INTERACTIONS BETWEEN VESTIBULAR, PYRAMIDAL, AND CORTICALLY EVOKED EXTRAPYRAMIDAL ACTIVITIES

Abstract

The effects of interaction between vestibular, pyramidal, and cortically evoked extrapyramidal activities, as reflected by recordings from brain stem reticular formation and motoneurons of cervicothoracic and lumbosacral levels, have been studied in cats. The significant size of the spinal motoneuron responses to motor cortex stimulation remaining after low medullary pyramidotomy demonstrates the important contribution of impulses conducted in extrapyramidal channels relative to those mediated through pyramidal pathways. Activity transmitted in the most rapidly conducting pyramidal fibers arrives at cervicothoracic levels in advance of impulses conveyed extrapyramidally, but there is hardly any temporal separation of the volleys by the time they reach lumbosacral levels. Following pyramidotomy, interactions between vestibular, cortically evoked "pure" extrapyramidal, and "pure" pyramidal, and "pure" pyramidal activity elicited by stimulation of the medullary pyramid below the site of transection have been analyzed. Responses to motor cortex stimulation recorded from the brain stem reticular formation are markedly diminished by conditioning vestibular volleys, but interference with vestibular responses at the same recording site by preceding cortical shocks is weak. In the competition for access to the final common path, particularly at cervicothoracic levels, vestibular evoked activity dominates and the vestibular pattern of discharge is preserved or even supplemented at the expense of cortically evoked activity. A component of the response to vestibular stimulation is selectively exalted by conditioning pyramidal or extrapyramidal stimulation. The monosynaptic component of a lumbosacral segmental reflex response is markedly exalted during simultaneous high frequency motor cortex stimulation but suffers a protracted post-stimulatory inhibition. Pure extrapyramidal activation by high frequency motor cortex stimulation after pyramidotomy results in an initially strong but gradually weakening facilitation of the test reflex and a more rapid and profound post-stimulatory inhibition. Selective pyramidal stimulation gives rise to a marked enhancement of the segmental reflex activity which endures for some minites post-stimulatory. The significance of the brain stem as a source of the post-stimulatory inhibition is discussed in the light of the present and previous studies.