Descending pathways to the spinal cord, IV: Some factors related to the amount of cortex devoted to the corticospinal tract.

In the companion paper to this one (Nudo and Masterton: J. Comp. Neurol. 296:559-583, '90), we have presented data indicating that in each of 22 mammals, there are either 2 or 3 separate regions of neocortex contributing corticospinal fibers. In this paper, we describe the variation in the absolute size of these cortical regions, the total amount of neocortex contributing corticospinal fibers (CST cortex), and the total amount of neocortex (total cortex) in each of the animals. We then use strict statistical tests to examine the relationships between these measures and several other quantitative measures or descriptions of the animals' size, ancestral heritage, motor prowess, and ecological adaptation. The results show that the absolute amount of CST cortex is more closely related to the total amount of neocortex than to any other quantitative measure available. The further variation--that is, the variation in the amount of CST cortex relative to total neocortex--appears to have been random over the inferred ancestral lineages of most animals in the sample, but seems to have been almost absent along the anthropoid lineage. Because this constancy in the relative amount of CST cortex over a very long period of anthropoid ancestry is apparently unusual if not unique among mammals, it may contain a clue to the special role of the corticospinal tract among primates. Finally, the distribution of the CST among the 3 cortical regions in primates was found to be more closely related to their particular mode of ecological adaptation than to their particular combination of digital dexterity and hand-eye coordination.