The capacity of neurons of the central nervous system to grow throughout an animal's life has been a subject of continuing interest. Studies were undertaken in rat to ascertain if nerve fibers of the dorsal funicular cortico-spinal tract increase in diameter and/or number during the animal's growth.

Animals were divided into four experimental groups: (a) 4-weeks, 156 gm; (b) 10-weeks, 181 gm; (c) 10-weeks, 281 gm; (d) 9–months, 563 gm. Spinal cords were fixed by perfusion with 2.5% glutaraldehyde and the eighth thoracic segment immersed in 1% osmic acid. Counts and size measurements were made from photomicrographs of 2 μ sections.

The average number of myelinated nerve fibers in the tract was 13,723. There were no statistical differences in number of fibers between the various age and weight groups. Growth in this tract was related to increased fiber diameter with age concomitant with increased area of tract and of spinal cord. As the diameter of individual nerve fibers increased, myelin sheath and axis cylinder diameters increased proportionately.

The cortico-spinal tract of the rat grows by increase in nerve fiber diameter from 4-weeks to at least 9-months of age. It is suggested that by means of this mechanism of central nervous system growth, the nerve fiber can transmit information at a more rapid rate over greater distances as growth proceeds, thus allowing for sensory and motor coordination of the animal during changes in body size.