The contralateral input is predominantly inhibitory, although a period of conductive hearing loss can result in an increase in excitatory inputs. Recent studies, however, demonstrate a projection from the contralateral ear by way of the contralateral cochlear nucleus. The cochlear nucleus complex traditionally has been regarded as a strictly monaural structure, responding only to stimulation of the ipsilateral ear. Journal of Comparative Neurology 178: 661–678. Arnesen AR and Osen KK (1978) The cochlear nerve in the cat: Topography, cochleotopy, and fiber spectrum. DCN, dorsal cochlear nucleus AVCN, anterior ventral cochlear nucleus. The posterior ventral cochlear nucleus (PVCN), not shown here, lies deep to the cochlear nerve root (CNR). Each fiber forms major ascending (rostral) and descending (caudal) branches in the cochlear nucleus complex, resulting in maps of characteristic frequency from low frequency (LF) to high frequency (HF). This diagram from the right side of the cat shows auditory nerve fibers originating at four levels of the spiral ganglion. Cochlear nucleus complex and auditory nerve. et al., 1998), and suggests that synaptic properties are determined not just by the identity of the pre- or postsynaptic cell but in a synapse-specific manner.įigure 2. This is reminiscent of the varying synaptic properties among different neocortical excitatory synapses ( Markram, H. Thus, both pre- and postsynaptic physiology may vary in a cell-type-specific manner despite a common presynaptic source of input. These features are ideal for promoting integration of EPSPs generated by multiple auditory nerve fibers in nucleus angularis. Moreover, EPSPs in nucleus angularis are slower than in NM, due to their different postsynaptic membrane properties, and do not exhibit the acute presynaptic synaptic depression seen in calyceal synapses ( MacLeod, K. E., 2005b), despite their neurons having very different membrane firing properties ( Soares, D. Fast AMPA receptors appear to mediate transmission in both nuclei ( Raman, I. In birds, auditory nerve fibers terminate in two cochlear nuclei, the NM and the nucleus angularis, the latter being involved in sound localization using intensity cues ( Takahashi, T. J., 1999), suggesting very local control of synaptic properties.Įven among these auditory nerve-dominant neurons, synaptic function may vary. Interestingly, although synapses on both apical and basal dendrites are slower than VCN synapses, they employ different receptor subunits, which are differentially targeted to the two dendrites ( Rubio, M. Thus, it may be that the fast synaptic phenotype is restricted to cells whose primary excitatory input is the auditory nerve. Fusiform cells also receive glutamatergic contacts of granule cells on their apical dendrites, and these synapses also have slow kinetics. Synapses and AMPA receptors on basal dendrites of fusiform cells of the DCN exhibit much slower kinetics than synapses in the VCN or synapses onto interneurons in the DCN ( Gardner, S. What are the common physiological elements at these synapses? We have seen that synapses of the auditory nerve use glutamate receptors with rapid kinetics however, this is not always the case. Trussell, in The Senses: A Comprehensive Reference, 2008 3.33.11 Determination of Synaptic Information TransferĪuditory nerve fibers innervate diverse cell types each with very specific processing roles. In response to a complex signal, with a broad range of frequencies the temporal information could, in principle, provide better resolution of the stimulus spectrum than possible through place information alone. Fibers are capable of encoding information concerning stimulus frequency in terms of the temporal properties of the discharge, in addition to the cochlear place identity of the fiber. A distinctive property of AN fibers is their ability to temporally synchronize (phase-lock) their discharge to a restricted portion of the stimulus cycle. This place coding of frequency is translated into discrete populations of AN fibers responding to individual frequencies shown in Fig. At low sound pressure levels the basilar membrane responds most vigorously to low frequencies at its apex and to high frequencies at the base. As a consequence of the mechanical properties of the organ of Corti, complex sounds are decomposed into a spectral series of signals distributed along the cochlear partition.
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