Lecture 35: Cortical Circuits 2 Flashcards
What does not significantly increase with age? What is the implication of this?
the number of neurons in the brain
learning and memories are generally not thought to be due to the formation of new neurons
What is synaptic plasticity?
the change that occurs at synapses that enables them to alter the strength of communication between neurons
What is Hebb’s rule?
neurons that fire together wire together
crucial to understanding the cellular basis of learning and memory
What happens when axon A is near enough to excite cell B and repeatedly or persistently takes part in firing it?
some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased
What occurs during long-term potentiation?
synaptic connections become stronger (ie synaptic strength increases)
What happens during long-term depression?
synaptic connections become weaker (ie synaptic strength decreases)
What is the role of spike timing dependent plasticity?
adjusts the strength of connections between neurons according to the timing of the synaptic input
What are some examples of spike timing dependent plasticity?
if A fires before B, then the response is potentiated resulting in LTP
if B fires before A, then the response is depressed resulting in LTD
What time scale does spike timing dependent plasticity operate on?
on a millisecond time scale in vitro
e.g. if A fires less than 50ms before B, then LTP occurs and if B fires less than 50ms before A, then LTD occurs
What time scale does synaptic potentiation during behaviour occur on?
longer time scales than previously recorded in the brain slice
What can plasticity occur in response to?
synaptic input that arrives seconds before and after cell firing e.g. if cell A fires within 2 seconds of cell B
Where has behavioural timescale synaptic plasticity been illustrated?
only in the hippocampus
What are the forms of synaptic plasticity?
the strength of existing synaptic connections changes
dendritic spines may change morphologically by either growing or shrinking
What does synaptic input from synaptically coupled neurons generate?
a synaptic potential (EPSP)
What does the EPSP depend on?
many factors, including receptor selectivity and density
What can the amplitude of the EPSP undergo?
LTP or LTD (because it is plastic)
What occurs following pairing of pre-synaptic extracellular stimulation with a post-synaptic action potential?
the synaptic response (EPSP) in the postsynaptic neuron is increased from baseline (LTP)
How can the morphology of post-synaptic spines change?
can change in size following certain patterns of synaptic input (because they are plastic)
How can new connections (spines) be formed?
instantly
spines formed in response to the release of glutamate (via uncaging) near a dendritic branch
How often does a new spine rapidly form following glutamate uncaging?
in ~10% attempts
What can structural plasticity rapidly generate?
new connections in a dendritic location where there wasn’t a spine previously
Why are there only a handful of direct examples of plasticity during learning in vivo?
this is due to the difficulty in recording from small structures (spines, axons) in vivo
What has the advent of advanced experimental techniques enabled?
plasticity and learning to be measured in vivo -> namely two-photon microscopy and genetic fluorescence indicators
How does two-photon microscopy work?
two-photon excitation arises from the simultaneous absorption of 2 photons by a fluorophore in a single event -> enables fluorescence imaging to be performed deep into tissue
What alters the morphology of spines in the barrel (vibrissae) cortex?
altering synaptic input to spines (via sensory deprivation)
When does the number of new spines formed daily increase?
during motor learning
What does learning increase?
new connections between neurons
When does the synaptic activity of cortical neurons change?
following learning of sensory-association
How can plasticity of the synaptic response to sensory input be measured?
by measuring the change in calcium between pre-(naïve) to post-(expert) learning
What is modulated following learning in vivo?
synaptic strength
What is the hippocampus thought to drive?
long-term memory formation and retrieval
What is the cortex known to be a major player in?
working memory, especially the prefrontal cortex
What is working memory?
the temporary holding of information
enables us to dynamically change and manipulate stored information
What are examples of working memory?
following instructions, responding in conversations, remembering phone numbers
How can working memory be tested?
using a ’delay’ task where working memory must be used during a delay imposed between the stimulus and response
What are the benefits of a behavioural ‘delay’ task?
the ALM has increased action potentials during the delay/working memory period
