Cochlear nerve projections to the small cell shell of the cochlear nucleus: The neuroanatomy of extremely thin sensory axons

Labeling cochlear nerve fibers in the inner ear of chinchillas with biotiny lated dextran polyamine was used to trace the thin fibers (Type II), which likely innervate outer hair cells. These axons, 0.1-0.5 mu m in diameter, w ere distinguished from the thicker Type I, fibers innervating inner hair ce lls, and traced to small-cell clusters in the cochlear nucleus. This study provided two major new insights into the outer hair cell connections in the cochlear nucleus and the potential significance of very thin axons and syn aptic nests, which are widespread in the CNS. 1) EM serial reconstructions of labeled and unlabeled material revealed that Type II axons rarely formed synapses with conventional features (vesicles gathered at junctions). Rath er, their endings contained arrays of endoplasmic reticulum and small spher ical vesicles without junctions. 2) Type II axons projected predominantly t o synaptic nests, where they contacted other endings and dendrites of local interneurons (small stellate and mitt cells, but not granule cells). Synap tic nests lacked intrinsic glia and, presumably, their high-affinity amino acid transporters. As functional units, nests and their Type II inputs from outer hair cells may contribute to an analog processing mode, which is slo wer, more diffuse, longer-lasting, and potentially more plastic than the di gital processors addressed by inner hair cells. (C) 1999 Wiley-Liss, Inc.