Ee. Gilliam et Sj. Goldberg, CONTRACTILE PROPERTIES OF THE TONGUE MUSCLES - EFFECTS OF HYPOGLOSSALNERVE AND EXTRACELLULAR MOTONEURON STIMULATION IN RAT, Journal of neurophysiology, 74(2), 1995, pp. 547-555
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 pro
duced a retrusion of the tongue, whereas stimulation of the medial bra
nch produced a protrusion. 2. The average retrusive twitch tension evo
ked 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) t
han the protrusive twitch tension (1.05 g) elicited by medial branch s
timulation. The tetanic tension produced by lateral branch stimulation
(36.82 g) was significantly greater (P less than or equal to 0.007) t
han the whole nerve tetanic tension (28.23 g). The greater tension eli
cited 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 inn
ervating 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 stimula
ted. The twitch contraction time of the tongue in response to whole XI
Ith nerve or lateral branch stimulation was not significantly differen
t. 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 (F
I of 0.76). 5. Extracellular stimulation of motoneurons within the XII
th nucleus elicited protrusive or retrusive movements with the medial
branch of the nerve intact. Protrusive movements were evoked by stimul
ation at sites generally ventral in the nucleus, but rostral to the ob
ex only, and were thought to be caused by genioglossus motoneurons. Th
e average protrusive twitch tension elicited was 47.1 mg with a 16.0-m
s twitch contraction time and an Fl of 1.1. Retrusive movements were e
voked by stimulation at sites throughout the length of the XIIth nucle
us, generally ventrally, and were thought to be caused by intrinsic mu
scle motoneurons. The average retrusive twitch tension was 67.1 mg wit
h 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 moto
neurons within the XIIth nucleus elicited only retrusion with the late
ral branch of the nerve intact. This retrusion was elicited from nucle
ar stimulation sites generally dorsal in the nucleus, but primarily ro
stral to the obex, and was presumed to be caused by styloglossus/hyogl
ossus motoneurons. The average retrusive twitch tension was 86.0 mg wi
th a 13.5-ms twitch contraction time and an FI of 1.4. This stylogloss
us/hyoglossus-motorneuron-evoked twitch tension was significantly stro
nger than that elicited by stimulation of presumed genioglossus (P les
s than or equal to 0.0001) or intrinsic (P less than or equal to 0.000
1) motoneurons. The styloglossus/hyglossus-elicited twitch contraction
time was significantly faster than that elicited by stimulation of ge
nioglossus (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 respo
nse to whole nerve or nerve branch stimulation.