1Department of Speech Pathology and Audiology, University of Iowa, Iowa City, Iowa 52242; 2Department of Speech, Language, Hearing Science, University of Colorado, Boulder, Colorado 80309; 3Wilbur James Gould Voice Research Center, Denver Center for Performing Arts, Denver, Colorado 80204; and 4Division of Otolaryngology, University of Utah School of Medicine, Salt Lake City, Utah 84132
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ABSTRACT |
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Luschei, Erich S., Lorraine O. Ramig, Kristin L. Baker, and Marshall E. Smith. Discharge characteristics of laryngeal single motor units during phonation in young and older adults, and in persons with Parkinson disease. The rate and variability of the firing of single motor units in the laryngeal muscles of young and older nondisordered humans and people with idiopathic Parkinson disease (IPD) were determined during steady phonation and other laryngeal behaviors. Typical firing rates during phonation were ~24 s/s. The highest rate observed, during a cough, was 50 s/s. Decreases in the rate and increases in the variability of motor unit firing were observed in the thyroarytenoid muscle of older and IPD male subjects but not female subjects. These gender-specific age-related changes may relate to differential effects of aging on the male and female voice characteristics. The range and typical firing rates of laryngeal motor units were similar to those reported for other human skeletal muscles, so we conclude that human laryngeal muscles are probably no faster, in terms of their contraction speed, than other human skeletal muscles. Interspike interval (ISI) variability during steady phonation was quite low, however, with average CV of ~10%, with a range of 5 to 30%. These values appear to be lower than typical values of the CV of firing reported in three studies of limb muscles of humans. We suggest therefore that low ISI variability is a special although not unique property of laryngeal muscles compared with other muscles of the body. This conceivably could be the result of less synaptic "noise" in the laryngeal motoneurons, perhaps as a result of suppression of local reflex inputs to these motoneurons during phonation.
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INTRODUCTION |
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Although there is currently a great deal of
information about the firing characteristics of motor units in limb and
hand muscles of the human body (see Table 3), there is little
information about the behavior of human laryngeal motor units (LMUs).
Chanaud and Ludlow (1992) reported on LMU behavior related to quiet
respiration, Faaborg-Andersen (1957)
illustrated a few records of LMUs
during phonation, and Larson and colleagues (1987)
demonstrated small brief changes in the fundamental frequency of vocal fold vibration (Fo) that immediately followed the discharges of LMUs. There
has not been however a systematic study of firing rates and interspike interval (ISI) variability of LMUs during steady-state phonation. This
information could allow a comparison with similar measurements that
were made on other human muscles. Typical firing rates would provide
suggestive evidence about the contraction speed of the human laryngeal
muscles, which has been thought to be "fast" on the basis of direct
measures in cats (Hirose et al. 1969
) and dogs
(Hast 1966
; Mårtensson and Skoglund
1964
). There have been no in vivo direct measures of the
contraction time of human laryngeal muscles (with a force transducer
attached directly to the stimulated muscle). The techniques necessary
to stabilize the position of the larynx and isolate the pull of the
stimulated muscles cannot be used in a human. Thus less direct methods
must be used to estimate the speed of contraction of human laryngeal
muscles. Kempster and colleagues (1988)
measured the time course of
changes in Fo after electrical stimulation of laryngeal
muscles of humans who were phonating at a steady Fo. These
authors suggested a contraction time of ~35 ms in the human, which is
twice as long as the contraction time reported in the thyroarytenoid
(TA) muscle of the dog and cat. It should be noted however that the
indirect method used by Kempster et al. may be affected by factors that
are not represented in the methods used with animals, e.g., the
nonisometric condition of the contracting muscle in the human.
Knowledge of typical firing rates in human laryngeal muscles could help
clarify whether there is a species difference in this characteristic of
laryngeal muscles. If, for example, human laryngeal and limb muscles
fire spontaneously at similar rates during normal activity, then one
could infer that these muscles have similar contraction speed. The ISI
variability could potentially be related, in current models
(Matthews 1996
), to the mechanisms of repetitive firing
and synaptic noise in laryngeal motoneurons.
