Summary plots from morphometric analyses of PSD size (μm2), synaptic vesicle density (number per μm2), mitochondrial volume fraction (%), and spherical bushy cell size (μm2). Number of asterisks (one or two) indicates groupings of statistical significance (P < 0.01 for PSD size and P < 0.002 for SV density, Kruskal-Wallis tests). There was no difference among the cohorts with respect to mitochondria volume fraction (P = 0.37, Tukey-Hanson HDS tests), and only SBCs of normal hearing cats differed in size from those of the other cohorts (P < 0.001, Tukey-Hanson HDS tests). The symbol (*/**) for PSD size from the 3-month-old contralateral implanted cats indicates that this group was statistically between the two groups. These plots indicate the strong effect that cochlear implants (CIs) have on the structure of synaptic endings when activated in young animals. Only ipsilateral data are shown for the inactivated control cat.