Topography of spiral ganglion projections to cochlear nucleus during postnatal development in cats.

A fundamenntal organizational principle of the central auditory system is that virtually all areas are tonotopically organized. However, we know very little about the timing or mechanisms that are responsible for the development of this organization. When cats are born, their auditory nervous systems are extremely immature, and their hearing thresholds are very high. Until postnatal days 7-10 (P7-10), cats have behavioral and physiological thresholds which are near or above the pain threshold for adults and also have poor frequency selectivity. Physiological thresholds for auditory nerve fibers and cochlear nucleus neurons are typically above 100-120 dB SPL (sound pressure level re 20 microPa). Three weeks later (at approximately P31), the sensitivity and frequency discrimination (tuning) of these neurons approximate adult values. This study examines the development of the tonotopic projections from the spiral ganglion to the cochlear nucleus during the period in cat development in which the auditory system undergoes the transition from being essentially nonfunctional to having adult-like function. With the animals heavily anesthetized, the cochleas were surgically exposed in kittens ranging in age from P6 to P45. Focal injections of Neurobiotin (NB) were made into Rosenthal's canal, labeling a small cluster of cells in the spiral ganglion of each cochlea. The projections of these labeled cells were visualized as frequency-specific bands of labeled axons and terminals in all major subdivisions of the cochlear nucleus. The thickness of these bands (i.e., the dimension of the bands orthogonal to the isofrequency representation and across the frequency gradient) were measured and compared to similar projections in adults. As in adult cats, the thickness of the bands varied only slightly with the location of the injection site (frequency representation) over a range of 1-7 mm from the cochlear base (45-13 kHz). Moreover, band thickness did not vary significantly with age. These data indicate that the tonotopic organization of spiral ganglion projections to the cochlear nucleus is as precise in kittens as young as P6 as it is in adults.