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Portrait of Hippocampus

Anatomy · Brain & nerves

Hippocampus

Greek 'seahorse'

Brain · limbic system, temporal lobe

A curved, seahorse-shaped fold of cortex tucked into the floor of the temporal horn of each lateral ventricle, deep in the medial temporal lobe. It is essential for forming new long-term memories and for spatial navigation, and it is one of the first regions damaged in Alzheimer's disease. One is found in each hemisphere.

How to spot them

  • ▸A curled, seahorse-shaped ridge of grey matter along the floor of the lower horn of the lateral ventricle
  • ▸Lies deep in the medial temporal lobe, just behind the amygdala
  • ▸Its outline is a C-shaped arc following the temporal horn, with a lobed head at the front
  • ▸A tight double-layer of cells: Ammon's horn and the dentate gyrus interlock like two curled scrolls

What to know

Named by the Bolognese anatomist Julius Caesar Aranzi in 1587 after a small marine animal

Hippocampus — Greek for 'seahorse'.

Its removal from both sides in 1953 left the patient H.M. unable to form new long-term memories

Hippocampus — Henry Molaison; surgery by William Scoville, studied by Brenda Milner.

London taxi drivers who had passed 'the Knowledge' were found to have an enlarged posterior part

Hippocampus — Eleanor Maguire's 2000 study.

Its 'place cells' fire when an animal is in a particular spot, work that shared a 2014 Nobel Prize

Hippocampus — John O'Keefe, with May-Britt and Edvard Moser.

Loss of volume here is among the earliest changes in Alzheimer's disease

Hippocampus — early memory loss reflects hippocampal atrophy.

Its main subdivision, Ammon's horn, is named after an Egyptian god with a ram's horns

Hippocampus — cornu Ammonis, abbreviated CA1 to CA4.

One of the few regions where new neurons are believed to form in the adult brain, in the dentate gyrus

Hippocampus — adult neurogenesis.

Key ideas

  • It consolidates short-term memory into long-term: patients with damage can remember old events and skills but cannot lay down new ones (anterograde amnesia).
  • It contains a map of space, built from place cells, which together with grid cells in the neighbouring entorhinal cortex makes a positioning system.

Quick-fire facts

  • •'Hippocampus' comes from the Greek for seahorse; Aranzi named it in 1587
  • •There is one in each temporal lobe, on the floor of the inferior horn of the lateral ventricle
  • •Patient H.M. (Henry Molaison) had both removed in 1953 to treat epilepsy; he could no longer make new declarative memories, though he could learn motor skills
  • •The 2014 Nobel Prize in Physiology or Medicine went to John O'Keefe, May-Britt Moser and Edvard Moser for the brain's positioning system
  • •Eleanor Maguire showed in 2000 that London taxi drivers have a larger posterior hippocampus
  • •Ammon's horn (cornu Ammonis) is divided into fields CA1 to CA4; CA1 is especially vulnerable to lack of oxygen
  • •The dentate gyrus is one of the few regions where new neurons are thought to arise in adults, though how far this happens in humans is still debated
  • •It is part of the limbic system and the Papez circuit
  • •Chronic stress and elevated cortisol are linked to reduced hippocampal volume

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Sources