%0 Journal Article %A LD Liberman %A MC Liberman %T Cochlear efferent innervation is sparse in humans and decreases with age %D 2019 %R 10.1523/JNEUROSCI.3004-18.2019 %J The Journal of Neuroscience %P 3004-18 %X The mammalian cochlea is innervated by two cholinergic feedback systems called the medial (M) and lateral (L) olivocochlear (OC) pathways, which send control signals from the brainstem back to the outer hair cells and auditory-nerve fibers, respectively. Despite countless studies of the cochlear projections of these efferent fibers in animal models, comparable data for humans is almost completely lacking. Here, we immunostained the cochlear sensory epithelium from 23 normal-aging humans (14 males and 9 females), aged 0 — 86 years, with cholinergic markers to quantify the normal density of MOC and LOC projections, and the degree of age-related degeneration. In younger ears, the MOC density peaks in mid-cochlear regions, and falls off both apically and basally, whereas the LOC innervation peaks near the apex. In older ears, MOC density decreases dramatically, whereas the LOC density does not. The loss of MOC feedback may contribute to the age-related decrease in word-recognition in noise, however, even at its peak, the MOC density is lower than in other mammals, suggesting the MOC pathway is less important for human hearing.SIGNIFICANCE STATEMENTThe cochlear epithelium and its sensory innervation are modulated by the olivococochlear (OC) efferent pathway. Although the medial (M)OC reflex has been extensively studied in humans, via contralateral-sound suppression, the cochlear projections of these cholinergic neurons have not been described in human. Here, we use immunostaining to quantify the MOC projections to outer hair cells and lateral (L)OC projections to the inner hair cell area in humans aged 0 to 89 yrs. We show age-related loss of MOC, but not LOC innervation, which likely contributes to hearing impairments, and a relative paucity of MOC terminals at all ages, which may account for the relative weakness of the human MOC reflex and the difficulty demonstrating a robust functional role in human experiments. %U https://www.jneurosci.org/content/jneuro/early/2019/10/18/JNEUROSCI.3004-18.2019.full.pdf