We broke the genetic code when we unraveled the human genome, but that most
certainly is not the end of the research that needs to be done on us, the
species that prides itself on being the top of the food chain.
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.”
