What are extra-temporal connections to the MTL?
- Papez Circuit
- Frontal Lobes
- Dienecephalon
What is the Limbic System and what does it regulate?
Limbic System = Amygdala + Papez Circuit
- Emotional expression and experiences
What is the role of the Amygdala in memory? What happens if it’s leisoned?
Role of Amygdala
- Memory for emotionally arousing experiences (Not necessarily fear)
- Fear conditioning
- Rich representations of emotional experiences
Lesioned
- Loss of conditioned fear
- Impairment of new fear learning
- Reduced memory for emotionally laden events (Not necessarily fear)
What are the parts of Papez Circuit (in order?
- Hippocampus
- Fornix
- Mamillary Body - Part of Hypothalamus
- Anterior Thalamus Nuclei - Part of Thalamus
- Cingulate Gyrus
[Closed Circuit]
What is the role of the Papez Circuit in memory? Lesion to which part is memory most reliably imparied?
Declarative Memory
General View
- Relational memory/encoding, similar to MTL
- Most reliable when hippocampus or ATN are leisoned. But other parts also cause relational damage
- Damage tend to be spread anyways
What does the Frontal Lobe contain?
- Posterior Frontal Lobe
- Motor and premotor cortex
- Motor programming
- Anterior Frontal Lobe
- Prefontal cortex
- Cognitive Control Processes
Leision of Frontal Lobes in Memory?
- Impairment in developing and implementing strategies for appropriate memory encoding and retrieval
- Impairment in remembering contextual details (DLPFC)
- E.g., Sources of information, chronological order of memories
- Confabulation (VLPFC)
- Production of statements involving bizarre distortions of memory
Where are the rich connections in frontal lobes connected to?
Rich reciprocal connections:
- ) Within frontal lobes itself
- ) With MTL (hippocampus, neocortical association areas, etc)
(With hippocampus via fornix)
What is the Dienecephalon?
Dienecephalon = Thalamus + Hypothalamus
What does the hypothalamus contain? Hence?
- Mamillary bodies (In Papez Circuit)
- Impairment = loss of declarative memory (relational)
What does the thalamus contain?
- ATN (Papez Circuit)
- Mammillo-thalamic tract (MMT): Connects ATN with hipocampus
- Medio Dorsal Nuclei (MDM) / Dorsal Medial Nuclei
- Internal Medullary Lamina (IML) / Midline
Which parts of leisons of thalamus most likely to cause memory loss?
Anterior and Medial
- More likely
- Most connected to frontal lobes
- Anterior (part of Papez circuit)
- Medial
Posterior and Lateral
- Less likely
Damage of mammillo-thalamic tract (connects ATN to hippocampus) and ATN affects what type of memory?
Declarative (Relational memory)
Damage of Medio Dorsal Nucleus (MDN) and/or Internal Medullary Lamina (IML) damage; but spared mammillo-thalamtic tract (MTT) affects memory how?
Specific retrieval difficulties
- Impaired Recall
- Preserved Recognition
- Top Down cannot recall, but can recall under prompt
Damage of Medio Dorsal Nucleus (MDN) and Intralaminar/Midline Nucleus affects memory how?
Medio Dorsal Nucleus
- Deficits in selecting the appropriate information to be retrieved
- ‘Active retrieval’
Intralaminar/Midline
- Deficits seen in semantic memory, memory retrieval
Summarise lesions of the 3 areas.
- Papez’s circuit lesion
- Impaired relational memory/encoding (Similar to MTL)
- Frontal Lobes
- Impaired ability to organise the encoding, retrieval and maintenance of memories
- Dorso-medial, intralaminar and midline nuclei
- Impaired memory due to reduced mental flexibility and inappropriate selection of information retrieval
Biochemistry: What does learning require?
Synaptic Plasticity
Biochemistry of synapses change to alter the effect on post-synaptic neuron
What is LTP?
Long-term increase in excitability of a neuron (B) to a particular synaptic input caused by repeated high frequency activity of that input (A)
What is Hebb’s Law? What does it describe?
When an axon of cell A…excites cell B by repeatedly firing it, change takes place in 1 or both cells to increase A’s efficiency
- Describes long-term potentiation
Evidence for LTP?
- Baseline EPSP measured for single electrical stimulus
- 100 electrical stimuli delivered rapidly
- Increased EPSP for subsequent single electrical stimulus
- LTP
Biochemically, how does LTP cause synaptic changes? Describe the changes at the level of the pre and post-synaptic.
(Glutamate)
Pre-Synaptic
- Increased glutamate by pre-synaptic terminal button
Post-Synaptic
- New receptors
- Increased receptor sensitivity to glutamate
- Increase protein synthesis in post-synaptic dendrites
Sites of LTP (Where it occurs)?
- ) Hippocampus (esp. dentate gryus + CA1), entorhinal cortex
- ) Others: Frontal Lobes, Thalamus, Amygdala, Visual Cortex
What are 3 other mechanisms of neural plasticity?
- ) Long term depression
- ) Habituation
- ) Sensitization
How does Long Term Depression affect neural plasticity?
Low frequency stimulation at synapse decrease synaptic strength
