Exam 3: Learning and Memory

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Synaptic Plasticity

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Synaptic Plasticity

100 trillion snapses

  • can be weak or strong

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synaptic connectivity constantly changes in response

to activity and other factors

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During development: synaptic connections

provides the basic “wiring” of the brain’s circuits

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Throughout rest of life: synaptic connections

basis of “learning

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Synaptic plasticity

may last from milliseconds to years

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•short-term plasticity

change in the amount of NT released by the presynaptic neuron in response to an AP

  • ability to change it's funciton •related to altered Ca++ levels in the axon terminal

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• facilitation, augmentation, and potentiation

all forms of enhanced NT release -the neuron is releasing more NT than it would

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• depression

a decrease in NT release •related to a decrease in the available NT vesicles

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long-term plasticity

(~30 min to lifetime) -Long-term potentiation (LTP) / Long-term depression

  • initially caused by post-translational modifications of existing proteins • e.g., upregulating glutamate receptors

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long-term plasticity later sustained by.....

changes in gene expression creating new proteins

  • • physical changes, including growth of new synapses etc (“structural”)

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Learning

the process of acquiring new information

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Memory

the process by which we store and retrieve / recall information

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3 types of memory

sensory / short-term / long-term

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3 processes of memory

encoding, storage, retrieval

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• Encoding / Consolidation:

information from the environment is transduced into neural “codes” within the brain (sensation)

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• Storage

retaining the information over time

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• Retrieval

pulling the information out of storage

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Encoding process

is a big mass of “interconnected” biological tissue • parallel and serial networks within massively parallel networks • sensations cause changes in spatiotemporal activity patterns of the cortex – we experience this as our “perception” of reality

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Neurons and other areas that are active at the same time tend

to become more strongly connected (and vice versa)

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Donald Hebb

"fire together / wire together” via functional and structural changes -• activity in 1 area is more likely to induce activity in connected areas

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• Encoding/ Consolidation

transduction into neural code (spatiotemporal patterns of cortical activity)

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Storage (biological basis or learning) -

long-term synaptic changes in cortical connections -• the biological basis of learning depends upon the ability of neurons to modify their synaptic connections within neural circuits based on experience

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Storage involves the retention of....

Involves the retention of “information” through networks of associations –Connections between cortical areas –Auto-associative neural networks

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Retrieval

• Gaining access to and being able to use information stored in memory • Reactivating cortical patterns

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• Retrieval (memory) happens when

we re-experience similar spatiotemporal patterns in cortical activity

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• Retrieval cue

any stimulus that is used to activate retrieval • Multiple cues work best • State (internal) and context (external) are important cues

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Retrieval

• a neural network can process large amounts of information with only a small group of viable neurons • will yield output based on partial input • Many stimuli will induce memory of a person • One stimuli will elicit many memories

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• Sensory (immediate)

artifact of sensory processing

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• Short-term / working memory

functional changes (reverberating circuits, more / less NT)

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• Long-term

physical changes (more / less terminals and dendrites) -• a behavioral continuum, but with distinct physiological processes

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Sensory (Immediate) memory

• Holds incoming sensory information for very brief periods of time (seconds or less) • Artifact of sensory processing and the elicited patterns of cortical activity –“iconic” = visual sensory register –“echoic” = auditory sensory register

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iconic

visual sensory register

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echoic

auditory sensory register

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Characteristics of Short term memory

duration, capacity

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• Duration

• Duration = information only remains in STM for a short period of time (seconds to minutes)

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Capacity

amount of information that can be held in STM is limited -Depends on the type of stimulus and the number of meaningful units, called chunks • ~5-7 chunks of info

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Working memory (a type of STM)

• Small amounts of information held in memory for short periods of time – until distracted

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Visual encoding (Working memory)

an image is formed in the mind • Activity in visual cortex

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Phonological encoding (working memory)

auditory; based on sound • Activity in auditory cortex

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Working memory

– higher order cognition - math etc • we can be consciously aware of the information, actively “keeping” it in STM (“working memory”)

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working memory components

central executive, visual spatial memory, auditory memory

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• “Central executive

Prefrontal cortex is directly connected to sensory areas, controlling these processes and directing activity within the brain

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– Visual-spatial memory:

visuo-spatial sketchpad” involves forming mental images.

