Category Archives: connectivity

On The Wiring Diagram

From reference 1

The human brain has roughly 100 Billion neurons and each neuron has between 1000 and 10000 synapses (connections), thus approximately 500 Trillion synapses. This makes the problem of determining the connectivity, or the wiring diagram of the brain absurdly complex. This is one of the most fundamental problems confronting neuroscientists today because the solution to many problems of how the brain works would be made much easier if we simply knew the structure that it is built on.

A recent piece of computational research (published a wonderful PLOS journal) suggests a novel statistical method to identify which synapses of a given neuron are active at a given time. The author of this study simulated the output of many single neurons when a particular subset of it’s synapses were active. This characterization was based on the number of action potentials the neuron fired in response to the activity of these many specific synapses. Next, the author examined the changes in the output when a single additional synapse was activated along with the baseline subset. He found that if he simulated the addition of one synapse ~80 times, he could measure significant changes in the output of the simulated neuron such that it was possible in subsequent tests to reliably predict when this synapse was active.

The authors suggestion is that taking this technique out of the computer and into the world of real brains (or small slices of brain, as is commonly employed), would facilitate the task of elucidating the numerous connections in the brain. While this is true, it must be said that this method is good for asking the following question: Which neurons are connected to one neuron that I know very well? In other words, somebody interested in applying this work would have to have one neuron of interest and then stimulate every other neuron that might be connected to it in order to determine the connectivity. In this sense, the approach is a far cry from revealing the wiring of the brain, but it certainly does help.

References:
1. Bhalla US. (2008) How to record a million synaptic weights in a hippocampal slice. PLoS Comput Biol. 4(6):e1000098.