Ultrastructural study of the human spiral ganglion.

Electron microscopic study of the human spiral ganglion was conducted on 17 ears from 12 individuals aged 9 months to 92 years. Two types of neurons, large and small, with distinct cytological characteristics were found. Both types of neurons were myelinated and unmyelinated; however, a majority of the population was unmyelinated (94%). The distribution of myelinated neurons varied greatly among individuals, though a slight increase in their population was noted in aged individuals, the highest count being 28% in a specimen from an individual 75 years old. The small neurons constituted 6% of the population, and their cytoplasms were highly filamentous in both myelinated and unmyelinated types. The findings of this study provide no evidence as to the functional significance of the myelination of spiral ganglion cells.