learning/memory behavioral neuro pt.2

Synaptic plasticity

ability of a synapse between 2 neurons to change in strength

Hebbian plasticity

Cells that fire together, wire together

(General principle underlying LTP)

Aplysia as a model system for studying plasticity

-Simple nervous system

-Easily charcterized/studied behavioral reflexes can be modified through learning

Habituation

decrease in behavioral response to a stimulus after repeated exposure to that stimulus over-time

Habituation of the gill-reflex

Touching or applying water pressure to the siphon of Aplysia results in activation of
the gill withdrawal response

-After multiple rounds of siphon stimulation, the gill-withdrawl response weakens

Sensitization

Non-associative learning type, where behavioral responses are amplified in reaction to repeated stimulus

Sensitization of gill-withdrawl

After habituation, pairing a noxious/electrical stimulus to the tail while applying
pressure to the siphon reinstates the gill withdrawal reflex and increases the
strength of the response compared to baseline

-This sensitization is long-lasting; would take alot longer for the response to habituate again

Post-synaptic mechanisms

change in number or conductance of post-synaptic receptors, activation or inactivation of ion channels

Pre-synaptic mechanisms

Change in Ca2+ influx to the synaptic terminal which would change the amount of NT release

Seratonin in gillwithdrawl response

Seratonin(5H-T) release from L29 neuron (neuron that recieves electrical stimulus applied to tail) leads to changes in the synaptic terminal of the sensory neuron

What does 5-HT/Seratonin act at?

A GPCR on the sensory neuron, activates second messenger cascades —> depolarization of the sensory cell, more NT released onto MN —> sensitization of the gillwithdrawl response

Long-term sensization

can be elicted by applying multiple shocks (noxious stimuli) to the tail rather than just 1, the senitizing effect is long lasting (weeks)

CREB acts as..

a transcription factor, promoting expression of certain genes

WHat does CREB lead to

further increase in PKA activity, activates C/EBP (another transcription factor) that promotes growth of new synapses

What brain regions are important for CC?

Cerebellum and the amygdala

Pavlovian conditioning/associative conditioning

pairing with a conditioned stimulus or CS (bell tone) with an unconditioned stimulus (meat) so that eventually the animal responds to the CS with the response intrinsic to the US (salivation) this is the CR

Associative learning/CC in Aplysia

In aplysia, pairing an air/water puff to the siphon with a tail or headshock, here the timing of stimuli presentation and pairings is what distinguishes conditioning from sensitization,

-The CS is soecific, so only innocous pressure to the siphon but not the head/tail will elict the response

What happens if you inhibit protien synthesis in animals trained in classical conditioning or a maze

No impairment in learning/memory 1 hr after training

Impairments evident if you test animals 24 hr after training

Long-term potentiation

Rapid onset, activity dependent, long-lasting change in synaptic strength, can be elicted in the hippocampus and to some extent in cortex.

LTP procedure

induced by high frequency stimulation (100Hz tetanus), transverse slices of hippocampus bathe in acritical cerebrospinal fluid

LTP what is stimulated?

Schaffer collaterals (projections from CA3) and record externally from CA1 field (population or field potential)

When stimulating schaffer collaterals…

See a negative response in extracellular recording of CA1 population

With stronger input/increase in tetanus number…

you see an enhancement of response (early/late LTP)

Mechanisms of LTP

NMDA receptors act as coincidence detector, expressed on the CA1 neurons

What do NMDA receptors do in LTP

Allow Ca2+ influx when pre/post synaptic cells are activated simultaneously

(coincidence of glutamate release/depolarization)

What does Ca2+ activate in LTP?

kinases, short and long term effects leading to potentiation of the synapse

Short-term

covalent modifications of pre-existing protiens

(phosphorylation of channels/receptors)

Long-term

CREB, nuclear transcription, changes in protein synthesis

AMPA receptor cycling in LTP

AMPA receptors are inserted into post-synaptic membrane

AMPA receptor cycling in LTD

Decrease of AMPA receptors into post-synaptic membrane

Where is polymodal information from association cortex sent?

To the entorhinal cortex and then to the hippocampus

Superficial layer of entorhinal cortex

projects to hippocampus, hippocampus projects back to deep layers of entorhinal cortex, and then back to superfical layers

Pattern seperation

Disassociation of similar scenes thought to occur at dentate gyrus

Pattern completion

memory retrival with pattern completion, occurs at CA3 level, due to recurrent connections between CA3 neurons.

CA1 monitor processing

recieves input processed by the indirect pathway and non-processed information directly from entrohinal cortex

Indirect pathway of CA1 monitoring

Denrate gyrus and CA3

Proximal dendrites recieve input from

DG/CA3

Distal dendrites recieve input from

entrohinal cortex

-Allows CA1 to act as a comparator

Hippocampal place cells

discovered through in vivo recordings in awake behaving rodents

Cordination of popilations of place cells…

Make up cognitive maps of the enviroment

Some cells show

metric/scaled representations of the enviroment

Hippocampus

critical for consolidation of memories, forming new long-term memories

Declaritive/spatial memory use..

The hippocampus

You can’t form new memories without…

the hippocampus

Cortex role in memory storage

storage of long-tetm memories; damage to certain cortical areas can cause retrograde amnesia

Striatum role in memory

Important for procedual memory; the striatum is apart of the basal ganglia

What is the striatum composed of?

Caudate nucleus/Putamen

If striatum is damaged how does that affect learning of tasks/skills?

Patient may remeber having learned the task before, but they will never improve on it

Cerebellum role in memory

regulating skeletal muscles, so it’s critical in classical conditioning that involves muscle responses

Patient H.M

bilateral exision of medial temporal lobes (removing hippocampus)

-Anterograde amnesia

-Procedual memory intact

Tracing shapes

procedual memory requiring the striatum

Morris water maze

Hippocampal dependent learning/memory (spatial naviagtion task)

Radial arm maze

spatial cues —> hippocamus dependent

light stimulus —> striatum dependent associative learning/memory