Department of Physiology, The University of Arizona Health Sciences Center, Tucson, Arizona 85721-0093
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ABSTRACT |
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Mateika, J. H., E. G. Essif, C. DelloRusso, and R. F. Fregosi. Contractile properties of human nasal dilator motor units. J. Neurophysiol. 79: 371-378, 1998. The technique of intramuscular microstimulation was used to activate facial nerve fibers while acquiring simultaneous twitch force measurements 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 were 47.9 ± 1.8 ms, 42.6 ± 2.1 ms, 28.6 ± 1.8 s1, 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 but were faster than the values recorded from human toe extensor motor units. 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. Force-frequency curves were recorded from eight ND motor units. The force produced by the eight 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 eight 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 ~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 TF 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.
The techniques of intramuscular and intraneural microstimulation have been used in previous investigations to examine the contractile properties of motor units recorded from human hand and foot muscles (Elek and Dengler 1995 Subjects
After written informed consent had been obtained, a total of 37 subjects (22 male and 15 female), ranging in age from 20 to 30 yr, each participated in one experimental session that was designed to measure the contractile properties of ND motor units. The length of the session did not extend beyond 1.5 h because of the discomfort that was associated with the apparatus and methods utilized to measure motor unit forces. On a second occasion 23 of the 37 subjects returned to the laboratory to measure the maximal voluntary contraction (MVC) force (see critique of the methods in DISCUSSION) of the ND muscles. Of the 14 subjects that did not participate in the second experimental session, 4 subjects could not voluntarily flare their nares and 10 subjects elected not to participate.
Experimental setup and protocol
The apparatus used to measure ND motor unit TFs has been described in detail in a previous publication (Fuller et al. 1995
Data collection
Once a motor unit was identified in the surface EMG, the action potential was amplified and filtered (300 Hz to 10 kHz; model 1700, A-M systems, Everett, WA) and the corresponding TF was amplified (model S72-25, Coulbourn Instruments, Allentown, PA) before being sampled on-line (sampling frequency = 1.7 kHz) with a commercially available data acquisition software package (EGAA, R. C. Electronics, Santa Barbara, CA). Ensemble averaging was then performed to determine the force that was associated with the discharge of a motor unit. The stimulus pulses were used to trigger an averaging program (EGAA, R. C. Electronics) that sampled force for 200 ms after the onset of each trigger pulse. By using this method of analysis, the force signal that was time locked to the trigger pulse became more distinct as the number of averaged triggered sweeps increased. Approximately 30-100 sweeps were required to generate a well-defined force tracing. During data collection the subjects often held their breath, for no more than 15 s, to prevent respiratory artifact from being introduced into the force recordings. After recording the TF we decreased the stimulus current below the activation threshold of the motor unit and recorded the force tracing for an additional 30-100 sweeps to ensure that the TF measured previously was abolished.
Data analysis
The TF tracings obtained with the technique of ensemble averaging were analyzed off-line for peak amplitude, CT and HRT. The force was differentiated to obtain the rate of rise of force. The peak rate of rise was normalized to the peak force (normalized maximum contraction rate of force, MCR) to determine the maximal contraction rate of a motor unit. This latter measure provides an index of contraction rate independent of the differences in muscle size and strength that may exist between subjects. A similar procedure has been used by Thomas et al. (1990) Contractile properties of nasal dilator motor units
A total of 98 ND motor units was recorded from the 37 subjects that participated in the first experimental session. The average number of motor units that were obtained from each subject was 3 ± 1, respectively.
Correlations between contractile properties
Figure 4 demonstrates the relationship between TF and MCR, CT, and HRT. Note that TF was not significantly correlated with MCR, CT, or HRT. Although not presented in the figure, similar plots and correlation values were obtained for the motor units that were recorded from the 23 subjects in which TF was standardized as a percentage of each individual's MVC force. The average MVC force recorded for the 23 subjects that participated in the second session was 242 ± 15.6 mN and the values ranged from90 to 354.3 mN.
