Abstract
Experiments in several species have identified direct projections to the medial geniculate nucleus (MG) from cells in subcollicular auditory nuclei. Moreover, many cochlear nucleus cells that project to the MG send collateral projections to the inferior colliculus (IC) (Schofield et al., 2014). We conducted three experiments to characterize projections to the MG from the superior olivary and the lateral lemniscal regions in guinea pigs. For experiment 1, we made large injections of retrograde tracer into the MG. Labeled cells were most numerous in the superior paraolivary nucleus, ventral nucleus of the trapezoid body, lateral superior olivary nucleus, ventral nucleus of the lateral lemniscus, ventrolateral tegmental nucleus, paralemniscal region and sagulum. Additional sources include other periolivary nuclei and the medial superior olivary nucleus. The projections are bilateral with an ipsilateral dominance (66%). For experiment 2, we injected tracer into individual MG subdivisions. The results show that the subcollicular projections terminate primarily in the medial MG, with the dorsal MG a secondary target. The variety of projecting nuclei suggest a range of functions, including monaural and binaural aspects of hearing. These direct projections could provide the thalamus with some of the earliest (i.e., fastest) information regarding acoustic stimuli. For experiment 3, we made large injections of different retrograde tracers into one MG and the homolateral IC to identify cells that project to both targets. Such cells were numerous and distributed across many of the nuclei listed above, mostly ipsilateral to the injections. The prominence of the collateral projections suggests that the same information is delivered to both the IC and the MG, or perhaps that a common signal is being delivered as a preparatory indicator or temporal reference point. The results are discussed from functional and evolutionary perspectives.
Highlights
Second we describe the distribution of labeled cells after injections of tracer confined to individual subdivisions of the medial geniculate nuclei (MG)
These projections originate from the SOC, NLL, and adjacent tegmental regions and are more extensive than described in any previous medial subdivision (MGm) (GP689). (F,G) Single- and double-labeled cells in the left medial nucleus of the trapezoid body (MTB) after injections of FluoroGold into the left MG and green beads into the left inferior colliculus (IC) (GP597)
We have shown previously that MTB projections to the IC originate from elongate cells (Schofield, 1994), so collateral projections to the IC and MG must arise from these cells
Summary
The general view of the ascending auditory pathways includes divergent projections from the cochlear nucleus to multiplebrainstem nuclei and a re-convergence of projections from most of these nuclei to the inferior colliculus (IC). The IC provides the ascending input to the medial geniculate nucleus (MG), the main auditory center in the thalamus and source of projections to auditory cortex. Bilateral lesions of the brachia do not eliminate sensory input to the auditory cortex or prevent various auditory behaviors. A series of behavioral studies in cats with similar lesions showed clearly that the animals could localize and orient to a sound source and learn frequency discriminations (Goldberg and Neff, 1961; Jane et al, 1965; Casseday and Neff, 1975). The animals’ performance in these tasks was much degraded compared to unlesioned animals, and they learned discriminations only with difficulty. These behavioral results demonstrate clear auditory function in the absence of Frontiers in Neuroanatomy www.frontiersin.org
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