Cochlear Nuclear Complex Initial Transformation of Sound Information Reaching the Brain

The cochlear nuclei, located bilaterally at the ponto-medullary junction, are the first synaptic stations in the central auditory pathway. The cochlear nuclei are obligatory relays, meaning that all information flowing from the cochlea must here encounter at least one synapse before being transmitted to higher auditory centers. In humans, as in all other mammals studied, two major cochlear nuclei are recognized: a dorsal cochlear nucleus (DCN) and a ventral cochlear nucleus (VCN). The latter is often subdivided into anterior and posterior divisions. Auditory nerve fibers penetrate this nuclear complex, bifurcate in an orderly (tonotopic) way, and terminate on neurons in both the DCN and the VCN. In addition to its major afferent supply from the inner ear, the cochlear nuclei receive input from a variety of other sources both intrinsic and extrinsic to the complex. Extrinsic sources include the other major brain stem nuclei. The role of these inputs in modulating the activity produced by primary afferent input from the ears is essentially unknown.

Cochlear nuclear neurons have been categorized based on their synaptic structure, cell morphology, pharmacology, connectivity, and acoustic response properties. Incoming spike trains that reach CN neurons are transformed by combinations of synaptic convergence and physiology, intrinsic membrane properties of target neurons, and feedback of excitation and inhibition via intrinsic or extrinsic circuitry. Cells of each category tend to extract particular features of sound that are transmitted by auditory nerve fibers, such as intensity, spectral composition, modulation amplitude and frequency, low-frequency time structure, and the timing of acoustic transients. Information received about human speech, for example, is thus distributed in such a way that one cell type preserves the temporal representation of vowels, whereas another cell type represents vowel information best in its rate of discharge. Similarly, other cell types extract formant transitions and still others transients (e.g., voice onset). Temporal information needed to detect ITD is preserved by still other cell types in the VCN having a highly specialized synaptic relationship (end bulb of Held) with auditory nerve fibers. Transformed information is sent to higher auditory centers for further processing over three pathways (stria).

Most axons leaving the VCN form a broad pathway that crosses the brain stem ventrally as the ventral stria or trapezoid body. A much smaller pathway, the intermediate acoustic stria, also leaves the VCN and joins the trapezoid body as it approaches the superior olivary complex, where many of its axons terminate. Axons of projecting neurons of the DCN form the dorsal acoustic stria, which reaches primarily the contralateral dorsal nucleus of the lateral lemniscus and the central nucleus of the inferior colliculus. At the level of the cochlear nuclei the input from the two ears has, for the most part, remained separated. One of the next levels of processing takes place in the superior olivary complex, where the inputs from the two ears converge and interact to encode sound direction.

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