Myelination of brainstem axons during the critical period

The influence of sensory activity on myelination has been extensively studied. Particularly the auditory system is equipped with molecular and anatomical features supporting speed and precision to optimally encode the temporal features of sound where myelination plays a critical role in this process. In altricial mammals, cochlear inner hair cells display spontaneous electrical activity before hearing onset. The pattern and firing rate of these cells are crucial for the correct maturation of the central auditory pathway. The activity of medial olivocochlear system (MOC) leads to an accurate development of precise central connectivity by modulating spontaneous activity in the immature inner ear. While the relationship between sound stimuli and myelination in mature animals was studied, having shown that synaptic activity promotes myelin thickening, the impact of spontaneous activity on myelination during the critical period was not explored. In this study, an ultrastructural study was performed using serial block-face electron microscopy in a wild type (WT) mouse and a mouse model with reduced spontaneous activity (α9KI). Analysis of morphological parameters shows that both the number of myelin sheaths as well as myelin thickness are reduced in the α9KIadult mice. These data suggest that spontaneous activity is involved in brainstem myelination, thereby influencing auditory physiology.