How many neurons does the brain really have?

13 Apr

Our brain is a fantastically complicated network of neurons. There is controversy about how many neurons are contained in the brain. Some estimates exceed a trillion. More textbooks claim that the brain has 100 billion neurons. Given how difficult it is to count such a large number of such small objects, can we have any confidence in these estimates?
A technique developed by Suzana Herculano-Houzel and Robert Lent of the University of Rio De Janeiro seems to have given us the best answer to date. Their technique involved taking whole brains and dissolving them into a suspension. The dissolving process broke down most cell structures but kept the nucleus of each cell intact.
A suspension is a mixture of particles in a solvent, where the particles are large enough to settle out. However this suspension of dissolved brain was kept uniform by intense agitation. Since each cell in the brain has exactly one nucleus, the total number of cells would be equal to the number of nuclei. The researchers drew off a sample and count the number of nuclei. Now knowing the density of the cells in the the sample, they multiplied the density by the total volume of the dissolved brain. This yielded the total number of brain cells.
One final problem remains, not all brain cells are neurons, there are many supporting cells, called glia, in the brain. Fortunately, it is possible to distinguish between these neural and glial nuclei with an antibody. The antibody binds only to the nuclei of neurons. Thus, it was possible to determine the percentage of total brain cells are neurons. That percentage of the total number of brain cells was the number of neurons.
From this procedure our best estimate of total neurons in the adult male human brain is 86.1 billion.

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