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NROC61 - Medina article notes


Department
Neuroscience
Course Code
NROC61H3
Professor
Le Boutillier

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Medina
-memories believed to be initially and temporarily stored in hippocampus and lat
er transferred to cortex
for persistent storage during consolidation, but cortex may have crucial role in
initial steps of memory
formation and hippocampus may not be disengaged from memory processing as early
as has been proposed
-memory takes time to stabilize/consolidate
-Ribot gradient: memories might be processed over time
-Muller&Pilzecker: consolidation - post-acquisition processes that stabilize mem
ories
-newly formed memory undergoes a lingering process, becoming stronger and resili
ent over time until it is
insensitive to disruption
-newly learned material consolidates over time, memory trace is vulnerable to di
sruption only for a limited
period after training
-post-training treatments can enhance memory storage
-the time course of consolidation of a memory trace varies depending on learning
task parameters and on brain
structure being analyzed
-memory consolidation is not a single process - divided into 2 phases in vertebr
ates
1) a protein and RNA synthesis-independent phase that lasts min to 1-3h (STM)
2) a protein and RNA syntehsis-dependent phase that lasts several hrs to days, w
eeks, or longer (LTM)
-property of LTM is its sensitivity to protein synthesis inhibitors
-during memory formation, protein synthesis is req'd to transform newly learned
info into stable synaptic
modifications
-LTM reqs de novo protein synthesis around time of training/during first few hrs
post-training
-cell assemblies work together to represent info in the brain
-no single memory 'center' exists, diff regions of brain are involved in memory
processing dealing w/ diff
types of info of the new learned material, so consolidation of memories is not a
single unitary event
depending on a single region of brain
-need to look at temporal contributions of diff regions of brain involved in pro
cessing a learned experience
-in diff species, diff types of memory req multiple and distinct neural circuits
= existence of multiple
memory systems
-consolidation of memories undergoes temporal changes = consolidation occurs ove
r an extended period of time
-graded retrograde amnesia caused by hippocampal damage can extend across severa
l yrs in humans =
consolidation of memories is gradual and protracted
-mammals: many types of memories depend on hippocampal processing during first d
ays to few weeks, this
hippocampus-directed process of stabilization eventually ends and memories becom
e hippocampus-independent
Cellular and systems consolidation
2 types of memory consolidation
1) one is fast, involves early molecular & cellular events occuring early after
training and lasting no more
than several hrs to a few days in particular brain regions engaged in acquisitio
n and early processing of

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new info (synaptic/cellular consolidation)
2) other is slow, entails participation of neocortical regions and their interac
tions w/ MTL that
reorganize recently learned material (systems consolidation)
-stabilization of memory trace achieved by gradually binding together the multip
le cortical regions that
store memory for a whole event
-systems-level consolidation lasts weeks-months, but when consolidation of new i
nfo involves interactions
w/ already stored associative 'schema' systems consolidation may be v rapid
-cellular/synaptic consolidation involves diff molecular events incl activation
of several signaling cascades
in specific brain regions
-attention has focused on structures of MTL, esp some subregions of hippocampus
-activation of cascades in hippocampus is initiated by receptor activation incl
AMPA, metabotropic and
NMDA glutamate receptors, BDNF and monoamine receptors followed by recruitment o
f second messenger systems
and activation of diff protein kinases and phosphatases
-leads to synthesis of proteins req'd for structural & functional changes
-cellular consolidation is transition of memory from a gene expression & protein
synthesis dependence to
independence
-consolidation of LTM implies growth of new connections and rearrangements of ex
isting ones
-these early molecular requirements for consolidation are just initial steps of
a series of events that
evolve over an extended period in diff brain regions, these events may be gatewa
y to systems consolidation,
and even cellular consolidation
-temporal evolution of memory processing after initial stages is unknown
-several reports describe changes in activation state of signalling cascade mole
cules and gene expression
pathways that take place in structures such as hippocampus and occur & persist f
or hrs/days after learning
-time course of these molecular events goes beyond what is considered end of cel
lular consolidation phase
-others say that completion of cellular consolidation phase in hippocampus takes
days or even weeks
-so, we do not know when consolidation phase ends and when storage process begin
s, incl possibility that both
phases may overlap to some extent
-time dynamics for cellular & system consolidation are v different
-for many forms of declarative memory, info loss in retrograde amnesia is larger
for recent memories than for
remote memories (Ribot gradient), use systems consolidation to explain this
-systems-level consolidation is mechanistically & temporarily distinct from mole
cular & cellular events
underlying cellular consolidation
-but systems and cellular consolidation are interdependent b/c the synaptic cons
olidation product (stable
memory trace in hippocampus) must last long enough to permit systems consolidati
on processes to work
-systems consolidation is not well understood compared to cellular consolidation
-systems con implies that contribution of hippocampus and MTL to memory con grad
ually wane and neocortex

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comes to support stable long-term storage of info; it involves progressive disen
gagement of MTL structures
and inc recruitment of cortical regions w/ time; as reorganization of neocortica
l network finishes,
hippocampus etc is not needed any more
Lesion, imaging, & pharmacological evidence for system consolidation
-hard to distinguish remote & recent memory btwn species & learning tasks
-recent memory: a stabilizing memory that undergoes and completes cellular con i
n brain regions involved in
acquisition of a particular learning experience; recent memories last no more th
an a few days
-remote memory: the stabilized memory trace that outlasts these initial stages a
nd thus becomes consolidated
at the system level in same &/ diff distributed brain circuits than those engage
d in recent memory processing;
memory that can be retrieved several days/weeks after training
-In humans: hippocampal lesions produce temporally graded retrograde amnesia for
majority of episodic forms
of memory; extensive MTL damage affects recent allocentric spatial and semantic
memories depending on extent
of lesion
-In animals: disruption of hippocampal function hinders recent memory w/o affect
ing remote memory
exception: hippocampal lesions produce ungraded (flat) retrograde amnesia for sp
atial memory in Morris water
maze training, so in apparent contradiction w/ what is seen in humans, storage a
nd/or retrieval of certain
spatial memories depends on functional integrity of hippocampus
-memory formation & spatial discrimination is associated w/ activation of severa
l diff cortical & subcortical
brain regions
-diff structures of rat brain incl hippocampus, amygdala, entorhinal cortex, & p
osterior parietal cortex are
sequentially engaged in memory retrieval of a one-trial inhibitory avoidance tas
k
-learning associated changes in metabolic activity in various cortical regions d
epend on task parameters
and time interval btwn training and test sessions = system-level con is in charg
e of memory con and
maintenance over time
-systems con supported by findings that anterior cingulated cortex (ACC) plays
critical role in remote
memory for contextual fear conditioning, a hippocampus dependent learning task
Frankland:
-Zif-268 (activity dependent gene) expression inc in ACC after remote memory tes
ts, similar findings obtained
in prelimbic and infralimbic regions of prefrontal cortex
-these changes not seen in a null CaMKII mutation which has cortical LTP impairm
ents and specific deficits
in remote memory
-reversible inactivation of ACC using targeted delivery of lidocaine hindered re
mote memory
-electroporation of small interference RNA for 2B subunit of NMDA receptor into
ACC blocked recent (24h)
contextual fear memory
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