Human brain composition

To regard the human brain as unique requires considering it to be an outlier: an exception to the rule, whatever that rule is. This makes little sense, however, in light of evolution. If we go to such great lengths to affirm, and teach, that evolution is the origin of diversity in life, and to find trends and laws that apply to kingdoms, phyla and orders as a whole, why then insist that whatever scaling rules apply to other primates must not apply to us? In view of the vexing size inferiority in brain size and of the lack of information about what our brains are actually made of – and how that compares to other brains, particularly those of whales and elephants – resorting to a quest for uniqueness may have seemed as a necessary, natural step to justify the cognitive superiority of the human brain.

Recently, a novel quantitative tool developed in our lab has finally made the numbers of neurons and non-neuronal cells that compose the brains of various mammals, humans included, available for comparative analysis. This review will focus on such a quantitative, comparative analysis, with emphasis on the numbers that characterize the human brain: what they are, how they have been viewed in the past, and how they change our view of where the human brain fits into the diversity of the mammalian nervous system.

The Human Brain in Numbers

How many neurons does the human brain have, and how does that compare to other species? Many original articles, reviews and textbooks affirm that we have 100 billion neurons and 10 times more glial cells ( ; ; ; ; ; ), usually with no references cited. This leaves the reader with the impression that the cellular composition of the human brain has long been determined. Indeed, an informal survey with senior neuroscientists that we ran in 2007 showed that most believed that the number of cells in the human brain was indeed already known: that we have about 100 billion neurons, outnumbered by about 10 times more glial cells – but none of the consulted scientists could cite an original reference for these numbers (Herculano-Houzel and Lent, unpublished observations). Curiously, the widespread concept that neurons represent about 10% of all cells in the human brain might be one of the arguments behind the popular, but mistaken, notion that we only use 10% of our brain.The reason for such lack of references is that indeed there was, to our knowledge, no actual, direct estimate of numbers of cells or of neurons in the entire human brain to be cited until 2009. A reasonable approximation was provided by , from the compilation of partial numbers in the literature. These authors estimated the number of neurons in the human brain at about 85 billion: 12–15 billion in the telencephalon, 70 billion in the cerebellum, as granule cells (based on ), and fewer than 1 billion in the brainstem. With more recent estimates of 21–26 billion neurons in the cerebral cortex and 101 billion neurons in the cerebellum, however, the total number of neurons in the human brain would increase to over 120 billion neurons.As to the 10 times more numerous glial cells in the human brain, that seems to be the case only in subcortical nuclei such as the thalamus (17 glial cells per neuron) and the ventral pallidum (12 glial cells per neuron; ). In the gray matter of the cerebral cortex, glial cells outnumber neurons by a factor of ; ). Given the relatively small number of glial cells reported for the human cerebellum, where they are outnumbered by neurons by at least 25:1, the only possible explanation for the ubiquitous quote of 10 times more glial than neuronal cells in the entire human brain would be the presence of nearly one trillion glial cells in the remaining structures alone – an unlikely scenario, since these structures represent

Why Bother with Cell Numbers?

Across species, the number of neurons and their relative abundance in different parts of the brain is widely considered to be a determinant of neural function and, consequently, of behavior. Among mammals, those species with the largest brains, such as cetaceans and primates, have a greater range and versatility of behavior than those with the smallest brains, such as insectivores ( ; ). Among birds, those that are larger-brained (corvids, parrots and owls) are also considered the most intelligent. A recent comparison of several parameters, including brain size, relative brain size, encephalization, conduction velocity and estimated numbers of neurons led two authors to conclude that the “factors that correlate better with intelligence (across species) are the number of cortical neurons and conduction velocity, as the basis for information processing”. Indeed, within non-human primates, a recent meta-analysis concluded that the best predictor of the cognitive abilities of a species is absolute brain size, not relative size nor encephalization quotient (EQ; ).However, the correlation between absolute brain size and cognitive abilities breaks down when species of similar brain size are compared across orders. Monkeys, for instance, possess brains that are much smaller than those of ungulates, but the higher cognitive and behavioral flexibility of monkeys over ungulates is anecdotally evident to any observer who compares the ingenious and complex abilities of macaques to those of cows or horses, even though the latter have 4–5× larger brains than macaques. For similar-sized brains, rodents also perform more poorly than primates: With a brain of only 52 g, the behavioral, social and cognitive repertoire of the capuchin monkey is outstanding compared to the capybara, a giant Amazonian rodent, even though the latter has a larger brain of 75 g. This is reminiscent of the most striking and troubling discrepancy regarding brain size and cognitive abilities: that between humans and larger-brained species such as whales and elephants. If the latter have brains that are up to six times larger than a human brain, why should we be more cognitively able? Answering this question requires a direct examination of the numbers of neurons that compose the brains of humans and other species.

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Q&A

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Do humans only use 10% of their brain for actual psychological processes? | Yahoo Answers

No, it's *all+ used for sub-conscious processing and autonomic care and functioning of the body.

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