Lecture 16: Learning and Memory l Flashcards
learning
the process by which experiences change our nervous system and hence our behaviour
memories
changes in our nervous system that occur as a result of learning.
Can be transient or durable, explicit or implicit, personal or impersonal
memory retrieval
Accessing memories
neuronal plasticity
the ability of the nervous system to change and adapt
what is the cellular basis of long-term memory?
neuronal plasticity
what do researchers use to identify neuronal plasticity?
intrinsic excitability & synaptic strength
intrinsic excitability
the number of action potentials a neuron exhibits in response to an influx of positive current
synaptic strength
the amount of positive (or negative) current that enters the postsynaptic neuron when a presynaptic cell has an action potential.
synaptic plasticity
A change in the strength of the synaptic connection between two neurons
how is intrinsic excitability determined?
by the number and type of ion channels (leak channels and voltage-gated channels) expressed by the neuron
what happens if a neutron starts making fewer potassium leak channels?
its resting membrane potential will be slightly depolarized, which means the neuron will be more excitable in general (will exhibit more action potentials)
Long term potentiation (LTP)/ Long term depression (LTD)
enduring changes in synaptic strength
main difference between LTP & LTD
LTP makes the synapse stronger; LTD makes the synapse weaker
EPSPs
Membrane depolarizations that are driven by neurotransmitter release and postsynaptic receptor activation.
presynaptic plasticity changes
the amount of voltage-gated calcium channels on the presynaptic membrane influences how many vesicles will be released following an action potential.
postsynaptic plasticity changes
the amount of neurotransmitter receptors influences the sensitivity of the postsynaptic cell to neurotransmitter.
two types of synaptic plasticity changes
presynaptic and postsynaptic
habituation
reduced physiological or behavioural responding to a repeated stimulus.
sensitization
Increased sensitivity to a stimulus
apylsia
an invertebrate sea slug with a simple nervous system. It has a large gill for respiration, and a siphon through which it expels water.
habituation in apylsia
Repeated light touching of the siphon will reduce the magnitude of the reflex until the Aplysia completely ignores this stimulus.
sensitization in apylsia
the sea slug’s response to an electrical shock often becomes greater with additional exposures.
does the sensory neuron of aplysias become less sensitive to the touch?
No, it depolarizes the same amount
Has the excitability of the sensory neuron changed in apylsias with the gill withdrawal reflex?
Yes, fewer action potentials (1 vs 2) occur when the siphon is touched
Has the synaptic connection weakened between the sensory and motor neurons as a result of the gill withdrawal reflex?
yes
Has the motor neuron become less excitable as a result of the gill withdrawal reflex?
No, it spikes the same amount when depolarized.
Has the synaptic connection weakened between the motor neuron and gill as a result of the gill withdrawal reflex?
No, the gill is as sensitive to an action potential in the motor neuron as before.
how are cell excitability and synaptic strength measured?
through brain slice recordings
structure of neural networks
number of nodes and layers as well as how each node gets activated
objective function of neural networks
the goal; for example, to label things in the input or to predict what the next input will be or to identify the best action given the input
learning function of neural networks
method of adjusting the strength of each connection to better achieve the objective function
Long-term potentiation (LTP)
Long-term increase in the strength of the connection between two neurons (i.e., increased synaptic strength).
how is LTP induced
Repeated high-frequency (tetanic) stimulation of the inputs to a neuron
where is LTP induced
LTP is often initiated on the postsynaptic side (with more neurotransmitter receptors) but retrograde signaling of nitric oxide (NO) can drive presynaptic modifications (e.g., more vesicles of neurotransmitters).
Long-term depression (LTD)
Long-term decrease in the strength of the connection between two neurons (i.e., decreased synaptic strength).
how is LTD induced
Persistent low-frequency stimulation of the inputs to a quiet neuron
where is LTD induced
often initiated on the postsynaptic side (with less neurotransmitter receptors) but retrograde endocannabinoid signaling can drive presynaptic modifications (e.g., less calcium-influx per action potential).
LTP and LTD are a function of ____
the number of times the synapse was activated as well as whether the postsynaptic neuron fired at those precise times
what MUST happen for LTP to occur?
the release of neurotransmitter must coincide with a substantial depolarization of the postsynaptic cell
what happens when there is high frequency axon stimulation
often causes postsynaptic neurons to spike (summation of EPSPs brings the neuron across threshold).
what happens where there is low frequency axon stimulation
often not sufficient to get a postsynaptic neuron to spike.
NMDA receptor
Ionotropic glutamate receptor with a large pore in the centre
NMDA receptors are important for ___
learning and memory
what happens when NMDA receptors bind to glutamate and open
magnesium ions (Mg2+) try to pass through its pore, but they get stuck in it and block all current flow. The Mg2+ blockage only occurs when the membrane potential is below threshold (< -40mV), such as when the cell is at rest.
what happens when the receptors of NMDA are depolarized?
Mg2+ ions will not try to enter though the NMDA receptor, and thus they won’t clog the pore.
what gates NMDA current flow?
both glutamate and membrane voltage.
when do Na and Ca enter the cell through NMDA receptors
only when these receptors are bound to glutamate and Mg2+ is not clogging the pore.
AMPA receptor
the glutamate receptor that mediates most excitatory fast synaptic currents in the brain. It is ionotropic and opens upon glutamate binding. It lets in sodium ions which causes EPSPs that depolarizes the neurons
CaMKII (Type II calcium-calmodulin kinase)
an enzyme activated by calcium influx through NMDA receptors. It plays a role in the intracellular signaling cascade that establishes long-term potentiation, by increasing the number of postsynaptic AMPA receptors (in excitatory glutamatergic synapses).
where is LTP initiated
usually on the postsynaptic side
role of nitric oxide in long-term potentiation
act as a retrograde messenger (released from postsynaptic membrane and detected by presynaptic membrane) to promote LTP.
associative long-term potentiation
The increase in synaptic strength that occurs in weak synapses when they are active right around the time when stronger inputs cause the postsynaptic neuron to spike.
what happens if the activity of strong synapses is sufficient to trigger an action potential
the dendritic spike will depolarize the membrane of dendritic spines, priming NMDA receptors so that any weak synapses active at that time will become strengthened
what happens if the weak and strong stimulus are applied at the same time
the synapses activated by the weak stimulus will be strengthened
Hebb’s rule
The cellular basis of learning involves the strengthening of synaptic connections that are active when the postsynaptic neuron fires an action potential. (fire together, wire together)
perceptual learning
Learning to recognize stimuli as distinct entities.
motor learning
Learning to make skilled, choreographed movements. Procedural learning.
relational learning
Learning relationships among individual stimuli. Stimulus-Stimulus learning.
stimulus-response learning
Learning to perform a particular behavior when a particular stimulus is present. Includes classical and instrumental conditioning.
Information processing model
Encoding (learning) -> consolidation (memory) -> storage (memory) -> retrieval (memory)
what part of the brain is involved in perceptual learning?
Largely dependent on the neocortex & sensory association areas
what part of the brain is inovlved in motor learning?
Involves different brain areas involved in movement
what part of the brain is involved in relational learning?
Largely dependent on the hippocampus and neocortex
what part of the brain is involved in stimulus-response learning?
Involves different brain areas depending on the stimulus and response
