Auditory brainstem in the mole (Mogera): nuclear configurations and the projections to the inferior colliculus

Anatomical organization of the central auditory system in the mole was studied at the lower brainstem levels. The cyto-, myelo-, and chemoarchitectures were examined in Nissl, myelin, and acetylcholinesterase stained materials, and then the origins of the ascending afferents to the inferior colliculus (IC) were identified by injecting wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the unilateral IC and processing the tissue according to the standard retrograde tracing techniques. The results indicate that the auditory nuclei and pathways in the lower brainstem of the mole conform to the basic plan common to many other mammals. Nevertheless, several characteristic features are evidenced in the present study: (1) in the cochlear nucleus (CochN), granule cell fields are very large in both the ventral (VCN) and dorsal (DCN) nuclei; among several populations of neurons, fusiform cells in the DCN, multipolar cells in the VCN and DCN, and small spherical cells in the VCN project to the IC directly, (2) in the superior olivary complex (SOC), the medial nucleus (MSO) is well developed in comparison with that in the hedgehog, the opossum, the mouse, and the rat, although the general configuration of the SOC is similar to that in those mammals, most strikingly, the MSO projects to the IC bilaterally in the mole, and (3) the nuclei of the lateral lemniscus (NLL) show a great development and consist of three well-differentiated parts of the dorsal, intermediate, and ventral nuclei. The projections from these subnuclei to the IC conform to the basic mammalian plan.