Proprioceptive afferents in facial nerves of some insectivores

Abstract

Fiber-caliber analysis of the facial nerve was made in the Japanese lesser shrew-mole and the Japanese shrew-mole possessing muscle spindles in the snout muscles and in the shinto shrew with no spindles. Using an enlarged photographic scale ( x 250) prepared from the object-micrometer (1/100 mm), the measurement of the fiber-caliber was made on the enlarged photograph ( X 1250) of the cross paraffin section treated with a modified myelin-sheath staining procedure by Pettersen et al. ('70). The facial nerves of the shrew-moles contained fibres of 1 to 10 p in diameter, while the facial nerve of the shrew consisted of fibers of 1 to 6 p . The spectra of the former have a slight bimodality with the highest peak at 2 P and smaller one at 6 P. The spectrum of the latter has a unimodality with a greater peak at 2 r . Each peak revealed a log-normal distribution curve. Statistically, it can be said that there is a significant difference in the caliber-spectra between the facial nerves supplying the snout muscles with muscle spindles and without spindles.For testing whether the skew of the spectrum can be the result of a truly bimodal distribution, the left facial nerve of the Japanese shrew-moles was cut at the site beneath the auditory capsule. Complete degeneration of spindle innervation in the snout muscles was observed histologically in animals seven days after operation.