Learning and Memory: Neural Mechanisms

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Learning and Memory Neural Mechanisms
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18 Learning and Memory Neural Mechanisms

• Changes in Synapses May Be Mechanisms of Memory
  Storage
• Theories about neural circuits locate memory
  storage in one or more plastic synapses.
• A synapse is plastic if it can change the
  strength with which it affects its target.

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18 Changes in Synapses May Be Mechanisms of
Memory Storage

• Different circuits underlie memories
• A neural chain can be a monosynaptic reflex
• Simple neural circuits can receive input from
  superordinate circuits, also called modulatory
  circuits
• Cell assemblies are complex networks of neurons

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Figure 18.1 Sites of Synaptic Plasticity in
Neural Networks (Part 1)
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Figure 18.1 Sites of Synaptic Plasticity in
Neural Networks (Part 2)
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18 The Nervous System May Form and Store
Memories in Various Ways

• Physiological changes at synapses may store
  information.
• Changes can be presynaptic, or postsynaptic, or
  both.
• Changes can include increased neurotransmitter
  release, or effectiveness of receptors.
• Changes in the rate of inactivation of
  transmitter would increase effects.
• Inputs from other neurons might increase or
  decrease neurotransmitter release.

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Figure 18.2 Synaptic Changes That May Store
Memories (Part 1)
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Figure 18.2 Synaptic Changes That May Store
Memories (Part 2)
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18 The Nervous System May Form and Store
Memories in Various Ways

• Structural changes at the synapse may provide
  long-term storage.
• New synapses could form or some could be
  eliminated with training.
• Training might also lead to synaptic
  reorganization.

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Figure 18.2 Synaptic Changes That May Store
Memories (Part 3)
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18 The Nervous System May Form and Store
Memories in Various Ways

• Some conditions are required to induce
  memory-related changes.
• Hebbian synapses grow stronger when the
  presynaptic neuron repeatedly causes the
  postsynaptic cell to fire.
• The synapse must be active and effective at
  causing firing.

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18 The Nervous System May Form and Store
Memories in Various Ways

• The dual-trace hypothesis memory formation
  involves
• A brief transient memory storage process
• A stable change in the nervous system

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18 Cerebral Changes Result from Training

• Experiments with lab animals used three
  environmental conditions
• Standard condition (SC)
• Impoverished (or isolated) condition (IC)
• Enriched condition (EC)

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Figure 18.4 Experimental Environments to Test
Effects of Enrichments on Learning and Brain
Measures (Part 1)
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18 Cerebral Changes Result from Training

• Animals housed in enriched condition (EC)
  developed
• Increased acetylcholinesterase (AChE) activity
• A heavier cerebral cortex, due to cortical
  thickness

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18 Cerebral Changes Result from Training

• Enriched experience
• Promotes better learning and problem solving
• Aids recovery from conditions such as
  malnutrition
• May protect against age-related declines in
  memory
• (Donaldson) May protect against drug-related
  fetal insult (prenatal cocaine (PCOC) exposure)

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18 Cerebral Changes Result from Training

• Learning in EC can produce
• More dendritic spines in the cortex
• Increased BASAL dendritic branching, indicating
  new synapses
• Filopodia, which grow in response to electrical
  activity and may become dendritic spines or
  branches
• (Donaldson) Increased DAT activity, especially in
  response to stress, again in PCOC rats

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Figure 18.5 Measurement of Dendritic Branching
(Part 1)
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18 Cerebral Changes Result from Training

• Experience can also increase the size of
  synaptic contacts.
• These changes occur in the cortex and are related
  to long-term memory.
• One hypothesis is that information is stored in
  the cortex but processed for memory storage in
  other brain areas.

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18 Invertebrate Nervous Systems Show Plasticity

• Aplysia is used to study plastic synaptic changes
  in neural circuits.
• The advantages of Aplysia
• Has fewer nerve cells
• Can create detailed circuit maps in invertebrate
  ganglia little variation between individuals

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Figure 18.6 Characteristic Behaviors of Aplysia
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18 Invertebrate Nervous Systems Show Plasticity

• Habituation is studied in Aplysia.
• Squirts of water on its siphon causes it to
  retract its gill.
• After repeated squirts, the animal retracts the
  gills less it has learned that the water poses
  no danger.

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18 Invertebrate Nervous Systems Show Plasticity

• The habituation is caused by synaptic changes
  between the sensory cell in the siphon and the
  motoneuron that retracts the gill.
• Less transmitter released in the synapse results
  in less retraction.
• Over several days the animal habituates faster,
  representing long-term habituation.
• The number of synapses between the sensory cell
  and the motoneuron is reduced.

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18.7 Synaptic Plasticity Underlying Habituation
in Aplysia (Part 1)
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18 Mammalian memories of Different Durations
Form by Different Neurochemical Mechanisms

• LTP of neural response and depends on activation
  of NMDA glutamate receptors, which increases AMPA
  receptors and NT release.
• The formation of LTM requires protein synthesis.
• It occurs in two waves about 1 hour after
  training, and at about 5 to 8 hours after
  training.
• Inhibitors of protein synthesis at these stages
  prevent LTM formation.

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18 Memory Formation Can Be Modulated

• Emotions appear to enhance the formation of
  memory.
• Physiological correlates of an emotion may
  potentiate cellular mechanisms of memory
  formation.
• Many substances affect memory formation.
• Epinephrine affects memory formation by
  influencing the amygdala.
• Giving patients with traumatic memories
  propranolol (NE antagonist) has shown some effect
  of lowered stress.

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18 Some Brain Measures Correlate with
Age-Related Impairments of Memory

• Learning and memory decline with age.
• Older subjects show less cortical activation
  during memory retrieval.
• Loss of ACh neurons or neuronal connections may
  explain impairment.
• (Donaldson) Loss of myelin may underlie deficits.
• Aged rats that perform poorly on tests also have
  reduced ACh activity.
• This decline is only seen when aging is
  associated with impaired performance not solely
  with age.

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Figure 18.15 Active Brain Regions during
Encoding and Retrieval Tasks in Young and Old
People
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18 Take Home Message

• Synaptic changes - neural mechanisms of memory
  (neural, superordinate and cell assembly)
• Hebbian synapse
• Training and environment - structural changes
• Plasticity in invertebrate species (Aplysia)
• LTP in hippocampus (glutamate NMDA and AMPA
  receptors)
• LTM - involves protein synthesis
• Age-related decline - but can be thwarted!