The dentate gyrus is a region within the brain's hippocampus. It is part of a group of neural tissues associated with the creation of certain kinds of memory, with habit formation, and with learning various skills. Unlike many other neuronal types, new cells within this part of the hippocampus grow throughout much of an organism's life. In neurodegenerative diseases, the cells of the dentate gyrus atrophy and die, contributing to the memory loss associated with these ailments.
The hippocampus, where the dentate gyrus is located, is in the cerebral cortex of many higher animals. In humans, it is on both sides of the temporal lobe, where it forms circuits with other parts of the limbic system that moderate associations between memory, emotion, and, via the olfactory bulb, smell. The dentate gyrus is one of four hippocampal regions, distinguished from each other by the types of neurons that compose them and by their respective functions. All parts of the hippocampus are connected to outside brain regions by neuronal circuits extending throughout the cerebral cortex.
Anatomically, the dentate gyrus contains three layers of cells. The granule cells, the most important of these neuron layers, fire electrically whenever the region is activated by input from neighboring neural groups. It receives direct communication from the neurons of the adjacent regions of the hippocampus, with its primary source of activation coming from the entorhinal cortex, a central relay point for many neuronal networks that enable the consolidation of new memories and habit formation.
As is true throughout the hippocampal formation, damage to or destruction of the dentate gyrus often prevents the growth of new neurons during the critical period of memory formation. Much of the laboratory research that has examined this process has studied spatial memory, the ability of an animal to navigate around a maze or recall the location of objects within a room. In part, this is because much experimental data on hippocampal physiology has been done with rodents, for which maze tasks have been designed. Despite the limitations of rodent models, there is also significant clinical evidence for something similar in humans, suggesting that the formation of new neurons in this region correlates with learning new locations and with navigating back to a place for the first time.
Neuronal growth, also called neurogenesis, can be interrupted by certain chemicals. Stress hormones like cortisol are released during times of fear and anxiety, and can block the formation of new neurons within the dentate gyrus. Alzheimer's disease causes atrophy of hippocampal neurons, and neurogenesis stops as the illness progresses. Some research on neurodegenerative disease — and on memory loss in the elderly — focuses on the processes that cause or prevent neurogenesis, in an effort to develop new drugs and treatments to control these better.
