apply nootropics to enhance memory (turns out they also enhance LTP)

29
Q

what are the two main inputs to purkinje cells?

what happens when stimulating a parallel fibre alone vs both he climbing and parallel fibres together?

A

Parallel fibres - synapse with a large number of purkinje cells, but only one synapse with each purkinje cell (so weak stimulation of multiple)

Climbing fibres - synapses with only one purkinje cells but many synapses - so strong stimulation of an individual purkinje cell

Stimulating one parallel fibre -
You get relatively stable EPSPs

Stimulation of BOTH the climbing and parallel fibres -
Reduces EPSP amplitude - this is LTD

30
Q

what is meant by the term ‘input specific’ when talking about LTD here?

A

Input specific - LTD occurs at only the one synapse between the parallel fibre and purkinje cell in question (not all the purkinje cell’s synapses)

31
Q

why is LTD used at purkinje cells in the cerebellum?

A

involved in motor learning - input from the climbing fibre may indicate motor error, e.g. going to grab something and missing as a child, so you want to weaken this connection

32
Q

a big difference between LTD mechanisms and LTP ones?

A

LTD does not involve NMDARs

33
Q

what happens, mechanistically, when you activate a parallel fibre?

A

Parallel fibre release glutamate activating:

  1. AMPA receptors on the purkinje = EPSP
  2. mGluRs - triggering phospholipase C cascade (the enzyme converts PIP2 → DAG + IP3, IP3 causes intracellular Ca2+ release and DAG activates PKC)

***IF you only activate parallel fibre (not climbing fibre as well) PKC activity is not sufficient for LTD

34
Q

what happens when you activate the parallel fibre and climbing fibre at the same time?

what mechanisms are involved?

A

Release of glutamate (added to that from the parallel fibre)

Depolarises the purkinje cell, opening the VG Ca2+ channels

Influx of calcium, ‘potentiates’ the PKC in the cell as a result of the parallel fibre, and PKC starts to work more

NEED coincident activation of both intracellular signalling pathways

MECHANISMS -
PKC phosphorylates AMPAR GluR2 subunit (different site from LTP)
Causes endocytosis (internalisation of AMPARs) reducing current/amplitude of EPSP. This can be shown by using endocytosis inhibitors

35
Q

so you’ve had simultaneous activation of climbing and parallel fibres, glutamate release from both etc…

PKC activated/potentiated…

what does PKC do to cause LTD?

A

causes internalisation of AMPARs

PKC phosphorylates AMPA GluR2 subunit (different site from where they are phosphorylated in LTP)

Causes endocytosis (internalisation of AMPARs) reducing current/amplitude of EPSP. This can be shown by using endocytosis inhibitors

36
Q

LTD was also investigated at the CA3-CA1 synapse (Schaffer collateral synapses)

how, and what was a little confusing?

A

Stimulated presynaptic neuron with LOW frequency stimulation (1-5 Hz), for much longer
(Can be an undoing of LTP, but does not have to be)

confusing at first - seemed to be Ca2+ dependent, but so is LTP…

37
Q

at the CA3-CA1 synapse, both LTD and LTP appeared to be calcium dependent.

how does this work?

A

Degree of activation of NMDA receptor on the Ca1 neuron (and therefore level of intracellular Ca2+) defines whether the synapse undergoes LTP or LTD

Weak activation = small inc. in Ca2+ = LTD (calcineurin a phosphatase that dephosphorylates the AMPARs)
Strong activation = large inc. in Ca2+ = LTP

38
Q

what effects do small increases in Ca2+ levels cause and what effects do large increases cause?

A

Phosphatases are required for LTD, while LTP requires kinases

Small increases in Ca2+ from NMDA-R trigger more phosphatase action and reduce AMPA-R efficacy (LTD)

Large increases activate more protein kinases, which increase AMPA-R efficacy (LTP)

39
Q

Is LTP + LTD what makes up memory?

A

Causes permanent synaptic changes so, yes to some extent
However there are other, larger changes distributed around the brain when you memorise something

40
Q

what experiment on memory was done in marmoset monkeys and what did it show?

A

memory alters morphology -

One group in a boring cage, not much going on

Other group in a more stimulating environment, toys etc…
This other group - developed more skills/memories - had much more advanced dendritic trees (more spines etc..) in certain cells