VESTIBULAR AND PROPRIOSPINAL INTERACTIONS AND PROTRACTED SPINAL INHIBITION BY BRAIN STEM ACTIVATION

Abstract

The motor outflow in response to vestibular stimulation shows significant differences when recorded from consecutively lower levels of the spinal cord. The progressive decline and disappearance of the earliest component of the response is presumed to reflect attenuation of the vestibulospinal connections along the caudal extent of the neuraxis. Vestibular responses at lumbosacral levels reflect activity mediated predominantly via reticulospinal pathways or their functional continuations as propriospinal intersegmental relays. Interactions are analyzed and a comparison is made of the influences upon lumbosacral motor pools by activation of the vestibular, segmental, and intersegmental propriospinal relay systems. The importance of tonic and phasic suprasegmental control of spinal reflex activity is investigated. In decerebrate preparations, single shock activation of the descending vestibular or propriospinal mediating systems evokes a pattern of facilitatory and inhibitory excitability changes of lumbosacral ventral horn cells. The prominent inhibitory influence of intersegmental propriospinal activity can be eliminated by high spinal transection. The protracted inhibition of local and long spinal reflex activities following high frequency vestibular or brachial plexus stimulation is abolished by a small localized incision in the mid-line at the caudal limit of the medulla, interrupting continuity between medial reticular formation and spinal segments.