Function of the Uncrossed Efferent Olivo-cochlear Fibres in the Cat

Abstract

RECENT anatomical evidence obtained in the cat indicates that two sets of nerve fibres actually contribute to the efferent bundle (Kasmussen) which forms the anastomosis of Oort between vestibular and cochlear nerves and which travels centrifugally towards the organ of Corti in the inner ear. Approximately three-quarters of the fibres are crossed and originate from small neurones adjacent to the accessory nucleus of the oliva superior on the opposite side of the brain-stem¹. The remaining quarter of these efferent axons originates from neurones located on the dorsal aspect of the principal S-shaped nucleus of the oliva superior on the same side². Many data have been collected on the function of the crossed component^3–6, which can be activated at its decussation under the floor of the fourth ventricle (Fig. 1, F) and the inhibitory effect of which on auditory input is quite powerful, ranging up to a −25 db equivalent attenuation of sound energy⁶. A special feature of this pure inhibitory pathway is that the (long) axons available for stimulation synapse directly on to afferent auditory terminals and hair cells with no interposed interneurone^2,7. The uncrossed efferent fibres also establish direct connexions with the inner ear structures⁷, but their functional properties are still unknown.