Limbic System

Question 1

Limbic system fxn (general)

Answer 1

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

Question 2

Characteristics of declarative memory

Answer 2

• 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

Question 3

Characteristics of procedural memory

Answer 3

• 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

Question 4

Case of HM

Answer 4

• 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

Question 5

Types of memory (temporal classifications)

Answer 5

• 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)

Question 6

Anatomy of the hippocampus

Answer 6

• 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

Question 7

Characteristics of long term potentiation (LTP)

Answer 7

• 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

Question 8

Conditions under which LTP occurs

Answer 8

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

Question 9

LTP & associative learning

Answer 9

• 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

Question 10

Molecular mechanisms of LTP

Answer 10

• 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

Question 11

Molecular mechanisms of learning

Answer 11

• 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

Question 12

Amyloid hypothesis of Alzheimer’s disease

Answer 12

• 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

Question 13

Limbic system and amygdala role in emotion

Answer 13

• 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

Question 14

Characteristics of conditioned flavor acersions

Answer 14

• 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]