Sunday, September 27, 2009

Fact or Fiction? There ten times more glia than neurons in the brain.

"We only use 10% of our brains" is one of the most commonly held brain myths. But as the Neuroscience for Kids website tells us,
There is no scientific evidence to suggest that we use only 10% of our brains.
Neuroscientists, of course, already know that. But are there other false beliefs about the brain? Brain Mythology is a new column in the journal Brain Structure and Function, edited by Laszlo Zaborszky and Karl Zilles. In the first installment, Hilgetag and Barbas (2009) ask the question, Are there ten times more glia than neurons in the brain?
Neuroscience students take it for granted that there are many more glia than neurons in the brain. Neuroscience textbooks state with confidence: “Although there are many neurons in the human brain…, glia outnumber neurons by tenfold” (Bear et al. 2006) or, not to be outdone, even by “10–50 times”, as claimed in another text (Kandel et al. 2000). This fact is happily invoked by gliologists to promote the status of their field.
Damn those status-conscious gliologists! They've been leading us astray!
Given this well-accepted figure, we were surprised when our cell counts in the prefrontal cortex of the rhesus monkey turned up a glia-to-neuron ratio (GNR) of just about 1 (Dombrowski et al. 2001). There was some regional variation, but no prefrontal area had a GNR larger than 1.2. Maybe the proportion of glia is very different in other cortical regions or other parts of the brain, so that the overall ratio for the whole brain is much larger than 1? Classic studies, however, conducted by O’Kusky and Colonnier (1982) in the opposite pole of the brain, the visual cortex, had reported an even lower GNR of 0.5.

From Fig. 5A (Dombrowski et al., 2001). The ratio of glia/neurons in prefrontal areas or subdivisions of areas.

The origin of the glial dominance myth is not clear at all. Hilgetag and Barbas conclude by mentioning that...
Since the number of synapses increases faster than the number of neurons in larger brains, this affiliation of glia with the multitude of neural connection points may help explain... For example, in large brains such as the human brain... there may be as many as 1.4 astrocytes for each neuron, up from 0.33 in the rodent cortex (Nedergaard et al. 2003). Even that ratio, however, is still a long way from the myth of 10 times more glia than neurons, in any species.
Those species include humans of course, who are like monkeys with a nearly perfect 1:1 neuron:glia ratio [as noted by Jason Snyder].


Bear MF, Connors B, Paradiso M (2006). Neuroscience—exploring the brain, 3rd edn. Lippincott Williams & Wilkins, Philadelphia.

Dombrowski S, Hilgetag CC, Barbas H (2001). Quantitative architecture distinguishes prefrontal cortical systems in the rhesus monkey. Cereb Cortex 11:975–988.

Hilgetag, C., & Barbas, H. (2009). Are there ten times more glia than neurons in the brain? Brain Structure and Function, 213 (4-5), 365-366. DOI: 10.1007/s00429-009-0202-z

Kandel ER, Schwartz JH, Jessell TM (eds) (2000). Principles of neural science, nerve cells and behavior, 4th edn. McGraw-Hill, New York, p 22.

Nedergaard M, Ransom B, Goldman SA (2003). New roles for astrocytes: redefining the functional architecture of the brain. Trends Neurosci 26:523–530

O’Kusky J, Collonnier M (1982). A laminar analysis of the number of neurons, glia, and synapses in the adult cortex (area 17) of adult macaque monkeys. J Comp Neurol 210:278–290.

Image via EduSpaces.

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