Morphological aspects of the vestibular nerve of the squirrel monkey

A morphological study of the vestibular nerve of a primate, the squirrel monkey, revealed a systematic organization of neurons according to fiber diameter. At the center of the cristae, thick fibers align in two parallel rows along the long axis; in the periphery of the cristae, thin fibers predominate. Within the superior vestibular nerve the thick fibers of the anterior and horizontal semicircular canals and utricle are situated in the anterosuperior area, the thin fibers in the posterior part. A similar segregation occurs with fibers of the posterior ampullary nerve and the saccule within the inferior vestibular nerve. The fibers continue to segregate at the root entry zone where they lose their Schwann's cell ensheathing and begin to divide into secondary central branches. There is a correspondence between the volume of the ganglion cell and the fiber diameter; hence, a segregation of ganglion cells according to their size also takes place in the vestibular nerve. The organization of the primate's vestibular nerve is similar to that found earlier in an anuran. The relevance of the phylogenetic preservation of the vestibular nerve's anatomical organization is discussed in terms of current physiological findings, which support the differential role of fibers of different diameters in vestibular function.