Why Do Axons Differ in Caliber?

CNS axons differ in diameter ( d ) by nearly 100-fold (∼0.1–10 μm); therefore, they differ in cross-sectional area ( d 2 ) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d 2 . We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning...