Hippocampus

UBERON:0001954 · FMA:9771 · Atlas One / Scale 05 / Tissue

Bilateral archicortical (3-layer) structure in the medial temporal lobe. CA1–CA4 pyramidal neurons and dentate gyrus granule cells form the trisynaptic entorhinal-hippocampal circuit. Critical for episodic memory encoding (declarative), spatial navigation (place cells/cognitive map), and emotional memory. Site of canonical LTP (Bliss & Lømo 1973), adult neurogenesis (dentate gyrus SGZ), and the earliest tau pathology in Alzheimer’s disease.

Overview

The hippocampus is a paired, seahorse-shaped archicortical (three-layered) structure located in the medial temporal lobe, forming a prominent elevation on the floor of the inferior horn of the lateral ventricle. Approximately 3–4 cm long in each hemisphere, it is connected to overlying cortex and subcortical structures via the fornix (white matter arch to mammillary bodies, septal nuclei, and hypothalamus). Together with the dentate gyrus, subiculum, and entorhinal cortex, it forms the hippocampal formation — the primary limbic structure for declarative memory encoding, spatial navigation, and emotional memory modulation.

The critical importance of the hippocampus was established by the case of H.M. (Henry Molaison, 1953): bilateral hippocampal resection for intractable epilepsy produced profound anterograde amnesia (inability to form new declarative memories) while preserving procedural memory, intelligence, and remote memories — cleanly dissociating hippocampus-dependent from hippocampus-independent memory systems.

John O’Keefe’s 1971 discovery of hippocampal place cells (neurons firing selectively at specific locations in an environment) established the hippocampus as the neural substrate of the mammalian cognitive map — work recognised by the 2014 Nobel Prize in Physiology or Medicine (shared with Edvard and May-Britt Moser for grid cells in entorhinal cortex).

Anatomy — Subfields and Cytoarchitecture

Subfield	Pyramidal neuron size	Key inputs	Key outputs	Distinct features
CA1	Medium, densely packed	Schaffer collaterals (CA3), TA path (EC layer III)	Subiculum, EC layer V, prefrontal cortex	Canonical LTP model synapse; most vulnerable to ischaemia (Sommer sector); critical for memory output
CA2	Large, densely packed	Mossy fibres (DG), TA path, lateral EC	CA1, CA3	Social memory; relatively ischaemia-resistant; RGS14-enriched; resistant to standard LTP induction
CA3	Large, loosely packed	Mossy fibres (DG), perforant path (EC layer II)	Schaffer collaterals → CA1; commissural fibres	Autoassociative recurrent network; pattern completion; high intrinsic connectivity; epilepsy-prone
CA4 / Hilus	Mossy cells, interneurons	DG granule cell axons	DG (feedback inhibition)	Mossy cell loss → temporal lobe epilepsy after status epilepticus
Dentate Gyrus	Granule cells (~10 µm); ~1.2 million/human hippocampus	Perforant path (EC layer II)	Mossy fibres → CA3 thorny excrescence synapses	Pattern separation (sparse, orthogonal coding); adult neurogenesis (SGZ); powerful detonator synapses onto CA3

Anatomy — Trisynaptic and Temporo-Ammonic Circuits

TRISYNAPTIC ENTORHINAL-HIPPOCAMPAL CIRCUIT Input: Neocortical/limbic sensory information --> Entorhinal Cortex (EC) layer II stellate cells --> EC layer II mossy cells SYNAPSE 1 -- PERFORANT PATH (PP) Medial EC (MEC, grid cells) --> molecular layer of Dentate Gyrus [spatial/metric input] Lateral EC (LEC) --> DG outer molecular layer [non-spatial/object input] SYNAPSE 2 -- MOSSY FIBRES (MF) DG granule cells --> CA3 proximal dendrites (thorny excrescence synapses) Giant boutons (~4 um, multiple active zones) -- DETONATOR synapses Contain GluR2-lacking AMPAR (Ca2+-permeable) + kainate receptors (GluK2/K5) LOW stimulation: GABAergic interneuron inhibition dominant HIGH stimulation: detonator excitation of CA3 recurrent network SYNAPSE 3 -- SCHAFFER COLLATERALS (SC) CA3 pyramidal axon collaterals --> CA1 stratum radiatum dendrites AMPAR (fast EPSP) + NMDAR (Mg2+-gated coincidence detector) --> Canonical site of LTP induction (Bliss & Lomo, 1973) TEMPORO-AMMONIC (TA) PATHWAY (direct cortical bypass): EC layer III --> CA1 stratum lacunosum-moleculare (apical dendrites) Active during sleep (sharp wave-ripples); pattern completion and memory consolidation OUTPUT: CA1 --> Subiculum --> EC layer V --> Neocortex (memory consolidation) CA1 --> Prefrontal cortex (direct projection; working memory, context) CA1/subiculum --> Hypothalamus (via fornix; HPA axis, spatial context)

Function — Memory Systems

Episodic memory encoding. The hippocampus binds spatially and temporally separate elements of experience (what, where, when) into coherent episodic memories by rapidly indexing neocortical representations. The complementary learning systems (CLS) framework posits: hippocampus performs fast, one-shot learning of specific episodes (high plasticity, interference-prone) while neocortex extracts slow statistical regularities (semantic knowledge, stable). The index theory proposes that the hippocampus stores pointers to neocortical patterns rather than the patterns themselves.

Spatial navigation and cognitive map. Place cells (CA1/CA3 pyramidal neurons) fire selectively when the animal is at a specific location (“place field,” 0.5–5 m in humans). Grid cells (EC layer II/III) fire at multiple locations in a hexagonal lattice — a metric coordinate system. Head-direction cells (presubiculum, anterior thalamus) encode heading angle. These three cell types form an integrated navigation system that has been recorded in humans during virtual reality navigation tasks (Ekstrom et al., 2003).

Hippocampal oscillations and memory consolidation.

θ oscillations (4–8 Hz)

During active locomotion and REM sleep; driven by medial septal cholinergic/GABAergic pacemaker. θ phase-gates synaptic plasticity: LTP preferential at θ peak; LTD at θ trough. ACh enhances θ power via M1/M3 on pyramidal neurons and M2 on interneurons. Disrupted in early Alzheimer’s disease.

Sharp wave-ripples (SWR, 80–200 Hz)

During slow-wave sleep and quiet wakefulness; originate from CA3 synchronous bursts → CA1 ripples (PV⁺ basket cell oscillations). Replay recent experiences in compressed, accelerated form → hippocampal-to-neocortical memory transfer → systems consolidation. Disrupting SWRs impairs memory consolidation.

Plasticity — LTP and Adult Neurogenesis

Long-Term Potentiation (LTP) at Schaffer Collateral → CA1 synapses:

INDUCTION (E-LTP, Bliss & Lomo 1973): HFS (100 Hz, 1 s) of Schaffer collaterals --> AMPAR-mediated depolarisation --> NMDAR Mg2+ block relieved --> Ca2+ influx through NMDAR --> CaMKII T286 autophosphorylation (constitutively active) --> GluA1 Ser831 phosphorylation --> AMPAR conductance increase --> AMPAR exocytosis to PSD (Rab11/NSF-dependent) --> Silent synapse recruitment (AMPAR insertion into NMDAR-only synapses) --> Structural spine enlargement (F-actin remodelling, BDNF/TrkB/cofilin) CONSOLIDATION (L-LTP, >1 h): PKA/MAPK/ERK --> CREB Ser133 --> gene transcription --> Arc: regulates AMPAR endocytosis to limit LTP spread --> BDNF: TrkB --> MAPK/ERK + PI3K/Akt --> local dendritic protein synthesis --> CPEB: polyadenylates CaMKII-alpha, Arc mRNAs in dendrites --> New protein synthesis --> stable structural LTP, new synapses LTD (Low-frequency, 1 Hz, 900 pulses): Moderate NMDAR Ca2+ --> PP2B/calcineurin dominates --> PP1 --> GluA2 Ser880 dephosphorylation --> GRIP release --> AMPAR endocytosis --> Spine shrinkage; mGluR-LTD (Gq-IP3-PKC) also prominent in CA1

Adult neurogenesis in the dentate gyrus. Neural stem cells in the SGZ (type 1 radial glia-like cells: Sox2⁺/GFAP⁺/nestin⁺) → asymmetric division → type 2 intermediate progenitors (DCX⁺) → newborn granule cells (Prox1⁺) → integrate into trisynaptic circuit within 4–6 weeks. Newborn neurons have a critical window of enhanced plasticity (4–6 weeks post-mitosis) in which NMDAR-dependent LTP thresholds are lower than mature granule cells.

