Descending Central Auditory Pathway — Structure and Function

Abstract

The presence of many descending fibers in the auditory pathway has been known for many years and the complexity of the descending auditory pathway is well recognized. Recent findings revealing the role of the olivocochlear bundle in the control of cochlear micromechanics have opened a new era in the investigation of the role of descending fibres in the auditory pathway. The advent of axonal transport techniques for tracing neuronal connections stimulated reinvestigation of the connections of the olivocochlear bundle originally described by Rasmussen (1946, 1953). New concepts have emerged from the work of Warr and Guinan (1979), who defined two populations of the olivocochlear efferents: lateral and medial olivocochlear neurons. These two populations may be distinguished with respect to their origin in different parts of the superior olivary complex (SOC) and differing projections to the cochlea. The mediator of the olivocochlear bundle is undoubtedly acetylcholine, as was originally proposed by Rasmussen (1960). However, recently also other possible mediators were found with immunohistochemical methods such as enkephalins (Fex and Altschuler, 1981; Eybalin et al., 1984), aspartate and GABA (Fex et al., 1986).