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Energetic costs and speed limits in neocortical axons

Samuel S.-H. Wang, Kimberly H. Harrison, Jennifer R. Shultz, Mark J. Burish, and Matthew W. Wagers

Princeton University

In large brains, action potentials must travel for long distances, potentially increasing the time taken for information processing. This scaling problem is particularly apparent in mammalian brains, which vary in diameter by nearly 100-fold. Here we present evidence for adaptations for speed and energetic efficiency in long-distance axons of the neocortex. Axonal propagation speed can be increased by making axons wider and by adding myelin sheaths. We find that across species, the mean axon diameter and the degree of myelination increase steeply with brain size, indicating that conduction is faster in larger brains. The largest axons are proportional in width to brain diameter, suggesting that minimum cross-brain conduction time is conserved among mammals. Myelination also reduces the energetic cost of generating action potentials. As a result, the estimated per-volume cost of spike activity scales as the -0.30±0.09 power of body size, consistent with previously observed scaling of metabolic rates. Increases in axon size also account for the disproportionate growth in white matter volume, which for large brains can occupy nearly half the neocortex. Brain tissue is energetically expensive, and thus improvements in processing speed may be limited by the metabolic costs of an expanding white matter architecture.