Functions of the Hippocampus

The hippocampus, so named because its shape vaguely resembles that of a seahorse, is a symmehical shncture located inside the medial temporal lobe on both sides of the human brain. It is a curved sheet of cortex folded into the medial surface of the temporal lobe. In hansverse sections from rodent brain, the hippocampus has the appearance of tw o interlocking Cs vdth three distinct sub-fields, the dentate gyrus, the hippocampus proper (cornu 

In animals, the hippocampus is among the phylogenetically oldest parts of the brain. It occupies most of the ventroposte-rior and ventrolateral walls of the cerebral cortex in rodents. However, the hippocampus occupies less of the telencephalon in proportion to cerebral cortex in primates, especially in humans. The significant development of hippocampus volume in primates correlates with overall increase of brain mass and neocortical development. 

One striking feature of hippocampal circuitry is the pattern of afferent termination. Major hippocampal afferents originating from entorhinal cortex and ipsilateral and contralateral hippocampal subfields synapse on the dendrites of the principal cells in a laminated pattern. For instance, hippocampal commissural and associafional fibers synapse within the proximal one-third of the granule cell dendritic field, which is close to the cell body layer. The massive perforant path fibers terminate topographically in the outer two-thirds of the dendritic field. Afferents also have a laminar organization in the hippocampal proper. 

Affa ents to the Dentate Gyrus—The Perforant Path 

As emphasized by Cajal (1911) and later corroborated by Lor-ente de No (1934), the main input to the dentate gyrus is from the entorhinal cortex (but also perirhinal cortex, among others) by way of a fiber system called the perforant path. It is the major input to the hippocampus. The axons of the perforant path arise principally in layers II and III of the entorhinal cortex, with minor contributions from the deeper layers IV and V. Axons 

Efferents from the Dentate Gyrus—The Mossy Fibers 

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Scientists are only now beginning to discover why ketamine works the way it does. NDM A receptors in the human brain are densely localized in the cerebral cortex and hippocampus, areas that are important for higher functions like memory creation and retrieval. Ketamine, by blocking NDMA receptors, effectively disables the normal functioning of the hippocampus, accounting for short-term disorientation and memory loss. 

In explaining the function of the hippocampus—the brain structure in the medial temporal lobe of our brains—she says, That is what is active right now as you are trying to learn what I am telling you. Removal of the hippocampus leads to profound amnesia, and it is an amnesia where you cannot learn new information. She offers the example of Tom Hanks as Mr. Short-Term Memory on the old Saturday Night Live episodes. He walks out the door and crows, Nice to meet you completely unaware that they had met moments earlier. 

Few antagonists exist at present but the distribution of the peptide with high levels in cortex, hippocampus, amyglada and spinal cord may give some clues to potential functions of the peptide. 

From this brief description, it is apparent that the brain is really an assembly of organs all of which are structurally and functionally interconnected. Undoubtedly one of the most important areas for the psychopharmacologist is the so-called limbic system which is concerned with emotion. This region consists of the hippocampus (concerned with memory... 

A typical neuron consists of a cell body and an axonal projection through which information in the form of an action potential passes from the cell body to the axonal terminal. Information is received by the cell body via a complex array of dendrites which make contact with adjacent neurons. The structural complexity and the number of dendritic processes vary according to the type of nerve cell and its physiological function. For example, the granule cells in the dentate gyrus of the hippocampus (a region of the brain which plays a role in short-term memory) receives and integrates information from up to 10 000 other cells in the vicinity. 

Emx genes are important for the genesis of the hippocampus (Pellegrini et ah, 1996) and point mutations of Emx2 are associated with schizencephaly (Brunelli et ah, 1996). Thus, these homologues of early segmentation genes in Drosophila serve fundamental functions in the development in the human brain. 

Electromicroscopy of the presynaptic terminals of injured cortex treated with NGF shows a slight hypertrophy with an increase in synaptic differentiations of the membrane. The synaptic changes are larger and more pronounced in the cerebral cortex than in the cholinergic system. Biochemical changes have also been observed that indicate an improvement in the presynaptic functions of the cholinergic projections in the hippocampus and cerebral cortex. 

Over 30 structural imaging studies suggest that major depression is associated with a 5-10% loss of volume in the hippocampus, although some studies have not replicated this finding. Depression and chronic stress states have also been associated with a substantial loss of volume in the anterior cingulate and medial orbital frontal cortex. Loss of volume in structures such as the hippocampus also appears to increase as a function of the duration of illness and the amount of time that the depression remains untreated. 

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