Brain and Behaviour (7): Learning & Memory

Declarative Memory

Explicit memory including episodic and semantic memory

What brain regions are involved in Declarative Memory?

• Hippocampus

• Entorhinal cortex

Nondeclarative Memory

Implicit memory, involving skills and conditioning. Can be triggered unconsciously.

What brain regions are involved in Nondeclarative Memory?

• Cerebellum

• Amygdala

What process drives learning & memory formation?

Through strengthening & weakening of synaptic connections

What is Hebb’s rule?

Cells that fire together, wire together

What is Long-Term Potentiation (LTP)?

A persistent strengthening of synapses based on activity patterns → increase in signal transmission. Key in learning & memory formation.

Properties of Long Term Potentiation

• Temporal: Summation of inputs reaches a stimulus threshold → LTP.

• Input Specific: LTP at one synapse is not propagated to adjacent synapses

• Associative: Simultaneous stimulation of a strong and weak pathway will induce LTP at both pathways. (Spatial summation)

How do hippocampal lesions affect learning and memory?

Morris Water Maze: Rats with healthy hippocampi learn to find the hidden platform in water. Whereas, rats with lesions continue to struggle to find the platform even after multiple trials.

What neurotransmitter is crucial for LTP?

Glutamate

What receptor allows Ca²⁺ influx during LTP induction?

Ionotropic Glutamate receptors (e.g. NMDA Receptor) that allow the flow of ions (Ca²⁺ mainly). They are important in LTP.

NMDA Receptor

A type of ionotropic glutamate receptor that allows Ca²⁺ influx during synaptic transmission. Important in LTP. AP5 acts as an antagonist on the receptor.

Describe the synaptic transmission during LTP?

1. At rest: NMDA receptors are blocked by Mg²⁺; Glutamate activates AMPA receptors → Na⁺ enters → depolarisation;

2. During depolarisation: Glutamate binds AND Mg²⁺ is removed→ NMDA receptors open → Ca²⁺ enters.

3. Ca²⁺ activates CaMKII, which:

   • Adds more & boosts existing AMPA receptors

     → Result: Stronger synapse & LTP

What role does Nitrous Oxide (NO) play in LTP?

Acts as a retrograde messenger to increase presynaptic glutamate release.

Late Phase LTP

Long lasting (days, months) LTP requiring protein synthesis for new synapses, which allows consolidation of long term memories.

Tetanic Stimulation

Artificial high-frequency burst of electrical signals (e.g. 100 Hz for 1 second) applied to a neuron; commonly used in experiments to induce Long-Term Potentiation (LTP)

Long-Term Depression (LTD)

long-lasting decrease in synaptic strength triggered by low-frequency stimulation.

What brain rhythm is physiologically related to LTP?

Theta rhythm (4–12 Hz). Disruption in these waves → deficits in learning tasks.

How can LTP be enhanced?

• Genetically: Increased N42B (NMDA) receptors

• Living in enriched environments (Jankowsky et al: learning deficits in mice are mitigated by enrichment)

What is the effect of Age on LTP?

Decreases LTP

What can mitigate the effects of age on memory?

Winocur (1998): Aged mice in enriched environments showed better memory and fewer deficits than those in impoverished conditions.

What are the neural mechanisms associated with fear conditioning?

Involves synaptic connections in the amygdala:

1. Strong US Input (shock) → postsynaptic depolarisation. Paired with:

2. Initially weak CS (tone) but is strengthened via depolarisation and NMDA receptor activation → Long Term Potentiation