Excitation of lateral vestibular neurons by peripheral afferent fibers.

Abstract

Lateral vestibular (Deiters'') neurons have been studied with extracellular recording in decerebellated cats under chloraloseurethan anesthesia. Deiters'' cells were activated antidromically through electrodes placed under the cord at L3-L4 and at C3-C4. Most of the cells studied were neurons projecting to the lumbar part of the spinal cord (L cells). Histological localization of single units has shown that a majority of L units are in the caudal half of the nucleus, as previously demonstrated anatomically. Most cells discharged "spontaneously", at frequencies ranging from 10 to 80 impulses/sec. This spontaneous activity was very regular. Of 74 cells tested, 53 were facilitated by stimuli to peripheral nerves; a few cells were inhibited. Evidently Deiters'' cells can be strongly facilitated by peripheral impulses by means of pathways that do not traverse the cerebellum. When single shocks were used for conditioning, mixed and cutaneous nerves were more effective than muscle nerves in facilitating a Deiters'' cell. Many nerves, ipsilateral and contralateral, facilitated a given cell. The pattern of this convergence appears similar for all L cells. The convergence is not different under Pentobarbital anesthesia, and resembles convergence observed by other workers in animals with the cerebellum intact. Facilitation was obtained with mixed or cutaneous nerves with single shocks as weak as 1.3 times the threshold of the largest fibers in the nerve (1.3T). Shocks 8 or 9T, or stronger, were required with muscle nerves. The use of triple shocks for stimulation increased the facilitation produced by muscle nerves and decreased the threshold of the muscle nerve effect to 5-10T, and occasionally lower. Group I muscle afferents do not seem to contribute to the facilitation. Experiments with chronic lesions of the spinal cord indicate that the facilitation is due in part to fibers that ascend in the ventral or ventro-lateral white matter. These fibers probably include spinoolivary and spinoreticular fibers. It is suggested that the generalized facilitation produced in Deiters'' cells by activity in peripheral afferents may provide an excitatory background that can be modified by other, more organized, inputs to this nucleus.