The neural control of movement has long been a topic of critical scientific and clinical concern. Study of spinal cord motor systems has historically attracted the most attention and the majority of our theories of motor control have come from that research. Three specific experiments, derived from the clinical and research issues developed from these earlier studies, have been designed to examine the precisely regulated eye movement motor control system of the primate. The first experiment, using the lateral rectus muscle set at isometric tension, will examine: a) the functional range and types of extraocular motor units; b) motor unit recruitment by determining the response threshold of motoneurons to stimulation of an excitatory monosynaptic input; c) the relationship between stimulation frequency and tetanic tension (kt value) versus the relationship between motoneuron firing frequency and fixated eye position (K value) in order to further explore recruitment order; d) muscle unit force using stimulation parameters based on actual motoneuron firing patterns; e) compartmentalization or organization of muscle units using separate electromyogram electrodes placed in the orbital and the global layers of the lateral rectus muscle. The second experiment will be the first to examine the contribution of a single motor unit to an identified movement using the magnetic search coil technique coupled with muscle unit tension recordings. This study will be identical to the first in terms of neuronal stimulation parameters, but the lateral rectus muscle will remain attached to the globe and the animal will be placed in a magnetic coil system to measure the eye movement excursions and fixations elicited by stimulation of a single motoneuron. Last, compartmentalization of the abducens nerve and lateral rectus muscle will be examined since such organizational patterns often suggest movement or task oriented behavior within areas of a single muscle. These data will provide the first direct insight into the contributions of individual motor units to specific movements and could furnish the basis for models of movement control. Since surgical and pharmacological interventions at the muscle and muscle tendon are used to correct extraocular disorders, these studies may provide a new dimension to our clinical armamentarium at the level of the nerve and muscle.
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