Axodendritic and dendrodendritic synapses within outer spiral bundles in a human

Axodendritic and dendrodendritic synapses have been described at the level of the outer spiral bundle (OSB) (Nadol, J.B., Jr., 1983. Laryngoscope 93, 780-791; Bodian, D., 1978. Proc. Natl. Acad. Sci. USA 75, 4582-4586). The objectives of this study were to quantify these synaptic interactions and to describe their ultrastructural morphology in a young human subject. The temporal bone of an 8-month old infant was processed for transmission electron microscopy and semiserial section reconstructions of the three OSBs were performed. The nerve fibers ((NFs)) forming the OSBs were found to segregate into two morphological groups: (1) vesicle-rich and neurofilament-poor (VR/NP); (2) vesicle-poor and neurofilament-rich (VP/NR). Synapses between VR/NP and VP/NR NFs and synapses between two VP/NR NFs were quantified. Presumed axodendritic synapses (i.e. between VR/NP and VP/NR NFs) were numerous and their numbers decreased from the first towards the third row. Presumed dendrodendritic synapses (i.e. between two VP/NR NFs) were also frequent but their numbers did not vary significantly among different rows. The presence of axodendritic synapses may provide the morphological basis for modulation of the function of the type II spiral ganglion cells (type II's) by the olivocochlear efferent system. Similarly, numerous presumed dendrodendritic synapses may provide a morphological substrate for interaction between dendrites of type II's.