Force-frequency curves
Figure 5 (top) shows the force produced by a ND motor unit in response to stimulus trains of 15, 25, and 35 Hz. Note that the force reached a plateau at 15 and 25 Hz but did not exhibit this characteristic at 35 Hz in this example. The failure to reach a plateau was the result of the relatively brief duration of the 35 Hz stimulus train, which was insufficient to allow a distinct plateau in force. As mentioned above, the duration of the 35 Hz stimulus train was employed because the subjects experienced some discomfort in response to stimulation in the 30-40 Hz range. The example in the top of Fig. 5 also shows that the peak force was higher at 35 Hz than at 25 Hz, which is different from the average response (bottom right). This difference reflects the intersubject variability in the force-frequency data and the difficulty in obtaining these data (see critique of the methods, DISCUSSION). Force-frequency curves obtained from two subjects (bottom left and bottom middle) and the average force-frequency curve for the eight motor units (bottom right) also serve to illustrate the intersubject variability in the force-frequency responses. For example, a small increment in force (5.4%) between 1 and 10 Hz is shown in the bottom left tracing, whereas an 11% increase in the same frequency range is shown in the bottom middle tracing. Similarly, the average force-frequency curve (bottom right) shows that an average of 12.7% of peak force was obtained at 1 Hz, which was greater than the smaller relative forces (5.4 and 6.5%) obtained in the individual examples. However, on average (bottom right) there was a relatively small increase in force when the frequency varied between 1 and 10 Hz, but the increments became more prominent as the frequency increased between 10 and 20 Hz; beyond 30 Hz the force increments diminished or plateaued as the stimulus frequency increased. The mean TF of the eight motor units was 0.91 ± 0.3 mN and the average tetanic force was8.1 ± 1.8 mN, hence the average TF was equal to 12.7% of the tetanic force. Fifty percent of the tetanic force was achieved at an average frequency of 16.4 ± 1.7 Hz. The mean slope of the regression lines that were fit to the steep portion of each curve (Fig. 5, shaded area in left and middle panels) was 5.15 ± 0.5% peak force/Hz. The slope has been presented as "% peak force/Hz" to compare our data with that of human toe extensor and thenar motor units (Macefield et al. 1996 The technique of intramuscular microstimulation was used to activate facial nerve fibers while acquiring simultaneous TF measurements to measure and describe for the first time the contractile properties and force-frequency responses of human ND motor units. The present investigation showed that most ND motor units have fast contractile properties and that the CTs and HRTs recorded from ND motor units are not correlated with twitch force. The results obtained from the force-frequency responses also revealed that rate coding may play a significant role in the generation of force produced by the ND muscle.
Critique of the methods
The technique of intramuscular microstimulation was used in an attempt to avoid the methodological limitations that are often reported to be associated with the steady voluntary activation of motor units coupled with spike-triggered averaging. The limitations that have been reported include 1) motor unit twitch forces that are distorted by partial fusion of twitch responses (Calancie and Bawa 1986 Contractile properties of nasal dilator motor units
The contractile properties of upper airway muscle motor units have not been adequately described because of the technical difficulties associated with measuring the force generated from the discharge of single motor units in human and animal upper airway muscles. Consequently, the contractile properties of motor units in the upper airway muscles have been inferred primarily from whole muscle studies. The average TF, CT, and HRT have been measured for the genioglossus, sternothyroid, and sternohyoid muscles in cats and rats (Gilliam and Goldberg 1995 Correlations between physiological parameters
This investigation revealed that neither CT nor HRT were correlated significantly with TF. A weak or absent correlation between these variables has also been reported for motor units recorded from human first dorsal interosseus (Thomas et al. 1986 Force-frequency relationships
The force-frequency curves that were recorded in the present investigation support the findings outlined above, which suggest that most ND motor units have fast contractile properties (see Contractile properties of ND motor units). The mean frequency required to achieve half-maximal force was 16.4 Hz, which is greater than the value recorded for human toe extensor motor units (9.6 Hz) (Macefield et al. 1996 Conclusions
In the present investigation we used the technique of intramuscular microstimulation to characterize the contractile properties of ND motor units. The results demonstrated that most of these units have fast contractile properties and that rate coding may play a significant role in the gradation of force produced by the ND muscle. In addition, 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.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
; Macefield et al. 1996
; Stein and Yang 1990
; Taylor and Stephens 1976
). In studies that have examined human thenar (Thomas et al. 1990
, 1991a
), first dorsal interosseus (Thomas et al. 1986
) and toe extensor (Macefield et al. 1996
) motor unit forces a weak or absent correlation between contraction time (CT) or half relaxation time (HRT) and twitch force (TF) was reported. Furthermore, no relationship between contractile rate and fatigability was observed for human thenar motor units (Thomas 1991b).
