July 28, 2011
With some 70 billion neurons and hundreds of thousands of kilometres of circuits, the human brain is so complex that, for many years, it seemed impossible to reconstruct the network in detail. Each neuron is linked to about a thousand others by means of finely branched projections called dendrites and axons, and communicates with them using electrical signals. The connections between the cells are critical for brain function, so neuroscientists are keen to understand the structure of these circuits – the connectome – and to reconstruct it in a three-dimensional map. Since no computer is powerful enough yet for the task, researchers are dependent on the human eye. However, the sheer number of cellular connections contained in even the tiniest fragment of tissue makes the undertaking seem pointless – unless it is shared among a large number of people.
Moritz Helmstaedter, Kevin L. Briggman and Winfried Denk, scientists at the Max Planck Institute for Medical Research in Heidelberg, have now successfully tested this procedure. They developed a special software tool called RESCOP which summarises the results of several annotators to yield an overall picture. In this way, and with the support of over 70 students from Heidelberg University, they reconstructed a network of over 100 neurons from the retina in full detail.
The students used the KNOSSOS software developed by the team in Heidelberg to trace the connections between the neurons. It is no coincidence that the program is named after Crete’s legendary palace, renowned for its elaborate labyrinth: “Tracing the connections in the brain is at least as hard as finding your way out of a mythological labyrinth”, explains Moritz Helmstaedter.