Afferents from the hindbrain auditory system to the nuclei of the lateral lemniscus were analyzed by the use of orthograde and retrograde axon-tracing techniques. Three divisions of the nuclei of the lateral lemniscus, a dorsal, an intermediate, and a ventral division are discussed. The dorsal nucleus of the lateral lemniscus is a recipient of afferents from cells located mainly in the superior olivary complex and the contralateral dorsal nucleus of the lateral lemniscus. It receives direct afferents from only a few cells in the cochlear nuclei. In sharp contrast, the ventral nucleus of the lateral lemniscus is the recipient of afferents from many cells in the contralateral ventral cochlear nucleus and from only a few cells in the superior olivary complex. Further, it receives no afferents from cells in the contralateral nuclei of the lateral lemniscus. The intermediate nucleus of the lateral lemniscus receives afferents from some cells in the cochlear nucleus and the superior olivary complex. It is unique among the three nuclei of the lateral lemniscus in that it receives a substantial projection from the medial nucleus of the trapezoid body.
The ascending auditory projections to central nucleus of inferior colliculus and its ventrolateral and dorsomedial subdivisions (ICVL and ICDM) have been studied in cat using both pressure and electrophoretic injections of horseradish peroxidase (HRP). The results indicate that the predominant ascending projections to inferior colliculus originate in (1) contralateral cochlear nucleus, (2) contralateral and ipsilateral lateral superior olive, (3) ipsilateral medial superior olive, (4) ipsilateral ventral nucleus of the lateral lemniscus, (5) ipsilateral and contralateral dorsal nucleus of the lateral lemniscus, and (6) contralateral inferior colliculus. In addition, ipsilateral cochlear nucleus, ipsilateral and contralateral intermediate nucleus of the lateral lemniscus, ipsilateral, and to a lesser extent contralateral, periolivary nuclei project to inferior colliculus. Of these nuclei, the lateral superior olive projects exclusively to ICVL and ipsilateral cochlear nucleus and contralateral inferior colliculus project mostly, if not exclusively, to ICDM. Many of these projections demonstrate a cochleotopic organization and frequency a nucleotopic organization as well. A cochleotopic organization of the projections is apparent for cochlear nucleus and superior olivary complex. A nucleotopic organization suggests that the heaviest terminations of contralateral inferior colliculus are medial and dorsal in inferior colliculus, of medial superior olive are dorsal and lateral, of superior olivary complex are rostral, of cochlear nucleus are caudal, and of ventral nucleus of the lateral lemniscus are caudal.
Four varieties of mammals whose medial superior olives range from large to none at all were tested for their ability to localize single, brief tone pips at various frequencies. Although each animal could localize high-frequency tone pips, their ability to localize middle-and low-frequency tone pips corresponded to the size of their medial superior olive (MSO). Since this latter range of frequencies is the one in which binaural phase-difference cues predominate, this anatomical-behavioral correspondence supports the idea that MSO is the chief binaural time-analyzing center for sound localization.
The presence of ascending auditory inputs from the posteroventral cochlear nucleus (PVCN) to olivocochlear neurons was examined in guinea pig by using the combination Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde and horseradish peroxidase (HRP) retrograde tract-tracing technique. By labeling the somata of olivocochlear neurons after injection of HRP into the cochlea and simultaneously labeling terminal endings of PVCN efferent neurons after injection of PHA-L into PVCN, we observed neuronal connections between these two elements within all regions of the superior olivary complex known to contain olivocochlear neurons. These regions include the superior paraolivary nucleus, medial nucleus of the trapezoid body, lateral superior olive, and periolivary regions. All possible projection patterns regarding side of input and output of both large (four combinations) and small (two combinations) olivocochlear neurons were observed. However, the most frequently observed pattern was the PVCN projection to a contralaterally located and contralaterally projecting, large olivocochlear neuron. Thus the most prevalent pattern demonstrated a feedback pathway that crossed the brainstem twice. Additional patterns demonstrated pathways that fed back to the same cochlea as well as pathways that fed forward to the opposite cochlea.
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