Electron microscopical evidence for common inner ear and lateral line efferents in urodeles

The efferents of the lateral line system and the inner ear were examined in urodeles using retrograde labelling with horseradish peroxidase (HRP). The Golgi-like filling thus achieved allowed tracing of the axons of efferent cells from one inner ear to the other and from the lateral line nerves into both inner ears. Electron microscopic examination of the inner ear revealed HRP label only in vesicle filled terminals on hair cells traditionally considered as efferent synapses. These data confirm earlier claims of common bilateral inner ear and lateral line efferents. In addition, the efferent nature of retrogradely labelled rhombencephalic cells is proven by their continuity with ultrastructurally identified efferent synapses in the inner ear.