Applications of neuronal labeling techniques to the study of the peripheral auditory system

Horseradish peroxidase (HRP) techniques have recently been applied to the study of the peripheral auditory system. These techniques make it possible to trace single, auditory-nerve fibers from their peripheral terminals on sensory cells to their central terminations within the cochlear nucleus. For the large, myelinated axons of type-I neurons contacting inner hair cells, HRP can be injected intracellularly through the same electrode used to record neural activity, allowing correlations to be made between structure and function on a single-cell basis. From these data, a cochlear frequency map has been constructed, and morphological correlates of differences in spontaneous discharge have been identified. For the small, unmyelinated axons of type-II neurons contacting outer hair cells, single-unit recordings have not yet been made; however, these neurons have been labeled by gross extracellular injections of HRP. Extracellular injections have also been used to study the peripheral terminations of single efferent neurons projecting to the outer hair cell region. The afferent fibers to the outer hair cell (OHC) region contact primarily OHCs from the third row, while efferent fibers primarily contact cells from the first row. Single efferent fibers innervate OHCs over a significantly larger cochlear length than single afferent fibers of either type. Possible functional interpretations of these morphological differences are discussed.