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 < or = 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 < or = 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 < or = 0.007) or lateral branch (13.36 ms, P < or = 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.(ABSTRACT TRUNCATED AT 250 WORDS)