Synaptic Plasticity
= ability of synapses to change as a result of experience
- neurons change physically, as a result of learning
- Hebb (1949): “Neurons that fire together, wire together”
Synaptic Plasticity
- Long-Term Potentiation
= an effect in which synaptic transmission becomes more effective as a result of recent activity
- Associative LTP = if two neurons are conjointly active, synapse between them is potentiated
Synaptic Plasticity
- Implementing LTP in a neuron
- response changes in postsynaptic receptors
- changes in presynaptic neuron
- structural changes in postsynaptic neuron
Synaptic Plasticity
- Implementing LTP in a neuron
- Relationship of LTP to Learning
- evidence:
- drugs that block LTP can impair animal’s ability to learn
- rats that have been genetically bred to have enhanced LTP, show better learning than normal rats
Synaptic Plasticity
- Implementing LTP in a neuron
- Long-Term Depression
= occurs when synaptic transmission becomes less effective with experience, weakening connections between neurons
- opponent porcess of LTP
- ‘neurons that don’t fire together, become disengaged’
Different Types of Neural Circuits
- monosynaptic neural circuit
- modulatory circuits ( = simple neural circuits that receive input from superordinate ones, e.g. motor system)
- cell assemblies ( = complex network of neurons; here, several plastic synapses help form a neural network)
Synaptic Changes that may store Memories
- Physiological Changes at Synapses Store Information
(1) increase in size of postsynaptic receptor membrane causes larger response to same amount of transmitter release
(2) interneuron modulates polarization of axon terminal and causes release of more transmitter molecules per nerve impulse
Synaptic Changes that may store Memories
- Structural Changes at Synapses provide Long-Term Storage
(1) neural circuit that is used more often increases number of synaptic contacts
(2) more frequently used neural pathway takes over synaptic sites formerly occupied by less active competitor
Synaptic Changes that may store Memories
- Conditions to induce memory-related changes at synapses
- Hebb: suggested that functional relationship presynaptic and postsynaptic neuron could change if A frequently took part in exciting B
- Hebbian synapses: in absence of complex circuit, hippocampal synapses show conditional changes that could mediate aspects of associative learning
- dual-trace hypothesis = formation of a memory involves first a relatively brief transient process: learning experience sets up activity that tends to reverberate though the activated neural circuits; this activity holds the memory for a short period; if sufficient, the activity helps build up a stable change in the nervous system - a long-lasting memory trace
Synaptic Changes that may store Memories
- computing what to remember requires circuits of neurons
- various kinds of learning require specialized circuits to compute particular features of what is to be remembered
Changes in Brain from Training
- interaktiv with enriched environment has measurable effects on brain measures, on stress reactions and on learning
- conditions: standard condition (SC), impoverished/isolated condition (IC), enriched condition (EC)
- experience in EC environment promotes better learning and problem-solving in variety of tests
- environment alters expression of large number of genes
- environment helps in recovery from or compensation for a variety of conditions
Changes in Brain from Training
- New Synaptic Connections From Learning
- enriched experience appears to lead to development of increased number of synaptic contacts and richer, more complex intracortical networks
- new synaptic contacts in specific brain regions in response to learning and formation of long-term memory
- measurable changes can be induced in brain by experience
Roles of NMDA and AMPA receptors in induction of LTP in CA1 region
- hippocampus has several regions and one of them is CA1, which has two kinds of glutamate receptors
- AMPA receptors: bind glutamate agonist –> glutamate first activates these receptors
- NMDA receptors: selective ligand –> do not respond until enough AMPA receptors are stimulated, and neuron is partially depolarized
Roles of NMDA and AMPA receptors in induction of LTP in CA1 region
- LTP is induced via cascade of neurochemical steps
- many of these steps are seen whenever a signal leads cells to change the kinds of compounds they synthesize or the rate of synthesis; proteins form signaling cascades that allow neurons to adapt to input they are getting; these cascades link activity with connectivity
- how? IEGs (‘immediate early genes’) produce transcription factors (=proteins) that control the expression of other so-called ‘late genes’
Hippocampus as Spatial Map
- hippocampus provedes rest of the brain with spatial reference map –> activity of cells in such map would specify direction in which the rat was pointing, relative to environmental land marks and occurrence of particular tactile/visual stimuli while facing that direction
Cellular Consolidation
- in the way. synaptic plasticity occurs, it appears that it goes in two stages:
(1) synaptic transmission between two vocative neurons becomes more efficient –> not permanent and wars off after some hours
(2) for long-term memory, synaptic changes that would underlie formation of new memory trace, would have to be more stable and long-lasting
Cellular Consolidation
- Relationship Genes and Long-Term Plasticity
synaptic plasticity: 2 stages
(1) synaptic transmission between two neurons becomes more efficient
(2) long-term plasticity: genes
- Replication (DNA->DNA); transcription (DNA->RNA); translation (RNA->Protein)
- genes are responsible for producing more vesicles filled with glutamate (presynaptic neuron) or for producing more glutamate receptors (postsynaptic neuron)
Cellular Consolidation
- Definitions
- synaptic consolidation/late-phase LTP = form of memory consolidation seen across all species and long-term memory tasks; in this case, long-term memory is one that lasts for at least 24 hours
- standard model of synaptic consolidation = suggests that alterations of synaptic protein synthesis and changes in membrane potential are achieved through activating intracellular transduction cascades
- spacing effect = distributed learning has been found to enhance memory consolidation; if we do it during course of 24 hours, it decreases the rate of forgetting compared to massed learning and enhances relational memory consolidation
- protein synthesis = plays important role in formation of new memories; P.S. inhibitors given after learning weaken memory; effects of P.S. inhibitors also inhibit LTP
Is the Hippocampus more than a Tutor?
- Standard theory of memory consolidation
–> hippocampus drops out after tutoring
Is the Hippocampus more than a Tutor?
- Multiple Trace Theory
–> hippocampus always remains part of the memory trace, if episodic)
