Central Projections of Spiral Ganglion Neurons

Neurons of the spiral ganglion exhibit a complex yet precise organization for delivering acoustic information from the mammalian inner ear to the brain. These neurons display a range of anatomical and physiological specializations for accurate encoding of sound features, and many of the characteristics observed in the periphery are reflected in the pattern of central projections of the auditory nerve into the cochlear nucleus. The dominant organizational principle of the auditory system is tonotopy, in which the topographic ordering of frequency from low-to-high along the sensory epithelium is replicated throughout the auditory pathway. Overlying this tonotopic organization is a second layer of complexity relating to spontaneous discharge rate, activation threshold, average rates of activity, and sound intensity coding. In the cochlear nucleus, different levels of spiral ganglion cell activity are associated with different termination patterns, even within an isofrequency lamina, and can produce morphological differences in ending structure. Ending morphology and distribution also differ with respect to target cell types and physiological response properties in the cochlear nucleus, suggesting that particular classes of connections code different aspects of the acoustic signal. Ultimately, neural activity initiated by hair cells is sent along divergent, parallel pathways to converge and recombine into percepts of the sound environment. The evolutionary persistence of features that enhance acoustic processing is a reminder that auditory specializations promote species survival.