Postnatal growth of nerve fibres in the pyramidal tract of the rabbit

The postnatal growth in diameter of corticospinal nerve fibres was studied in the medullary pyramid of the rabbit in order to make comparisons with the development of functional properties, which has been demonstrated by previous workers. During the first postnatal days the as yet unmyelinated fibres underwent a distinct increase in size. The first signs of myelin ensheathment were seen in 5-day-old animals, the ensheathed fibres varying between 0.4 and 0.7 mu in diameter. The growth in size of the largest myelinated fibres was more rapid during the second week and thereafter continued at a slower rate to a final size of 4--5 mu. The number of myelinated pyramidal fibres increased most rapidly during the first postnatal month and reached around 90,000 (unilaterally) in the adult. The vascular density initially increased, reaching a peak at 3 weeks and thereafter decreased. Comparison with physiological data showed that the initiation of myelination does not coincide in time with any striking decrease in response latency but rather seems to allow for a continued steady decrease. At a size of 3.5-4 mu the fibres appear to have attained the capability to follow rapid rates of stimulation.