ANATOMY AND PROJECTION PATTERNS OF THE SUPERIOR OLIVARY COMPLEX IN THE MEXICAN FREE-TAILED BAT, TADARIDA-BRASILIENSIS-MEXICANA

The superior olivary complex (SOC) is the first station in the ascendi ng auditory pathway that receives binaural projections. Two of the pri ncipal nuclei, the lateral superior olive (LSO) and the medial superio r olive (MSG), are major sources of ascending projections to the infer ior colliculus. Whereas almost all mammals have an LSO, it has traditi onally been thought that only animals that hear low frequencies have a n MSG. Recent reports, however, suggest that the medial part of the SO C in bats is highly variable and that at least some bats have a well-d eveloped MSG. Thus, the main goal of this study was to evaluate the cy toarchitecture and connections of the principal superior olivary nucle i of the Mexican free-tailed bat, with specific attention directed at the MSG. Cell and fiber stained material revealed that the LSO and the medial nucleus of the trapezoid body (MNTB) are similar to those desc ribed for other mammals. There are two medial nuclei we refer to as do rsomedial periolivary nucleus (DMPO) and MSG. Tracer experiments exhib ited that the DMPO receives bilateral projections from the cochlear nu cleus, and additional projections from the ipsilateral MNTB. The DMPO sends a strong projection to the ipsilateral inferior colliculus. Posi tive staining for acetylcholinesterase indicates that the DMPO is a pa rt of the olivocochlear system, as it is in other animals. The MSG in the free-tailed bat meets many of the criteria that traditionally defi ne this nucleus. These include the presence of bipolar and multipolar principal cells, bilateral innervation from the cochlear nucleus, a st rong projection from the ipsilateral MNTB, and the absence of choliner gic cells. The major difference from traditional MSG features is that it projects bilaterally to the inferior colliculus. Approximately 30% of its cells provide collateral projections to the colliculi on both s ides. Functional implications of the MSO for the free-tailed bat are c onsidered in the Discussion. (C) 1994 Wiley-Liss, Inc.