The development of the auditory neurons and their connectivity was studied by early anatomists such as Gustaf Retzius, Fredrich-Christian Rosenthal, Alfonso Corti, Rudolph Albert von Kölliker, and Rafael Lorente de Nó. Indeed, the synapse between the hair cell and the sensory neurons was defined already by Retzius (1884) and made him one of the first adopters of the newly formulated neuron theory proposed by S. Ramon y Cajal. Retzius’ silver impregnations showed a sharp stop at hair cells and thus led him to believe that the proposed discontinuity of sensory neurons and hair cells was in line with the neuron theory, which contrasted with the prevailing reticular theory of a syncytial nerve net (Retzius, 1884; Grant, 2011). Retzius’ studies also presented the first detailed account of the development and cellular organization of the organ of Corti, the hearing organ of mammals. In addition, they clarified much of the evolution of the human organ of Corti from the basilar papilla found in other tetrapods (Retzius, 1884; Fritzsch et al., 2013). At the end of the 19th century, Retzius’ ear studies provided the most detailed and comprehensive analysis of any vertebrate organ system development and evolution. Based on his standing in science at the time, Retzius was nominated for the Nobel Prize 11 times, without ever receiving it.

It took about 50 years before another great neuroanatomist, R. Lorente de Nó, could surpass the achievements of Retzius. As summarized in his 1981 volume (Lorente de Nó, 1981), Lorente de Nó in the 1930s showed, in great detail, how spiral ganglion neurons project to the organ of Corti (Fig. 1.2) and how the central projections into the cochlear nuclei are cochleotopically organized (Fig. 1.3). This beautiful work, unfortunately, followed the perception of Lorente de Nó’s teacher, Ramon y Cajal, in assuming that there is no efferent system to the ear. Because of this bias, Lorente de Nó showed both afferent and efferent fibers in the organ of Corti in a bewildering complexity of a rich variety of spiral ganglion neurons (Fig. 1.2). In contrast to the baffling differences in peripheral projections, the central projections drawn by Lorente de Nó (Fig. 1.3) showed a much greater uniformity. The discovery that efferents to the ear exist (Rasmussen, 1953; Simmons et al., 2011; Sienknecht et al., 2014) and that spiral ganglion neurons comprise only two distinct types (Spoendlin & Schrott, 1989) overturned much of the early interpretations of Lorente de Nó. However, these different interpretations do not invalidate the information presented in the exquisite drawings that followed in great detail those of his teacher, S. Ramon y Cajal. Indeed, it took 100 years before modern techniques were developed that matched or exceeded the resolution of the Golgi technique to study spiral ganglion neurons (Figs. 1.1, 1.2, 1.3) and their peripheral and central projections. The present SHAR volume provides insights that go well beyond those early stages of the neurobiology of the spiral ganglion by adding molecular and physiological dimensions that will inform future clinical applications how to combat hearing impairment.

Beyond these early neuroanatomical insights, research over the last 30 years has shown that loss or damage of hair cells and/or spiral ganglion neurons is the leading cause of congenital and acquired hearing loss affecting millions of humans (Yamasoba et al., 2013). The most common therapeutic strategies for hearing loss are based on either using hearing aids to increase hair cell stimulation in moderate hearing loss or utilizing cochlear implants (CIs) as an electronic substitute for hair cells in cases of severe hearing loss resulting from severe hair cell loss (Zeng et al., 2011). However, both scenarios depend on the presence of functional spiral ganglion neurons to achieve a positive outcome. Indeed clinical data suggest that the function of CIs depends critically on the remaining number of spiral ganglion neurons (Nadol et al., 1989; Reiss et al., 2012). Thus, the development of methodologies that could be used in the maintenance, repair, and regeneration of spiral ganglion neurons in a damaged ear have significant implications for future advances in cochlear implant technology and the treatment of neurosensory hearing loss.