Hippocampus

What is the hippocampus?

A curved structure formed as an inward fold of the cerebral cortex in the medial temporal lobe.

What does the name hippocampus mean?

Seahorse.

What is another historical name for the hippocampus?

Cornu Ammonis (CA), a name still used for parts of its cellular structure.

What is the hippocampal formation?

The hippocampus and associated structures through which information flows into and out of the hippocampus, particularly via the entorhinal cortex.

What is the main route for information entering and leaving the hippocampus?

The entorhinal cortex.

What happened to Patient H.M. after bilateral medial temporal lobe resection?

He developed severe memory impairment, particularly a grave loss of recent memory.

What type of amnesia did Patient H.M. experience?

Severe anterograde amnesia and moderate retrograde amnesia affecting approximately the previous 2–3 years.

What is anterograde amnesia?

Difficulty forming/committing new events to long-term memory.

What is retrograde amnesia?

Loss of memories from before the onset of the amnesia.

What cognitive abilities were relatively preserved in Patient H.M.?

Perception, abstract thinking, reasoning, motivation, personality and IQ were largely preserved.

What happened to Patient H.M.'s working and implicit memory?

Working memory and implicit memory were intact.

What type of memory was particularly impaired in Patient H.M.?

Formation of new declarative memories.

What are the two types of declarative memory highlighted in the lecture?

Semantic and episodic memory.

What did Patient H.M.'s mirror-drawing performance demonstrate?

His performance could improve with practice even though he could not remember having practised the task.

What does H.M.'s mirror-drawing performance demonstrate about memory?

Different memory systems exist; implicit learning can occur despite severe impairment in forming new declarative memories.

What role does the hippocampus have in memory encoding?

It is central to encoding new episodic memories and likely semantic memories too.

Is the hippocampus required equally for all memory retrieval?

The lecture states it appears essential for recent memory retrieval but is likely not required for retrieval of older/remote memories, although this remains debated.

What is the entorhinal cortex's role in hippocampal information flow?

Information flows from the superficial layers of the entorhinal cortex into the hippocampus.

What is the perforant pathway?

The pathway carrying information from the entorhinal cortex into hippocampal structures.

Where do entorhinal cortex layer 2 projections go?

To the dentate gyrus (DG) and CA3.

Where do entorhinal cortex layer 3 projections go?

To CA1 and the subiculum.

What is the basic order of the hippocampal trisynaptic circuit?

Dentate gyrus → CA3 → CA1 → subiculum.

What type of cells are found in the dentate gyrus?

Granule cells, which are glutamatergic neurons.

What is the pathway from dentate gyrus granule cells to CA3 called?

The mossy fibres.

What is the hilus of the dentate gyrus?

A region through which granule-cell axons forming the mossy fibres project; it may sometimes be called CA4.

What are mossy cells?

Glutamatergic cells located in the hilus that can provide input to granule cells.

What type of cells are found in CA3?

Pyramidal cells, which are glutamatergic excitatory neurons.

What inputs does CA3 receive?

Input from the dentate gyrus via mossy fibres and from the entorhinal cortex.

What are Schaffer collaterals?

Axons from CA3 pyramidal cells projecting to CA1.

What other connections do CA3 pyramidal cells make?

They form recurrent collaterals that can contact other CA3 pyramidal neurons.

What type of cells are found in CA1?

Pyramidal cells, which are glutamatergic excitatory neurons.

What inputs does CA1 receive?

Input from CA3 via Schaffer collaterals and from the entorhinal cortex.

Where do CA1 pyramidal-cell axons generally project?

Towards the subiculum, although some can project directly to the entorhinal cortex.

What is the trisynaptic circuit?

Entorhinal cortex → dentate gyrus → CA3 → CA1 → subiculum, with the main three synaptic stages involving DG, CA3 and CA1.

What is thought to underlie memory encoding at the cellular level?

Synaptic plasticity, involving long-term changes in synaptic strength.

What was the significance of Patient H.M. for understanding the hippocampus?

His severe anterograde amnesia after medial temporal lobe surgery provided evidence for a role of the hippocampal region in forming new long-term memories.

What was discovered about H.M.'s brain after his death?

His resection was less extensive than initially believed; the left hippocampus was relatively spared, and a lesion in the left orbitofrontal cortex was also found.

What evidence from Alzheimer's disease supports a role for the hippocampus in memory?

Early pathology often affects the entorhinal cortex and hippocampus before widespread cortical pathology.

What are place cells?

Hippocampal neurons, particularly in CA1 and CA3, whose activity increases when an animal is in a particular location.

Where were place cells first reported?

In the rat hippocampus in the 1970s by O'Keefe and Dostrovsky.

What are grid cells?

Cells in the entorhinal cortex that create a hexagonal spatial pattern across an area.

How are place cells and grid cells related?

Hippocampal place cells are closely connected to entorhinal grid cells involved in representing spatial location.

What evidence shows that the hippocampus contributes to spatial memory?

Hippocampal lesions impair spatial memory in rats performing the Morris water maze.

What other condition is associated with spatial memory impairment in the lecture?

Alzheimer's disease.

What did research on humans such as London taxi drivers provide evidence for?

Evidence supporting a role for the hippocampus in spatial memory/navigation.

Are episodic memory encoding and spatial navigation necessarily separate hippocampal functions?

No. The lecture suggests they are likely not separate, distinct functions.

What are the four major hippocampal subdivisions you need to know?

Dentate gyrus, CA3, CA1 and subiculum.

What are the two major functional roles of the hippocampus highlighted in the lecture?

Encoding episodic memories and spatial navigation.