The Journal Of Comparative Neurology
Auditory efferents originate in the central auditory system and project to the cochlea. Although the specific anatomy of the olivocochlear (OC) efferents can vary between species, two types of auditory efferents have been identified based upon the general location of their cell bodies and their distinctly different axon terminations in the organ of Corti. In the mouse, the relatively small somata of the lateral (LOC) efferents reside in the lateral superior olive (LSO), have unmyelinated axons, and terminate around ipsilateral inner hair cells (IHCs), primarily against the afferent processes of type I auditory nerve fibers. In contrast, the larger somata of the medial (MOC) efferents are distributed in the ventral nucleus of the trapezoid body (VNTB), have myelinated axons, and terminate bilaterally against the base of multiple outer hair cells (OHCs). Using in vivo retrograde cell body marking, anterograde axon tracing, immunohistochemistry, and electron microscopy, we have identified a group of efferent neurons in mouse, whose cell bodies reside in the ventral nucleus of the lateral lemniscus (VNLL). By virtue of their location, we call them dorsal efferent (DE) neurons. Labeled DE cells were immuno-negative for tyrosine hydroxylase, glycine, and GABA, but immuno-positive for choline acetyltransferase. Morphologically, DEs resembled LOC efferents by their small somata, unmyelinated axons, and ipsilateral projection to IHCs. These three classes of efferent neurons all project axons directly to the cochlea and exhibit cholinergic staining characteristics. The challenge is to discover the contributions of this new population of neurons to auditory efferent function.