Olivocochlear neurons sending axon collaterals into the ventral cochlear nucleus of the rat

The olivocochlear projection constitutes the last stage of the descending auditory system in the mammalian brain. Its neurons reside in the superior olivary complex (SOC) and project to the inner and outer hair cell receptors in the cochlea. Olivocochlear neurons were also reported to send axon collaterals into the cochlear nucleus, but controversies about their number and about species differences persist. By injecting the fluorescent retrograde axonal tracers diamidino yellow and fast blue into the cochlea and the ventral cochlear nucleus (VCN), we studied the distribution and number of olivocochlear neurons with and without axon collaterals into the VCN of the rat. We found that olivocochlear neurons residing in the lateral superior olive (LSO), the intrinsic lateral olivocochlear cells (intrinsic LOCs), do not send axon collaterals into the VCN. By contrast, a majority, and possibly all, olivocochlear neurons residing in the ventral nucleus of the trapezoid body (VNTB), the medial olivocochlear cells (MOCs), do have such axon collaterals. These cells may thus affect processing in the ascending auditory pathway at the level of the receptors and concurrently at the level of the secondary sensory neurons in the cochlear nucleus. Belonging to the lateral olivocochlear system, shell neurons reside around the LSO and form a third group of olivocochlear cells (shell LOCs). Like intrinsic LOCs, they innervate the inner hair cells, but like MOCs they do, by means of axon collaterals, project into the VCN. These findings have implications for understanding both auditory signal processing and the plasticity responses that occur following loss of cochlear function.