Structure · one in each half of the brain

Hippocampus

Also called Hippocampal formation (with nearby parts such as the dentate gyrus and subiculum)

The seahorse-shaped memory maker that turns your day into lasting memories.

Open in 3D

The hippocampus is a curved ridge of brain tissue tucked deep inside each temporal lobe. It is essential for forming new memories of events and facts, like what you ate this morning or where you parked. It works like a "save" button: it records the sights, sounds and other details of a moment as a memory, then helps the rest of the brain store it for the long term. It also builds a mental map of places, so you can find your way around.

Where it is

Deep inside each temporal lobe, on its inner side. It curves along the floor of the lower horn of the lateral ventricle. You have two, one on each side.

What it does

Think of it as a busy newsroom editor who gathers the day's stories, ties the details together, and sends the important ones to the long-term archive.

Inside the hippocampus

• Cornu Ammonis (CA fields)Ammon's horn: the curled band of the hippocampus where memories take shape.
• Dentate gyrusThe hippocampus's front door, and one of the few spots where new neurons may form.
• SubiculumThe hippocampus's main exit, sending its processed signals out to the rest of the brain.

Works with

• Entorhinal cortexThe entorhinal cortex is its main doorway from the rest of the cortex. It also holds "grid cells", whose firing forms a regular pattern like the coordinates on a map.
• FornixThe fornix is its main output cable, carrying its messages to the mammillary bodies and other deep areas.
• AmygdalaIts next-door neighbor, the amygdala, boosts the storage of emotional events, which is why exciting or scary moments tend to stick.
• HypothalamusIt has direct links to the hypothalamus. When stress hormones run high, these links calm the hypothalamus and help wind down the stress response.
• Mammillary bodiesThe mammillary bodies relay its signals onward to the thalamus, as part of a memory loop called the Papez circuit.

When it goes wrong

For learning only, not medical advice. If you have a health concern, talk to a doctor.

Surprising facts

• London taxi drivers have long needed expert navigation skills for their job. Brain scans showed that the back part of their hippocampus was larger than in similar people who did not drive taxis, and the longer someone had driven a taxi, the larger it tended to be.
• In 1971, John O'Keefe and a student discovered "place cells" in the rat hippocampus: neurons that fire when the animal is in a particular spot. Many years later, O'Keefe won a Nobel Prize.
• It is one of the very few parts of the adult brain that can make new neurons, at least in most mammals studied so far. The new cells are born mainly in its dentate gyrus.

By the numbers

Where the name comes from

From the Greek "hippokampos", a seahorse, made from "hippos" (horse) and "kampos" (sea monster). The Italian anatomist Julius Caesar Arantius proposed the name in 1587, because the structure looked to him like a seahorse.

Learn it in a lesson

• MemoryThe hippocampus and its circuit. A short guided lesson in 3D.

Sources

1. Neuroanatomy, Hippocampus, StatPearls, NCBI Bookshelf
2. 18.3 Explicit Memories: Episodic and Semantic Memories, OpenStax, Introduction to Behavioral Neuroscience
3. Where are memories stored in the brain?, Queensland Brain Institute, University of Queensland
4. 12.2 Neural Mechanisms and Circuitry of the Stress Response, OpenStax, Introduction to Behavioral Neuroscience
5. Hippocampus and cornu ammonis: mythonyms that prevail in Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Histologica, Frontiers in Neuroanatomy
6. The Anatomy of the Hippocampus, Cerebral Ischemia (Exon Publications), NCBI Bookshelf

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