Functional Tradeoffs in Axonal Scaling: Implications for Brain Function

Abstract

Like electrical wires, axons carry signals from place to place. However, unlike wires, because of the electrochemical mechanisms for generating and propagating action potentials, the performance of an axon is strongly linked to the costs of its construction and operation. As a consequence, the architecture of brain wiring is biophysically constrained to trade off speed and energetic efficiency against volume. Because the biophysics of axonal conduction is well studied, this tradeoff is amenable to quantitative analysis. In this framework, an examination of axon tract composition can yield insights into neural circuit function in regard to energetics, processing speed, spike timing precision, and average rates of neural activity.

Regional variation, hemispheric asymmetries and gender differences in pericortical white matter

“…The positive correlation between MTR with GM volume is consistent with a disproportionate increase in the fraction of myelinated fibers as brain size increases (Barres and Raff, 1999;Wang, 2008). The positive correlation of MTR with age is consistent with observations that myelination continues after puberty throughout much of early adult life (Ge et al, 2002).…”

Beyond faithful conduction: Short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

“…Local neurons and fast projection neurons with myelinated axons can have axonal conduction delays in the sub-millisecond range (Bartos et al , 2002; Evarts, 1965; Takahashi, 1965), whereas even some central projections with axons several centimeters long can show delays of tens of milliseconds or more (Aston-Jones et al , 1985a; Faiers and Mogenson, 1976). Across mammalian species with vastly different brain sizes, and therefore different axonal projection distances, the degree of myelination and the statistical distribution of axon diameters point toward evolutionary trade-offs in construction costs, metabolic costs, spatial constraints, conduction delay, and temporal precision (Wang, 2008; Wang et al , 2008). Small unmyelinated axons are costly in terms of metabolic rate because they require a continuous distribution of channel and pump molecules.…”

Microstructures of Learning: Novel methods and approaches for assessing structural and functional changes underlying knowledge acquisition in the brain

“…In myelinated axons, a diameter of 0.7 mm appears to optimize the energy per transmitted bit of information (Perge et al, 2009). The presence of large axons, whose diameter correlates with brain size between species in contrast to mean diameters of myelinated and unmyelinated axons (Wang, 2008;Wang et al, 2008) indicates they must support another type of function than small axons that motivate the extra energy required to use them.…”