Hippocampus

At a glance

English

Hippocampus

Latin (TA2)

Hippocampus

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the hippocampus

Deep in the medial temporal lobe lies the hippocampus, a key member of the limbic system. It is a strip of old, simplified cortex (archicortex) that has rolled inwards during development, so that it now bulges into the floor of the lateral ventricle's inferior (temporal) horn. From the surface of the brain it is hidden by the parahippocampal gyrus, which wraps around it, and it can only be seen once the ventricle is opened. Its name, Greek for seahorse, describes the curled shape in a section; the old term cornu Ammonis (Ammon's horn) gives the abbreviation CA used for its fields. Its front end is broad and grooved, the head or pes hippocampi, near the amygdala; the body and narrower tail curve up and back under the splenium of the corpus callosum.

In cross-section it shows two interlocking C-shaped strips. The hippocampus proper (CA1 to CA4) is a single dense layer of pyramidal neurons, and the dentate gyrus, with its layer of small granule cells, caps the CA4 end. Together with the subiculum these make up the hippocampal formation, and some authors also include the entorhinal cortex. Its ventricular surface is covered by a white layer, the alveus, whose fibres gather into the fimbria and continue as the fornix.

Most of its input comes from the entorhinal cortex in the parahippocampal gyrus, which gathers information from wide areas of association cortex. Fibres of the perforant path run to the dentate gyrus; mossy fibres from the granule cells pass to CA3; and Schaffer collaterals from CA3 reach CA1, which sends output to the subiculum and back to the entorhinal cortex. Further output leaves in the fornix for the septal nuclei and the mammillary bodies, forming part of the Papez circuit. The medial septal nucleus drives its theta rhythm. These circuits are needed to lay down new declarative memories of facts and events and to consolidate them in the cortex; 'place cells' here map the surroundings and support navigation.

Removal of both hippocampi, as in the patient H.M., leaves a lasting inability to form new memories. The hippocampus shrinks early in Alzheimer disease, is the usual focus of temporal lobe epilepsy (hippocampal sclerosis), is very sensitive to low oxygen, and is affected by prolonged stress.

Function and supply

Function

Forms new declarative memories and consolidates them, supports spatial memory and navigation, and helps regulate emotional and stress responses.

Blood supply

Hippocampal branches of the posterior cerebral artery, with the anterior choroidal artery supplying its head; drained by the basal vein.