Brain
Hippocampus
The brain's memory indexer
Overview
The hippocampus, named after the Greek for seahorse due to its curved shape, is essential for converting short-term memories into long-term ones (memory consolidation) and for spatial navigation. It lies deep within the medial temporal lobe and is one of the first regions damaged in Alzheimer's disease.
Function
- Converts short-term memories to long-term storage
- Spatial navigation and cognitive mapping
- Pattern separation and completion
- Contextual fear memory and extinction
- One of few brain areas generating new neurons in adulthood (neurogenesis)
Key Facts
- Patient HM (Henry Molaison) lost both hippocampi and could form no new episodic memories
- London taxi drivers have enlarged posterior hippocampi from navigating complex routes
- The hippocampus contains place cells and grid cells that act as a GPS system
- Chronic stress shrinks hippocampal volume via cortisol damage
Key Substructures
- CA1: primary output subfield, highly vulnerable to hypoxia and ischemia
- CA3: pattern completion; origin of the Schaffer collateral pathway to CA1
- Dentate gyrus: pattern separation; major site of adult neurogenesis
- Subiculum: main output region projecting to entorhinal cortex and prefrontal cortex
- Entorhinal cortex: gateway interface carrying input and output to the hippocampal circuit
Clinical Notes
- Bilateral damage causes anterograde amnesia: inability to form any new episodic memories
- One of the earliest brain regions destroyed in Alzheimer's disease
- Chronic stress causes measurable hippocampal volume loss via cortisol-induced neuronal damage
- Temporal lobe epilepsy most commonly originates in hippocampal and parahippocampal structures