LONG-TERM EFFECTS OF SECTIONING THE OLIVOCOCHLEAR BUNDLE IN NEONATAL CATS

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate it s role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordin gs were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic ev aluation of the density of OCB fascicles in the tunnel of Corti, and s elected cases were analyzed via electron microscopy. In the most succe ssful cases, the deefferentation was virtually complete. ANFs from suc cessfully lesioned animals exhibited significant pathophysiology compa red with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), t he frequency at which ANFs are most sensitive, were elevated across th e CF range, with maximal effects for CFs in the 10 kHz region. Frequen cy threshold or tuning curves displayed reduction of tip-to-tail ratio s (the difference between CF and low-frequency ''tail'' thresholds) an d decreased sharpness of tuning. These pathological changes are genera lly associated with outer hair cell (OHC) damage. However, light micro scopic histological analysis showed minimal hair cell loss and no sign ificant differences between normal and deefferented groups. Spontaneou s discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs . Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mec hanical processes.