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

SummaryThe 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.

No Easy Target: Anatomic Constraints of Electrodes Interfacing the Human Cochlear Nucleus

Background: Auditory brainstem implants have failed to produce consistent clinical results comparable to those with the cochlear implant, both with surface and penetrating electrodes. Objective: To determine neuromorphological constraints of the auditory brainstem implant interface. Methods: The size, shape, surface depth, and spatial orientation of 33 human cochlear nuclei in 20 brainstem specimens obtained at autopsy were systematically analyzed in 792 slices each with a thickness of 8 µm. Three-dimensional renderings of the cochlear nucleus complex were obtained from a true-to-scale model, and the resulting photographic views were arranged according to the axes of the brainstem. Results: The dimensions of the ventral and dorsal cochlear nuclei in the axial, coronal, and sagittal planes correlated linearly with each other. There were no significant side differences. Maximum dimensions of the whole cochlear nuclear complex were 8.01 × 1.53 × 3.76 mm. The appearance of the ventral and dorsal nuclei combined resembles a distorted X shape from a lateral view and an angulated wedge shape when viewed from above. Slanted into the depth of the brainstem above the facial nerve entrance, the superior boundary of the ventral nucleus is located more than 7 mm off the surface of the brainstem on average. Conclusion: In the absence of appropriate surface landmarks and imaging guidance, to gain tonotopic access to the human cochlear nucleus with surface and depth electrode remains a major challenge. Due to its location close to the surface, the dorsal cochlear nucleus is vulnerable to surgical manipulation and to tumors.