Exploring Memory and the Hippocampus

Aug 8, 2024

Understanding Memory and the Hippocampus

Memory Types

  • Short-term Memory: Information retention for approximately 30 seconds.
  • Long-term Memory: Information retention for a lifetime.

Studying and Memory Retention

  • Studying activates various pathways in the brain.
  • Continued activation of these pathways leads to memory retention.

The Hippocampus

  • Located in the medial temporal lobe in both hemispheres.
  • Coordinates the storage and retrieval of memories.
  • Major structures include:
    • Dentate gyrus
    • CA1 region
    • CA3 region

Memory Processing Pathway

  1. Signal Entry: Signals enter through the dentate gyrus from the internal cortex (via perforant pathway).
  2. Neuronal Interaction:
    • Granule neurons in dentate gyrus synapse with pyramidal neurons in CA3.
    • CA3 neurons connect to CA1 pyramidal cells via Schaffer collaterals.
  3. Information Flow: Signals pass to the cingulate gyrus and back to the internal cortex.

Long-term Potentiation (LTP)

  • Definition: Cellular and molecular basis for memory; a long-lasting enhancement in signal transmission between two neurons after repeated stimulation.
  • Key Area Studied: CA3-CA1 synapse involving Schaffer collateral axons.
  • Mechanism:
    • Involves NMDA and AMPA receptors on the postsynaptic cell.
    • Glutamate binding leads to depolarization; calcium influx is pivotal for LTP.

NMDA and AMPA Receptors

  • AMPA Receptors:
    • Permeable to sodium ions.
    • Open with glutamate binding, allowing sodium influx.
  • NMDA Receptors:
    • Permeable to sodium and calcium.
    • Blocked by magnesium; requires significant depolarization to open.

Action Potentials and LTP

  • Low-frequency action potentials lead to minimal glutamate release and insufficient LTP.
  • High-frequency action potentials cause substantial glutamate release, allowing LTP to occur.
  • AMPA receptor activation leads to greater postsynaptic depolarization, displacing magnesium blockade on NMDA receptors.

Calcium's Role in LTP

  • Calcium acts as a secondary messenger, activating intracellular cascades.
  • Phases of LTP:
    • Early Phase:
      • Brief calcium increase leads to insertion of new AMPA receptors at CA3-CA1 synapse.
      • Changes last a few hours.
    • Late Phase:
      • Prolonged calcium influx increases transcription factors, leading to gene expression and new protein synthesis.
      • Results in lasting synaptic changes; can last 24 hours to a lifetime.

Synaptic Plasticity

  • Brain's ability to change as it learns.
  • Formation of new synapses between CA3 and CA1 neurons strengthens connections.

Importance of LTP

  • LTP is not a mechanism but an outcome of increased neuronal activity.
  • Strengthens connections allowing greater depolarization events from low-frequency action potentials.

Memory Processing Beyond the Hippocampus

  • Memory processing involves multiple brain regions, including parts of the cerebral cortex.
  • Continuous activation leads to high-frequency action potentials, strengthening specific pathways for better recall during exams.