); 2) they can be activated voluntarily or in response to respiratory related stimuli (Connel and Fregosi 1993
; Redline and Strohl 1987
); 3) they do not cross a joint, which suggests that the ND muscle has few muscle spindles or tendon organs (Hasan and Stuart 1984
); and 4) they are not subjected to large inertial forces (Fuller et al. 1995
). We have also chosen to investigate the ND muscle because, unlike other human upper airway muscles, the facial nerve (which innervates the ND muscle) and the ND muscle are readily accessible so that contractile properties of single motor units can be obtained with minimally invasive methodology. Therefore the primary purpose of this investigation was to describe the contractile properties of a previously unstudied muscle and to compare the findings to those previously obtained for muscles of the human hand and foot.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
). Briefly, each subject wore a custom designed headpiece. A force transducer (model ELO-06*-5, Entran Sensors and Electronics, Fairfield, NJ) was fastened to one end of a steel rod and positioned in a micromanipulator that was attached to the headpiece (Fig. 1). The transducer was placed perpendicular to the right external nares and positioned against the ND muscle. Optimal placement of the transducer was established by determining the position that was associated with the greatest force output during a maximum voluntary "flaring" of the nares. If the subjects could not flare their nares they performed a voluntary "sniff" maneuver in an attempt to measure the maximum force output. However, because the diameter of the transducer was smaller than the ND muscle of most subjects, the force produced during either voluntary maneuver was not considered to be an accurate measure of the maximum attainable force (see critique of the methods in DISCUSSION for further explanation). Two insulated wires with a 4 mm tip exposure were placed on the surface of the skin on either side of the force transducer (Fig. 1) and were used to record the surface ND muscle electromyograph (EMG) response. The surface EMG was used to identify single motor units that were activated by intramuscular microstimulation.
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FIG. 1.
Schematic diagram of apparatus used to measure nasal dilator (ND) muscle twitch forces. A displacement force transducer is attached to end of a steel rod, which is inserted into a micromanipulator that has several degrees of freedom so that angle of transducer with respect to nares is easily adjusted. Micromanipulator is attached to a support beam, which is adjoined to mask. Approximate position of electromyograph (EMG) and intramuscular electrodes are shown on frontal and lateral view of nose. i.m. electrode, intramuscular electrode.
, tip to tip distance = 125 µm; World Precision Instruments, Sarasota, FL) was inserted into the ND muscle (Fig. 1) and stimulus pulses with a duration of 0.1 ms and a frequency of 2 Hz (Grass model S48) were used to stimulate facial motor nerve branches located in the muscle. The average stimulus intensity required to activate the motor units was 6.8 ± 0.7 V. Once a motor unit was identified the stimulus intensity was lowered until the threshold for activation was established. We then gradually increased the stimulus intensity above the activation threshold to ensure that the amplitude of the EMG signal remained constant within a given voltage range. In this investigation, the voltage range over which the EMG signal remained constant was >3 V. If the EMG signal increased continuously the contractile properties were not measured, because Taylor and Stephens (1976)
reported that this response is characteristic of direct muscle fiber stimulation.
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FIG. 2.
A: sample recording of a unitary EMG potential (bottom) and corresponding motor unit force (top), which was elicited by intramuscular microstimulation. B: sample recording showing that unitary EMG potential (bottom) was abolished (top) after reducing stimulating current below threshold.
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FIG. 5.
