Figure 5.

Scaling of per-action potential metabolic costs. a, The estimated metabolic cost per fiber of generating an action potential for unmyelinated axons, for myelinated axons, and for all white matter axons averaged. b, The estimated metabolic cost per unit weight of white matter of generating an action potential in all fibers, G_AP, decreases as the −0.31 ± 0.03 power of body weight (○). Direct measurement of white matter metabolic rates per unit time, G_t, measured by autoradiographic methods (●; for data sources see the supplemental material, available at www.jneurosci.org), gives a power law slope of −0.32 ± 0.06. c, The same quantities plotted against brain size. The G_AP and G_t axes are on logarithmic scales and are aligned with one another by a factor of 14.3 spikes/s.