Hippocampal Structure and Function

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Questions and Answers

Which of the following statements accurately describes the function of the dentate gyrus in the hippocampus?

  • The dentate gyrus receives input from the entorhinal cortex and projects to the CA3 region. (correct)
  • The dentate gyrus is responsible for the formation of new memories and the consolidation of existing memories.
  • The dentate gyrus plays a role in regulating emotional responses and stress levels.
  • The dentate gyrus receives input from the CA3 region and projects to the CA1 region.

Which of the following is NOT a characteristic of Long-Term Potentiation (LTP)?

  • LTP is associated with a rapid induction phase and a long-lasting maintenance phase.
  • LTP can be induced by weak stimulation of a pathway if it is paired with strong stimulation of another pathway. (correct)
  • LTP is specific to a particular pathway, meaning it only affects the synapses that were strongly activated.
  • LTP requires simultaneous activation of presynaptic and postsynaptic neurons, adhering to Hebb's rule.

What is the role of NMDA receptors in the induction of LTP?

  • NMDA receptors act as coincidence detectors, allowing calcium influx only when the presynaptic neuron is releasing glutamate and the postsynaptic neuron is depolarized. (correct)
  • NMDA receptors directly activate protein kinases, leading to phosphorylation of downstream targets.
  • NMDA receptors are responsible for the long-term maintenance of LTP by increasing the number of AMPA receptors at the synapse.
  • NMDA receptors are responsible for the release of neurotransmitters from the presynaptic neuron.

Which of the following statements accurately describes the role of CA3 pyramidal neurons in the trisynaptic circuit?

<p>CA3 pyramidal neurons receive input from the dentate gyrus and project to the CA1 region via Schaffer collaterals. (C)</p> Signup and view all the answers

Which of the following statements accurately describes the relationship between the CA1 region and the Schaffer collaterals?

<p>Schaffer collaterals originate in the CA3 region and project to the CA1 region. (C)</p> Signup and view all the answers

What is the primary consequence of sustained low-frequency stimulation in relation to synaptic function?

<p>Induction of long-term depression (LTD) (D)</p> Signup and view all the answers

Which of the following accurately describes the relationship between Ca2+ influx and LTP?

<p>LTP is dependent on short bursts of high Ca2+ influx. (C)</p> Signup and view all the answers

What role do protein synthesis inhibitors play in LTP?

<p>They block the maintenance of LTP. (B)</p> Signup and view all the answers

What is an engram in the context of hippocampal function?

<p>A collection of synapses that encode specific memories. (C)</p> Signup and view all the answers

What distinguishes long-term potentiation (LTP) from long-term depression (LTD) in terms of synaptic stimulation paradigms?

<p>LTP is induced by short high-frequency stimulation, while LTD is due to prolonged low-frequency stimulation. (D)</p> Signup and view all the answers

Flashcards

LTP (Long-Term Potentiation)

A long-lasting enhancement in synaptic strength following high-frequency stimulation.

AMPA Receptors

Glutamate receptors that mediate fast synaptic transmission and are inserted during LTP.

LTD (Long-Term Depression)

A long-lasting decrease in synaptic strength due to low-frequency stimulation.

Role of Ca2+ in LTP

High influx of calcium ions triggers activation of pathways that strengthen synapses.

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Hippocampal Engrams

Synaptic structures that encode memories through plasticity, made of connected neuron ensembles.

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Hippocampus structure

The hippocampus consists of a trisynaptic circuit involving input from the entorhinal cortex, granule cells in the dentate gyrus, CA3 region, and CA1.

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Long-Term Potentiation (LTP)

LTP is the strengthening of synapses based on their recent activity, leading to improved memory formation.

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Hebb's Rule

Hebb's Rule states that if an axon of cell A is near enough to excite cell B, changes both occur to increase efficiency.

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Associative LTP

Associative LTP occurs when weak synaptic stimulation paired with strong stimulation results in strengthened connections.

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Study Notes

Hippocampal Structure and Function

  • The hippocampus, found in all mammalian orders, possesses a trisynaptic circuit with three excitatory synaptic stations.
  • Input arrives from the perforant path (entorhinal cortex axons), projecting to granule cells (dentate gyrus).
  • This path leads to the CA3 region via mossy fibers.
  • Then, Schaffer collaterals connect CA3 to CA1.
  • Other brain areas also have connections.
  • Granule cells (dentate gyrus) and pyramidal neurons (CA3, CA1) are key components.
  • CA3 has complex spines and projects to CA3/CA2 (associational) or CA1 (Schaffer collateral).
  • CA1's stratum radiatum houses Schaffer collateral synapses with normal spines.
  • This circuit is well-suited to spatial navigation, integrating context and content.
  • Interneurons, including 21 types in CA1, provide crucial inhibition.

Synaptic Plasticity (LTP & LTD)

  • Synaptic plasticity involves changing synaptic strength (e.g., Long-Term Potentiation and Long-Term Depression).
  • Hebbian learning: Repeated activation strengthens connections (synaptic efficiency increase). Information encoding must synchronize with memory timescales.
  • LTP (Long-Term Potentiation): Repeated strong activation of a synapse increases its strength. This is
    • Specific: to a pathway.
    • Fast: Induction occurs rapidly (seconds to minutes), sustained for hours.
    • Coincidence-dependent: Synaptic activity must occur simultaneously (Hebb's rule).
    • Associative LTP: Weak stimulation on one pathway can induce LTP in a different pathway if paired with strong stimulation in the first one (Pavlovian conditioning).
  • LTP Mechanisms:
    • NMDA receptors: act like coincidence detectors. Strong depolarization (high activity) removes Mg2+ blockage of NMDA channels.
    • Ca2+ influx activates signal cascades (protein kinases like CAMKII and PKC).
    • CAMKII phosphorylates downstream targets, enhancing AMPA receptor sensitivity.
    • More AMPA receptors are inserted into the synapse.
  • Maintenance of LTP: Requires synthesis of new proteins (LTP lasts hours). Blocking protein synthesis disrupts lasting LTP.
  • LTD (Long-Term Depression): Weak or infrequent synaptic activity weakens synaptic strength.
    • NMDA-dependent: Postsynaptic Ca2+ influx plays a role.
  • LTD is driven by sustained low-level Ca2+ influx. Lower Ca2+ influx activates protein phosphatases, removing AMPA receptors.
  • LTP and LTD Balance: High-frequency LTP and prolonged low-frequency LTD are unlikely in a physiological setting, as these require specific stimulation protocols.

Hippocampus and Memory

  • Hippocampal Engrams: Synaptic plasticity forms memory engrams, ensembles of synaptically connected cells encoding specific memories.

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