HSM330 article 2 Summary

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Ryerson University
Health Services Management
HSM 330
Daolun Chen

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 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 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 -other researchers found similar results as Frankland by using a 5 arms discrimi nation training -using multi-trial Morris water maze in mice, Teixeira foudn that Acc is critica lly involved in processing remote spatial memories, but expected disengagement of hippocampus did not occur -there was no evidence for a graded effect on recent vs remote spatial memory af ter inactivation of dorsal hippocampus -fMRI in humans showed that remote recognition memory performance correlated wel l w/ a dec in hippocampal activity and an inc in ventral medial prefrontal cortex activity -largest dec in hippocampal activity occurred btwn days 1 and 2, an interval of time w/o a dec in memory performance -interactions btwn hippocampus & cortex after acquisition of new info lead to pr ogressive establishment of enduring memories in distributed cortical networks that are independent of hippo campus -alternatively, for certain learning tasks, new info is consolidated and stored in neocortical structures from time of training and this is needed for memory recall -but, hippocampus may contain stable memory index of neocortical circuits where detailed info is actually stored and/or it may contain part/all of info that is stored simultaneously in h ippocampal and neocortical areas -ACC plays distinct functional roles incl error detection, attention, sensory pe rception, motor control, & effortful recall - is ACC involved in memory storage/retrieval? answer is unkn own -ACC is interconnected w/ prefrontal cortical regions and w/ sensory, motor, and limbic corticies -so maybe ACC is impt node of a largely cortical network that coordinates retrie val of remote memories stored in distributed cortical circuits -however, just b/c a region is active during remote memory retrieval does not me an this region underlies the retrieval, and reversible inactivation of hippocampus, amygdala, entorhinal, ACC , prefrontal, and posterior parietal cortex on recent and remote memories do not demonstrate that memories a re stored in those regions -these stuctures merely play a part in circuits relevant to memory retrieval Rudy speculates that activation of ACC during retrieval of remote memories does not reflect the impact of systems con processes but instead reflects a process of forgetting -this assumption is based on fact that old memories are harder to retrieve than recent ones and thus they would req. an additional activation of regions of prefrontal cortex to be retrie ved -in support of this, Rudy found that contextual fear conditioning undergoes a cl ear forgetting process consistent w/ contention that regional brain changes reflect attempts of cortica l networks to deal w/ degredation of memory trace w/ time -neocortex can rapidly consolidate new info, so cortical networks may participat e in encoding of info (a function assumed to be carried out by
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