Cochlear nucleus small cells use olivocochlear collaterals to encode sounds in noise

ABSTRACT Understanding communication signals, especially in noisy environments, is crucial to social interactions. Yet, as we age, acoustic signals can be disrupted by cochlear damage and the subsequent auditory nerve fiber degeneration. The most vulnerable—medium and high-threshold—auditory nerve fibers innervate various cell types in the cochlear nucleus, among which, the small cells are unique in receiving this input exclusively. Furthermore, small cells project to medial olivocochlear (MOC) neurons, which in turn send branched collaterals back into the SCC. Here, we use single-unit recordings to characterise small cell firing characteristics and demonstrate superior intensity coding in this cell class. We show converse effects when activating/blocking the MOC system, demonstrating that small-cell unique coding properties are facilitated by direct cholinergic input from the MOC system. Small cells also maintain tone-level coding in the presence of background noise. Finally, small cells precisely code low-frequency modulation more accurately than other ventral CN (VCN) cell types, demonstrating accurate envelope coding that may be important for vocalisation processing. These results highlight the small cell–olivocochlear circuit as a key player in signal processing in noisy environments, which may be selectively degraded in aging or after noise insult.