Hippocampus

Cellular structure and main components

Anatomically and histologically, several key regions and cell populations are distinguished in the hippocampus:

• Hippocampal fields (CA1–CA3): their basis is formed by pyramidal cells.
• Dentate gyrus: granular cells are located here.
• Conducting elements: perforant path, mossy fibers, and Schaffer collateral branches, which form synapses with the dendrites of CA3 field pyramids.
• Accessory structures: alveus of the hippocampus (alveus hippocampi) and fimbria (fimbria hippocampi).

An important role is played by interneurons of the hippocampus and cingulate gyrus. They are not tied to a specific cell layer. Biochemically, these cells contain not only the main inhibitory mediator GABA, but also calcium-binding proteins and various neuropeptides.

System of afferent fibers

The hippocampus receives signals from multiple sources, forming an extensive afferent network:

1. Entorhinal fibers. This is the main source of information. The entorhinal area (Brodmann area 28, allocortex) lies in the parahippocampal gyrus, acting as the "gateway" between the neocortex and the hippocampus. Most fibers form the perforant path running through the subiculum (subiculum). The fibers are glutamatergic (excitatory) and terminate on the distal branches of the dendrites of pyramidal and granular cells.
2. Septal fibers. They originate from the medial septum and the diagonal band of Broca. Cholinergic axons branch diffusely, whereas GABAergic ones branch strictly on the inhibitory interneurons of the hippocampus.
3. Commissural fibers. They connect the right and left hippocampi. They originate from CA3 pyramids and mossy cells of the dentate gyrus gate, and terminate on the proximal dendrites of neurons on the opposite side.
4. Brainstem afferents. Catecholaminergic fibers from various brainstem nuclei that distribute diffusely within the hippocampus.

Trisynaptic pathway and inhibitory control

The transmission of the excitatory signal (glutamate) in the hippocampus is strictly ordered and goes through three sequential stages:

• First synapse: The entorhinal cortex transmits the impulse to the granular cells of the dentate gyrus.
• Second synapse: From the granular cells via the mossy fiber system, the signal travels to the pyramidal cells of the CA3 field.
• Third synapse: Axons of CA3 cells give off Schaffer collaterals, which excite the pyramidal cells of the CA1 field.

At all three stages, this cascade is controlled by GABAergic inhibitory interneurons. They are localized on the cell bodies of excitatory neurons (basket cells), on the initial segments of their axons (chandelier-like cells), or directly on the dendrites.

Efferent pathways and functional connections with the amygdala

Output signals are formed by axons of the CA1 field, which head toward the subiculum (subiculum). From there, the fibers gather into the fimbria of the hippocampus and pass into the fornix (fornix) — the main efferent tract. Bypassing the diencephalon, the fornix reaches the mamillary bodies (corpus mamillare), thereby connecting the hippocampus to the hypothalamus and the autonomic nervous system.

Closely connected to the hippocampus is the amygdala (corpus amygdaloideum). Its medial and lateral zones belong to the limbic system. The main efferent pathway of the amygdala is the stria terminalis (stria terminalis), which splits and projects to the septum pellucidum area, the rostral hypothalamus, and the habenular nuclei.

Functions of the amygdala: upon stimulation, it induces a state of acute affect (anger, aggression), which is always accompanied by autonomic shifts (increased blood pressure, tachycardia, tachypnea). Its nuclei also modulate feeding and sexual behavior, and through the dorsal nucleus of the thalamus, the amygdala interacts with the orbitofrontal cortex.

What else to study on this topic

• Cytoarchitecture and borders of hippocampal fields CA1–CA3.
• Morphological differences of inhibitory interneurons: basket cells vs. chandelier cells.
• Amygdalar projections: topography of the stria terminalis and connections with habenular nuclei.
• Neurochemical profile of septal afferents: cholinergic and GABAergic influences on the hippocampus.

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

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