There are characteristic changes in the voice of both elderly people
(Liss et al. 1990; Murty et al. 1991
;
Ramig and Ringel 1983
; Wilcox and Horii
1980
) and people who have idiopathic Parkinson Disease (IPD)
(Dromey et al. 1995
; Gamboa et al. 1997
;
Gentil et al. 1995
; Perez et al. 1996
).
The changes in the larynx and its control system that are the primary
causes of the "old voice" and the weak and breathy voice of the
person with IPD are not established, however. These voice changes with
age and disease could conceivably be related in part to changes in the
rate and variability of the firing of laryngeal motoneurons. It was
reported that the rate of motor unit firing in limb muscles is
decreased and the ISI variability is increased in elderly individuals
(Nelson et al. 1984
; Soderberg et al.
1991
). Dengler and colleagues (1986)
observed an increase in
variability but not a change of firing rate of single motor units in
the first dorsal interosseus muscle in individuals with IPD compared
with healthy control subjects. Titze (1991)
has shown, in a
biomechanical model of voice production, that the rate and variability
of the firing of laryngeal motoneurons could have large influences on
the stability of Fo, which would therefore influence the
stability of the perceived pitch of the voice. In addition, Rossi and
colleagues (1996)
presented evidence for decreased fatigability and
hypertrophy of type 1 muscle fibers in tibialis anterior of people with
Parkinson disease.
Summarizing, we addressed two primary questions in this study. First, are the rate and ISI variability of LMUs during steady phonation similar to these firing characteristics of motor units of other human skeletal muscles during steady contraction? Second, do the firing characteristics of LMUs change with age and/or with IPD?
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METHODS |
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The data analyzed for this report come from a more general study of laryngeal muscle behavior in young and elderly nondisordered subjects and people with IPD. Although the electromyographic (EMG) electrodes used in this general investigation were designed to record wide-field EMG signals to provide a quantitative estimate of the degree of muscle activation during a variety of speech and phonation behaviors, they also provided, in many subjects, very usable records of single motor unit activity. Such records were adequate, with careful marking, to determine the rate and variability of LMU firing during steady phonation.
Subjects
Recordings that contained isolated single motor units responses were obtained from 31 subjects. These subjects were grouped into three categories, depending on age and disease status. Characteristics of the ages are shown in Table 1. The "young" and "older" subjects were all healthy, free of any neurological disease, and had normal voices for their age. The "older" group was composed of those nondisordered subjects who were 60 yr of age or older and had the characteristics of an "aged" voice, i.e., hoarseness, reduced loudness, unsteadiness, and evidence of mild glottal incompetence. This determination was based on audio perceptual and videoendoscopic screening. The IPD subjects were all diagnosed with IPD and were under the care of a neurologist. All were taking anti-Parkinson's drugs and were using their medications in their typical manner at the time of the recordings. Although these IPD subjects were "mild" to "moderate" in the general symptoms of IPD, they were selected based on audio perceptual and videoendoscopic screening to have classic characteristics of voice in IPD, e.g., weak breathy voices and mild to moderate glottal incompetence (vocal fold "bowing").
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There were two IPD subjects less than 60-yr-old, one 50 and the other 58. It should be noted, however, that there were no LMUs recorded from the 50-yr-old IPD subject that could be used for the quantitative analysis presented in the RESULTS (STEADY PHONATION GROUP). Therefore there is no overlap in the age ranges of the young and IPD groups in terms of the quantitatively based conclusions of this report. On the other hand, the ages of the older and IPD groups are very similar. Therefore, changes observed in the IPD group may also reflect changes associated with the aging process.
The experimental protocol was approved by the Human Subjects Research Committee of the University of Colorado-Boulder.
Overview of the intrinsic muscles of the larynx
The intrinsic laryngeal muscles studied in this report were the
TA, the cricothyroid (CT), and the lateral cricoarytenoid (LCA). The TA
and LCA are laryngeal adductors. They close the glottis tightly during
swallowing, coughing, and for maneuvers such as the valsalva. They are
also active during speech; they bring the vocal folds to a closed
position, which allows the air stream to cause vocal fold oscillation.
