Connectome

As Konnektom (English Connectome ) is defined as the totality of the compounds in the nervous system of an animal. His studies at various scales is dedicated to the Konnektomik (English connectomics ), a branch of neuroscience.

Since the compounds of a nerve cell in determining their function play a central role in their investigation is a traditional subject of biological research. The two terms " Konnektom " and " Konnektomik " but there is only since 2005. Among his establishment of the approved in September 2010 Human Connectome Project added, under which the National Institutes of Health research into the human Konnektoms with a total of nearly 40 million U.S. promote dollars.

Coining the term

The term " Konnektom " comes from the neuroscientist Olaf Sporns (Indiana University) and Patric Hagmann ( École Polytechnique Fédérale de Lausanne) back who had proposed it in 2005 independently.

Through conscious reference to the existing terms genome and proteome, which is the body of genetic information or the proteins in an organism, is the name " Konnektom " to express that the individual compounds can be understood only in their mutual relation to each other, similar to individual genes or proteins in the body interacting with one another. Similar to the proteome of the Konnektom is not static, but subject to constant changes due to neural plasticity.

The Konnektom on different scales

The Konnektom research focuses on the network properties of the nervous system. The network to be examined is in this case generally represented by a graph in terms of graph theory, as is the amount of so-called abstract nodes connected by edges. What specific meaning of the nodes and edges depends on the order on which the nervous system is considered. The possible orders of magnitude are roughly divided into micro- scale, mesoscale and macroscale.

Micro scale: The Konnektom as a network of nerve cells

At the finest level, the micro- scale, there is a nervous system neurons that are interconnected by synapses. An investigation of the nervous tissue on this scale requires its representation with a resolution in the micrometer range. The nodes in the network graph in this case correspond to the individual neurons. A complete reconstruction of the Konnektoms at the micro level is only succeeded in the nematode Caenorhabditis elegans. His nervous system consists of 302 neurons that form around 5000 synapses, 2000 neuromuscular junctions and 600 gap junctions. It was described in 1986 on the basis of an electron- microscopic study of serial sections through a team led by biologist John White.

A similar comprehensive study of more complex organisms is not possible in the present state of the art, since the number of its neurons typically is in the billions. Based on estimates, it is assumed, for example, that the human brain contains about 10 billion neurons that form around 100 trillion synapses; In comparison, the human genome contains only a little more than 3 billion base pairs. In this scale, it is not possible to manually keep track of all axons in successive electro- microscopic sectional images. Therefore, specialized for this task methods of machine vision are developed, the quality so far but still not quite reach the manual segmentation. In addition, already represents a technical challenge to store and process the amount of data

Mesoscale: Cortical columns and layers

The cerebral cortex is organized cortical columns, associations of a few hundred or thousand neurons, with an overall diameter of about 80 micrometers. They are characterized by joint afferent nerve connections are mutually connected and particularly strong, especially in the primary sensory areas clearly marked. They are a fundamental processing unit of the cortex.

In addition to this vertical organization can be histologically furthermore distinguish horizontal layers; due to their number one divides the cerebral cortex in the isocortex ( six layers) and the Allocortex (three to five layers). While brain research has achieved considerable progress in the understanding of the micro and macro scale in the course of their recent history, there are still few approaches to investigate the functioning of neural networks on the middle level.

Macro scale: Relationship between brain areas

Because of their anatomical characteristics or their function is different brain areas can be distinguished, the macro-level are the nodes of the network graph for viewing the Konnektoms. These centers are connected by long nerve fibers, which form the white matter. Due to findings that were obtained by using special preparation and staining procedures, the general course of many large bundle of nerves at the macro level is known. The aim of current research (as of 2012) is to examine the individual Konnektom living subjects using imaging procedures and clarify influences of genetic factors, normal aging processes, as well as learning processes and diseases. Important contributions of many smaller research projects presents the Human Connectome Project between 2010 and 2015, the largest coordinated research program in this field dar. The research are at present essentially the following three methods:

The diffusion imaging provides a direct examination of the white matter. It measures the Brownian molecular motion of water molecules. Since this is restricted by the microstructure of the nerve fibers that allows them to be an estimate of the local direction of preferential orientation of the nerve fibers. Tractography methods reconstruct from such data algorithmically the course of major nerves. There have been proposed various methods to derive from the diffusion data a network graph: One way is to ( the nodes of the graph that is ) to see a map cortical areas of the brain; Gong et al. thus defining 78 regions. A finer subdivision in node 500 to 4000 can be achieved by a random grouping of each 8-64 contiguous voxels which have, however, no anatomical importance in this case. Two nodes are connected by an edge if the tractography has a connection to the respective areas is reconstructed; Edge weights can be derived from the number of reconstructed curves. If the shared Konnektom a whole group of subjects to be presented, an edge is only set when the connection has been reliably reproduced within the group.

In addition to the diffusion imaging, are the network graph in the context of Konnektomik as an expression of "structural" Connectivity considered, including functional magnetic resonance imaging (fMRI ) is used. In particular, correlations in the BOLD signal is an indirect expression of brain activity, regarded as evidence of a connection of the corresponding brain regions. Particularly common are measurements at rest ( "resting state" ); the findings observed networks are therefore called " resting state networks". From the functional imaging is hoped that in the context of a particular Konnektomik representation polysynaptic compounds containing about a change in the thalamus and are therefore not reliably detected with diffusion imaging. Unlike tractography methods, although the fMRI shows the associated brain regions, but not the course of the nerve connection itself; in return, they suffer less crossing or fanning fiber tracts that limit the accuracy and reliability of tractography method.

A third approach performs systematic correlation to the thickness of various regions of the cortex back to a compound of the correlated areas. This thickness can be estimated from conventional T1 - weighted images of magnetic resonance imaging. Since these correlations can be calculated only by the comparison between subjects, results in this case no individual Konnektom, but a common graph for all subjects in the study group.

The Konnektom in the context of functional specialization and integration

Theory of functional specialization and integration According to the specific functions of the brain require integration of specialized brain areas. A major motivation for the study of Konnektoms is the assumption that the connections of the neurons contribute significantly to each other on both issues. For example, the function of the visual cortex is primarily determined by its afferent connection to the sensory cells in the eye of a hand. On the other hand, we know from experiments with split-brain patients that the fiber tracts in the corpus callosum for the integration of visual impressions of the left visual field are essential to the language centers.

While the functional specialization is now explored and occupied relatively thoroughly by methods of electrophysiology and functional imaging, it is hoped that the study of Konnektoms particular further insights into the far less detailed understood mechanisms of functional integration.