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Nervous System: Unit IV > Limbic System > Flashcards

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Limbic system fxn (general)

parts of the brain that deal with learning, memory, and emotion


Characteristics of declarative memory

  • definition
    • ability to recollect events or facts that have a specific or temporal spatial context
    • semantic knowledge
  • hippocampus is important in formation of declarative memory
  • HM's lesion ==> deficit in declarative memory
  • long term declarative memories stored @ neocortex


Characteristics of procedural memory

  • ability to learn new motor skills
  • HM's lesion did not impact his procedural memory
  • cerebellum, striatium, and frontal cortex are important in procedural memory


Case of HM

  • surgeon resected medial temporal lobe structures bilaterally to reduce HMs epileptic seizures
    • hippocampus
    • entorhinal cortex
    • amygdaloid complex
  • decreased seiures +
  • anterograde amnesia
    • could remember memories from before surgery, but no new facts
  • deficit in declarative memory + normal procedural memory


Types of memory (temporal classifications)

  • short-term memory = lasts fractions of seconds - seconds
    • takes place in sensory cortex
  • working memory = lasts seconds - minutes
    • e.g. cooking: did you add salt already?
    • takes place in frontal lobe @ areas of exectutive function
    • test for working memory: digit span
  • long-term memory = lasts days to years
    • stored @ cortex (various areas)


Anatomy of the hippocampus

  • dentate gyrus = inner layer spiral semicircle of neuron cell bodies
  • ammon's horn = surrounds dentate gyrus; spiral semicircle of neuronal cell bodies
    • 4 types of cell bodies; most importants = CA3 & CA1
  • entorhinal input ==> perforant path ==> synapse @ dentate gyrus & CA3
  • dentate gyrus axons ("Mossy fibers") ==> synpase @ CA3
  • CA3 axons ("Schaeffer collaterals") ==> synapse @ CA1
  • hippocampal output = CA1 + CA3 ==> form Fornix


Characteristics of long term potentiation (LTP)

  • strong repetitive stimulation (e.g. via mossy fibers or schaeffer collaterals) ==> modification of activity of synapses
  • e.g. pyramidal neurons @ CA3 or CA1 have a baseline EPSP response to a given population of axons
    • if a subset is given a tetanus (brief burst of high frequency stimulus) ==> larger EPSPs @ synapses stimulated by tetanus


Conditions under which LTP occurs

  1. only with synapses that have received vigourous stimulation (i.e. via glutamate NT)
  2. only if the stimulation ==> cell depolarization @ the postsynaptic neuron


LTP & associative learning

  • associative learning = associating several cues w/facts or objects
  • during the process of learning, person undergoes motivated rehearsal of task/association ==> vigorous stimulation of a specific subset of postsynaptic cells ==> LTP
  • LTP then allows the postsynaptic cell to be depolarized more easily ==> stronger association & the need for fewer cues to bring forth a memory
  • experiments @ mice:
    • more difficulty using fewer cues w/impaired CA3 areas
    • ==> LTP @ CA3 = important for associative memory


Molecular mechanisms of LTP

  • NMDA = coincidence detector @ postsynaptic cells
    • @ resting potential: NMDA channel is blocked by Mg2+, so it cannot respond to glutamate
    • @ depolarized conditions (i.e. additional electrical stimulus from elsewhere): Mg2+ ions expelld ==> opening of channel ==> influx of Ca2+
  • NMDA-associated Ca2+ influx ==> activated calmodulin ==> stimulation of CAMKII ==>
    • incorporation of AMPA (excitatory) receptors @ postsynaptic membrane
    • phosphorylation of AMPA receptors ==> enhanced response to glutamate


Molecular mechanisms of learning

  • LTP
  • synaptic plasticity
    • synapses are not static
    • learning involves restructuring of neuronal circuits via synapse formation and destruction
  • possibly adult neurogenesis
    • e.g. @ olfactory bulb, hippocampus, cerebellum


Amyloid hypothesis of Alzheimer's disease

  • General: amyloid beta protein ==> assembles as plaques w/in brain ==> cognitiive impairment through loss of synapses + neurodegeneration
  • Proteolysis of APP (membrane associated protein) by beta and gamm secretase ==> neurtoxic A(beta) ==> loss of LTP/synapses/etc.
  • developing therapies target reduction of A(beta) protein production and deposition


Limbic system and amygdala role in emotion

  • emotional limbic system = amygdala, cingulate gyrus, mediodorsal nucleus of the thalamus, ventral basal ganglia, insular cortex and hypothalmus
    • (structures surrounding region of third ventricle)
  • role in emotions: fear and reward; sadness or elation
    • e.g. amygdala of monkeys removed ==> altered feeding, sexual activity, lack of fear
  • fear conditioning (e.g. associating weak sound w/aversive stimulus like foot shock in mice) ==> 
    • fear reaction to conditioned stimulus
    • mediated by changes in neuronal circuits @ lateral and central nuclei of the amygdala


Characteristics of conditioned flavor acersions

  • CFA = form of associative learning = individual develops food aversion for a food that is associated with malaise
  • associative learning occurs even though the flavor stimulus and the aversive stimulus (feeling sick) do not occur simultaneously
  • food exposure ==> ACh release @ insular cortex (taste cortex) ==> phosphorylation of NMDA receptors ==> impacts response to fibers from amygdala 
    • malaise ==> vagus nerve ==> amygdala
    • [if ACh is blocked @ insular cortex ==> NO CFA occurs]