Our 100 Trillion Neural Connections: Mapping the Human Connectome

John McManamy Health Guide
  • Who are we? Our genome comprises about 20,000 genes, about the same as a worm. Perhaps we are more than our genome.

     

    This was a proposition MIT physicist and neuroscientist Sebastian Seung put to his audience in a 2010 TED Talk. Dr Seung is a leading proponent of the “connectome,” the sum of our neural connections, how the parts of our brain work together. 

     

    Does our mind, our identity, reside in our connectome?

     

    Back in the seventies and eighties, scientists mapped out the connectome in the c. elegans worm. This worm has a mere 300 neurons organized into 7,000 connections. By contrast our brains comprise 100 billion neurons organized into some 100 trillion connections.

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    It’s all about scale. According to a 2011 piece in Scientific American (pay for article here), a single neuron cannot do much. But string a few hundred neurons together and a primitive nervous system emerges. Billions of neurons lead to immensely mind-boggling arrays.

     

    I have heard brain scientists use the world wide web as a metaphor. The suggestion is that our brains function according to the same dynamics as complex self-organizing systems such as the world wide web or the stock market or the environment.

     

    Scientific American describes an experiment involving 1,600 synthetic neurons in a toy brain. In the first part of the experiment, the researchers limited these neurons to short-distance connections. Only random sparking occurred.

     

    Then the researchers connected every neuron to every other neuron. This produced a predictable on-off pulsing.

     

    Everything changed when the researchers created a network of both short and long-distance connections. The firing neurons created nonlinear patches of activity that swirled about the brain, colliding into other patches. 

     

    Nonlinear suggests a type of “power law curve” that governs the dynamics of heartbeats and seismic activity, where major tremors are exponentially more rare than minor ones. This was borne out in another experiment involving electric readings of pieces of brain tissue grown in petri dishes. 

     

    In 2011, at the Ninth International Conference on Bipolar Disorder, I listened to Nora Volkow, head of the National Institute on Drug Abuse, talk about scanning brains in their resting state.

     

    These scans yielded seven or eight interconnecting hubs consisting of dense networks of neurons. The 2011 Scientific American article picks up on the action:

     

    Such “small world” networks of hubs may help our brains process information more rapidly and allow the organ to maintain its structural integrity efficiently.

     

    Then there is the daunting prospect of drilling deep into the thicket of our ten trillion neural connections.This was the focus of Dr Seung’s TED Talk. We lack the technology to connect all those dots, or even a tiny percentage of a percentage, but we stand to gain significant insights from mapping out even the smallest sample. 

     

    Perhaps we are the sum of our connectome. Or maybe not. But we can put together a good hypothesis that serious mental illness is the result of parts of our connectome - neural networks, hubs - in collapse.

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    The NIH is currently engaged in a $40 million, five-year effort called the Human Connectome Project. This may be the first time you heard the word, connectome. It certainly won’t be the last.  

Published On: March 16, 2014