After reading the paper mentioned here, I have a much better understanding of what it is really about. I you remember my original post, a blog post in The Guardian made a big deal of the fact that based on the results of the paper, the human brain seems to have about 86 billion neurons on average, as opposed to the widely reported figure of about 100 billion (for a brief discussion of these big numbers, go here).
Indeed, the number of neurons per brain reported in the paper does not seem to be significantly different from 100 billion, and correctly so, not a big deal is made of that in the paper. Now, the main subject of the paper, explicitly stated in its text (and even in the title) is not neurons, but glial cells.
Our brain is not composed of neuronal cells only. It also contains other type of cells, collectively called glia or glial cells, which are thought to be about ten times more abundant than neurons in a typical brain. This 10:1 glia:neuron ratio is the one usually cited in the general literature (see below).
The Azevedo et al., (2009) paper provides additional, original information and a summary of previously reported work showing that different brain regions may display different glia:neuron ratios. On average, it seems that the glia to neuron ratio in whole human brains is more like 1/1; however, depending on the brain region examined there is a range of glia:neuron ratios from about 17:1 down to 1:1 or even less.
Surely additional work needs to be done to get a more accurate picture of the exact ratios and what do they actually mean, but at the very least, on average the brain’s complexity at least doubles cellwise when we take glia into account.
Moreover, when they were originally discovered, glial cells were thought to play just a structural or support role in the nervous system; in fact, the word glia means “glue”. However, glial cells are much more than that; we now know that they are quite the active partners in brain physiology. They work together with neurons so there is little doubt that glia affect many aspects of nervous system function. I am sure that there will be more interesting discoveries down the road!
The take-home message: Take science-related hyperbolic news with a grain of salt, and whenever possible, go straight to the source to find out what the main point was.
IF YOU WANT TO KNOW MORE
Azevedo FA et al., (2009) Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 2009 Apr 10;513(5):532-41.
Fields RD (2010) The Other Brain. Simon and Schuster.
Fields RD (2004) The Other Half of the Brain. Scientific American 290: 54-61.
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