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The human brain in numbers: a linearly scaled-up primate brain.

The human brain has often been viewed as outstanding among mammalian brains: the most cognitively able, the largest-than-expected from body size, endowed with an overdeveloped cerebral cortex that represents over 80% of brain mass, and purportedly containing 100 billion neurons and 10x more glial cells. Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. However, our recent studies using a novel method to determine the cellular composition of the brain of humans and other primates as well as of rodents and insectivores show that, since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain. These studies also showed that the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size. Additionally, the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals. In what regards absolute numbers of neurons, however, the human brain does have two advantages compared to other mammalian brains: compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates; and, among economically built primate brains, it is the largest, hence containing the most neurons. These findings argue in favor of a view of cognitive abilities that is centered on absolute numbers of neurons, rather than on body size or encephalization, and call for a re-examination of several concepts related to the exceptionality of the human brain.

Pubmed ID: 19915731 RIS Download

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Comparative Mammalian Brain Collections

This web site provides browsers with images and information from one of the world''s largest collection of well-preserved, sectioned and stained brains of mammals. Viewers can see and download photographs of brains of over 100 different species of mammals (including humans) representing over 20 Mammalian Orders. Also available are examples of stained sections from a wide variety of brains of special interest, including Humans, Chimpanzees, Monkeys, various Rodents and Carnivores, California Sealion, Florida Manatee, Big Brown Bat, American Badger, American Raccoon, Yellow Mongoose, Zebra, Cow, and the Atlantic Bottlenose Dolphin. A complete list of all available specimens is available. How brain evolution has occurred is discussed. Viewers will learn why these collections are important, why and how they were assembled, and why it is important to protect, preserve and maintain them. Moreover, a variety of issues in brain science are discussed. For users who are interested in using any of our images for educational or research purposes, you have our permission to use them. But, they are not to be published and copyrighted since this would prohibit others from using the same images. At any rate, we request that you identify them as from the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections, as well as from those at the National Museum of Health and Medicine. Also, we request that you refer to the Web Site where you obtained them, as well as the fact that preparation of all these images and specimens have been funded by the National Science Foundation, as well as by the National Institutes of Health.

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