Centrifugal Auditory Pathways

Centrifugal auditory pathways also include efferent projections to the sensory hair cells of the cochlea and to the muscles of the middle ear. The cochlear efferent fibers originate from a group of neurons on the medial side of the contralateral superior olive and pass to the cochlea via the crossed olivocochlear bundle and the cochlear division of the vestibulocochlear nerve. They are joined by a smaller number of fibers, which originate in the ipsilateral superior olive. The olivocochlear efferent pathway comprises the medial olivocochlear system (MOCS) and the lateral olivocochlear system (LOCS). The MOCS has large cell bodies in the medial and anterior olivary regions and innervate the outer hair cells of the cochlea. The LOCS has small cell bodies in and around the lateral superior olive and innervate the afferent dendrites beneath the inner hair cells of the ipsilateral cochlea. The large outer hair cell endings are primarily cholinergic, whereas the axodendritic synapses beneath the inner hair cells contain acetylcholine, dopamine, enkephalins, and other peptides. The efferent fibers produce hyperpolarization in the cochlear hair cells and afferent nerve terminals, thereby decreasing the afferent response produced when sound reaches the cochlea. Fibers innervating the muscles of the middle ear originate in the trigeminal motor nucleus and the facial nucleus (the tensor tympani muscle and the stapedius muscle). By contracting, these muscles decrease the transmission of sound vibrations from the eardrum to the oval window by way of the ossicles (incus, malleus, and stapes).

Several functions have been proposed for the centrifugal auditory pathways. One possibility is that efferent impulses can suppress the auditory nerve afferent responses to sound, thus preventing damage from too strong a stimulus. The middle ear muscles contract during loud noises and self-initiated vocalization, thereby helping to prevent saturation or damage of the cochlear receptors. Sound-activated efferent fibers in the olivocochlear bundle may additionally contribute to the suppression of sensory input that could saturate the central nervous pathways. A related mechanism, possibly also mediated by olivocochlear fibers, is improved auditory discrimination by the attenuation of loud background noise.

The phenomenon of selective attention to auditory signals is likely also to be an effect of the centrifugal auditory pathways. This “attentional filter” is absent in de-efferented humans. Evidence also shows that habituation to repeated auditory stimuli occurs with inhibition of the cochlear nuclei.

Finally, efferent olivocochlear pathways participate in auditory discrimination. Neurons at higher levels of the auditory pathway tend to respond to transient changes in auditory input rather than to steady signals. Centrifugal inhibition may be a factor in eliminating responses to steady signals, thus accentuating sensitivity to transient ones. Together with the inhibition that takes place within each level of the auditory system, it may also contribute to the processes that sharpen neuronal responses by restricting the ranges of the frequencies to which each neuron responds.

Only gold members can continue reading. Log In or Register to continue