Corpus: Hippocampus

The hippocampus is a paired brain structure that is part of the limbic system. It is primarily involved in memory formation. Here, information is probably transferred from the short-term memory (primary memory) to the long-term memory.

The hippocampus is located on the inner edge of the caudal temporal lobe. It belongs to the archicortex and is its largest part.

Morphology[Bearbeiten]

The hippocampus consists of several structures. This is why it is also referred to as a "hippocampal formation" or "formatio hippocampi". In cross-section (frontal), a distinction is made between three areas:

• dentate gyrus
• Cornu ammonis (Ammon's horn) and
• subiculum.

The "actual" hippocampus ("hippocampus proprius") with its four fields CA1 to CA4 corresponds to the ammonic horn. The rostral end of the hippocampus forms a paw-like structure in the temporal lobe, the pes hippocampi.

If one compares the curled cortical structure of the hippocampus with a seahorse, then the dentate gyrus corresponds to the distal part of the "tail". The subiculum is the transition area from the three-layered, archicortical hippocampus to the six-layered neocortex. It lies between the CA1 region of the ammonial horn and the entorhinal cortex.

Topography[Bearbeiten]

The hippocampus is located in the medial temporal lobe and borders laterally on the inferior horn of the lateral ventricle. Rostrally is the corpus amygdaloideum. The caudal end of the corpus callosum represents the posterior upper boundary. With the fornix, it extends below the bar in an arc to the corpora mamillaria.

The indusium griseum, which lies on the bar, is often seen as a continuation of the hippocampus, as the hippocampus is initially located on the bar during development and only later moves caudally as a result of hemispheric rotation.

Afferents[Bearbeiten]

The hippocampus receives its afferents from the entorhinal area via the tractus perforans. The tractus perforans borders on the stratum moleculare of the dentate gyrus and the cornu ammonis. Dendrites of the granule cells and pyramidal cells project into the stratum moleculare. Further afferents come from the thalamus, the cingulate gyrus as well as from the corpus amygdaloideum and neocortex areas.

Efferents[Bearbeiten]

The efferents (axons of the pyramidal cells) run in the peripheral alveus, which then continues into the fornix via the fimbria hippocampi. The fornix ends in the corpora mammillaria (see: Papez neuron circle) and also gives off fibres to the hypothalamus, the corpus amygadoideum and the septal nuclei.

Information flow[Bearbeiten]

To put it simply, the majority of information processed in the hippocampus takes the following route: Information reaches the dentate gyrus via the perforating tract. From there, further processing takes place in the cornu ammonis before the information reaches the fornix via the subiculum and is passed on to further structures.

The hippocampus is part of the archicortex. A distinction is therefore made between 3 or 4 layers, in contrast to the six-layered neocortex.

Layers of the dentate gyrus[Bearbeiten]

There are 3 layers in the dentate gyrus:

• Stratum moleculare: Carries afferents from the regio entorhinalis, analogous to the cornu ammonis
• Stratum granulare: Contains the cell bodies of the granule cells
• Stratum multiforme: Contains, among other things, the axons of the granule cells (mossy fibres). These are connected to the pyramidal cells in the stratum lucidum in CA3 (see below).

Layers of the cornu ammonis[Bearbeiten]

From the inside to the outside there are the following 4 layers:

• Stratum moleculare: Carries afferents from the regio entorhinalis (via the tractus perforans) as well as dendrites of cells from the stratum pyramidale
• Stratum radiatum: So-called Schaffer collaterals terminate there, via which pyramidal cells from CA3 can activate pyramidal cells from CA1.
• Stratum pyramidale: Contains the cell bodies of the pyramidal cells. It is subdivided into the fields CA1, CA2, CA3 and CA4 (end plate).
  • The CA3 field contains the stratum lucidum between the stratum radiatum and the stratum pyramidale. The stratum lucidum contains mossy fibres that form en passant synapses with the pyramidal cell dendrites.^[1]
• Stratum oriens: Contains, among other things, basal dendrites of the pyramidal cells and basket cells. These inhibit the pyramidal cells.

As the hippocampus is an important component for memory formation, the destruction of both hippocampi prevents information from being transferred from short-term to long-term memory. This plays a role above all in Alzheimer's disease: it is assumed that the destruction of neurons in the hippocampus (and the neighbouring entorhinal area in the parahippocampal gyrus) leads to the characteristic memory disorders.

Furthermore, the neurons of the hippocampus are prone to spontaneous discharges, which can lead to epileptic (complex focal) seizures in tumours, for example.

It is assumed that underactivity of the hippocampus and parahippocampal gyrus neurones can contribute to the development of schizophrenia.

• Aumüller et al, Duale Reihe Anatomie, Thieme Verlag, 2nd edition, 2010
• Trepel, Neuroanatomie - Struktur und Funktion, Urban & Fischer, 5th edition, 2012