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
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)
• 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