Projections of thin (type-II) and thick (type-I) auditory-nerve fibers into the cochlear nucleus of the mouse

Abstract

Injections of horseradish peroxidase into the mouse spiral ganglion were used to label type-I and type-II afferent fibers. Axons presumed to be from type-II spiral ganglion cells because of their small diameter (< 0.7 μm) and lack of nodes of Ranvier were traced to their terminations in the cochlear nucleus. Thicker fibers presumed to be from type-I ganglion cells were also reconstructed. Type-I and type-II axons labeled by basal turn injections bifurcate together in the dorsal part of the auditory nerve root, forming a branch that ascends into the anteroventral cochlear nucleus and a branch that descends into the posteroventral cochlear nucleus. Type-I fibers formed many collaterals ending in terminal swellings whereas type-II fibers were almost unbranched. Swellings from type-I and type-II fibers were often formed alongside one another. Examples of this proximity include terminal swellings of root collaterals in the auditory nerve root, as well as type-II en passant swellings and type-I terminal swellings throughout the ventral cochlear nucleus. The projections are dissimilar, however, since every type-II fiber projects at least one collateral to the granule-cell lamina. These collaterals usually end in neuropil forming the border between the ventral cochlear nucleus and the granule-cell lamina. In this border region, the type-II terminals overlap with those of branches from thick axons of the olivocochlear (efferent) bundle. Type-II fibers also differ from type-I fibers by only rarely coursing into the dorsal cochlear nucleus and by forming very few terminal swellings. En passant swellings, however, are numerous on type-II fibers, with ellipsoidal-shaped swellings prominent in the nerve root and angular and complex-shaped swellings common nearer the terminals. We suggest that the latter swellings are associated with type-II synapses whereas the ellipsoidal swellings represent non-synaptic structures.