Stress, memory, navigation

• Basics of memory

  • once information is learned, it must be stored in the brain; memory
  • there are many diff kinds of memory
  • we devise a hierarchy
  • this is not how the brain does it

  
• From episodic to semantic

  • episodic memories are autobiographical
  • yesterday I saw the sunset
  • semantic memories are rules abt the world
  • the sun sets in the evening
  • always derived from personal experience
  • how are episodic memories turned into semantic memories?
  • involves hippocampus
  • involves repeated exposures to relevant events
• Basic process of memory

  • events happen in the world and are perceived
  • the brain does some processing
  • the info is stored (in working memory)
  • the memory id consolidated into reference memory and stored there
  • when using it, it must be retrieved
• Where in the brain?


  where does learning happen?

  • diff types of learning happen in diff places
  • cerebellum is important for some types of simple association

  where is memory stored?

  • the memory trace itself is called an engram
  • hard to locate
  • memories are stored all over cortex
• Important brain areas


  -Basal ganglia

  • important for habit formation, implicit learning
  • learning a new skill

  -anterior temporal lobe

  • important for semantic memory
  • knowing facts abt the world

  prefrontal cortex

  • lots of things; value, complex concepts

  amygdala

  • emotional memories, memory consolidation

  hippocampus

  • episodic, spatial, consolidation, relational learning
• The hippocampus

  • very important for memory
  • precise function is debated
  • involved in;
  • turning working memories into reference memories
  • acquiring relational memories
  • where the important feature of the learning is a relationship between two or more objects
  • episodic memories
  • turning episodic memories into semantic memories
  • learning spatial things (which tend to be rational)
  • other (olfactory memory)
• Damage to hippocampus

  • patient HM had both his hippocampi removed
  • had severe memory deficits, including:
  • anterograde amnesia
  • not able to form new long-term memories
  • retrograde amnesia; loss of previous memories
  • loss of episodic memory and conversion to semantic
  • never learned to recognize Milner
  • intact working memory
  • good implicit learning
• Basics of learning

  • animals learn to associate events that reliably happen after each other with each other
  • every time u hear cookie, u get cookie
  • one neutral event (the word) and one innately positive/negative event (cookies)
  • animals begin to respond to the neutral event (the conditional stimulus) not jus the innately positive one (unconditioned stimulus)
  • u start salivating when u hear the word
  • the conditioned response doesn't have to be same as the unconditioned response
• Hebbian synapses

  • brain is made of neurons
  • memories have to consist in long term changes to neuron
  • changes happen in synapse
  • Donald Hebb proposed;
  • neurons that fire tg, wire tg
  • everytime the two connected neurons fir tg, the synapses between them gets stronger
  • learning consists of strengthing and weakening synapse
  • associations are strong (easily excitable) synapses
• Long-term potentiation

  • after repeated bursts of stimulation, synapses become easier to excite
  • mostly occur in glutamate synapses (excitatory)
  • 2 types of glutamate receptors
  • AMPA; ionotropic, allows Na+ in
  • NMDA; ionotropic, allows Na+, Ca+ in, blocked by Mg+
  • step 1;
  • glutamate released
  • AMPA-R open, Na+ enters
  • NMDA-R blocked
• LTP induction

  • Step 2:
  • strong stimulation (from 2 neurons)
  • lots of glutamate
  • AMPA-R opens, Na+ enters
  • Na+ depolarizes cells
  • depolarization removes Mg+ from NMDA-R
  • NMDA-R opens, Ca+ (and more Na+) enters
  • Ca+ works inside cells
  • step 3:
  • Ca+ activates CaMKll
  • CaMKll activated CREB
  • CREB regulates gene expression
• LTP maintenance

  • Step 4:
  • CREB changes the expression of genes which:
  • increase the number of NMDA and AMPA receptors at that dendrite
  • AMPA receptors become easy to open
  • more dendrites form onto the same axon branch
  • postsynaptic cell will receive more EPSPs from the same presynaptic neuron
  • retrograde messages change the presynaptic terminal
  • cell is more likely to fire
  • release more transmitter (glutamate) per event
• Inhibition

  • If Synapses jus had the LTP, they would all get stronger and stronger
  • need a way to also inhibit firing
  • long-term depression (LTD)
  • involves excitatory glutamate receptors (AMPA, NMDA)
  • Occurs in hippocampus and cerebellum
  • results from low-frequency repeated stimulation
  • cell reduces density of AMPA-R and phosphorylates them
• Navigation

  • animals and humans need to find their way to food, work, back home...
  • need a cognitive map
  • the map must allow:
  • navigation between any two points (shortcuts)
  • using diff landmarks
  • the map needs to be:
  • unified; all the info in one representation
  • allocentric; not dependent on where I am
  • flexible; allowing calculation of vectors between points
  • spatial info is relational
  • my house in 3 blocks north of the church
  • the map is in the hippocampus
• Hippocampus connection

  1. sensory cortex -> entorhinal cortex
  2. entorhinal cortex -> dentate gyrus
  3. dentate gyrus -> CA3
  4. CA3 -> CA1
  5. CA1 -> subiculum
• Cells of HF

  • Cells in HF represent aspects of the environment
  • place cells (CA1); always firing at the same place
  • head direction cell (EC); fire when facing a direction
  • boundary cells (EC & Sub.); fire when close to walls
  • Grid cells (EC); fire on grid
• Remapping

  • when in a new space, need new map
  • cells can remap when the space changes
  • completely, partially, not at all, rate, stop firing
  • new places, grid alignment
• Coordination

  • events in the brain need to be coordinated
  • firing of cells related to CS and US
  • brain has global rhythms of subthreshold stimulation
  • generated by trisynaptic circuit
  • abt 3-10 Hz
  • rhythm raises and lowers membrane potential
  • like an EPSP for all cells at once
  • makes it easier or harder to fire
  • causes firing to synchronize
• Summary

  • all diff cells, coordinated, create a representation of the world
  • places, directions, object, time
  • can also focus on task (leaving, returning)
  • we use this representation
  • to navigate
  • to remember
  • to learn