the histological evidences for developmental alternations in the transmitting time of impulses along the thalamocortical tract

change in transmitting time of impulses along axons is traditionally attributed to two parameters: the myelin formation and the diameter of neurite, both rising during the postnatal development. in the previous study, we showed that conduction velocity of the fibers projecting from the thalamus to the layer iv of the somatosensory (barrel) cortex increases as a function of age. however, the conduction velocity change is not parallel outside and inside of the barrel cortex. here, we tried to find a probable relationship between disparity of the conduction velocities and the extent of myelination of the thalamocortical pathway. our results indicate that myelin is evident on the extra-cortical but not intra-cortical fibers at ages >10 days. at the older age, however, myelin wholly covers the fibers both outside and inside of the cortex, more considerably in the former. our results demonstrate that difference in the conduction velocities of the extra-cortical and intra-cortical axons, at least to more extent, can be attributed to myelination dissimilarity along the thalamocortical fibers