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Medial Temporal Lobe

=Anatomy= The medial temporal lobe, located within the temporal lobe, is made up of hippocampal region and the adjacent parahippocampal gyrus. The structures contained by the hippocampal region are the dentate gyrus and the subiculum. The parahippocampal gyrus is formed by the entorhinal, perirhinal and parahippocampal cortices. The parahippocampal gyrus and the hippocampal region are connected by the entorhinal cortex, which lies medially against the subiculum.

=Function= The structures within the medial temporal lobe are generally thought to be necessary for declarative memory ; memory for facts and personal experiences. The hippocampus in particular is thought to be involved in the creation of new memories and the detection of novel stimuli. There have been suggestions that it is the perirhinal cortex which is vital for the recognition of objects, while the hippocampus is involved more in the memory of paths and places.

=Pathology=

Epilepsy
Temporal lobe epilepsy appears to be have a profound affect on several structures within the medial temporal lobe and is often associated with hippocampal sclerosis; damage and cell loss of the hippocampus. Temporal lobe epilepsy is also associated with a substantial decrease in hippocampal neurogenesis ; the creation of new neurons within the hippocampus. Though new cells are still created, there is evidence they have trouble differentiating; specializing into the required neuron type. This is especially problematic, as the dentate gyrus within the hippocampus normally engages in neurogenesis throughout our lifespan. This inability for new neurons to properly differentiate, has been suggested as a possibility for the persistence of seizures, learning and memory dysfunction, and depression associated with chronic epilepsy. However, there can be some uncertainty about whether epilepsy is the result of damage to the hippocampus or whether the abnormalities in the hippocampus are the result of the epileptic seizures. While artificially induced epileptic seizures in animals have been shown to result in severe damage to the hippocampus and entorhinal cortex, the effect can be inconsistent and is only a small part of the mayor damage the other structures in the brain.

Alzheimer's Disease
As memory is the main function associated with the medial temporal lobe, it has been the focus of many studies relating to Alzheimer's disease. One of the regions that is particularly vulnerable and affected earlier in Alzheimer's disease, is the entorhinal cortex; part of the entorhinal-hippocampal circuit, necessary for encoding specific forms of memory. In a postmortem study counting the neurons in the entorhinal cortex of deceased individuals, it was shown that even the participants with the mildest clinically detectable form of dementia, who still retained the neuropathological diagnosis of Alzheimer's disease, already had 32% fewer neurons in their entorhinal cortex than controls. Participants with the most severe cognitive impairment had lost as much as 90% of the neurons in the entorhinal cortex necessary for the entorhinal-hippocampal circuit. It has been suggested that it is the loss of this connection between the entorhinal cortex and the hippocampus that could be responsible for memory defects associated with Alzheimer's disease.

Exercise has been shown to be able to increase the volume size of the anterior hippocampus; which can lead to improving spacial memory. Fitness is also suggested to be able to prevent volume loss of the hippocampus. One attempt to use deep brain stimulation increased the participants' spacial memory when stimulating the entorhinal cortex, rather than the hippocampus.

This strong link between the medial temporal lobe and Alzheimer's disease is believed by some to be the key to predicting Alzheimer's disease in patients. MRI studies show that a volume reduction in the entorhinal cortex, more than the hippocampus, can be an early sign of Alzheimer's disease pathology. Some suggestions have been made that entorhinal cortex, specifically in the right hemisphere, can predict the conversion rate from Mild Cognitive Impairment to Alzheimer's disease with a concordance of 93.5%.

Schizophrenia
MRI and postmortem studies indicate a reduction in the temporal lobe in patients with schizophrenia. While the causes of schizophrenia and its symptoms are still heavily debated, there are some suggestions that that the medial temporal lobe is involved; with lots of research focusing on the hippocampus. While there have been indications of a reduction of the size of the hippocampus in patients with schizophrenia, smaller hippocampal volumes have also been noted in patients showing early symptoms of schizophrenia; suggesting the size reduction is not a secondary effect of the illness or the treatment thereof. There have also been suggestions that this hippocampal volume reduction could be the result of a genetic predisposition to schizophrenia. These hippocampal anomalies could be responsible for the long-term memory impairments associated with schizophrenia. Schizophrenia appears to be associated with asymmetry in the medial temporal cortex as a result of volume reductions in various structures. Postmortem research revealed that Schizophrenia patients had lower volumes in the left hemispherical parahippocampal and fusiform gyri than their right. This is a reversal of asymmetry when compared to control subjects. Such asymmetry can also be prevalent in the entorhinal cortex. While schizophrenia correlates with volume reductions of the entorhinal cortices in both hemispheres, the right one appears to be especially effected. Asymmetric reduction of entorhinal cortex volumes, and miswiring of entorhinal connections could be contributing factors to some cognitive disturbances associated with schizophrenia.

=References=