FUNCTIONAL ORGANIZATION OF DESCENDING VESTIBULAR INFLUENCES

Abstract

A study is presented of interaction of vestibular and proprioceptive mechanisms as they may relate to posture and locomotion. Interaction between these 2 systems is analyzed by stimulating vestibular nerve fibers in the inner ear, and mixed and motor and cutaneous nerves in the leg, while recording from lumbosacral ventral roots in decerebrated cats. Single vestibular nerve shocks elicit a 2-peak motoneuron response which appears along both ventral quadrants of the spinal cord. Impulses leading to the 1st peak undoubtedly descend the vestibulospinal tract and probably connect monosynaptically with ventral horn cells; they are bilaterally facilitatory to both extensor and flexor reflex patterns. The 2d peak probably represents a response conveyed by the reticulospinal pathway; these impulses are delayed at their start in the bulb and are both facilitatory and inhibitory in their spinal effects. Prolonged stimulation of the vestibular nerve yields long-lasting depression of spinal reflexes due to an active inhibitory influence descending the cord which can be abruptly interrupted by local freezing of the cord. Vestibular evoked ventral root responses are blocked by prior stimulation of most spinal nerves but are greatly enhanced locally by slight movement of the joints of the corresponding foot. Since vestibular excitation alone increases motor discharges to both extensors and flexors in the same limb, reinforcement of this effect by afferent discharges arising from the foot will contribute to an increased stability and strength of the corresponding limb during standing, walking and jumping behavior. The foot afferent augmentation of ventral root responses may provide a basis for the positive supporting ("magnet") reaction.