Functional and morphometrical maturation of the brainstem auditory pathway.

The correlation between the functional and morphological maturation of the auditory pathway was studied in preterm and term infants, children and adults. As to the auditory brainstem response (ABR), peak latencies and I-V interpeak latencies (central transmission) gradually decreased during the third trimester and the first 2 years postnatally. The calculated pontine auditory conduction velocity (PACV) showed dramatic development, which may indicate a more precise auditory function. The PACV value at the age of 2-4 years was almost the same as that of adults. In a histomorphometrical study, the density of nerve cells in the cochlear nucleus and the inferior colliculus was found to decrease with age, that in the inferior colliculus decreasing more slowly. Myelination in the lateral lemniscus proceeded from the late fetal to the infantile period, and the myelin sheaths of large diameter increased mainly in the infantile period. Thus, on studying ABR in combination with the quantitative histomorphometrical investigation, the development of PACV was found to be related to the maturation of nerve cells in the upper nuclei corresponding to each latency as well as myelination of small and large fibers in the auditory pathway. PACV, which can be calculated by studying ABR and magnetic resonance imaging, may be used to assess more accurately the brainstem function in individual patients.