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

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 (less than 0.7 microns) 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.