An example of force produced by a single motor unit in response to stimulus trains of 15, 25, and 35 Hz (top). Bottom, left and middle: 2 force-frequency curves recorded from 2 separate subjects. Shaded areas, estimated steep portion of force-frequency curve; lines within this area, calculated regression lines (using method of least squares). Bottom right: average (mean ± SE) force-frequency curve obtained from 8 ND motor units in response to stimulus trains ranging in frequency from 1-40 Hz.
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FIG. 3.
Histograms showing distribution of contraction times (top), half-relaxation times (top left), maximum contraction rate of force (bottom left), and twitch forces (bottom right) for 98 motor units.
and Kossev et al. (1994)
. Subsequent to this analysis, TF amplitudes were expressed as a percentage of the maximal values obtained during voluntary flaring of the nares, for those subjects that participated in both experimental sessions.
) as a measure of a motor unit's capacity to increase force by rate coding.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
1, and 1.3 mN, respectively. Figure 2B shows the latency of the action potential recording (bottom) and that the recording was abolished (top) after reducing the stimulating current below threshold.
1, and 1.06 ± 0.1 mN, respectively. Note that two motor units exhibited atypical MCRs, as compared with the values recorded from the remaining 96 motor units. In these two cases the peak rate of rise of force that was measured for each unit (39.9 and 14.7 mN·s
1) was coupled with a small twitch force (0.33 and 0.11 mN), which resulted in an increased normalized maximum contraction rate.
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FIG. 4.
Maximum contraction rate of force (top), contraction time (middle), and half-relaxation time (bottom), as a function of twitch force. Note that no significant correlations were identified.
; Thomas et al. 1991a
) (see DISCUSSION).
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
; Nordstrom et al. 1989
); 2) synchronization of simultaneously firing motor units that may lead to overestimation of twitch forces (Milner-Brown et al. 1973
); and 3) the bias toward sampling slow, fatigue-resistant motor units (Calancie and Bawa 1986
).
; Taylor and Stephens 1976
) that direct muscle fiber stimulation is characterized by an EMG response that increases continuously in amplitude, while retaining the same latency and waveform shape, as the stimulus intensity is increased. Therefore we did not record the contractile properties of any motor units that displayed these characteristics.
). It is also plausible that the smaller diameter force transducer did not accurately measure the twitch forces generated by the single motor units. However given the close proximity of the EMG and stimulating electrodes to the force transducer, we are confident that the smaller force transducer sampled from the area of the ND muscle in which motor units were activated.
; Salomone and Van Lunteren 1991
; Van Lunteren and Dick 1989
). In addition, similar measurements have been recorded from the genioglossus (Scardella et al. 1993
) and ND muscles (Fuller et al. 1995
) of humans. For all of the species studied to date, the CTs and HRTs measured for upper airway muscles have been found to be substantially shorter than the values measured for other respiratory (McKenzie et al. 1992
; Salomone and Van Lunteren 1991
; Van Lunteren and Strohl 1986
) and limb muscles (Buchtal and Schmalbruch 1980
; McKenzie et al. 1992
). These results have lead to the suggestion that the upper airway muscles have relatively fast contractile properties and therefore are comprised primarily of fast-twitch motor units. This hypothesis is supported by the results obtained in the present investigation, because most of the motor units recorded were characterized by faster contractile properties than those typically reported in human limb and respiratory muscles (McKenzie et al. 1992
).
; Thomas et al. 1990
). However, the average magnitude was 0.44% of the MVC force, which lies within the range reported for human motor units from other small muscles with fast contractile properties (Galganski et al. 1993
; Stephens and Usherwood 1977
). In addition, the CTs and HRTs that were recorded ranged from 20 to 168 ms and the distribution of these measures was skewed to the right of the average values, which were 48 and 43 ms, respectively. The higher average CT compared with HRT that was recorded for the ND motor units is not typical of human limb muscles. However, the range of values as well as the mean values recorded for the ND motor unit contractile properties are similar to the values observed in the masseter (Goldberg and Derfler 1977
), thenar (Thomas et al. 1990
) and first dorsal interosseus muscles of humans (Kossev et al. 1994
). By using the technique of spike-triggered averaging, the mean CT recorded from human masseter motor units was 49 ms (range 38-69 ms) (Goldberg and Derfler 1977
). Similarly, Kossev et al. (1994)
reported that the mean CT and HRT recorded from first dorsal interosseus motor units was 47.3 ms (range 30-135 ms) and 33.9 ms (range 24-130 ms), respectively. Last, Thomas et al. (1990)
reported that the mean CT and HRT measured for human thenar motor units was 49.9 ms (range 35-85 ms) and 59.4 ms (range 20-110 ms).
