What is the hippocampus? Anatomy and Duties

The hippocampus is an area of ​​the brain associated with memory. It got its name from the Greek word that means this, because its shape resembles a seahorse. It is located in the inner part of the temporal lobe . It is part of the limbic system responsible for regulating emotional responses . It is responsible for creating and preserving long-term memories. It also has a role in space recognition and orientation.


The hippocampus connects with the rest of the brain through a structure called the entorhinal cortex. The entorhinal cortex is located below the anterior part of the hippocampus. The hippocampus itself is subdivided into the cornu ammonis (CA1-4), dentate gyrus, and subiculum.

The subregions of the hippocampus are connected by two major neural circuits: the trisynaptic circuit and the monosynaptic circuit. The trisynaptic circuit transmits signals from the entorhinal cortex to the dentate gyrus via the perforating pathway that crosses the subiculum. The signals then reach CA3 from the dentate gyrus via the mossy fiber tract (named after the excessive branching of the axons). Finally, signals are carried from CA3 to CA1 by bundles of axons known as Schaffer collaterals. The circuit is completed by projections towards the subiculum and entorhinal cortex. Monosynaptic inputs bypass the dentate gyrus and CA3 and transmit data directly from the entorhinal cortex to CA1.

While the cerebral cortex has six layers, the CA areas consist of three layers. It uses pyramidal cells as principal excitatory cells. Pyramidal cells are neurons that resemble a pyramidal shape due to the spread of their dendrites. The CA3 region of the hippocampus contains a large excitatory recurrent collateral network, that is, axons excite input fibers or dendrites. This constitutes CA3’s largest source of input.

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The dentate gyrus is morphologically different from the CA areas and densely contains neurons called granule cells because their bodies are relatively small. The dentate gyrus is one of two areas in the brain known to contain neuronal stem cells. These cells can differentiate into new neurons throughout adult life.

The hippocampus receives input from regulatory (modulatory) neurotransmitter systems such as the serotonin, norepinephrine, and dopamine systems . It also receives cholinergic inputs from the medial septum that regulate the hippocampal physiological state. The medial septum plays a role in regulating theta rhythm , which is one of the critical vibrational rhythms of the hippocampus . The disappearance of this region or the associated theta rhythm impairs hippocampus function.

Hippocampus Functions

Theories about the functions of the hippocampus relate to space perception and memory. A 1971 study showed that hippocampus cells generate action potentials when mice pass through certain environments or “place areas.” It has therefore been suggested that the hippocampus was used to create a map of the environment. Later visual navigation studies in humans also revealed a strong relationship between the hippocampus and spatial navigation. The memory hypothesis emerged in 1957 when it was observed that the removal of the hippocampus led to the loss of the ability to form new memory (especially declarative memory associated with places and events ).

Although there is almost universal acceptance among scientists about the importance of the hippocampus for memory, the exact processes by which the hippocampus supports memory is a matter of debate. Some studies suggest that the hippocampus hides objects and places by relating them to certain experiences. Other research suggests that the hippocampus is particularly active during conscious recall or during mental time travel during recall. Other studies have shown that the hippocampus supports rapid learning by preventing similar memories from interfering (for example, remembering where the car was parked today rather than yesterday). There is also the view that the hippocampus, like the index on the back of a book, holds together the elements associated with experience, but does not hide the experience itself.

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Controversy continues as to whether long-term memories are ultimately independent of the hippocampus and the adequacy of the cortex to support recall. This view is known as the standard model of system consolidation. In contrast, the multi-track theory argues that the hippocampus is essential for recalling long-term episodic (content-rich) memories, but not for semantic or summary memories. In fact, the structure, function, and connections of the hippocampus are not uniform along its long axis. Its anterior part mostly connects with the amygdala and orbitofrontal cortex. It is thought to be mainly related to emotion and stress regulation. The posterior part is in contact with the retrosplenial and posterior parietal cortices. It is thought to be mainly related to cognitive and spatial processes.

Hippocampal Dysfunction

One of the first studies on hippocampal dysfunction was the observations made in 1953 in a patient named HM, whose medial temporal lobes, including the hippocampus and its surroundings, were surgically removed for the treatment of epileptic seizures. Although his cognitive functions remained mostly intact after the surgery, it was observed that he could not form new memories about events and facts (anterograde amnesia). Such amnesia is typical for diseases that cause hippocampal dysfunction. For example, Alzheimer’s disease Severe cell loss is seen in the hippocampus, which is associated with memory impairment in the early stages of the disease. Stress and depression are associated with loss of new cell generation ability in the dentate gyrus, loss of dendritic appendages, and decreased dendritic branching throughout the hippocampus. Hippocampal dysfunction may also be present in schizophrenia and related disorders. Hippocampus damage and anterograde amnesia are common in people whose brain is deprived of oxygen. Finally, the hippocampus is a place where epileptic seizures are concentrated and can cause hippocampal sclerosis (pathological loss of cells).

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