Regulation of neurogenesis: Enhanced by aerobic exercise (⇧VEGF, ⇧BDNF), enriched environment, learning. Suppressed by chronic stress (⇧glucocorticoids → ↓SGZ proliferation, ⇧apoptosis), ageing, alcohol, irradiation, and untreated depression.

Pathology

Alzheimer's Disease

The hippocampus and entorhinal cortex are the earliest sites of neurofibrillary tangle (NFT) deposition (hyperphosphorylated tau, Braak stages I–II) and neuronal loss. Amyloid-β oligomers impair LTP, activate complement-mediated synaptic pruning, and trigger TREM2-driven neuroinflammation. Clinically: hippocampal volume loss on MRI (entorhinal cortex + CA1) → anterograde amnesia years before other domains. Biomarkers: CSF Aβ₁–₄₂↓/p-tau⇧; amyloid PET; tau PET (entorhinal signal earliest). DMT: lecanemab, donanemab (anti-Aβ); symptomatic: donepezil, memantine.

Temporal Lobe Epilepsy (TLE) / Hippocampal Sclerosis

Selective neuronal loss in CA1 (Sommer sector), CA3, and hilar mossy cells/PV⁺ interneurons following prolonged febrile seizures or status epilepticus → reactive gliosis → hippocampal sclerosis. Loss of inhibitory interneurons → CA3 recurrent hyperexcitability → spontaneous temporal lobe seizures. Commonest cause of medically intractable epilepsy. Surgical resection (temporal lobectomy) → ~65–70% seizure-free at 2 years; MRI volumetry and EEG concordance required for localisation.

Depression and Hippocampal Atrophy

Recurrent MDD is associated with hippocampal volume reduction (~8–19% in severe/chronic cases) driven by glucocorticoid-mediated suppression of BDNF/TrkB/CREB signalling and adult neurogenesis. Neurogenesis hypothesis of depression (Santarelli et al., 2003): SSRIs require intact hippocampal neurogenesis for behavioural efficacy in animal models. Aerobic exercise (⇧BDNF, ⇧IGF-1 → ⇧SGZ proliferation) partly reverses atrophy. Ketamine (NMDAR antagonist → rapid AMPAR potentiation + BDNF release) achieves antidepressant effect within hours vs. 2–4 weeks for SSRIs.

PTSD

Hippocampal volume reduction (~6–8%) in PTSD vs. trauma-exposed non-PTSD controls — may be a predisposing factor. Impaired hippocampal contextual memory encoding → failure to discriminate safe from dangerous contexts → generalised fear and impaired fear extinction (vmPFC–hippocampus circuit). Treatments: trauma-focused CBT, EMDR; MDMA-assisted psychotherapy (Phase 3); propranolol reconsolidation interference (experimental).

Transient Global Amnesia (TGA)

Sudden, self-limiting anterograde amnesia (hours), often precipitated by Valsalva manoeuvre, emotional stress, or cold water. DWI MRI shows small hippocampal CA1 lesions. Mechanism: venous congestion → hippocampal ischaemia in CA1 (Sommer sector). Benign and self-resolving; no specific treatment required.

Limbic Encephalitis

Autoimmune encephalitis targeting limbic structures: Anti-NMDAR encephalitis (NR1 IgG, often ovarian teratoma) → acute psychosis, stereotypies, autonomic instability; 80% recovery with immunotherapy. Anti-LGI1 → faciobrachial dystonic seizures, hyponatraemia. Anti-CASPR2, anti-AMPAR, anti-GABA-B: each with distinct phenotypes. All: ⇧CSF protein, limbic FLAIR hyperintensity, EEG slow waves → urgent immunotherapy.

Cross-Atlas Connections

Part ofBrain (medial temporal lobe) Part ofNervous System ContainsPyramidal Neurons (CA1–CA3) + Granule Cells Site of canonicalLTP Synapse (Schaffer collateral) Tiled byAstrocyte Surveyed byMicroglia Trisynaptic circuit runs onGlutamate Inhibitory oscillations viaGABA interneurons Attenuated by chronicCortisol (GR/MR) Neurogenesis enhanced bySerotonin (SSRI)

References

Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021. Elsevier
Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022. NCBI Bookshelf
Bliss TV, Lømo T. Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. J Physiol. 1973;232(2):331–356. doi:10.1113/jphysiol.1973.sp010273 · PubMed 4727084
O’Keefe J, Dostrovsky J. The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat. Brain Res. 1971;34(1):171–175. doi:10.1016/0006-8993(71)90358-1 · PubMed 5124915