The tongue is essential in normal oromotor function, and of preeminent importance in the production of human speech. Tongue dysfunction is associated with many human clinical syndromes. Yet we do not understand how the nervous system enables the precise control of tongue movements. Central to this understanding is study of the ways in which the smallest functional units of the motor system - motor units - are combined and activated to produce different tongue movements. This study parts from previous investigation in focusing on the motor units of the tongue body, for it is in the rules of their activation that the complexity and diversity of tongue movements lie. The long-term goals for this study are to determine the principles by which the nervous system first selects and then activates tongue body motor units to produce tongue movement. To achieve these goals this study merges techniques for making detailed measures of motor unit properties and behavior with methods for activating and recording from the tongue during motor behaviors. In the decerebrate rat, motor units will be isolated and characterized with micropipettes and their activity assessed during different reflex-evoked tongue movements. The results of these studies will meet three general aims. First, the composition of active groups of motor units will be studied to determine the principles by which the nervous system selects motor units for a behavior. Second, the sequencing of active motor units will be studied to test the extent to which motor unit order is determined by tongue body location or other physiological attributes. Third, the correlation of motor unit activation patterns and tongue movements will be studied to determine the ways in which motor units code for tongue movement. Because all active tongue movements are reducible to these three features of motor unit organization - selection, sequencing, activation - these studies will provide the first detailed understanding of the neural strategies for the control of tongue movements. This understanding is essential if we are to develop effective therapies for the recovery of tongue function in human disease.
