Cognitive processes, such as learning and memory, are crucial for humans. In the latter, the hippocampus plays an important role. This is one of the regions that make up the hippocampal formation.
The hippocampal formation is a prominent C-shaped structure. It is located in part of the lateral ventricle of the brain. In itself, the hippocampus is made up of three subsets (CA1 – CA3). But is hippocampal formation reduced exclusively to the hippocampus?
Anatomical analysis of the hippocampal formation
In the 16th century, the anatomist Arantius first spoke of the appearance of human hippocampal formation. He gave it the name of seahorse, which derives from the Greek word for a sea horse.
However, hippocampal training is not limited exclusively to the hippocampus. It also includes the dentate gyrus, the subiculum and the entorhinal cortex.
Thus, the formation of the human hippocampus measures about five centimeters. In the middle, we can find the uncus. It is usually shaped like a leg and varies from brain to brain.
The architecture of the hippocampal formation
The dentate gyrus
The dentate gyrus is the part that is located in the middle of the cerebral cortex. It is a trilaminate cortical region. In the hippocampal formation, it 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 full of granule cell soma. The spinous dentrites of these cells branch out at the level of the dentate molecular layer. The granule cells and the molecular layers together make up the fascia dentata .
The third innermost layer of the dentate gyrus is called the polymorphic layer or hilum. Nearby, a fraction of the pyramidal cell layer is enclosed by the granule cell layer.
The hippocampus can be divided into three fields: CA1, CA2 and CA3. These fields have a main cell layer: the pyramidal cell layer. The area that separates it from the ventricular lumen is formed by axons of pyramidal cells and is called alveus. Historically, the division of this region was done according to:
- The stratum lucidum.
- The stratum radiatum.
- And the stratum lacunosum / moleculare.
The CA3 stratum lucidum consists of fibers that form synapses with dentrites above the pyramidal cell layer. The CA2 layer is relatively compact. In addition, it presents a layer of pyramidal cells. Its edges are difficult to determine.
The CA1 layer, on the other hand, is a subdomain of the hippocampus. The layer of pyramidal cells in this region can subdivide into an outer layer and an inner layer.
The CA1 layer and the subiculum form a transition zone. The subiculum can be divided into different layers:
- On the surface, we see a thick molecular layer where the dentrites of the subicular pyramidal cells develop. This layer of pyramidal cells can in turn be divided into two subcategories: the external and the internal.
- Cells in the outer layer contain an accumulation of lipofuscin pigment in their apical dentrites.
- The presubiculum consists of a surface layer that contains modified pyramidal neurons.
- The parasubiculum contains a cell layer that is difficult to differentiate from the presubiculum.
The term “entorhinal cortex” is synonymous with Brodmann’s area. This extends across the face to the middle of the amygdala.
This area is organized in thin layers. Its organization is a little different from the rest of the areas that make up the brain. We can especially distinguish six layers.
Hippocampal intrinsic circuit
Hippocampinic information connectivity follows a unidirectional, glutamatergic path that is part of a closed circuit. In this intrinsic chain of connections, the dentate gyrus is very important. It receives most of the information that is transmitted from the entorhinal cortex.
The extrinsic hippocampal circuit consists of:
- Several cortical areas
- The tonsillar complex
- The septal medial region
- The thalamus
- The supramillary region
- The monoaminergic nucleus of the brainstem
Thus, it would appear that the hippocampus receives sensory information from a variety of cortical regions.
These projections serve primarily to introduce sensory information into hippocampal formation.
The fimbria and the fornix form the classic efferent system of hippocampal formation. There are also great connections between the hippocampal formation and the amygdala. Other subcortical connections to the hippocampal formation include the medial septal complex. Finally, the connections that occur between the hippocampal formation and the hypothalamus are established through the subiculum.
So we were able to see that the hippocampal formation is a complex set of areas that include the hypothalamus. Even though much of this research has been done on animals, it is evident that the regions described here primarily form the human hippocampal formation.