Figure 6.

Age-related loss of afferent synapses was increased in de-efferented ears, and both LOC and MOC systems appear to contribute to survival. A, Mean outer hair cell (OHC) loss (±SEMs) at 45 weeks in control and de-efferented groups. B, Mean counts of afferent synapses in the inner hair cell (IHC) area (± SEMs), normalized to the values in young (8 week) control ears to estimate percentage survival. OC Lesion group in A and B is defined as shown in Figure 2A. The differences in synaptic survival between the OC Lesion group and age-matched controls was highly significant (p ≪ 0.01) at all frequency regions. C, The correlation, in the 22 kHz region at 45 weeks, between LOC survival (from Fig. 2A) and cochlear-nerve synapse survival. Each point is from a different ear; the best-fit straight line is shown (r = 0.73). Data are included from controls and all ears targeted by OC surgery (n = 42); i.e., both ears after midline cuts and the ipsilateral ear after LSO lesions (Fig. 3A). D, Correlation coefficients, as a function of cochlear frequency, between cochlear-nerve synaptic loss and either MOC or LOC survival. The dashed circle shows the value derived from the data in C.