The new goal is to learn how all 85 billion neurons in the human brain are wired up, and we have a chap called Jeff Lichtman working in a laboratory at Harvard University who is systematically slicing up brains to see if he can work out the human wiring diagram. And that is not an easy job. This is called the human ‘Connectome’ Project.
Jeff Lichtman began many years ago by slicing up mouse brains to see if he could demonstrate all the electrical connections in our rodent friends. The initial results were promising, so he shifted the research to the human brain. Incidentally, to complete every part of the mouse brain study he calculated how long it might take to image every slice of a 1 centimeter mouse brain. The answer was 7000 years!
Hoping that he could complete the study in his own lifetime, he began the human research. The human brain is another story. There are 85 billion neurons in the 1.4 kilograms of squishy tissue between our ears (though I have met some whom I am sure have a six inch stainless steel bar to hold their ears apart).
Each ‘electrical’ neuron has a cell body (the grey matter) and long, thin extensions called dendrites and axons (the white matter) that reach out and link to others. Most neurons have lots of dendrites that receive information from other nerve cells, and one axon that branches on to other cells and sends information out. On average, each neuron forms 10,000 connections, through synapses (connections) with other nerve cells. Altogether, Lichtman estimates there are between 100 trillion and 1000 trillion connections between neurons. That makes the ‘connectome’ study look fairly impossible as far as completion in a human life span timetable is concerned.
Whilst Lichtman’s study is to be commended for its thoroughness, there are others in the research field who claim that this degree of trying to identify all pathways is really too much. Olaf Sporns, the neuroscientist at Indiana University, who coined the term ‘connectome’ in 2005 says that, “If you want to study the rainforest, you don’t need to look at every leaf and every twig and measure its position and orientation. It’s too much detail.”
Sporns and other neuroscientists are focusing on what they believe is achievable today, under the $40 million Human Connectome Project.
One hurdle which must be overcome is the fact that all our brains are different in many details, though similar on a large scale. Neuroscientists have some idea of how brains are wired, but the variability between even healthy people is substantial.
The connectomes mapped out by the Connectome Project will (hopefully) show scientists what healthy brain wiring looks like. But is there a difference with dysfunctional brains, in psychiatric cases, for example?
“‘With a lot of psychiatric disorders, like schizophrenia, drug addiction, obsessive-compulsive disorder and depression, you can look at a brain scan and there’s nothing you can see that stands out as an abnormal hotspot on the scan,” Ed Bullmore, a neuroscientist at Cambridge University, says. That scanning is with the technology available today, such as MRI investigations.
Adding to the complexity, the cerebral cortex makes up 80 percent of the human brain, but holds only a fifth of its neurons. Tucked beneath the cerebral hemispheres, the cerebellar cortex occupies only a tenth of the brain’s volume but contains 80 percent of its neurons. Again, even with the naked eye, the brain’s lobes and lobules differ markedly from person to person.
These differences are vital in trying to understand the complex workings, and the Connectome Project is studying twins and their non-twin siblings. The researchers should also come to Suvarnabhumi airport, as many visitors appear to leave their brains there!
Finally Jeff Lichtman says, “All the normal functions of the brain, the storage of information about the world, our memories, the way we perceive the world, the behaviors we learn, are all probably encoded in connectivity.”