FINE-STRUCTURE OF THE CELL CLUSTERS IN THE COCHLEAR NERVE ROOT - STELLATE, GRANULE, AND MITT CELLS OFFER INSIGHTS INTO THE SYNAPTIC ORGANIZATION OF LOCAL CIRCUIT NEURONS

The small cell shell of the cochlear nucleus contains a complex integr ative machinery which can be used to study the roles of interneurons i n sensory processing. The cell clusters in the cochlear nerve root of the chinchilla provide the simplest example of this structure. Reporte d here are the neuronal architecture and synaptic organization of the three principal cell types and the three distinctive neuropil structur es that could be characterized with the Nissl and Golgi methods and el ectron microscopy. Granule cells were characterized by several dendrit es with claw-like terminals that received synaptic contacts from multi ple excitatory messy fiber rosettes. Given their relatively large numb er and their prolific parallel fiber synapses, the granule cells provi de a suitable substrate for a tangential spread of excitatory activity , which could build to considerable proportions. The mitt cells had a thickened, single dendrite, its terminal branches arranged in a shape reminiscent of a baseball catcher's mitt. The dendritic mitt enclosed an enormous, convoluted messy fiber rosette forming many excitatory sy napses on just one cell. This could provide for a discrete, comparativ ely fast input-output relay of signals. Small stellate cells had longe r, radiating dendrites that engaged the synaptic nests. These nests we re strung in long strands, containing heterogeneous synapses from puta tive excitatory and inhibitory inputs. Given the prevalence of the syn aptic nests, the small stellate cells appear to have the greatest inte grative capacity. They provide the main output of the synaptic nests. (C) 1996 Wiley-Liss, Inc.