PHYSIOLOGICAL DEVELOPMENT OF THE EFFERENT SYSTEM

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Although a great deal is known about the physiology of the olivocochlear reflex in adult mammals, very little is known about its developmental physiology. The organization of the lateral an medial olivocochlear bundle (OCB) projections within the cochlea appears to develop postnatally in the cat. In perinatal animals, anatomical studies have shown that efferents project exclusively to the region of inner hair cells, where, unlike in adults, some projections form direct axosomatic synapses. Efferent innervation to the outer hair cell region, on the other hand, is delayed for several days. Consequently, cochlear innervation is uniquely organized in perinatal animals, a condition that we propose to exploit physiologically. Adultlike innervaion patterns are thought to be acquired during the first several postnatal weeks. In the proposed investigation, both limbs of the olivocochlear loop will be studied neurophysiologically, using a variety of methods. First, acoustically-evoked responses of individual efferent fibers, recorded within the internal auditory canal, will be examined in developing cats to determine if the maturation of these neurons coincides with the development of primary afferent neurons or whether development is delayed for the more rostrally-located olivocochlear neurons. Additionally, the OCB will be electrically stimulated at the floor of the fourth ventricle, thus bypassing the auditory periphery, and allowing us to examine the intrinsic properties of OCB fibers during development. Lastly, factors influencing the development of the olivocochlear circuit will be investigated by examining the consequences of OCB stimulation and deefferentation on auditory nerve responses (individual fibers and evoked potentials) at appropriate developmental stages. The overall hypothesis to be tested is that the olivocochlear circuit is immature at birth in the cat and that certain response features, such as frequency selectivity, develop coincidently with the development of the auditory periphery and that a second set of response properties, such as maximum driven discharge rate and spike train regularity acquire adultlike characteristics over an extended postnatal period. The global, long-term purpose of the proposed research is to extend our understanding of auditory function in both immature and adult animals, and to provide a basis for future research aimed at concerns regarding the consequences of early exposure to potentially hazardous environmental agents and the amelioration of congenital hearing anomalies.