A quantitative profile of the synapses on the stellate cell body and axon in the cochlear nucleus of the chinchilla

One of the most numerous neurons in the cochlear nucleus is the type I stel late cell. Previous attempts to understand the structural basis for its sig nal coding assumed that integration of synaptic potentials arising from axo dendritic synapses should account for the generation of its response proper ties. However, the present study documents the importance of excitatory and inhibitory types of synapses on the soma and axon. Retrograde transport of cholera toxin B subunit, injected in the inferior colliculus of chinchilla s, was used to label exclusively type I stellate cells in the anteroventral cochlear nucleus. The relative distribution of terminal types by vesicle m orphology was pleomorphic > large spherical > flattened > smaller spherical . The somatic perimeter covered by endings ranged from almost none to nearl y half. More flattened-vesicle terminals contacted somata in the high-frequ ency than in the low-frequency region. Eight of twenty axons received endin gs that contained large spherical vesicles and made asymmetric junctions; h alf of these extensively apposed the initial segment, forming a collar of p resumed excitatory input. Thus, type I stellate cells are a heterogeneous g roup. Inhibitory synapses probably compose the majority of terminals. Some cells receive mostly inhibitory synapses near the presumed site of the spik e generator, but others also have a prominent excitatory input. These findi ngs call for a new look at the mechanisms for signal coding in stellate cel ls in the auditory system in particular and raise issues concerning the sto chastic nature of information processing in sensory systems in general.