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Auditory memory

phonological loop” involves repeating information to yourself

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long term memory

Vast “library” of stored information and memories (patterns of cortical activity) •Unlimited capacity and duration •distinct cellular processes from STM

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ways to get info into the LTM

Exposure and Rehearsal -Simple exposure to information, even many times, does not guarantee that it will get stored in LTM

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Levels of Processing

Information that is processed more deeply will be remembered more easily later on

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Declarative (Explicit) memories

relational knowledge –“explicit” - conscious, intentional memory retrieval –Divided into episodic and semantic memory.

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Episodic memory (Declarative)

where and when info was learned

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Semantic memory (Declarative)

general knowledge

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1 trial learning

  • Declarative memory

  • something you can remember for the rest of your life a single event remembered forever

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Procedural (implicit) memory (LTM)

skills and actions –“implicit” – memory can influence behavior without conscious awareness • - generally requires several trials to acquire

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Forgetting.. this that can cause

  • Encoding failure –Decay theory - not accessing on a regular basis –Interference –Motivated forgetting
 “Active” forgetting

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Retrograde amnesia

memory loss for events prior to onset.

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anterograde amnesia

memory loss for events after onset.

  • inability to learn new things after injury

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infantile amnesia

loss of childhood memories before the age of 2 or 3 years

  • hard time remembering things before the hippocampus is wired up

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• Neurons that fire together wire together (Hebb)

Multiple reactivation of spatiotemporal cortical patterns cause neuroplastic (functional / structural) changes to reinforce these patterns

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• Different types of info are encoded by

different brain regions / processes:

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procedural” info encoded by....

direct, multiple reactivations of sensory cortical patterns • along w/ basal ganglia & cerebellum

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declarative info encoded by

by the sensory cortical patterns + hippocampus • “emotional” info from a declarative memory by the amygdala

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the hippocampal formation

can be thought of as the highest level of association cortex -receives convergent inputs from all sensory areas

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Hippocampus in Humans

S-shaped curved structure in the medial temporal lobe

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Hippo campus in rodents

follows the curve of the lateral ventricle (more dorsal)

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Areas of the cortex incluse

cortex, dentrate gyrus, ammon's horn (areas CA1-CA4), subiculum/presubiculum/parasubiculum, output pathways frorm the fimbria-forniz

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Job of the Hippocampus

• inputs from the neocortex alter its circuitry to set up an “index” of cortical locations (long-term synaptic change) for the combined perceptual patterns that may make up a given memory

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Role of the Hippocampus

• role of hippocampus is to initially “tie together” specific stimuli (“relationships” / context)

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just like procedural memories, consolidation of perceptual associations into long-term memories in the cortex requires....

requires multiple activations -repeated or continual activation of a particular hippocampal index gradually strengthens the associations of cortical areas making up a memory

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consolidation of perceptual associations in the hippocampus

probably takes place during sleep - rapid, looped replay -hippocampus is relaying the experiences -relaying the day you had over and over again

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eventually, the cortex can generate the appropriate spatiotemporal patterns....

generate a memory without the hippocampus

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bilateral hippocampal damage....

disrupts learning, memory formation and retrieval of recent “declarative” memories -severe / global anterograde amnesia, presumably for the rest of the individuals life • temporally graded retrograde amnesia, at some point they can recall stuff

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what is spared with damage to the hippocampus

remote memories • immediate memory • perceptual / motor / cognitive functions (“procedural” memories) • may not remember learning the task, but when prodded to continue, they can perform at normal levels

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Lateralized hippocampal damage: left

verbal material such as story content

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lateralized hippocampal damage: righ

spatial tasks (localization of objects), facial recognition

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memories older than about three years are not affected by hippocampectomy because....

the hippocampus is not necessary for retrieval or storage of older memories -those patterns have been activated enough that they don't need the hippocampus

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Eric Kandel’s research with Aplysia shows...

