The hippocampal formation: structure and functions

Cognitive processes, such as learning and memory, are crucial for human beings. In them it plays an important role the hippocampus, one of the regions that comprise the hippocampal formation.

The hippocampal formation is a prominent C-shaped structure. It sits in the floor of a part of the lateral ventricle of the brain. The hippocampus itself consists of three main subfields (CA1 – CA3). But is the hippocampal formation reduced exclusively to the hippocampus?

Anatomical analysis of the hippocampal formation

In the 16th century, the anatomist Arantius (2) spoke for the first time about the appearance of the human hippocampal formation. He gives you the name hippocampus, which derives from the Greek word describing a seahorse.

However, the hippocampal formation is not exclusively reduced to the hippocampus. The hippocampal formation is also the dentate gyrus, the subicular complex, and the entorhinal cortex.

So, The human hippocampal formation has an approximate extension of five centimeters. In the middle part of the hippocampal formation we can find the uncus. Thus, it has a shape similar to that of a paw and varies greatly from one brain to another.

The architecture of the hippocampal formation

toothed turn

The dentate gyrus forms the most medial part of the cerebral cortex. At the cytoarchitectonic level, The dentate gyrus is a trilaminate cortical region. In the hippocampal formation, the dentate gyrus forms a typical C-shaped structure that is ventrally separated from the first part of the hippocampus and the subiculum by the hippocampal fissure.

The main cell layer of this structure is filled with granule cell soma. The spiny dendrites of these cells branch into the dentate molecular layer. The granule cells and molecular layers together represent the fascia dentata.

Third The innermost layer of the dentate gyrus is called the polymorphic layer or hilum.. Near it, a fraction of the pyramidal cell layer is enclosed by the granule cell layer.

The hippocampus

The hippocampus can be divided into three fields: CA1, CA2 and CA3. These fields mainly have one cell layer: the pyramidal cell layer. The surface that borders the ventricular lumen is formed by axons of the pyramidal cells and is called alveus. Historically, The division of this region has been made up of:

The stratum lucida. The stratum radiatum. Lacunosumoleculare layer.

The stratum lucidum CA3 It is composed of fibers that form synapses with proximal dendrites above the pyramidal cell layer of this stratum. The CA2 layer is relatively compact. In addition, it has a layer of pyramidal cells. Its borders are difficult to determine.

The CA1 layer, for its part, is a subfield of the hippocampus. (3). The layer of pyramidal cells in this region can be further subdivided into an outer and inner layer.

subiculum

The CA1 layer and the subiculum overlap at their edges, forming a transitional zone. Mainly, the subiculum can be divided into the following layers:

Superficially, there is a broad molecular layer where the dendrites of the subicular pyramidal cells extend. In turn, this layer of pyramidal cells can be divided into two sublayers: the external and the internal. The cells of the external layer contain an accumulation of lipofuscine pigment in their apical dendrites. The presubiculum consists of a superficial layer, which contains pyramidal neurons modified.The parasubiculum contains a cellular layer that is difficult to differentiate from the presubiculum.

entorhinal cortex

The term ‘entorhinal cortex’ is synonymous with Brodmann’s area. Primarily, this extends rostrally towards the middle part of the amygdala, and caudally towards the anterior limit of the lateral geniculate nucleus.

This area is organized into sheets. However, this organization is somewhat different from that of the rest of the areas that make up the brain. Mainly, six layers can be distinguished.

Connectivity

Hippocampal intrinsic circuit

Mainly, the connectivity of hippocampal information follows a unidirectional and glutamatergic (excitatory) path that is part of a closed circuit. In this intrinsic chain of connections, the dentate gyrus is very important. This receives most of the information that is transmitted from the entorhinal cortex.

Extrinsic connections

The extrinsic hippocampal circuit It is composed of:

Various cortical areas. The amygdaloid complex. The medial septal region. The thalamus. The supramammillary region. The monoaminergic nucleus of the brain stem.

So, it seems that The hippocampus receives sensory information from a variety of cortical regions.

Cortical connections

These projections mainly serve to introduce sensory information into the hippocampal formation.

Subcortical connections

The fimbria and fornix form the classical efferent system of the hippocampal formation. In addition, there are also great connections between the hippocampal formation and the amygdala. Thus, other subcortical projections to the hippocampal formation are those that include the medial septal complex. Finally, The connections that occur between the hippocampal formation and the hypothalamus are established through the subiculum.

Thus, we have been able to see that the hippocampal formation is a complex set of areas that include the hypothalamus. Although much of the research has been carried out with animals, it seems clear that the regions described here mainly form the human hippocampal formation.

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