ABSTRACT
The cochlear nucleus of the brainstem receives direct input from the inner ear and is therefore the first synaptic station of the central auditory system. The resident neurons of the cochlear nucleus in turn give rise to all ascending pathways. Consequently, the organization between incoming auditory nerve fibers and second-order neurons plays a key role in the central processing of sound. A corollary is that abnormalities in the cochlear nucleus are likely to have downstream effects throughout the system. The cochlear nucleus contains a variety of different cell types (see Chapters 18 and 19), each of which exhibits different somatic and dendritic properties (Osen, 1969; Brawer et al., 1974), associates with a particular constellation of afferent endings (Lorente de Nó, 1981; Cant, 1982; Smith and Rhode, 1989), projects to different targets (Van Noort, 1969; Warr, 1982a; Schofield and Cant, 1996a), and expresses different response properties to sound (Pfeiffer, 1966a; Evans and Nelson, 1973; Rhode et al., 1983a; b). The particular combinations of these various properties are thought to underlie separate functions in hearing. Within the anteroventral cochlear nucleus (AVCN), myelinated auditory nerve fibers produce one or several large axosomatic endings known as endbulbs of Held (Held, 1893; Ramón y Cajal, 1909). The endbulb is one of the largest synaptic endings in the brain (Lenn and Reese, 1966), exhibits an elaborately branched appearance in adult animals (Ryugo and Fekete, 1982), expresses an estimated 500 to 2000 synaptic active zones (Ryugo et al., 1996), and contacts a population of second-order neurons called spherical bushy cells (Brawer and Morest, 1975; Cant and Morest, 1979a; Ryugo and Fekete, 1982). These features reflect a highly secure synaptic interface, consistent with the suggestion that every presynaptic discharge produces a postsynaptic spike (Pfeiffer, 1966b). The postsynaptic spherical bushy cell exhibits rapid depolarizations and repolarizations, thereby maintaining the temporal fidelity of incoming signals (Romand, 1978; Oertel, 1983; Manis and Marx, 1991). In addition, spherical bushy cells project to the superior olivary complex (Cant and Casseday, 1986) where they form a circuit implicated in the processing of interaural timing differences (Yin and Chan, 1990; Fitzpatrick et al., 1997). Thus, this component of the auditory pathway faithfully preserves the temporal changes and transients of acoustic streams necessary for the localization of sound sources in space and for the comprehension of speech (Moiseff and Konishi, 1981; Takahashi et al., 1984; Blackburn and Sachs, 1990).
