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Hippocampus

life science Maturity 5-7 Next article →

The hippocampus is a vital component of the brain found in humans and many other vertebrates.

Sobo 1909 639
Sobo 1909 639

It serves as a major part of the hippocampal formation, which is located within the limbic system. This formation includes the hippocampus proper, the dentate gyrus, and the subiculum. The hippocampus is essential for the consolidation of information. This process moves data from short-term memory into long-term memory. It is also critical for spatial memory, which allows organisms to navigate their environments.

Blausen 0614 LimbicSystem
Blausen 0614 LimbicSystem

In the human brain, the hippocampus is an inward fold of the archicortex. This is one of the three regions of the allocortex. It is located in each hemisphere of the brain. The structure has direct neural projections to the neocortex. It also has reciprocal indirect projections from the neocortex. The hippocampus is a five centimeter long ridge of gray matter. It sits within the parahippocampal gyrus. This gyrus can only be seen when it is opened. The hippocampus stretches from the amygdala to the splenium of the corpus callosum. It is divided into subregions called the head, body, and tail.

Brainmaps-macaque-hippocampus
Brainmaps-macaque-hippocampus

The hippocampus operates through complex neural pathways. The major input to the hippocampus comes through the entorhinal cortex (EC). The EC acts as an interface between the neocortex and the hippocampus. Information can reach the CA1 area through two main pathways. One is the direct perforant pathway. This pathway originates in layer III of the EC. It forms synapses on the distal apical dendrites of CA1 neurons. The other is the indirect pathway, known as the trisynaptic circuit. In this circuit, axons from layer II of the EC project to the dentate gyrus. This is the first synapse. Then, information moves via mossy cell fibers to the CA3 area. This is the second synapse. Finally, axons called Schaffer collaterals move from CA3 to CA1. This is the third synapse.

CajalHippocampus (modified)
CajalHippocampus (modified)

Researchers study the hippocampus to understand neurophysiology. The structure is highly organized into distinct layers of neuronal cell types. This makes it an excellent model system. Scientists study a process called long-term potentiation (LTP). LTP is a form of neural plasticity. It is widely believed to be a main mechanism for memory storage. In rodents, scientists study the hippocampus to understand spatial navigation. Many neurons in rats and mice act as place cells. These cells fire bursts of action potentials when an animal enters a specific location.

Place Cell Spiking Activity Example
Place Cell Spiking Activity Example

These place cells interact with head direction cells and grid cells in the entorhinal cortex.

The history of studying the hippocampus is quite long. The Venetian anatomist Julius Caesar Aranzi first described the ridge in 1587. He compared it to a silkworm and then a seahorse. The name comes from the Greek words for horse and sea monster. In 1729, Johann Georg Duvernoy provided the first illustrations. Later, Jacob Winsløw suggested the name "ram's horn" in 1732. The surgeon de Garengeot used the term cornu Ammonis. This refers to the horn of Amun, an ancient Egyptian god. In 1861, a dispute called the "Great Hippocampus Question" arose. This was a debate between Thomas Henry Huxley and Richard Owen regarding human evolution.

Hippocampus and seahorse cropped
Hippocampus and seahorse cropped

Understanding the hippocampus is important for medical science. It is one of the first regions damaged by Alzheimer's disease. This damage causes early symptoms like short-term memory loss and disorientation. Other causes of damage include hypoxia, which is oxygen starvation. Encephalitis and medial temporal lobe epilepsy can also cause damage. Extensive, bilateral damage can lead to anterograde amnesia. This is the inability to form or retain new memories. The size of the hippocampus also relates to memory capacity. Species with greater spatial memory capacity tend to have larger hippocampal volumes.

The hippocampus is part of a much larger system. It is anatomically connected to regions involved in emotional behavior. These include the septal area and the hypothalamic mammillary bodies. It also connects to the anterior nuclear complex in the thalamus. The hippocampus is part of the limbic lobe. This lobe was first described by Broca in 1878. The limbic system includes the cingulate gyrus and the parahippocampal gyrus. While the hippocampus is small, its role is massive. In adult humans, each hippocampus is about 3.0 to 3.5 cm3. This is small compared to the neocortex, which is 320 to 420 cm3.

HIPO-33-533-g013
HIPO-33-533-g013
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🖼️ Images & Media (14)
File:Hippocampus and seahorse cropped.JPG
Hippocampus and seahorse cropped.JPG
File:Blausen 0614 LimbicSystem.png
Blausen 0614 LimbicSystem.png
File:Sobo 1909 639.png
Sobo 1909 639.png
File:Brainmaps-macaque-hippocampus.jpg
Brainmaps-macaque-hippocampus.jpg
File:CajalHippocampus (modified).png
CajalHippocampus (modified).png
File:Hippocampus coronal section176157.fig.004.jpg
Hippocampus coronal section176157.fig.004.jpg
File:Golgi Hippocampus.jpg
Golgi Hippocampus.jpg
File:Rats_and_cognitive_maps_and_maze.png
Rats_and_cognitive_maps_and_maze.png
File:Place Cell Spiking Activity Example.png
Place Cell Spiking Activity Example.png
File:HIPO-33-533-g013.jpg
HIPO-33-533-g013.jpg
File:Rat-hippocampal-activity-modes.png
Rat-hippocampal-activity-modes.png
File:eeg theta.svg
eeg theta.svg

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