Ultrastructural Study of the Synaptic Glomeruli in the Rat’s Cochlear Nucleus

The granule cell domain of the cochlear nuclear complex contains interneurons, which are the targets for nonprimary auditory inputs from the superior olivary complex, inferior colliculus, auditory cortex, cuneate and trigeminal nuclei of the somatosensory system. The cellular targets of the non-primary projections are unknown due to a lack of information regarding postsynaptic profiles in the granule cell areas. In the present paper, we examined the synaptic relationships between a heterogeneous class of large synaptic terminals, called mossy fibers and their targets within subdivisions of the granule cell domain. During the late stage of postnatal development, we observed heterogenous groups of complex synaptic glomeruli. Using electron microscopy, we provide evidence for ultrastructural features of dendrites that receive input from the mossy fibers. The distinct synaptic relations between mossy fibers and dendrites of microneurons further imply fundamentally separate roles in processing of acoustic signals.