A Thesis Presented to Princeton University in Partial Fulfillment for the Degree of Bachelor of Arts in Department of Molecular Biology.
Princeton University, 2003
A serious consequence of brain scaling is that components at different brain locations will be placed increasingly distant from one another. Consequently, action potentials will have to travel increasingly long distances, potentially placing limits on the speed of information processing. Here we present evidence from wide-field electron microscopic surveys that populations of large axons exist in the corpus callosum of five mammals whose brain diameters range from 9 to 55 mm. These axons support fast interhemispheric conduction, such that there is the possibility of an invariant cross-brain conduction time of 2-5 msec across species. Such timing invariance may be important for coordinating activity across the cerebral hemispheres (Swadlow 2000; Bush & Sejnowski 1996). The revised axon size spectra resulting from the inclusion of large axons reveals a conserved myelination threshold diameter, as others have predicted (Rushton 1951; Waxman & Bennett 1972), which suggests that myelination occurs such that white matter uses space optimally to achieve the fastest possible conduction velocities.
Earlier work (Schultz 2001; Burish 2002) has shown that allometric scaling laws observed in macroscopic parameters, such as white matter volume, can be accounted for by ultrastructural measurements. The present work both confirms this, and provides evidence that link these ultrastructural parameters with potential adaptations for the speed of information processing. We suggest that the morphometric axon size spectrum analysis we performed may well be extended to specific neocortical areas which have functional constraints on timing, such as primate area MT.
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