The vestibular system, whose sensory signals originate in the labyrinth of the inner ear, plays an important role in the maintenance of normal posture. An investigation into how this sensory/motor system transforms sensory information, especially head orientation with respect to gravity, into appropriate reflex signals to limb and trunk musculature, will lead to a better understanding of both the postural control reflex system and, more generally, on mechanisms employed within the CNS to process these, and similar, signals. Neurons in the lateral vestibular nucleus of the cat respond preferentially to lateral tilt (roll). Processing of fore/aft (pitch) information will be sought in other vestibular nuclei and brainstem structures; this should also reveal how pitch signals get down the spinal cord. A quantitative description of the response dynamics of primary vestibular afferents, especially those from the otolith organs, is needed to understand how incoming vestibular signals are processed and transformed at various levels of the vestibulospinal system. Such afferent responses to natural tilt stimuli will also clarify the relative contribution of the semicircular canals and otolith organs to these postural reflexes. The tilt reflex in animals without functioning semicircular canals has recently been modeled by a parallel pathway fearuring an inhibitory, high-pass filtered limb. One possible source for such a branch is through the cerebellum, which will be tested by determining neural and reflex responses in animals with cerebellar lesions. A combined modeling/experimental approach should permit an unraveling of the neural circuitry responsible for the observed transformations. Integration of vestibular and propriospinal (neck, trunk, limb) information takes place, in part, at the level of interneurons in the spinal cord. Describing the response dynamics and directional selectivity of both these interneurons and specific muscle groups will provide a better understanding of the vestibulospinal system's role in the control of body posture and orientation.
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