Projections of cochlear root neurons, sentinels of the rat auditory pathway.

In certain rodents, the root of the cochlear nerve contains a population of large neurons, known as cochlear root neurons (CRNs), an essential element of the primary acoustic startle pathway. To characterize the projections of the CRNs, we made stereotaxically guided, iontophoretic injections of biotinylated tracers into the cochlear nerve root of albino rats. CRN axons, which are remarkably thick, enter the trapezoid body, cross the midline, and ascend in the rostral aspect of the lateral lemniscus to reach the upper levels of the midbrain. As a group, CRN axons produce a characteristic pattern of profusely ramified collaterals that innervate specific brainstem regions. The main target of CRN axons is the contralateral pontine reticular formation, where collaterals terminate in the caudal pontine reticular nucleus (PnC) and, to a lesser degree, in the ventrolateral tegmental area, the oral pontine reticular nucleus, and the rostral and medial paralemniscal regions. Other targets of CRN axons include the lateral paragigantocellular nucleus of both sides, the ipsilateral facial motor nucleus and PnC, and the contralateral intercollicular tegmentum and superior colliculus. Notably, CRNs apparently do not innervate any of the nuclei of the auditory brainstem, as usually defined, even though their axons pass through or in close proximity to them. The fact that CRNs innervate several reticular and tectal structures that mediate auditory alerting and escape behaviors suggests that they are "early warning neurons," i.e., true sentinels of the auditory pathway.