The Brain from Inside Out

by György Buzsáki

There appear to be at least two mechanisms that allow scaling of neuronal networks while conserving timing mechanisms. The first mechanism compensates for the increase in neuronal numbers and the enormous numbers of possible connections by shortening the synaptic path length between neurons, defined as the average number of monosynaptic connections in the shortest path between two neurons. This problem is akin to connecting cities with highways and airlines to obtain an efficient compromise between the length of the connections and the number of intermediate cities required to get from city A to city B. Directly connecting everything with everything else is not an option in most real-world networks as the number of connections increases much more rapidly than the number of nodes to be connected. An efficient compromise can be achieved by inserting a smallish number of long-range short-cut connections into local connections. The resulting “small-world-like” architectural solution allows for scaling while keeping the average synaptic path length similar as brains increase in size. While such scaling prevents excessive volume growth, longer axons increase the travel time of action potentials. This would pose a serious problem for neuronal communication. Thus, a second mechanism is needed to compensate for these time delays. This requirement is solved by using larger caliber, better insulated, and more rapidly conducting axons in more complex brains.