Dual inhibitory of thalamocortical circuits

Speaker: Acsády László
Department:
Dept. of Cellular and Network Biol., Hungarian Ac.
Subject: Dual inhibitory of thalamocortical circuits.
Location: Queridozaal (Erasmus MC)
Date: 31-10-2016

Author: Kristian Blom

At the 31st of September I visited a seminar given by Acsády László, head of the Thalamus Research department at the Institute of Experimental Medicine of the Hungarian Academy of Sciences. Dr. László studied Biology at the ELTE university in Budapest. Thereafter he received a PhD in neuroanatomy at the same university. For his first postdoc he moved over to the Rutgers University in the USA, where he studied mossy fibers (which is a pathway of an input signal to the cerebellum) in the rat hippocampus.

Before we went to the real research, dr. László first explained a bit about the structure and function of the thalamus. The thalamus is a centrally-located brain structure that controls the flow of all information to the cortex. The size of the thalamus is comparable with a regular sized walnut. Over the years fifteen different thalamic nuclei (≠ cell nuclei) have been discovered in the thalamus, and each of these have their own function. Back in the days it was thought that sensory relay was the primary mode of thalamic information transfer, however nowadays it is known that thalamic information transfer is important for more reasons. Therefore it is very important to do research about information transfer in the thalamus.

Before I discuss the research problem, I have to explain what neural inhibition is and how it is mediated in the brain. Neural inhibition is nothing more than the blockade of activity and restriction of activity patterns in both space and time in the brain. In the cortex, inhibition is mediated by axon (part of neuron cell) terminals of interneurons that release gamma amino butyric acid (GABA) onto their synaptic targets. In terms of ‘signals and systems’ we can think of inhibition as a filter which either causes the input signal to penetrate less far into the brain or causes the signal to be shorter in time.  With that being sad, we can turn over to the research of dr. Laszlo.

As already sad, the thalamus controls the flow of all information to the cortex. In the cortex, the problem of scaling inhibition in space and time is solved by a variety of interneuron types. However, the thalamus lacks this variety of cell populations, but it still has to cope with similar dimensions of inhibition to regulate neural activity. So the question is: How does thalamic inhibition deals with variable spatio-temporal scales in the thalamocortical circuits? Dr. László provided two answers to this question: 1) The thalamic reticular nucleus is able to switch the scales of the its inhibitory activity in a behavior dependent manner, and therefore can regulate the spatial gain of thalamocortical information transfer. So, we see that intrathalamic inhibition regulates the spatial gain of an input signal. 2) A parallel inhibitory system is able to provide temporally precise inhibition. So, extrathalamic inhibition regulates the temporal gain of an input signal. How cool is that!? We see that the spatial and temporal ‘filters’ are regulated separately by different regions in the brain.

thalamus-nuclei-schematic
Figure 1 – A schematic image of the thalamus with its different nuclei. The thalamic reticular nucleus, which acts as the spatial inhibitor of input signals, is part of the ventral thalamus that forms a capsule around these nuclei. Source: http://www.edoctoronline.com/medical-atlas.asp?c=4&id=21820&m=2

For me it was rather difficult to follow this seminar, since a lot of terms which came by I never heard of before. Luckily I have a Sobotta (Atlas of anatomy) at home, so some of the terms (e.g. thalamocortical circuits and GABA) I could read back. However, besides my lack of knowledge in the field of neuroscience, I have to admit that the results of this research are quite awesome. In the future perhaps some of the following questions can be answered: How do the intrathalamic inhibition regions and extrathalamic inhibition regions communicate with each other to inhibit a signal properly? Or do they even communicate with each other? And why does the spatial inhibition comes first for an input signal (from thalamus to cortex), and later on in the extrathalamic region the temporal inhibition?

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