Dentate gyrus
Encyclopedia
The dentate gyrus is part of the hippocampal formation
Hippocampal formation
The hippocampal formation is a compound structure in the medial temporal lobe of the brain. There is currently no consensus concerning which brain regions are encompassed by the term, with some authors defining it as the dentate gyrus, the hippocampus proper and the subiculum; and others including...

. It is thought to contribute to new memories
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

 as well as other functional roles. It is notable as being one of a select few brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 structures currently known to have high rates of neurogenesis
Neurogenesis
Neurogenesis is the process by which neurons are generated from neural stem and progenitor cells. Most active during pre-natal development, neurogenesis is responsible for populating the growing brain with neurons. Recently neurogenesis was shown to continue in several small parts of the brain of...

 in adult rats, (other sites include the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

 and cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

).

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).

Structure

The dentate gyrus consists of three layers of neurons
Archicortex
Archicortex is categorized under allocortex. It is any cortex with fewer than six layers, specifically three layered hippocampal cortexes. It is necessary for hippocampus formation....

: molecular, granular, and polymorphic. The middle layer is most prominent and contains granule cell
Granule cell
In neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum , the dentate gyrus of the...

s that project to the CA3 subfield of the hippocampus. These granule cell
Granule cell
In neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum , the dentate gyrus of the...

s project mostly to interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...

s, but also to pyramidal cell
Pyramidal cell
Pyramidal neurons are a type of neuron found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons were first discovered and...

s 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
Entorhinal cortex
The entorhinal cortex is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC is the main interface between the hippocampus and neocortex...

, 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
Entorhinal cortex
The entorhinal cortex is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC is the main interface between the hippocampus and neocortex...

. The medial perforant path synapses onto the proximal dendritic area of the granule cell
Granule cell
In neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum , the dentate gyrus of the...

s, whereas the lateral perforant path does so onto the distal dendrites of these same cells.

Development

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. 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. The germinal sources of granule cells and their migration pathways 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. 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, 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
Epidermal growth factor
Epidermal growth factor or EGF is a growth factor that plays an important role in the regulation of cell growth, proliferation, and differentiation by binding to its receptor EGFR...

 receptor signaling pathway and Notch signaling
Notch signaling
The notch signaling pathway is a highly conserved cell signaling system present in most multicellular organisms.Notch is present in all metazoans, and mammals possess four different notch receptors, referred to as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. The notch receptor is a single-pass...

 pathway.

Function

The dentate gyrus is thought to contribute to the formation of memories
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

 and to play a role in depression.

Memory

The dentate gyrus is one of the few regions of the adult brain where neurogenesis
Neurogenesis
Neurogenesis is the process by which neurons are generated from neural stem and progenitor cells. Most active during pre-natal development, neurogenesis is responsible for populating the growing brain with neurons. Recently neurogenesis was shown to continue in several small parts of the brain of...

 (i.e., the birth of new neurons) takes place. Neurogenesis is thought to play a role in the formation of new memories
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

. 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.

Stress and Depression

The dentate gyrus may also have a functional role in stress and depression. For instance, neurogenesis
Neurogenesis
Neurogenesis is the process by which neurons are generated from neural stem and progenitor cells. Most active during pre-natal development, neurogenesis is responsible for populating the growing brain with neurons. Recently neurogenesis was shown to continue in several small parts of the brain of...

 has been found to increase in response to chronic treatment with antidepressant
Antidepressant
An antidepressant is a psychiatric medication used to alleviate mood disorders, such as major depression and dysthymia and anxiety disorders such as social anxiety disorder. According to Gelder, Mayou &*Geddes people with a depressive illness will experience a therapeutic effect to their mood;...

s. On the contrary, however, the physiological effects of stress, often characterized by release of glucocorticoid
Glucocorticoid
Glucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor , which is present in almost every vertebrate animal cell...

s such as cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...

, as well as activation of the sympathetic division of the autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

, have been shown to inhibit the process of neurogenesis in primates. Both endogenous and exogenous glucocorticoids are known to cause psychosis and depression
Clinical depression
Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities...

, implying that neurogenesis in the dentate gyrus may play an important role in modulating symptoms of stress and depression.

Other

Some evidence suggests that neurogenesis in the dentate gyrus increases in response to aerobic exercise. 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.

Spatial Behavior

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.

Blood Sugar

Studies by researchers at Columbia University Medical Center
Columbia University Medical Center
Columbia University Medical Center is an academic medical center that includes Columbia University's College of Physicians and Surgeons, College of Dental Medicine, School of Nursing and Mailman School of Public Health...

 indicate that poor glucose control can lead to deleterious effects on the dentate gyrus.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK