Rl. Snyder et Pa. Leake, TOPOGRAPHY OF SPIRAL GANGLION PROJECTIONS TO COCHLEAR NUCLEUS DURING POSTNATAL-DEVELOPMENT IN CATS, Journal of comparative neurology, 384(2), 1997, pp. 293-311
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 f
or the development of this organization. When cats are born, their aud
itory nervous systems are extremely immature, and their hearing thresh
olds are very high. Until postnatal days 7-10 (P7-10), cats have behav
ioral and physiological thresholds which are near or above the pain th
reshold for adults and also have poor frequency selectivity. Physiolog
ical thresholds for auditory nerve fibers and cochlear nucleus neurons
are typically above 100-120 dB SPL (sound pressure level re 20 mu Pa)
. Three weeks later (at approximately P31), the sensitivity and freque
ncy 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 t
he animals heavily anesthetized, the cochleas were surgically exposed
in kittens ranging in age from P6 to P45. Focal injections of Neurobio
tin (NB) were made into Rosenthal's canal, labeling a small cluster of
cells in the spiral ganglion of each cochlea. The projections of thes
e 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 ortho
gonal to the isofrequency representation and across the frequency grad
ient) were measured and compared to similar projections in adults. As
in adult cats, the thickness of the bands varied only slightly with th
e location of the injection site (frequency representation) over a ran
ge of 1-7 mm from the cochlear base (45-13 kHz). Moreover, band thickn
ess 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. (C
) 1997 Wiley-Liss, Inc.