CONTRACTILE PROPERTIES OF THE TONGUE MUSCLES - EFFECTS OF HYPOGLOSSAL NERVE AND EXTRACELLULAR MOTONEURON STIMULATION IN RAT

1. Stimulation of the whole hypoglossal (XIIth) nerve or its medial or lateral branch in the rat produced two major movements of the tongue as measured with a single force transducer attached to the tip of the tongue. Stimulation of the whole XIIth nerve or the lateral branch produced a retrusion of the tongue, whereas stimulation of the medial branch produced a protrusion. 2. The average retrusive twitch tension evoked by stimulation of the XIIth nerve (11.25 g) or the lateral branch (12.02 g) was significantly greater (P less than or equal to 0.0001) than the protrusive twitch tension (1.05 g) elicited by medial branch stimulation. The tetanic tension produced by lateral branch stimulation (36.82 g) was significantly greater (P less than or equal to 0.007) than the whole nerve tetanic tension (28.23 g). The greater tension elicited by stimulation of the lateral branch of the nerve when compared with the tension elicited by stimulation of the whole XIIth nerve was probably due to the absence of protrusive axons in the lateral branch of the nerve. Stimulation of the whole XIIth nerve activates axons innervating both protrusive and retrusive muscles, resulting in a weaker net retrusive force. 3. The contraction time of the tongue in response to medial branch stimulation (10.94 ms) was significantly faster than when the whole XIIth nerve (15.68 ms, P less than or equal to 0.007) or lateral branch (13.36 ms, P less than or equal to 0.05) was stimulated. The twitch contraction time of the tongue in response to whole XIIth nerve or lateral branch stimulation was not significantly different. 4. The tongue appears to be generally fatigue resistant in response to stimulation of the whole XIIth nerve [fatigue index (FI) of 0.67], the lateral nerve branch (FI of 0.67), and the medial nerve branch (FI of 0.76). 5. Extracellular stimulation of motoneurons within the XIIth nucleus elicited protrusive or retrusive movements with the medial branch of the nerve intact. Protrusive movements were evoked by stimulation at sites generally ventral in the nucleus, but rostral to the obex only, and were thought to be caused by genioglossus motoneurons. The average protrusive twitch tension elicited was 47.1 mg with a 16.0-ms twitch contraction time and an Fl of 1.1. Retrusive movements were evoked by stimulation at sites throughout the length of the XIIth nucleus, generally ventrally, and were thought to be caused by intrinsic muscle motoneurons. The average retrusive twitch tension was 67.1 mg with a 18.5-ms twitch contraction time and an Fl of 0.97. The difference in the evoked protrusive and retrusive twitch tension was significant (P less than or equal to 0.0001). 6. Extracellular stimulation of motoneurons within the XIIth nucleus elicited only retrusion with the lateral branch of the nerve intact. This retrusion was elicited from nuclear stimulation sites generally dorsal in the nucleus, but primarily rostral to the obex, and was presumed to be caused by styloglossus/hyoglossus motoneurons. The average retrusive twitch tension was 86.0 mg with a 13.5-ms twitch contraction time and an FI of 1.4. This styloglossus/hyoglossus-motorneuron-evoked twitch tension was significantly stronger than that elicited by stimulation of presumed genioglossus (P less than or equal to 0.0001) or intrinsic (P less than or equal to 0.0001) motoneurons. The styloglossus/hyglossus-elicited twitch contraction time was significantly faster than that elicited by stimulation of genioglossus (p less than or equal to 0.01) or intrinsic (P less than or equal to 0.0001) motoneurons. 7. The FIs in response to extracellular motoneuron stimulation were generally higher than those seen in response to whole nerve or nerve branch stimulation.