PSYC 310 Lecture Notes - Lecture 9: Subgranular Zone, Carbon-14, Memory Consolidation
7 May 2018
School
Department
Course
Professor
PSYC 318
Behavioural Neuroscience II
February 28th, 2018
Lecture 15/24: The influence of neurogenesis on memory
• Midterm is not really cumulative and includes all the lectures between MT1 and MT2
• Hippocampal circuitry
o How we believe memory recall works
o Conceptual overview of how many neuroscientists are thinking about the hippocampus
now
o In the cortex: everything gets funneled into the hippocampus through the entorhinal
cortex
o Entorhinal cortex uses grid cell activity which reflects location in space and as you switch
contexts grid cell activity remaps: has a different relationship between each room,
where animal is standing
o Combinatorial control of grid cells allows HC to form unique representation of every
moment in time
▪ CA3 is where we believe memory recall happens
▪ Every cell in CA3 is connected to all other cells – interconnection
▪ Memory recall because of this
▪ CA3: very easily induced synaptic plasticity: the locus of one-trial learning
o External inputs & Pattern completion
▪ A tone comes in through a pattern of neural activity, external input forms
synapses
▪ Spike in pattern of cells in CA3
▪ Passed on to downstream region
▪ When that tone representation is activated it will alert every other cell
▪ Sub-threshold synaptic activity: every cell receives signal but the euros ot
fire sub-threshold
▪ If a pattern comes in (2 simultaneous events occurring at the same time) then
they will induce an action potential
• Based on synaptic plasticity we think this is a fire together/wire
together event
• Interconnected: strengthened synapses
▪ In the future: you would only need one of these things to induce a firing, either
the tone or the light
• After a pattern of input is received they wire together and in the future
you only need one part of this pattern to be introduced to cause all the
previously fired cells to be fired again
• CA3 pattern completion
o HC forms snapshot of what just happened, in CA3 the wiring up recreates this pattern
o In the future: partial input comes in and in CA3 recalls what was last active in response
to some part of this pattern
o People call this pattern completion and we believe it occurs in CA3 region
o It is an unstable network
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▪ Seizure disorders/epilepsy arise from problems in this region of the HC when
neurons wire up too much and lead to super high levels of excitation
▪ So memories have to be stored very far apart at different neurons
o Pattern completion/pattern separation
▪ “tored eories ill eetuall oerge if there stored ear eah other
▪ If you want CA3 to recreate any previously active pattern then every unique
moment needs to be stored in a different section of CA3 neurons
▪ Is there a way to store memories far apart?
o Dentate gyrus:
▪ Give DG an input activity, then slightly change input activity
▪ Very slight change dramatically changes output firing of the dentate gyrus
▪ Appears that the dentate gyrus is capable of pushing the neural traces of
memory far apart
▪ Takes small changes and orthoganlizes them: takes small differences, amplifies
to make as large as possible
• Information processing in the hippocampus
o Is there any difference the DG can identify in some experience? If it can identify a
difference it uses this to orthoganlize and store in a different area of the CA3 so that
your events are distinct
o Pattern completion could be critical for memory recall to occur
o Pattern separation could be critical for memory storage
o There are more neurons in DG
▪ Anatomically and physiologically supports this hypothesis that information is
stored in a collection of cells that wire up together so memory recall can occur
o Pattern separation:
▪ Process of reducing interference from similar inputs using non-overlapping
representations
▪ Make patterns of memories more distinct than they were when the animal
perceived them
▪ Good pattern separation vs. poor
• Poor pattern separation: car parking
• People drive to work each day and often park in huge car parks
• Generally they remember where they parked that day, but unlikely to
remember where they parked 3 days ago
• Hypotheses about car-parking phenomenon:
o 1: Theres ot eough differee i eperiees of parkig eah orig to support
effective pattern separation, so each day the location of where you parked overwrites
the previous parking spot information
o 2: our pattern separation abilities are unlimited, to recall a memory you must recall a
cue that is uniquely associated with this event (e.g. it was raining that day) then you
might be able to isolate that event and remember it
▪ But DG might use whatever it can to activate a memory
▪ You dot osiousl ko hat the HC was using to differentiate these
episodes
▪ Whe oure trig to thik ak to a eet ou eed to osider if theres
anything that uniquely associates with that event
• Cue overload
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o The most effective retrieval cue for memory is one that uniquely associates with a
memory
▪ Greater # of associations cue acquires not efficient at retrieving desired
memory
o Pattern separation/completion in rodent task
▪ How valuable memory is: how often we misremember information
▪ Example: teach an animal something (like this room is horrible, you were
shocked last time you were in here)
• 1-2 weeks later bring the animal back to a similar room
• Some rodents will be afraid and start to exhibit freezing behavior, fear
the room
• It is a pattern completion event
• The CA3 region when animal looked at familiar wall it acted as cue that
recalled the memory in a pattern completion event
• Activates all cortical circuits that were active on the previous day
▪ All neural activity from previous experience reactivated
▪ When we look into the brain the cells active in HC or other part of cortex are
very different than the cells that were active when the rodent was previously
shocked in the first room
▪ Pattern completion/generalization vs. pattern separation/distinction
• Paper 18: theory of hippocampal neurogenesis and forgetting
o Neurogenesis is the birth of new neurons, neurons are generated from stem and
progenitor cells across brain, mostly happens during prenatal development when it
populates growing brain with neurons
▪ Usually it stops eventually
▪ Some labs claimed they still saw it, however
▪ Adult neurogenesis:
• It happens, in very particular places, and in most mammals
• Subgranular zone: SGZ which is part of the dentate gyrus of the HC
• Subventricular zone: SVZ which lines lateral ventricles
o Neurogenesis in humans
▪ Carbon 14 is radioactive: when a protein is made it incorporates carbon 14 from
wherever it gets it from (plant, food)
▪ % of carbon 14 reflects how much was in atmosphere at the moment, locked
into place and decays over time (when it was pulled out of the atmosphere)
▪ In most proteins and all of life forms they last 2 weeks, almost all proteins in
body carbon 14 reflects current atmospheric carbon 14
▪ There are some carbon molecules in your body that are there from birth and
never turned over:
• The carbon in the DNA of cells that do not divide, die, replicate
• The DNA in neurons
• Created he oure i o ad is eer haged for our etire life,
is very stable
• What is the % of carbon 14 in brain? You can guess the age that person
was born based on this
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