Metabolic, functional and volumetric constraints on brain architecture
Sam Wang, Princeton University

Across phylogeny, mammalian brains can vary over 100,000-fold in size.

What functional principles are conserved across this large size range? We are examining how neural components such as axons and dendrites scale up as brains become larger. One subject of our recent investigations is the neocortical white matter, which is composed entirely of axons. We find that from shrews to whales, the composition of the white matter shifts from compact, slow-conducting, and metabolically expensive unmyelinated axons to large, fast-conducting, and metabolically inexpensive myelinated axons. The observed axon size distributions can account for allometric power laws for scaling of metabolic rate with respect to brain size, and for the increase in white matter volume with respect to gray matter volume. The fastest axons scale in width to keep minimum cross-brain conduction times a brief and consistent 1-2 milliseconds. Thus, because of a select set of communicating fibers, brains do not pay large timing costs as brain size increases. These very thick axons account for a considerable fraction of the white matter volume. White matter architecture in large brains may reflect a design trade-off, within energetic constraints, between reducing volume and reducing conduction time.

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