Learning and Memory

Long Term Potentiation (LTP)

  • The process where a weak synaptic input pathway strengthens its connection by being activated at the same time as a strong, neighbouring input pathway

  • Long-term increase in excitability of neuron caused by repeated high-frequency activity of that input

  • Achieved at postsynaptic terminal via excitatory postsynaptic potentials (EPSPs)

  • Requires high frequency to create summation

Hebbian or Associative Learning

Hebb’s rule (Donald Hebb, 1949) → “Cells that fire together, wire together”

  • The theory → Any two cells or systems that are repeatedly active at the same time will tend to become ‘associated’ so that activity in one facilitates activity in the other.

  • This causes changes in structure or chemistry of synapse making them stronger and more likely to produce EPSPs

LTP and Hebbian Learning in Action

Hippocampus - Entorhinal cortex region

  1. Stimulate the perforant path axons

  2. High frequency train of stimuli to increase EPSPs amplitude

  3. Resulting EPSPs recorded in dentate gyrus

  4. Pre and postsynaptic cell become active at same time

Known as ‘population EPSP’ – synapses of perforant path axons and dentate granule cells.

  • Synaptic strengthening occurs when synapses are active, and membrane of the postsynaptic cell is depolarized

Glutamatergic System, NDMA Receptors and LTP

  • in the hippocampus → CA 1 field

  • NDMA receptor control calcium ion channel → cause post-synaptic depolarisation

    • Typically blocked by magnesium → prevents influx of calcium when activated by glutamate

  • Depolarising postsynaptic membrane ejects magnesium and calcium ions to enter through NMDA controlled receptor

    • Voltage dependent ion channel

Pre- and Post-synaptic changes due to LTP

Presynaptic

  • Increases amount of Glutamate → Nitric oxide, a gas neurotransmitter, communicates retrograde messages from one cell to another

  • Free floating nitric oxide activates nearby dendrites to produce LTP induction

Postsynaptic

  • Changes in size and shape of dendrites

    • e.g. enlargement of thin dendrites into mushroom shaped ones

    • Growth continues up to 19hrs later

Amnesia

Anterograde amnesia

  • Difficulty in learning new information

  • Deficit in complex relational memory

  • Damage to temporal lobes → patient HM

Reterograde amnesia

  • Inability to remember events before the brain damage occurred

Korsakoff’s syndrome

  • Severe anterograde amnesia

  • May be caused by alcoholism

Patient HM

What we can learn from this

  • Hippocampus is…

    • Not the location of LTM or retrieval of LTM - HM remembered events from early life and could talk

    • Not the location of STM - HM would not have been able to carry on conversations

    • Involved in converting STM to LTM

      • STM of event retained by neural activity, with LTM in permanent biochemical or structural changes e.g. LTP

Morris Water Maze

  • A behavioural test used in neuroscience, primarily with rodents, to study spatial learning and memory.

  • The procedure involves a pool of opaque water with a submerged escape platform. The animal must use external cues in the room to locate the platform, and its performance is assessed by tracking how quickly it finds the escape route over multiple trials.  

  • part 2 not uploaded yet