Organization of the disynaptic pathway from the anteroventral cochlear nucleus to the lateral superior olivary nucleus in the ferret

The medial nucleus of the trapezoid body (MNTB) is one of three major nucle i of the superior olivary complex and provides an important inhibitory inpu t from the contralateral ear to the lateral superior olivary nucleus (LSO) in the initial binaural pathway for coding interaural intensity differences . The major input to the MNTB from the contralateral anteroventral cochlear nucleus (AVCN) involves giant, calyx-like endings that have a one-to-one r elationship with cells in the MNTB as confirmed in the ferret in this study . The main objective of the present study was to define the subsequent orga nization of projections from cells receiving these calyx-like endings. Seve ral anatomical tracers (Phaseolus vulgaris leucoagglutinin, dextran-biotin, and biocytin) were used that are transported both anterogradely and retrog radely within neuronal projections in order to define the organization of M NTB connections with the LSO in the adult ferret. Analysis focused on deter mining the topography in both the transverse and longitudinal planes of the projections. Focal tracer injections in the LSO resulted in retrograde lab eling of a long, narrow column of cells in the MNTB. The orientation and lo cation of labeled cells was dependent on the medial-lateral position of the injection site. In the rostral-caudal dimension of MNTB, there was no such topographic relation between the injection site and the position of labele d cells. Labeled cells in the MNTB were distributed more or less evenly in a longitudinal column regardless of whether the injection site was restrict ed to the rostral, middle or caudal part of the LSO. In keeping with this p attern, tracer injections in the MNTB resulted in bands of labeled axons th at distributed endings throughout the rostral-caudal axis of the LSO. These bands or sheets varied in medial-lateral position relative to the location of the injection site, but lacked any such rostral-caudal gradient. Thus, overall the MNTB-LSO projections have a convergent-divergent pattern of org anization. While MNTB cells receive singular calyx-like endings from the AV CN, LSO cells receive projections from a long column of cells in the MNTB. Implications for processing interaural intensity differences are discussed.