Overview

Length≈4–5 cm

Cortex typeArchicortex (3 layers)

Most hypoxia-sensitive fieldCA1

The hippocampus, named for its resemblance to a seahorse in coronal section, is a curled sheet of allocortex about 4–5 cm long. It bulges into the floor and medial wall of the temporal (inferior) horn of the lateral ventricle. Its ventricular surface is covered by a layer of white matter, the alveus, which gathers into the fimbria and continues as the fornix. The hippocampal formation comprises:
• the dentate gyrus;
• the hippocampus proper (cornu Ammonis, CA1–CA4);
• the subiculum, which merges with the entorhinal cortex of the parahippocampal gyrus.

Information flows through the classic trisynaptic circuit: entorhinal cortex (layer II) → perforant path → dentate granule cells → mossy fibres → CA3 pyramidal cells → Schaffer collaterals → CA1 → subiculum → back to the entorhinal cortex and out through the fornix. Long-term potentiation at CA3→CA1 synapses (NMDA-receptor dependent) is the best-studied cellular model of memory. The hippocampus binds the elements of an experience into an episodic memory and, over weeks to years, transfers it to the neocortex (consolidation). Hippocampal place cells fire at specific locations, and together with entorhinal grid cells they form a cognitive map (Nobel Prize 2014).

Location: Medial temporal lobe, in the floor of the temporal horn of the lateral ventricle, posterior to the amygdala; its tail curves up behind the thalamus towards the splenium.

Parts & subdivisions

Dentate gyrus
Granule cells receiving the perforant path; one of the few proposed sites of adult neurogenesis (debated in humans); pattern separation.
CA3
Pyramidal cells with recurrent collaterals (auto-associative network; pattern completion).
CA1 (Sommer sector)
Main output field; the region most vulnerable to hypoxia/ischaemia (selective neuronal loss after cardiac arrest).
Subiculum
Transitional cortex; major source of fornix fibres.
Alveus, fimbria and fornix
Efferent white matter: alveus → fimbria → crus → body → column of fornix → mammillary bodies and septal nuclei.

Anatomy

Arterial supply: Hippocampal branches of the posterior cerebral artery and the anterior choroidal artery.
Histology: Three-layered archicortex: molecular layer, pyramidal (CA) or granule (dentate) cell layer, polymorphic layer.

Function

Encoding and consolidation of new declarative (episodic and semantic) memories
Spatial memory and navigation (place cells)
Contextual fear memory (with the amygdala)
Feedback regulation of the HPA stress axis (rich in glucocorticoid receptors)

Clinical significance

Anterograde amnesia

Bilateral hippocampal damage prevents the formation of new declarative memories, while short-term memory, older memories and procedural learning are spared. Causes include surgery (patient H.M., bilateral medial temporal resection 1953), HSV encephalitis, hypoxia, transient global amnesia and Alzheimer disease. Unilateral left lesions mainly impair verbal memory; right lesions impair visuospatial memory.

Hippocampal (mesial temporal) sclerosis

Neuronal loss and gliosis of CA1 and CA3 (often after prolonged febrile seizures in childhood) is the commonest cause of drug-resistant temporal lobe epilepsy. MRI shows an atrophic, T2-bright hippocampus.

Alzheimer disease

Hippocampal and entorhinal atrophy is early and is measured clinically on MRI (medial temporal atrophy score).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.