The long-term goal of this research is to understand how the nervous system controls stance and balance and, in particular, what role the vestibular system plays in this control. The ability to maintain stability during stance is fundamental for the successful performance of many functional motor tasks, and yet little is known about the mechanisms that the CNS uses to solve the complex control problems of stance. Dizziness is one of the most common complaints faced by clinicians, and symptoms of dizziness are often accompanied by problems with balance. A significant proportion of these problems of instability could be related to pathology, injury, or degeneration in the vestibular system. Therefore, it is vital to understand how vestibular inputs are used by the motor system to maintain stance and stability. The goal of this proposal is to examine the role of the vestibular canals and otoliths in the dynamic control of stance and locomotion through experimentation and computational modelling. Experiments will quantify the dynamics of unrestrained stance and treadmill locomotion in cats, before and after lesions of the vestibular apparatus. Two hypotheses underlie the proposed work: i) that dynamic stabilization of the head is required for maintaining a stable postural orientation to earth vertical and, ii) altered vestibular input results in hypermetria that arises from increased system gain.
The specific aims are 1) to quantify the frequency response characteristics of the balance control system of the unrestrained standing animal, by applying sinusoidal perturbations to the support surface. A systems analysis approach will be applied in analyzing the ground reaction forces under each limb of the cat, positions of the body segments (especially the head), joint torques, trajectory of the center of mass, and electromyographic activity from selected muscles of the neck, trunk, and limbs. 2) to determine patterns of coordination between head movement and limb movement during locomotion, 3) to quantify the excitability of local spinal circuits using low-threshold stimulation of cutaneous nerves. All these procedures will be carried out before and after lesion of the vestibular apparatus (either total bilateral labyrinthectomy, or bilateral plugging of the semicircular canals). The advantage of the animal model is the ability to precisely control the lesion, and to follow each animal during the acute phase and as it undergoes sensory-motor adaptation. Each animal will be tested in all behaviors and will serve as its own control. Finally, 4) to create a computational model of the cat balance control system, incorporating not only proprioceptors, but also the head-based sensors (vestibular, visual, and neck proprioceptors). The data and models generated by these studies will provide valuable insights into neuromotor control processes and will lead to new hypotheses and predictions about vestibular dysfunction as well as vestibular function under altered force field environments.
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