Anatomy of the ventral nucleus of the lateral lemniscus in rats: a nucleus with a concentric laminar organization.

The lateral lemniscus contains relay nuclei of the auditory pathway in which the neurons have been grouped into dorsal and ventral (VNLL) nuclei. The data about the cytoarchitecture of the VNLL are controversial and no agreement exists concerning its tonotopical organization. In this paper, the cytoarchitecture of VNLL and the spatial distribution of its neurons projecting to the central nucleus of the inferior colliculus (CNIC) have been studied by using different tracers. Rats were iontophoretically injected in the CNIC and grouped in three sets. Group 1 rats received large injections of biotinylated dextran amine (BDA). Group 2 animals received restricted single injections of BDA in the low-, medium-, or high-frequency regions of the CNIC. Group 3 rats were double injected, with horseradish peroxidase placed in the high-frequency region of the CNIC, and with biocytin in the low-frequency one. The distribution of retrogradely labeled neurons in the ipsilateral VNLL was three-dimensionally reconstructed by use of a computer microscope. The analysis of labeled neurons and Nissl material suggests that the VNLL contains flat stellate neurons. Labeled flat stellate neurons and fibers are oriented in parallel and form fibrodendritic laminae. The projection from the VNLL to the CNIC is topographically organized: neurons in peripheral laminae project to dorsolateral, low-frequency regions of the CNIC, and those of central laminae project to ventromedial, high-frequency regions. Each VNLL lamina forms a continuous ventrodorsal structure which resembles a helicoid.