The human brain as a linearly scaled-up primate brain
Azevedo et al., J Comp Neurol (2009)
The human brain is often considered to be 5-7x larger than expected for a mammal of our body size, with an especially enlarged cerebral cortex that represents over 80% of brain mass. Although the number of neurons is generally assumed to be a determinant of computational power across species, and despite the widespread quotes that the human brain contains 100 billion neurons and 10x more glial cells, the absolute numbers of neurons and glial cells in the human brain remained unknown up to this study. Here we determined these numbers by using the isotropic fractionator and compared them with the expected values for a human-sized primate.
How many cells in a human brain?
We find that the adult male human brain, weighing on average 1.5 kg, contains on average 86 billion neurons and 85 billion non-neuronal cells: the maximal proportion of glial to neuronal cells in the human brain, therefore, is of 1:1.
How many cells should the human brain have, as a primate?
According to the linear cellular scaling rules that were previously found to apply to primate brains (Herculano-Houzel et al., 2007), a primate of approximately 75 kg would be expected to have a brain weight of 1.3 kg, with 78 billion neurons and 94 billion non-neuronal cells. The values we found, of 86 and 84 billion respectively, fall within 10% of the expected from body mass.
According to brain mass, a primate brain weighing 1.5 kg should be composed of 94 billion neurons, with 22 billion in the cerebral cortex, 78 billion in the cerebellum, and 0.6 billion in the remaining areas. The numbers of cells found in the cerebral cortex (16 billion), cerebellum (69 billion) and remaining areas (0.7 billion) are therefore very close to the numbers expected for a generic primate of our brain size. This proximity indicates that the human brain conforms to the primate cellular scaling rules.
For the sake of comparison, a brain with 86 billion neurons built according to the cellular scaling rules that apply to rodents (Herculano-Houzel et al., 2006) would be expected to weigh 35 kg and to inhabit a body of 50 T. Conversely, with our 70 kg of body mass, our brain - if built as a rodent brain - would weigh only 145 g and hold a meager 12 billion neurons.
These comparisons show that the human brain is, indeed, almost 10x larger and richer in neurons than expected for a rodent of our body size - but has just as many neurons as expected for the primate that it is. The human brain, therefore, is a linearly scaled-up primate brain.
What about the specially large human cerebral cortex?
We find, as other authors have, that the human cerebral cortex represents 82% of total brain mass, which is larger than the relative cortical size of other primates. However, this enlarged cortex contains only 19% of all brain neurons - a ratio that is similar to that found in other primates as well as in rodents, regardless of brain size (Herculano-Houzel et al., 2006, 2007). A relatively larger cerebral cortex, therefore, is not a valid proxy for relatively more cortical neurons. Much to the contrary: the relative number of neurons in the cerebral cortex seems not to covary with brain size. We propose, therefore, that the neuronal basis for any cognitive advantage of the human brain over others should not be sought in the relative enlargement of the cerebral cortex, but rather in its large absolute number of neurons, which scales coordinately with the number of neurons in the cerebellum (Herculano-Houzel, submitted).
Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S (2009) Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol 513, 532-541.