lecture 14: synaptic plasticity Flashcards
what is synaptic plasticity
- a from of neuromodulation
- defined as the activity-dependent modification of the strength (or efficacy) of synaptic transmission
- short or long-lasting
- underpins learning and memory
examples of invertebrate models
- drosophila melanogaster
- aplysia californica
advantages of drosophila melanogaster
- small genome and rapid life cycle (drosophila is good for genetics)
- small nervous system (135,000 neurons)
- easy to identify circuits
advantages of aplysia californica
- small nervous system (20,000 neurons)
- large neurons (easy to impale and record from)
- easy to identify individual neurons and circuits
what was Eric Kandels discovery
- that short term memory results from a transient strengthening of pre-existing synaptic connections, due to the modification of pre existing proteins
- long term memory results from a persistent strengthening of synaptic connections brought about by alterations in gene expression, the synthesis of new proteins, and the growth of new synaptic connections
habituation
learning to ignore a stimulus that lacks meaning
- weakened neurotransmission
- decreased neurotransmitter release from sensory neuron
sensitization
enhanced response to stimuli
how does aplysia californica model learning and memory
through the siphon-gill withdrawal reflex
- tactile stimulus to siphon causes gill withdrawal
- this reflex undergoes habituation or sensitization (a simple form of learning) when a noxious stimulus is applied
tactile stimulus
a non-painful touch or pressure stimulus
noxious stimulus
a potentially damaging stimulus that may cause pain
difference between if the siphon is gently touched vs touched and then given an electrical shock
- when the siphon is gently touched = withdraws its gills for a brief period
- but if the “touch” is preceded by an electrical shock to its tail, the same gentle touch to the siphon = a longer period of withdrawal
- multiple shocks = its subsequent response to a gentle touch on the siphon is much larger and is retained longer
sensitisation to a noxious stimulus delivered to the tail requires:
- a sensory neuron that picks up the stimulus from the tail connecting to
- interneurons that terminate on the sensory neuron in the siphon-gill pathway
facilitation
increased synaptic activity which sensitisation depends on
summary of presynaptic facilitation
- stimuli results in increased sensitivity of the gill-withdrawal response
- noxious stimuli activate interneuron L29
- this enhances neurotransmitter release from the sensory neuron presynaptic terminal (short lasting change)
but with sufficient stimuli can also - alter gene expression (long lasting change)
role of L29 in short lasting change (presynaptic facilitation)
- L29 releases 5HT (serotonin)
- activates 5HT receptors on presynaptic nerve terminal
- stimulates formation of cAMP
- activates protein kinase A (PKA)
–> release of neurotransmitter
role of protein kinase A (PKA) in short lasting change (presynaptic facilitation)
- increases phosphorylation of K+ channels
–> phosphorylated K+ channels close
–> K+ does not exit terminal as rapidly
–> prolonging the action potential (limits repolarisation)
–> ca2+ channels increase their opening - increased mobilisation of synaptic vesicles
- increases phosphorylation of ca2+ channels
–> voltage-sensitive ca2+ channels open
–> greater ca2+ influx increases release of neurotransmitter - stronger signal to motor neuron
- enhanced gill withdrawal
number of tail shocks for short and long lasting change
short = single tail shock
long = multiple tail shocks
mechanisms for long lasting change (pre-synaptic facilitation)
- higher levels of cAMP
- activates PKA
- which now moves to the nucleus
–> altered gene expression
–> new proteins stimulate synapse growth
–> creating a strengthened network
hippocampus
the hippocampus is in the temporal lobe of the cerebral cortex
- plays important roles in learning and memory
process of the trisynaptic circuit of the hippocampus (rodent)
- entorhinal cortex –> denate gyrus (perforant path) synapses
- dentate gyrus –> CA3 (mossy fiber) synapses
- CA3 –> CA1 (schaffer collateral) synapses
what is the trisynaptic circuit
its a three synapse loop inside the hippocampus that processes and transfers information coming from the entorhinal cortex (EC), through the hippocampus, and back out
what are the three synapses in the trisynaptic circuit
- perforant path
- mossy fibers
- schaffer collaterals
long term potentiation (LTP)
- elicited in brain regions associated with memory
- synaptically-located
- specific to active synapses
- associative (ie. interactions between active inputs can influence its induction)
- persistent
- activity dependent increase in synaptic transmission
what is long term potentiation underpinned by
underpinned by increased glutamate receptor responsiveness and changes in gene expression
