Redwood Neuroscience
Title: “Speed limits in the neocortex: scaling
constraints from biophysics”
Sam
Wang
Department
of Molecular Biology
Abstract:
From shrew to whale, the mammalian neocortex displays
orderly scaling laws, suggesting that brain architecture is governed by general
functional design constraints. Neocortical white matter consists of myelinated axons, which conduct more quickly and consume
less energy per action potential than unmyelinated
axons, but are also wider and occupy much more volume. Here, using new and
existing measurements, we find that white matter satisfies two constraints:
maintaining a relatively constant cross-brain transmission time for the fastest
fibers, and scaling energy consumption to body size. Fast transmission is
accomplished by a few very thick axons, which occupy up to half of the white
matter of large brains. Axon size distributions can account for white matter
volume scaling with the +1.23 power of gray matter volume, and for a metabolic
rate scaling with the -0.32 power of body weight. These scaling laws may
reflect a trade-off among axon volume, speed, and energetics.