). This variation may exist because the ND muscles do not have tendons, whereas the extensor muscles of the human toe are comprised of long tendons. This anatomic difference may be important, because Hill (1951)
showed that increasing tendon length results in a reduction in TF and an increase in CT in whole muscle. However, as reported recently by Macefield et al. (1996)
, this is likely not the sole reason for this discrepancy, because Fuglevand et al. (1995)
showed that CTs recorded from long flexors of the fingers were faster than those reported for human toe extensors, even though both types of muscles are comprised of long tendons. The CTs and HRTs measured likely reflect a true difference in contractile properties between motor units of ND and human toe extensor muscles. The fast contractile properties of ND motor units may be functionally significant, given that the ND muscle is activated during brisk facial movements, speech, and respiration (Dempsey et al. 1996
).
), masseter (Goldberg and Derfler 1977
), toe extensor (Macefield et al. 1996
), and thenar muscles (Thomas et al. 1990
, 1991a
). In contrast, twitch amplitude and contraction time have been shown to be correlated for single motor units recorded from the human gastrocnemius (Garnett et al. 1979
) and tibialis anterior (Andreassen and Arendt-Nielsen 1987
) muscles. Similarly, most studies completed on animal limb muscles have typically shown that a significant and inverse correlation exists between CT or HRT and TF (Burke et al. 1973
; Kernell et al. 1983
; Zajac and Faden 1985
).
; Gillespie et al. 1986
). Finally, our preliminary data (C. Rankin, C. DelloRusso, and R. F. Fregosi, unpublished observations) on the morphological characteristics of the ND muscle is also consistent with this hypothesis. Thus one ND muscle obtained at autopsy was found to have over 85% fast-glycolytic fibers, on the basis of an actin-myosin ATPase staining technique.
). Thus the force produced by ND motor units is more sensitiveto changes in discharge frequency over the range of ~10-30Hz and less sensitive to changes in the range of 0-10 Hz; this difference is likely a result of the fast contractile properties of ND motor units. In addition to the above finding, the average slope of the steep part of the force frequency curves measured for the ND motor units (5.2% peak force/Hz) was greater than that measured for human toe extensor muscles (4.2% peak force/Hz) (Macefield et al. 1996
), but was within the range of slopes measured for human thenar motor units (5-7% peak force/Hz) (Thomas et al. 1991a
). These observations suggest that force gradation by ND motor units is more sensitive to changes in stimulation frequency than human toe extensor motor units. Therefore, rate coding may play a more significant role in the gradation of force produced by the ND muscle. The differences in the force-frequency relationship that exist between human toe extensor and ND muscles likely reflects the unique function of each muscle; the toe extensors are required primarily for postural functions whereas the ND muscles are normally activated during brisk facial movements.
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ACKNOWLEDGEMENTS |
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We thank Dr. William Cameron for the use of the RC Electronics A/D board, which was used in our data analysis.
This study was supported by National Heart, Lung and Blood Institute Grants HL-41790 and HL-51056 to R. F. Fregosi. J. H. Mateika was supported by the VIDDA foundation during the writing of this paper.
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FOOTNOTES |
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Present address of J. H. Mateika: Teachers College, Columbia University, Box 199, New York, NY, 10027.
Address for reprint requests: R. F. Fregosi, Dept. of Physiology, Gittings Building, The University of Arizona, Tucson, AZ 85721-0093.
Received 2 July 1997; accepted in final form 4 September 1997.
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