plasticity and learning Flashcards
recollection of ‘memory’: Wilder Penfield
- 1950s: Wilder Penfield
- A neurosurgeon mapping motor, sensory, language
functions of cerebral cortex for patients undergoing surgery
for epilepsy. - On rare occasions he found that electrical stimulation of the
‘temporal lobes’ produced an ‘experiental response’ – a
coherent recollection of an earlier experience - “…a mother told me she was suddenly aware, as my
electrode touched the cortex, of being in the kitchen
listening to the voice of her little boy who was playing
outside in the yard.
classifying learning and memory: the case of H.M.
- Henry Gustav Molaison
- Recognized as one of the most important patients in
the history of brain science. - 27 yr-old man who suffered from untreatable
seizures - Medial temporal lobes were removed bilaterally
- After surgery, seizures reduced but also major
deficits to memory systems - He could still learn new ‘skills’
- Good long-term memory for events prior to
surgery - Good command of language
- Good short-term memory (good for conversation….
but lasted <1 min). - No conversion of short to long term memory
- “….at this moment everything looks clear to me,
but what happened just before? That’s what
worries me. It’s like waking from a dream; I just
don’t remember.”
state the 2 classes of memory
- Non-declarative memory (implicity memory) - ‘skills’
- a memory that is recalled unconsciously
- training of reflexive, motor or perceptual skills
- neocortex, striatum, amygdala, cerebellum, reflex pathways - Declarative memory (explicit memory) - ‘knowledge’
- memory about objects, people, places, events, and what that info means
- highly flexible - bringing together associations between multiple elements
- medial temporal lobe
define plasticity
the capacity to change neuronal connections in an experience-dependent manner
how can we examine plasticity changes during learning
simple model systems
- flies, bees, octopus, and molluscs - aplysia sea slugs
simple forms of implicit memory
- learning about relationship between 2 stimuli or a stimulus and behavior - associative
- learning about properties of a single stimulus - non associative (habituation or sensitisation)
example of habituation
def: animal learns that the properties of a novel stimulus are harmless
- aplysia - gill is withdrawn in response to a tactile stimulus to the siphon (‘gill withdrawal reflex’)
short term habituation in aplysia
- lasts a few minutes
cellular basis - reduced number of synaptic vesicles released (fewer functional vesicles)
long term habituation in aplysia
- lasts up to three weeks
- cellular basis - structural change: fewer synapses (~1300 to ~700)
example of sensitisation
def: animal learns to respond more vigorously to a harmful stimulus and also to other harmless ones
aplysia - gill withdrawal response can be sensitised (heightened) by an electrical tail shock
short term sesnitisation in aplysia
- 1 tail shock
- enhanced gill withdrawal lasts several minutes
- more vesicles released to synapse
long term sensitisation in aplysia
- 5+ tail shocks
- enhanced gill withdrawal lasts days to weeks
- structural change of more synapses added from snesory neuron to motor (~1300 to ~2800)
declarative/explicit memory
- knowledge
- Memory about objects, people, places and
events. - Brings together associations between multiple
elements. - HM showed demonstrated the importance of
medial temporal lobe. - Lesion studies of monkeys localized the deficit in
declarative memory to damage to the
hippocampus.
long term potentiation in hippocampus
- Timothy Bliss (1973) demonstrated that these pathways are very sensitive to their previous activity history
- it is the strong simultaneous (coincident) activation of the presynaptic and postsynaptic neuron suring the tetanus that leads to ltp
- this is the basis of the associative nature of long term potentiation
define tetanus
a sustained and strong muscle contraction that occurs when nerve impulses stimulate the muscle fibers at a very high frequency
nmda receptor in LTP (n-methyl-d-asperate)
- a specific type of ionotropic glutamate receptor
- its biophysical properties provide a means to explain ltp induction
define LTP
a long-lasting increase in the strength of synaptic connections between neurons
CA3-CA1 LTP depends on NMDA-receptors
to allow ca2+ to enter the postsynaptic neuron, two conditions must be met:
1. the presynaptic neuron must be active and release glutamate
2. the postsynaptic neuron must be depolarised to remove mg2+ from the nmda pore
nmda acts likea coincidence detector - in the post synaptic terminal (CA1)
thus it is both glutamate-dependent and voltage-dependent
summarise LTP
- LTP is a mechanism thought to underlie
the encoding of learned associations - Relies on coincident activation of the
pre- and postsynaptic neurons - Exhibits specificity and associativity (see
previous slide) - Most extensively studied in CA3->CA1
glutamatergic connections in the
hippocampus - Encoding of memory (learning) involves
the strengthening of synaptic
connections
summarise nmda
- The NMDA receptor in the postsynaptic
terminal is the coincidence sensor - Its activity is both glutamate- and voltage-
dependent (via the Mg2+ block) - Therefore, it only allows Ca2+ to flow into the
postsynaptic terminal when there is strong
pre- and post synaptic activity - Ca2+ influx triggers a molecular cascade which
improves transmission by:
1. Inserting new AMPA receptors (a second
type of ionotropic receptor) into the
postsynaptic terminal.
2. Improving presynaptic vesicle release.
3. Building new synapses
define specificity
potentiation is specific to active synapses
