Pyramidal tract fiber spectrum in rats, with comments on cats and man.

Most CNS fiber spectrums are unimodal and strongly positively skewed, with many small and few large fibers. This study shows that the pyramidal tract (PT) fiber spectrum of a rat can be calculated as the sum of three distributions of myelinated axons, each derived by normal Gompertzian growth from three normal distributions of protoaxons. Histological measurement of the rat PT determined the values entered into the model, thus forcing a unique solution. The model was generalized to cats and man by assuming values for which no experimental data was available; the simulated PT fiber spectrums closely matched the observed PT fiber spectrums, in both species. It is concluded that normal Gompertzian growth is sufficient to account for the specific shape of the fiber spectrum, with no recourse to morphogenetic sculpting. The overproduction of cells during growth, and death of cells during development, may regulate the total number of neurons in different areas of cortex, but plays no role in determining the specific shape of the PT fiber spectrum.