Contractile properties of human nasal dilator motor units.
J.H. Mateika, E.G. Essif, C. DelloRusso and R.F. Fregosi.
Department of Physiology, The University of Arizona, Health Sciences
Center, Tucson, AZ 85721-0093.
APStracts 4:235N, 1997.
ABSTRACT
The technique of intramuscular microstimulation was used to activate facial
nerve fibers while acquiring simultaneous twitch force measurements in order
to measure the contractile properties and force-frequency responses of human
nasal dilator (ND) motor units. Twitch force amplitude (TF), contraction time
(CT), half-relaxation time (HRT) and the maximal rate of rise of force
normalized to the peak force (maximum contraction rate, MCR) were recorded
from 98 ND motor units in 37 subjects. The average CT, HRT, MCR, and TF was
47.9 1.8 ms, 42.6 2.1 ms, 28.6 1.8 s-1 and 1.06 0.1 mN, respectively.
Neither CT nor HRT were significantly correlated with TF. The average CT and
HRT were similar to values recorded for small muscles of the hand (Thomas et
al. 1990) but were faster than the values recorded from human toe extensor
motor units (Macefield et al. 1996). However, the lack of an association
between twitch force and CT or HRT was similar to the findings obtained for
both human hand and foot muscles (Macefield et al. 1996; Thomas et al. 1990).
Force-frequency curves were recorded from 8 ND motor units. The force produced
by the 8 motor units was recorded in response to stimuli delivered at 1, 5,
10, 15, 20, 25, 30, 35 and 40 Hz to assess force-frequency relationships. The
mean twitch force of the 8 motor units was 0.91 ( 0.3 mN and the average
tetanic force was 8.1 ( 1.8 mN. Therefore, the average twitch force was equal
to 12.7 % of the tetanic force. Fifty percent of the unit tetanic force was
achieved at an average frequency of 16.4 ( 1.7 Hz which is greater than the
value recorded for human toe extensor motor units (9.6 Hz). Thus, the force
produced by the ND motor units was more sensitive to changes in discharge
frequency over the range of approximately 10-30 Hz, and less sensitive to
changes in the range of 0-10 Hz because of their fast contractile properties.
The mean slope of the regression lines that were fit to the steep portion of
each force-frequency curve was 5.15 ( 0.5 % change in force/Hz. This value was
greater than the slope measured for human toe extensor muscles (4.2 % change
in force/Hz). These observations suggest that force gradation by ND motor
units is more sensitive to changes in stimulation frequency than human toe
extensor motor units. We conclude that most ND motor units have fast
contractile properties and that rate coding may play a significant role in the
gradation of force produced by the ND muscle. Furthermore, the findings of
this investigation have demonstrated that contractile speed and twitch force
in a human facial muscle are not correlated. This supports previous findings
obtained from human hand and foot muscles and suggests that there may be a
fundamental difference in the contractile speed-twitch force relationship
between many human muscles and most muscles of other mammals.
Received 2 July 1997; accepted in final form 4 September 1997.
APS Manuscript Number J551-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997