Hippocampus
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Direct answer
Curled like a seahorse in the floor of the inferior horn of the lateral ventricle, the hippocampus is the core of the hippocampal formation — hippocampus proper (Ammon's horn), dentate gyrus and subiculum — and the brain's gateway for new declarative memories. Its efferents sweep from the alveus to the fimbria, gather into the crus, body and columns of the fornix, and end in the mammillary body of the hypothalamus, the next station of the Papez circuit: hippocampus to fornix to mammillary body to anterior thalamic nucleus to cingulate gyrus and back through the entorhinal cortex. Its CA1 field (Sommer's sector) is exquisitely vulnerable to hypoxia and to epilepsy-related sclerosis, making the shrunken hippocampus the commonest finding in mesial temporal lobe epilepsy.
What you must remember
- Position: a longitudinal elevation in the floor of the inferior (temporal) horn of the lateral ventricle; its anterior end is expanded and grooved — the pes hippocampi — related to the uncus medially.
- Three parts of the formation: hippocampus proper with its CA1–CA4 fields, the dentate gyrus tucked in the concavity, and the subiculum as the transitional cortex with the parahippocampal gyrus.
- Efferent highway: pyramidal cell axons → alveus on the ventricular surface → fimbria → crus of fornix around the pulvinar → body beneath the corpus callosum → columns diving in front of the interventricular foramen → mammillary bodies (and direct fibres to the septal area).
- Papez circuit in order: hippocampus → fornix → mammillary body → mammillothalamic tract → anterior nucleus of thalamus → cingulate gyrus → cingulum → entorhinal (parahippocampal) cortex → hippocampus.
- Afferents: the entorhinal cortex (area 28) is the great funnel — cortical information of every kind reaches the hippocampus through it via the perforant path to the dentate gyrus.
- Memory role: bilateral hippocampal damage causes anterograde amnesia with intact intelligence, procedural memory and remote memory — the profile of the famous patient H.M.
- Vulnerability of CA1: hypoxia, hypoglycaemia and status epilepticus kill Sommer's sector first; hippocampal sclerosis is the pathological marker of mesial temporal lobe epilepsy and is best shown on coronal MRI.
- Eponymous remnants: the indusium griseum (a thin grey strand over the corpus callosum) and the medial and lateral longitudinal striae are hippocampal vestiges; the fasciolar gyrus connects the dentate gyrus to the indusium griseum around the splenium.
A seizure work-up that is pure anatomy
A 24-year-old has episodes beginning with a rising epigastric sensation and an old-memory flashback, progressing to staring, lip-smacking and automatisms for ninety seconds, with postictal confusion; MRI shows a small, bright, shrunken right hippocampus. Every element maps onto this structure. The aura is the entorhinal–limbic recruitment of the mesial temporal lobe; automatisms reflect spread to the amygdala; the sclerotic hippocampus with loss of CA1 pyramidal cells and gliosis is hippocampal sclerosis, the single commonest cause of refractory epilepsy in young adults and the lesion with the best response to surgery. Because memory materialises through the Papez circuit, the surgeon maps language-dominant hippocampal function before temporal lobectomy, and the classical long-term worry after bilateral resection, anterograde amnesia, is the direct price of losing the hippocampi, exactly as patient H.M. taught neuropsychology. Wernicke–Korsakoff amnesia tells the same story from a different direction: thiamine deficiency hits the mammillary bodies and dorsomedial thalamus, two stations of the same circuit, producing confabulating amnesia without touching the hippocampus itself.
Where students slip
The fornix gets reduced to "connects hippocampus to hypothalamus" — candidates forget its parts (crus, body, columns), its position under the corpus callosum, or its role as the posterior boundary of the interventricular foramen. The second slip is Papez order: mammillary body to anterior thalamic nucleus by the mammillothalamic tract (of Vicq d'Azyr) — students derail the circuit through the pulvinar or the medial dorsal nucleus. And the trap question comes reliably: Papez proposed the circuit for emotion, but its recognised function today is memory — answering "emotion" without that caveat costs the distinction.
Frequently asked questions
What are the components of the hippocampal formation?
The hippocampus proper (Ammon's horn with fields CA1–CA4), the dentate gyrus and the subiculum. The entorhinal cortex of the parahippocampal gyrus is the principal input via the perforant pathway.
Trace the efferent pathway of the hippocampus.
Pyramidal axons pass to the alveus, then the fimbria, forming the crus of the fornix; the two crura join as the body beneath the corpus callosum, and the columns of the fornix descend to terminate in the mammillary bodies, with additional fibres to the septal nuclei and hypothalamus.
What is the Papez circuit and its function?
Hippocampus → fornix → mammillary body → mammillothalamic tract → anterior thalamic nucleus → cingulate gyrus → cingulum → entorhinal cortex → hippocampus. Papez framed it for emotion, but it is now central to memory encoding and consolidation.
Why is the hippocampus vulnerable to hypoxia?
Its CA1 pyramidal neurons (Sommer's sector) have high metabolic demand and excitatory circuitry, so brief hypoxic or ischaemic insults — cardiac arrest, hypoglycaemia, status epilepticus — selectively destroy them. The result is hippocampal sclerosis and declarative memory impairment.
What happens after bilateral hippocampal destruction?
Profound anterograde amnesia: no new facts or episodes can be laid down, though intelligence, procedural skill and remote memory survive, as demonstrated by patient H.M. Unilateral damage, especially of the non-dominant side, is far better tolerated.