Lecture 17 - Learning and Memory 1 Flashcards
Is the brain wired?
¥ Developmental Neuroscience – neurocircuits or development? How the brain is wired
¥ Neuroanatomy – we all have the same brain but different experiences
1. Both suggest that nervous system is hard-wired - guided by cell-to-cell communication via both physical contact and chemical signalling between cells. e.g. spinal circuits for locomotion (already laid down from birth and then activated as we learn) that guide neurons to connect with each other to produce a fully wired brain
2. However – circuitry is modified/refined by environmental factors during ‘critical periods’ of development. e.g. activity-dependent synaptic plasticity of the visual cortex. Pathways needs to be reinforced to ensure that the pathway is constantly active.
3. Similar processes of synaptic plasticity (changes in strength of existing pathways) occur throughout life – learning and memory
4. Increasing evidence that some neuronal pathways can regenerate
Basic Principles of Learning and Memory:
¥ Multiple regions of the brain are involved
¥ Learning and memory involve changes in existing neural circuits
¥ Changes include altered synaptic strength and neuronal excitability (and changes in structure)
¥ Intracellular signalling pathways play a key role in effecting these changes
¥ Long-term memory requires new protein synthesis (‘permanent’) whereas short-term does not
Learning and Memory:
“A change in behaviour as a result of experience”
- Learning is ‘Adaptive’
the way you respond to an input considering another experience
¥ Learning – acquisition
¥ Memory – storage and retrieval
¥ Involves modification of existing circuits in the brain
Altered Stimulus Response relationship
e.g. Pavlovian (classical) conditioning
Types of Memory:
- Declarative – Symantec - individual events that have occurred in the past or in general facts (can pop up without thinking about it or have to think about it)
- Non-declarative – implicit – learning about skills and relationships, requires a lot of repetition (becomes the norm)
memory
¬ Working memory – seconds to minutes; capacity limited to about 7 items (looking at a telephone number and then having enough memory to remember it)
¬ Intermediate memory – to remember things for exams and then usually forgotten about
¬ Disruption of neuronal activity can affect memory loss
¬ Long-term memory has a structural change as it withstands memory loss
Amnesias:
- tell us something about how and where memories are laid down (and retrieved)
- often result from trauma
¥ Retrograde amnesia – memory loss for event prior to trauma (every memory can be destroyed)
¥ Anterograde amnesia – inability to lay down new memories (only old memories are remembered but can’t make any new ones)
Alzheimer’s Disease:
¥ First memories to go are short term (predicted that memories may be retrieved?)
Classical conditioning suggests a change in the strength/ efficacy of pathways in the brain:
¥ Unconditioned stimulus UCS = food
¥ Response R = salivation
¥ Conditioned stimulus CS = bell
¥ Learning: CS —– R (a ‘new’ pathway)
¥ Conducted experiment by Russian physiologist Ivan Pavlov (using dogs – dog salivating whenever man entered the room and the bell rang as this was when food was given – associated)
Activity-Dependent Synaptic Plasticity:
- Hebb’s Rule: “When an axon of cell A is near enough to excite a 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.” (1949)
- When pre- and post-synaptic neurons (pathways) are simultaneously and strongly or repeatedly activated, the synaptic connection between them becomes enhanced/strengthened
- ‘neurons that fire together wire together’
Rats which develop in sparse environments exhibit a smaller area of neuronal branching:
strengthening the brain is both functional and structural
- rats raised in a highly stimulated environment (lots of things to do) and others in low stimulated one
- image shows density of glutamate receptors in the hippocampus
- the density is greater in stimulated – more sensory input so more synapses are stimulated and stronger
- more neurotic branching
Neuronal Correlate for Activity-Dependent Synaptic Plasticity:
Long Term Potentiation (LTP) in the Hippocampus
LTP is finding a more branching which leads to a greater strength
Activity-dependent sign 50 where the synapse becomes stronger
¥ Hippocampus (living system) located deep in the brain within the temporal lobes (defects in epilepsy – affects memories)
¥ Part of the limbic system
¥ Involved in memory processing (acquisition of declarative memories)
¥ Important for spatial learning: London cab drivers have large hippocampi
¥ Severely affected in Alzheimer’s Disease
Hippocampal slice preparation for LTP:
Bliss & Lomo, 1973
¥ Brief high-frequency electrical stimulation of the input pathway produces long-lasting enhancement of the strength of the input-output synapse
- granule fibres go to CA3 cells which goes to CA1 cells which provides the main output by a synapse
- glutamate is used as the neurotransmitter
- activity-dependent plasticity
- detect activity by using a microelectrode – detect EPSP
Potentiation of EPSP amplitude in CA1 neuron:
measuring the amplitude
- at the tetanus frequency, a high stimulation is given to the input pathway
- then the normal stimulus is given
- this gives long-lasting enhanced stimulation
AMPA & NMDA Glutamate Receptors:
- on the post-synaptic cell
- agonists as two different receptors
- AMPA is ligand activated to make it open and sodium moves in and this causes depolarisation
- NMDA are usually blocked by magnesium ion, when glutamate binds, it cant cause the opening of the membrane channel as magnesium is in the way
- When the membrane is both depolarised and glutamate is added, magnesium moves out of the way and sodium and calcium ions can move in
Before tetanic stimulation
¥ AMPA receptors: ligand-gated Na+ channels
NMDA receptors: ligand-gated Ca2+ channels, blocked by Mg2+ ions (this Mg-block is voltage-dependent)
