We propose studies to examine the functional and structural organization of the hypoglossal nucleus and nerve with reference to the mechanical properties of tongue movement. A series of six interrelated, acute experiments on totally anesthetized rats and cats will be done. 1) The forces involved in tongue movement, mainly retrusion and protrusion, will be measured in response to stimulation of the whole Xllth nerve as well as to stimulation of the medial (protrusive division) or lateral (retrusive division) branch of the nerve. Antidromically evoked field potential profiles of the hypoglossal nucleus will also be studied using whole nerve and nerve branch stimulation. These examinations will broadly define tongue forces and nuclear organization. 2) The forces evoked by direct stimulation of the nucleus will be measured in order to more narrowly define the geography of the hypoglossal nucleus. It is expected that stimulation of specific areas will yield retrusion, protrusion as well as movements likely generated by contractIon of the intrinsic musculature. 3) The forces and nuclear profiles associated with the styloglossus and hyoglossus retrussor muscles will be examined. 4) Examination of single motoneuron properties and motor unit mechanical characteristics will be done. Motoneuron conduction velocity, rheobase, and input resistance will be correlated with motor unit twitch and tetanic attributes such as twitch contraction time, fusion frequency, twitch tension, maximum tetanic tension and fatigue. 5) Hypoglossal motor unit recruitment will be assessed through stimulation of cortical sites known to evoke various organized tongue movements. Responses of the antidromically identified cells from """"""""4"""""""" above will be examined with reference to their order and degree of activation. 6) Since it is probable that hypoglossal motoneurons fire with intricate patterns in order to produce diverse and rapid tongue movements, we will use various motoneuron stimulation frequencies to examine the possibility that motor unit force production may not always vary in a linear manner. These studies, then, should lay a firm foundation for understanding how the hypoglossal nucleus is functionally and anatomically organized with respect to the forces produced in the tongue. There have been no previous systematic investigations of tongue forces at either the whole muscle or motor unit level, to our knowledge. Such basic information is critical to our understanding and treatment of this important multifunctional organ, particularly with reference to the complex movement patterns used in speech.
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