Central projections of cochlear nerve fibers in the alligator lizard

The auditory (cochlear) ganglion cells of the alligator lizard (Gerrhonotus multicarinatus) give rise to two types of peripheral fibers: tectorial fibers, which contact hair cells covered by a tectorial membrane, and free-standing fibers, which contact hair cells without a tectorial membrane. To determine the central projections of these fibers, we applied intracellular and extracellular injections of horseradish peroxidase (HRP) to the peripheral component of the cochlear nerve. After histological processing with diaminobenzidine, individual cochlear nerve fibers could be traced through serial sections with the aid of a light microscope and drawing tube. The projection patterns formed two morphologically distinct groups. Neurons whose peripheral processes contacted tectorial hair cells in the cochlea projected to three divisions of the cochlear nucleus: nucleus magnocellularis lateralis (NML), nucleus magnocellularis medialis (NMM), and nucleus angularis lateralis (NAL). Neurons whose peripheral processes contacted free-standing hair cells projected primarily to the nucleus angularis medialis (NAM), although some also sent a single, thin branch to the NML; these neurons never projected to NAL or NMM. Morphometric comparisons of tectorial and free-standing fibers demonstrate that tectorial fibers have a larger axonal diameter, form a greater number of terminal swellings, and make proportionally more somatic contacts. By correlating the morphologically defined groups with previously reported physiologically defined groups, we conclude that different divisions of the cochlear nucleus are associated with separate frequency ranges and that stimuli in the different frequency ranges may be processed separately in the brain.