Morphology of the primary posterior plexus of the rat cochlear nucleus.

Golgi impregnation results confirm the existence of a plexus (previously described with horseradish peroxidase tracing techniques) located in the posteroventral cochlear nucleus. The bulk of axons involved in this plexus could be traced to their origin in the root, and were not related to the V-bifurcating fibres, though some of the posterior branches of the latter contributed to the plexus. The internal structure of the plexus, and the different types of endings are described. These findings are discussed in relation to previous Golgi studies, especially those referring to the neuron and axon population of this area.

Distribution of primary cochlear afferents in the bulbar nuclei of the rat: a horseradish peroxidase (HRP) study in parasagittal sections.

HRP was injected into the cochleae of 25 young albino rats in order to trace the primary afferents to the bulbar cochlear nuclei. Besides the classic V-shaped pattern and unconnected with it, HRP labelling revealed two plexuses stemming directly from the axons of the cochlear root. The plexuses cover the posterior area of the posteroventral cochlear nucleus (posterior plexus) and the anterolaterodorsal area of the anteroventral cochlear nucleus (anterior plexus). The fibres giving rise to these two plexuses were previously grouped in two bundles which have been called the posterior and anterior bundles, respectively. The origin of the anterior bundle is typically seen with the fibres stemming out at right angles; the origin and course of the posterior bundle, which characteristically cross over, is also a typical feature.

Distribution of primary afferent fibres in the cochlear nuclei. A silver and horseradish peroxidase (HRP) study.

Horseradish peroxidase, when injected intracochlearly, is transported transganglionically to the brain stem cochlear nuclei, thus providing an excellent method for tracing the central projection of the spiral ganglion neurons. Silver impregnation using the Cajal-de Castro method, which stains axons even when inside the bone, was used as a reference technique. The combination of both procedures led to the following conclusions. Primary cochlear afferents are found only in the ventral zone of the dorsal cochlear nucleus. In this area they cover the deep and fusiform cell layers. The molecular layer shows no HRP label. The higher concentration of primary cochlear afferents in the ventral cochlear nucleus appears in its central zone; wide areas in this nucleus are not labelled at all. A thin bundle of primary cochlear afferents runs parallel to, and beneath, the granular region.