The CT is considered a laryngeal adductor, although it primarily
elongates the vocal fold. It is particularly important in raising the
frequency of vocal fold oscillation and becomes very active during the
production of high-pitch phonation. For more detail on the laryngeal
muscular actions and aerodynamics of vocal fold vibration, see Titze
(1994).
Electrode placement
The electrodes used to record the laryngeal muscle activity were bipolar hooked-wire electrodes made from two strands (bifiler) of 0.002-in. diam insulated stainless steel wire. The bifiler wire was placed in the lumen of a 1.5-in. 25-gauge hypodermic needle. A flame was used to burn the insulation away for ~1.5 mm of each wire in the "hook." Hypodermic needles carrying the electrode wires were inserted through the CT membrane and directed laterally and rostrally in the submucosa to the TA muscle and laterally and slightly caudally to the LCA muscle. The belly of the CT was palpated through the skin of the neck, and electrodes were inserted into it, using care to make sure the electrodes did not enter the overlying strap muscles.
Electrode characterization procedures
The EMG activity from each electrode was observed during several behaviors to determine whether the electrode was in the intended muscle. For the TA and LCA, strong activity during swallow, valsalva, and high and loud phonation and absence of activity during rapid inspirations indicated a successful insertion. These two muscles behaved, in our experiments, in a highly parallel fashion. However, our designation of an "LCA" electrode was based on its simple adductor-like behavior and its being placed caudal to the TA electrode. There are no well-established EMG behavioral criteria for distinguishing LCA from TA. For the CT muscle, strong activation during high-pitch phonation, the suppression of background activity during low-pitch phonation, and the absence of EMG activity when the subject attempted to push the chin down against resistance indicated a successful CT insertion.
Recording procedures
Variable inductance plethysmograph bands were placed around the chest and abdomen of the subject to document relative changes in lung volume. The subject was seated in a dental chair, which was initially placed in a recumbent position to insert the electrodes. Data were collected, however, with the subject in a comfortable sitting position. A small head-mounted microphone (model C410, AKG Acoustics) was placed close to the mouth to record the voice. A sound level meter, placed 30 cm from the subject's mouth, was used to measure the sound pressure level of the voice. The EMG signals were amplified with optically isolated amplifiers (band-pass 30 Hz to 5kHz). All signals were anti-alias filtered at 2.5 kHz and then recorded on a DAT instrumentation recorder and/or digitized directly to computer disk. The A/D sample rate for both the DAT and the computer acquisition was 5 kHz/channel, which is somewhat low for optimal study of motor unit waveforms. We were, however, limited in our sampling rate because of our instrumentation. Although this sampling rate probably inflated the variability of the amplitude and duration measures of the LMU potentials, we do no think it had a significant effect on our measures of firing rate and ISI variability.
Characterization behaviors, such as swallowing, the valsalva maneuver, and phonating a high pitch were recorded at the beginning of the session and again at the end if the session if it had not been terminated by the loss of electrodes or at the subject's request. After the initial characterization behaviors, the subjects produced steady phonation at their self-selected normal pitch and loudness for as long as possible and subsequently at various combinations of loudness and pitch for ~2 s. Recordings were also made during reading of phrases, monolog (telling about the "happiest day of your life"), and rest breathing.
Data analysis
The entire record of each subject was inspected on a computer monitor by one of the authors (Luschei). Sections of the record where a motor unit in one of the muscles could be unambiguously identified were noted. After reviewing the entire record, separate computer files were made of a short section in each record where unit isolation was the best. Unit discharges in each file were marked by an analysis feature of the computer program, with a simple amplitude criterion set by the observer. The peaks of units above (positive-going peaks) or below (negative-going peaks) the amplitude criterion were detected and marked by a computer algorithm. Subsequently, each mark was checked by an observer against the actual unit waveform. Special attention was directed to unusually short or long intervals. In the vast majority of cases, these were due to the intrusion of another unit or a low-amplitude spike from the unit under study. Such marks were either deleted or added based on the unit waveform and with the assumption that the ISI of a tonically firing motor unit will not suddenly decrease to one-half or less or increase to double the preceding and following intervals. If, however, a suspected "extra" spike had the same waveform as the unit under study, the mark was not deleted. Similarly, if a suspected "missing" mark did not have an accompanying unit waveform that was reasonably close to the unit under study, a mark was not added.
