Hippocampal Formation: Structure And Function
Cognitive processes, such as learning and memory, are an integral part of being human. The hippocampus plays a fundamental role in these processes, which are part of the regions that form the hippocampus formation.
The hippocampal formation is a C-shaped prominent structure, located in the lower part of the lateral ventricle of the brain. The hippocampus itself consists of three smaller fields (CA1-CA3). However, the hippocampal formation consists of more than just this structure.
Anatomical analysis of the hippocampal formation
In the 16th century, the hippocampus was first mentioned by the anatomist Julius Caesar Arantius. He called it the hippocampus, which comes from the Latin word for seahorse.
However, the hippocampal formation does not consist exclusively of the hippocampus. This term also includes gyrus dentatus, subiculum and cortex entorrhinicus.
Thus, the hippocampal formation is about five centimeters long. In the middle part of the formation we also find the uncus, which is shaped like a potato, and varies in size and shape from brain to brain.
The Hippocampus Formation: Architecture
Gyrus dentatus
The gyrus dentatus is the most medial part of the cerebral cortex. In cytoarchitecture, the dentate gyrus is a trilaminate cortical area. In the hippocampal formation, the dentate gyrus, in its typical C-shape, will separate ventrally from the first part of the hippocampus and from the subiculum at the hippocampal cleft.
The main layer of cells in this structure is full of granular cell bodies. The apical dendrites of these cells have branches in the molecular layer of the dentate gyrus. The granular cell bodies and the molecular layers combined, represent the fascia dentatus.
Furthermore, the third and innermost layer of the dentate gyrus is the polymorphic layer, or hilum. Next to it, we find a fraction of the layer of pyramidal cells, which is surrounded by the granular cells.
Hippocampus
The hippocampus has three subfields called CA1, Ca2 and Ca3. They consist of one cell layer: The layer of pyramidal cells. The surface that joins the ventricular lumen, formed by the axons of pyramidal cells, is called the alveus. Historically, this region consists of:
- Stratum lucidum (not to be confused with the skin layer)
- Stratum radiatum
- Lacunosum molecular
Stratum lucidum, CA3, has fibers that form proximal dendritic synapses across the cell wall of pyramidal cells in this layer. The CA2 layer is relatively compact and also presents a layer of pyramidal cells, but it is difficult to define the outline.
Furthermore, since the CA1 layer is a subarea of the hippocampus, the layer of pyramidal cells in this region consists of both an inner and outer layer.
Subiculum
The CA1 layer and subiculum overlap at the edges, forming a transition area. Subiculum is mainly divided into the following layers:
- Superficially, there is a broad molecular layer where subicular pyramidal cell dendrites are located. In turn, this cell layer of pyramidal cells can be divided into two layers again: the outer and inner.
- Cells in the outer layer have an accumulation of lipofuscin pigments in their apical dendrites.
- The presubiculum consists of a superficial layer, which contains modified pyramidal neurons.
- The parasubiculum is a cell layer that is difficult to differentiate from the presubiculum.
Cortex entorrhinicus
The term “cortex entorrhinicus” is basically synonymous with Brodmann’s areas. It extends mainly rostally towards the middle part of the amygdala, and caudally towards the anterior commissure of the lateral knee body.
This area is somewhat different from the rest of the brain areas.
Connections in the hippocampal formation
Hippocampus’ integrated circuit
Connection in the hippocampal formation follows a one-way glutamate, or excitatory, pathway that is part of a closed circuit. In this integrated circuit of connections, the gyrus dentatus is very important, since it receives the most information and then transmits it on to the cortex entorrhinicus.
Extrinsic compounds
The extrinsic circuit in the hippocampus is formed by:
- Multiple cortical arteries.
- Amygdaloid complex.
- Medial septal nucleus.
- Thalamus.
- Supramammaillary nucleus.
- Monominergic nucleus in the brainstem.
This is apparently how the hippocampus receives sensory information from a number of cortical regions.
Cortical connections
These projections will mainly be used to introduce sensory information into the hippocampal formation.
Subcortical connections and hippocampal formation
Fimbriae and fornix form the classical efferent system of the hippocampal formation. In addition, there are also good connections between the hippocampal formation and the amygdala.
Finally, the connections produced between the hippocampal formation and the hypothalamus will be established through the subiculum.
As you can see, the hippocampal formation is a set of complex regions that include the hypothalamus. Although most research projects on this topic have studied animals, it seems clear that the areas described there are very similar to the human hippocampus formation.