  • learning is a physiological process -gill withdrawal reflex (“reflex arc”) • siphon is an organ that takes in sea water • lightly brushing siphon causes gills to withdraw

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snail model of habituation (Eric Kandel)

-40 BIG sensory neurons in siphon > 6 motor neurons in gill muscles (excitatory / glu)

  • plus some excitatory / inhibitory interneurons

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Kandel notices in the snail model of habituation

  • continued stimulation causes this withdrawal reflex to habituate (response stops); demonstrates learning by stopping doing stuff

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how is the animal learning that it does not need to be concerned?

-stimulation still produces APs in sensory neurons -BUT synaptic structures responsible for vesicle release are inactivated

  • mobilization of transmitter vesicles is decreased -voltage dependent Ca++ channels become inactivated with repeated stimuli -• each sensory neuron AP releases fewer NT vesicles

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decrease in response is learning

produces smaller EPSP in motor neurons & interneurons (functional change) • less likelihood of generating APs (and therefore gill withdrawal)

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transient functional change in neural circuit =

short term memory

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doing habituation repeatedly results

n a structural change of the axon terminals • some terminals totally retract

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long term memory

• change in protein synthesis mediated by genes that maintain axon terminal structures loss of input causes postsynaptic dendritic arbor to shrink • reduced numbers of synaptic connections

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forgetting

after several weeks of no stimulus, terminals start to grow back and system returns to baseline

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sensitization

an increase in reflex magnitude -shock tail > brush siphon > stronger gill -involves same set of neurons with addition of modulatory interneurons from tail (more complex)

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sensitization results in

release of more excitatory transmitter from sensory neurons onto the motor neurons (opposite of habituation)

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Serotonin (aka 5-HT) from facilitating interneurons

binds to receptors on siphon sensory neuron axon terminals and creates changes inside the neuron that increase release of NT vesicles

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At first: transient functional change (STM)

• repeated initiates a cascade of intracellular enzymatic events -CREB activates genes to synthesize proteins • neurotrophic factors (BDNF, NGF, etc )

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more NT and/or receptors

structural changes - more axon terminal branches, neural connections, etc. (LTM)

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for habituation and sensitizations -

LONG-TERM MEMORY REQUIRES MAKING NEW PROTEINS (prevented by drugs that inhibit protein synthesis)

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Back to human model - tri-synaptic pathway

essentially a one-way avenue through the hippocampus • widespread cortical inputs and outputs • hippocampal circuits set up an “index” of activity patterns that initially “assists” in the cross-cortical association process

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hippocampal circuits set up an “index” of activity patterns

that initially “assists” in the cross-cortical association process; now you can start to recall things easier and easier

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How to study the hippocampus and it' s changes

placing the hippocampus in a Petri dish and electrically stimulating axons in the pathway will produce EPSPs in the CA1 pyramidal neurons (output cells)

  • a high frequency (tetanic) burst of stimulation produces a long-lasting increase in EPSPs to subsequent, “normal” stimuli

  • so, efficacy of synapse is enhanced (“potentiated”) by a single “experience

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long term potentiation

the functional and structure of the neuronal synapses are changing in response to the EPSPS

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LTP in general

occurs between neurons that use the “excitatory” amino acid glutamate (glu) • several types of postsynaptic glu receptors: • N-methyl-d-aspartate (NMDA) receptor-channels • non-NMDA

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NMDA-type receptors

open ion channels BUT they are usually blocked by Mg++, so they don’t participate in EPSPs • NMDA receptors are “doubly-gated” ion channels (“associative”)

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To pop Mg++ pug out and open Glutamate channel requires

requires EPSP of a high enough level allowing Ca++ (among other ions) into the dendrite • requires both NT and voltage (EPSP)

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to avoid "saturation"

the opposing mechanism also exists (LTD)

  • synaptic strength being depressed/ weaken

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