Synaptic stripping in the human facial nucleus

Summary

An autopsy case of severe peripheral facial nerve paresis with disconnection of synapses from facial motor neurons is reported. A 77-year-old man presented with left-sided otitis media and subsequent development of facial nerve paresis. Three months later, the patient died of an acute gastrointestinal bleeding from a chronic duodenal ulcer. Gross inspection of the brain revealed non-stenosing arteriosclerotic vascular changes and a single small cystic lesion in the right putamen. Microscopically, marked chromatolytic changes were observed in the left facial nucleus. Immunocytochemistry for synaptophysin revealed a marked loss of afferent synaptic contacts from somatic and stem dendritic surface membranes of all chromatolytic motor neurons. Wrapping of a number of neurons by newly formed glial fibrillary acidic protein-positive astrocytic cell processes could be detected in the regenerting facial motor nucleus. In addition, expression of HLA-DR was increased on a small number of microglia and pertivascular cells. These changes were absent from the contralateral, normal-appearing facial nucleus. To our knowledge, this case provides the first evidence for disconnection of synapses following peripheral nerve lesioning in humans. Occurrence of synaptic stripping is likely to explain nuclear hyperexcitability and failure of recovery of complex fine motor movements that are commonly observed following peripheral injury to the facial nerve.