Hearing involves the integration of many brain functions in addition to the processing of vibrations in air. Sound detection initiates the operation of sound localization which uses two ears for acquiring binaural time and intensity information. Head-related transfer functions provide supplemental localization cues so knowledge of head and body position is important. Pinna orientation is significant for animals with mobile ears. Proprioceptive, vestibular, and visual information can be integrated with acoustic cues to determine relative motion between organism and the sound source. Once a sound is localized, it needs to be identified. Identification of a sound requires learning and memory. For example, sounds made by conspecifics will differ from those of a predator, and they can evoke different affective states. Not surprisingly, the auditory system is the recipient of diverse inputs, and we have shown that a number of inputs terminate in the granule cell domain (GCD) of the cochlear nucleus. The GCD contains a multitude of microneurons whose local circuit projections terminate on the principal projection neurons of the dorsal cochlear nucleus. Thus the GCD is well situated to influence ascending auditory information.
The dorsal cochlear nucleus (DCN) resembles a cerebellar folium in terms of structural organization, homologous cell types, and kinds of inputs. Afferents to these separate structures appear as bouton endings or mossy fibers (MFs) with the caveat that climbing fibers are restricted to the cerebellum. We are studying inputs to the cochlear nucleus with light and electron microscopy in order to identify postsynaptic targets that establish auditory circuits.
We use BDA, Pha-L, Cholera Toxin B, and various kinds of fluorescent dyes to study the afferent and efferent connections of cochlear nucleus. Electrophysiological recordings are often used to characterize the auditory responses and to guide tracer placement. Projections are studied by light and electron microscopy. Structures are visualized with 3-dimensional computer reconstructions. We are currently using 3D reconstruction techniques to assess the nature of mossy fibers in the cochlear nucleus that arise from pontine nuclei, cuneate nucleus, trigeminal nucleus, lateral reticular nucleus, and dorsal root ganglion of the second cervical vertebrae.
It is our working hypothesis that microneurons in the cochlear nucleus granule cell domain form distinct functional circuits with MFs of different origins. The data may also contribute to understanding the relationship between clinical tinnitus and the somatosensory system.