Brain: Dentate gyrus | ||
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Diagram of hippocampal regions. DG: Dentate gyrus. | ||
Coronal section of brain immediately in front of pons. (Label for "Gyrus dentatus" is at bottom left.) | ||
Latin | gyrus dentatus | |
Gray's | subject #189 827 | |
Part of | Temporal lobe | |
Artery | Posterior cerebral Anterior choroidal |
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NeuroNames | hier-161 | |
MeSH | Dentate+Gyrus | |
NeuroLex ID | birnlex_1178 |
The dentate gyrus is part of the hippocampal formation. It is thought to contribute to new memories as well as other functional roles.[1][2] It is notable as being one of a select few brain structures currently known to have high rates of neurogenesis in adult rats,[3] (other sites include the olfactory bulb and cerebellum).[4][5]
The dentate gyrus cells receive excitatory input from the entorhinal cortex, in the medial temporal lobe , through the perforant path into the molecular layer(Xavier GF, et al., 2009).This activates pyramidal cells among the CA4 and CA3 pyramidal neurons. CA 4 and 3 stand for Cornu Ammonis, areas in the neocortex(Xavier GF, et al., 2009). Via the Schaffer collaterals, contact CA1 pyramidal cells, allowing the Dentate Gyrus to be in a position to control the flow of information within the hippocampus(Xavier GF, et al., 2009).
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The dentate gyrus consists of three layers of neurons: molecular, granular, and polymorphic. The middle layer is most prominent and contains granule cells that project to the CA3 subfield of the hippocampus.[6] These granule cells project mostly to interneurons, but also to pyramidal cells and are the principal excitatory neurons of the dentate gyrus. The major input to the dentate gyrus (the so-called perforant pathway) is from layer 2 of the entorhinal cortex, and the dentate gyrus receives no direct inputs from other cortical structures. The perforant pathway is divided into the medial perforant path and the lateral perforant path, generated, respectively, at the medial and lateral portions of the entorhinal cortex. The medial perforant path synapses onto the proximal dendritic area of the granule cells, whereas the lateral perforant path does so onto the distal dendrites of these same cells.
The granule cells in the dentate gyrus are distinguished by their late time of formation during brain development. In rats, approximately 85% of the granule cells are generated after birth.[7] In humans, it is estimated that granule cells begin to be generated during gestation weeks 10.5 to 11, and continue being generated during the second and third trimesters, after birth and all the way into adulthood.[8][9] The germinal sources of granule cells and their migration pathways [10][11] have been studied during rat brain development. The oldest granule cells are generated in a specific region of the hippocampal neuroepithelium and migrate into the primordial dentate gyrus around embryonic days (E) 17/18, and then settle as the outermost cells in the forming granular layer. Next, dentate precursor cells move out of this same area of the hippocampal neuroepithelium and, retaining their mitotic capacity, invade the hilus (core) of the forming dentate gyrus. This dispersed germinal matrix is the source of granule cells from that point on. The newly generated granule cells accumulate under the older cells that began to settle in the granular layer. As more granule cells are produced, the layer thickens and the cells are stacked up according to age - the oldest being the most superficial and the youngest being deeper.[12] The granule cell precursors remain in a subgranular zone that becomes progressively thinner as the dentate gyrus grows, but these precursor cells are retained in adult rats. These sparsely scattered cells constantly generate granule cell neurons,[13][14] which add to the total population. Thus, granule cells in the dentate gyrus are possibly the only known population of neurons in the brain that are constantly increasing their numbers. In 2010, it was shown that the balance between neural stem cells (NSCs) and neural progenitor cells (NPCs) is maintained by an interaction between the epidermal growth factor receptor signaling pathway and Notch signaling pathway.[15]
The dentate gyrus is thought to contribute to the formation of memories and to play a role in depression.
The dentate gyrus is one of the few regions of the adult brain where neurogenesis (i.e., the birth of new neurons) takes place. Neurogenesis is thought to play a role in the formation of new memories. New memories could preferentially utilize newly-formed dentate gyrus cell, providing a potential mechanism for distinguishing multiple instances of similar events or multiple visits to the same location. A Study at the Human Nutrition Research Center on Aging showed that feeding blueberry extract to older rats for a short time frame increases neurogenesis in the dentate gyrus. This increased neurogenesis is associated with improved spatial memory, as seen through performance in a maze.[17]
The dentate gyrus may also have a functional role in stress and depression. For instance, neurogenesis has been found to increase in response to chronic treatment with antidepressants.[18] On the contrary, however, the physiological effects of stress, often characterized by release of glucocorticoids such as cortisol, as well as activation of the sympathetic division of the autonomic nervous system, have been shown to inhibit the process of neurogenesis in primates.[19] Both endogenous and exogenous glucocorticoids are known to cause psychosis and depression,[20] implying that neurogenesis in the dentate gyrus may play an important role in modulating symptoms of stress and depression.
Some evidence suggests that neurogenesis in the dentate gyrus increases in response to aerobic exercise.[21] It has been shown through several experiments that neurogenesis (the development of nerve tissues) often increase in the dentate gyrus of adult rodents when they are exposed to an enriched environment. [22]
Studies shown, that by destroying about 90% of the dentate gyrus (dg) cells in rats, had extreme difficulty in maneuvering through a maze which they had currently been through prior to the lesion. When being put through the test a numerous amount of times to give the rat time to see if it could relearn the maze, the results showed that the rats showed no improvement at all which indicated that their working memory is severely impaired. Rats have trouble with place strategies because they cannot consolidate what they have learned about the maze in the current run into the their working memory so the they can remember it later when maneuvering through the same maze a repeated number of times. Every time the rat enters the maze, it feels like it has seen it for the first time. [23]
Studies by researchers at Columbia University Medical Center indicate that poor glucose control can lead to deleterious effects on the dentate gyrus.[24]
Praag H. (1999). Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus. Nature Neuroscience 2, 266 - 270.
Gilberto Fernando Xavier; Valéria Catelli Infantozzi Costa .(2009).Dentate gyrus and spatial behaviour."Progress in Neuropsychopharmacology & Biological Psychiatry". 33 (5), pg. 762-773 .
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