Time course of axonal myelination in the human brainstem auditory pathway.

Structures in the human brainstem auditory pathway, from the proximal end of the cochlear nerve to the inferior colliculus, undergo myelination between the 26th and 29th fetal weeks. By the 26th week of gestation, axons in the cochlear nerve and brainstem pathways have acquired linear arrays of oligodendrocytes, and faint myelin sheaths can be distinguished. By the 29th week, definitive myelination is present in all auditory pathways, including the proximal end of the cochlear nerve, trapezoid body, lateral lemniscus, dorsal commissure of the lemniscus, commissure of the inferior colliculus and brachium of the inferior colliculus. Subsequent to the 29th gestational week, density of myelination increases in all pathways until at least 1 year postnatal age. The time of onset of myelination coincides with the onset of acousticomotor reflexes and brainstem auditory evoked responses, processes which depend on rapid, synchronized conduction of auditory impulses in the cochlear nerve and brainstem. The cotemporality in appearance of myelin, reflex responses, and evoked responses supports the idea that the 26th to 28th gestational weeks are a critical period in the onset of human central auditory function. The subsequent increase in myelin density is likely to be a factor in the steady decrease in ABR wave III-V latencies observed during the perinatal period.