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Entohinal Cortex, Image source: Wikipedia
A microscopic view of the entorhinal cortex. The bright spots are the
bodies of neurons. Source: DZNE/Charité – Universitätsmedizin Berlin,
A Head Brake ?— New insight into the brain
Scientists investigated how nerve signals are suppressed inside the entorhinal cortex of the brain. According to their observations, this center of neuronal inhibition leads nerve cells to synchronize their activity.
Researchers of the Charité – Universitätsmedizin, Berlin, along with the German Center for Neurodegenerative Diseases (DZNE), have new insights into the functioning of a region in the brain that is normally involved in spatial orientation, and damaged by Alzheimer’s disease.
Their results are now published in the journal Neuron.
The entorhinal cortex is a link between the brain’s memory centre, the hippocampus, and other areas of the brain. It is, however, more than an interface only transfering nervous impulses. The entorhinal cortex also has an independent role in learning and thinking processes.
This is particularly applicable to spatial navigation. “We know precious little about how this happens. This is why we are investigating in animal models how the nerve cells within the entorhinal cortex are connected with each other.“ says Prof. Dietmar Schmitz, a researcher at the Cluster of Excellence NeuroCure at the Charité – Universitätsmedizin Berlin and Site Speaker for the DZNE in Berlin. “
Signals wander inside the brain as electrical impulses from nerve cell to nerve cell.
In general, signals are not merely forwarded.
Rather, operation of the brain critically depends on the fact that the nerve impulses in some situations are activated and in other cases suppressed.
A correct balance between suppression and excitation is decisive for all brain processes.
“Until now research has mainly concentrated on signal excitation within the entorhinal cortex. This is why we looked into inhibition and detected a gradient inside the entorhinal cortex. This means that nerve signals are not suppressed equally. The blockage of the nerve signals is weaker in certain parts of the entorhinal cortex and stronger in others. The inhibition has, so to speak, a spatial profile,” explains Dr. Prateep Beed, lead author of the study.
When the brain is busy, nerve cells often coordinate their operation. In an electroencephalogram (EEG) – a recording of the brain’s electrical activity – the synchronous rhythm of the nerve cells manifests as a periodic pattern.
As Beed explains, it is also unclear whether these oscillations are only just a side effect or whether they trigger other phenomena.
"It is a moot question as to how nerve cells synchronize their behavior and how they bring about such rhythms.
"But it has been demonstrated that neuronal oscillations accompany learning processes and even happen during sleep. They are a typical feature of the brain's activity.
"In our opinion, the inhibitory gradient, which we detected, plays an important role in creating the synchronous rhythm of the nerve cells and the related oscillations.”
Dr. Prateep Beed, lead author of the study
In the case of Alzheimer’s, the entorhinal cortex is among the regions of the brain that are the first to be affected.
“In recent times, studies related to this brain structure have increased. Here, already in the early stages of Alzheimer's, one finds the protein deposits that are typical of this disease.
"It is also known that patients affected by Alzheimer’s have a striking EEG. Our studies help us to understand how the nerve cells in the entorhinal cortex operate and how electrical activities might get interrupted in this area of the brain.”
Dietmar Schmitz, researcher, Cluster of Excellence NeuroCure at the Charité – Universitätsmedizin, Berlin, and Site Speaker for the DZNE in Berlin, head of research
Local inhibitory microcircuits in the medial entorhinal cortex (MEC) and their role in network activity are little investigated. Using a combination of electrophysiological, optical, and morphological circuit analysis tools, we find that layer II stellate cells are embedded in a dense local inhibitory microcircuit. Specifically, we report a gradient of inhibitory inputs along the dorsoventral axis of the MEC, with the majority of this local inhibition arising from parvalbumin positive (PV+) interneurons. Finally, the gradient of PV+ fibers is accompanied by a gradient in the power of extracellular network oscillations in the gamma range, measured both in vitro and in vivo. The reported differences in the inhibitory microcircuitry in layer II of the MEC may therefore have a profound functional impact on the computational working principles at different locations of the entorhinal network and influence the input pathways to the hippocampus.
Inhibitory gradient along the dorso-ventral axis in the medial entorhinal cortex
Prateep Beed, Anja Gundlfinger et al., Neuron, DOI: 10.1016/j.neuron.2013.06.038
Original press releas: http://www.eurekalert.org/pub_releases/2013-09/ru-msa091013.php