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The Journal of Neurophysiology Vol. 82 No. 2 August 1999, pp. 648-663
Copyright ©1999 by the American Physiological Society
1Department of Otolaryngology, West Virginia University School of Medicine, Morgantown, West Virginia 26506-9200; 2Department of Biomedical Engineering and Center for Hearing Sciences, Johns Hopkins University, Baltimore, Maryland 21205; and 3Department of Physiology, Hebrew University-Hadassah Medical School and the Interdisciplinary Center for Neural Computations, Hebrew University, Jerusalem, Israel
Spirou, George A.,
Kevin A. Davis,
Israel Nelken, and
Eric D. Young.
Spectral Integration by Type II Interneurons in Dorsal
Cochlear Nucleus. J. Neurophysiol. 82: 648-663, 1999. The type II unit is a prominent inhibitory interneuron
in the dorsal cochlear nucleus (DCN), most likely recorded from
vertical cells. Type II units are characterized by low rates of
spontaneous activity, weak responses to broadband noise, and vigorous,
narrowly tuned responses to tones. The weak responses of type II units to broadband stimuli are unusual for neurons in the lower auditory system and suggest that these units receive strong inhibitory inputs,
most likely from onset-C neurons of the ventral cochlear nucleus. The
question of the definition of type II units is considered here; the
characteristics listed in the preceding text define a homogeneous type
II group, but the boundary between this group and other low spontaneous
rate neurons in DCN (type I/III units) is not yet clear. Type II units
in decerebrate cats were studied using a two-tone paradigm to map
inhibitory responses to tones and using noisebands of varying width to
study the inhibitory processes evoked by broadband stimuli.
Iontophoresis of bicuculline and strychnine and comparisons of two-tone
responses between type II units and auditory nerve fibers were used to
differentiate inhibitory processes occurring near the cell from
two-tone suppression in the cochlea. For type II units, a significant
inhibitory region is always seen with two-tone stimuli; the bandwidth
of this region corresponds roughly to the previously reported
excitatory bandwidth of onset-C neurons. Bandwidth widening experiments
with noisebands show a monotonic decline in response as the bandwidth
increases; these data are interpreted as revealing strong inhibitory
inputs with properties more like onset-C neurons than any other
response type in the lower auditory system. Consistent with these
properties, iontophoresis of inhibitory antagonists produces a large
increase in discharge rate to broadband noise, making tone and noise
responses nearly equal.
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