THE COCHLEAR NUCLEUS

The cochlear nucleus contains the circuits through which information about sound is coupled to the brain. In the cochlear nucleus, fibers of the auditory nerve contact neurons that form multiple, parallel representations of the acoustic environment. These circuits vary from simple synapses that preserve the timing of auditory events to complex neuropils that are sensitive to features that identify sounds. The parallel pathways each perform a different analysis of the auditory signal. Thus calculations such as the localization of a sound source in space or the identification of a sound are separated in the cochlear nucleus and performed in parallel as signals ascend through the brainstem auditory nuclei. A tonotopic map of sound is generated in the cochlea, shown schematically in Fig. 4.1. Sound causes a traveling-wave of displacement on the basilar membrane. The location of the largest displacement depends on the frequency of the sound, with low frequency sounds causing displacement at the apex and high frequencies at the base of the cochlea. The resulting map of stimulus frequency as a function of place along the basilar membrane is shown in Fig. 4.1 for the cat cochlea (for descriptions of cochlear anatomy and physiology, see Pickles, 1988; Patuzzi, 1996). Transduction of basilar membrane motion occurs in hair cells (Kros, 1996). Fluctuations of hair-cell membrane potential reflect motion of the basilar membrane. Depolarization in hair cells excites dendrites of auditory-nerve fibers (ANFs), synaptically evoking action potentials whose rate reflects the degree of excitation. Thus any given ANF responds to sound in proportion to the degree of basilar membrane motion at the point on the membrane innervated by the fiber. Each ANF responds to a limited range of frequencies. The frequency to which a fiber is most sensitive is its best frequency (BF). The best frequencies of ANFs vary systematically, forming a tonotopic map. Perceptually, position along the tonotopic map, or frequency of sound, corresponds to the sense of highness or lowness of the sound, which is like its position along a musical scale (Moore, 1997). In contrast with the visual and somatosensory systems, the tonotopic organization of the auditory system does not reflect location in space. The auditory system’s frequency map is more like the olfactory system’s map of odor molecular structure. There are two groups of hair cells and ANFs (Spoendlin, 1973; Kiang et al., 1982; Ryugo, 1992). Inner hair cells and type I ANFs form the major afferent pathway to the cochlear nucleus; each type I fiber innervates one inner hair cell. Type I fibers are myelinated, constitute 90-95% of the fibers in the auditory nerve, and are the primary afferent input to most of the cell types in the cochlear nucleus. Outer hair cells are innervated by type II fibers, which are unmyelinated and project to nonprincipal cell regions of the cochlear nucleus (Brown et al., 1988a; Brown and Ledwith, 1990). To date, responses to sound have not been recorded from type II fibers and it is not clear what role they play. Outer hair cells appear to participate in the mechanical response of the basilar membrane; loss of outer hair cells leads to a loss of sensitivity to soft sounds and a decrease in the sharpness of tuning (i.e. the frequency selectivity) of the basilar membrane (Patuzzi, 1996). In the rest of the chapter, the term "auditory nerve fiber" or ANF will refer to type I fibers only, unless otherwise stated.