The ISIs defined by these marks were transferred to a spreadsheet program, and a curve showing the ISIs as a function of time was created. The portion of this curve where the ISIs were not changing significantly over time and where the moving average of the ISI was lowest was selected (see Fig. 2) for calculation of the mean, SD, and CV of the period of steady firing. It should be noted that this definition of the period of "steady phonation" was based on the LMU's behavior. However, the Fo and gross EMG levels of the muscles being recorded were also steady during these periods. Of the units to be presented in RESULTS, the minimum number of ISIs for calculation of these statistics was 10, the maximum was 128, and the mean was 39.
The duration of the waveform of 10 motor unit potentials in each unit file was measured by an observer by marking the beginning and end of the waveform. We attempted to make these consecutive waveforms but skipped those that were disturbed by intrusion of smaller units. The peak-to-peak amplitude of the spike-like portion of each unit was estimated by measuring a waveform whose amplitude was approximately halfway between the maximum and minimum amplitude of other potentials from this unit during the record.
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RESULTS |
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Qualitative aspects of recruitment and maximal firing rates
In most cases, there was little opportunity to study the recruitment of LMUs. We were able to obtain, however, several records of identifiable motor units during slow gradual increases in the frequency of phonation (voluntary "pitch ascensions"). Figure 1 illustrates such a record. A single motor unit, identifiable in the initial portion of the CT EMG, begins firing at a rate of ~6 s/s and then increases its rate even before phonation begins (~0.5 s later than the first unit discharge). The firing rate continues to increase as Fo increases, but unit firing rate becomes stable (arrow) at an ISI of ~42 ms, although the Fo continues to increase. After ~2.5 s into the record, the single motor unit waveform can no longer be separated from the EMG interference pattern, so one cannot determine whether this unit made further increases in its firing rate as the voice increased to its maximum Fo. From our few records of this type, it appears that LMUs are capable of exhibiting gradual systematic increases in firing rate during a task involving gradual increases in the activity of a muscle and that they can achieve steady firing rates of >20 s/s.
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In two instances, we were able to observe a single motor unit during a cough. Figure 2 (right) illustrates such a case. Shortly after the data for the left hand side of Fig. 2 were recorded during sustained phonation, the subject coughed. Before any sound was produced by the cough, there was a brief episode of high-frequency firing in a motor unit from this same muscle. We presume that this occurred during a tight closure of the glottis during the buildup of the tracheal pressure. There was no sound produced during this period. This was followed by a series of cough sounds, each associated with one or two discharges from the motor unit. The minimum ISI during the period of high-frequency firing was 20 ms, and the first three couplets during the cough bursts were 31, 22, and 33 ms, respectively. Because the unit observed during the cough occurred shortly after the data of Fig. 2 (left) and because it had the same waveform, we think it is very likely that the units of Fig. 2 (right and left) were the same unit. There was an increase in the amplitude of the unit from 0.125 to 0.180 mV between these two records, but we observed amplitude changes of this magnitude when recording a continuous record of a single motor unit. One can conclude therefore that at least some LMUs can discharge at rates as high as 50 s/s. It should be noted, however, that we never observed ISIs, even for couplets, that were <20 ms, i.e., rates >50 s/s.
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Quantitative analysis of rate and variability
Quantitative determination of the rate and variability of LMU firing was based on relatively short periods of firing during steady phonation. A representative record is illustrated in Fig. 2, left. Because the first one-half of the record had shorter stable firing intervals, only the first 1.25 s of the record was used to calculate the mean and SD of the firing rates, which in this case were, respectively, 42.5 and 2.9 ms. Thus the CV was 7%. The voice was very steady during this interval; the CV of the Fo, which had a mean of 229 Hz, was only 0.6%.
A total of 122 units were identified that were of sufficient quality and record duration to characterize their pattern of firing. These were observed during a wide variety of behaviors. Most (n = 70) were observed during steady-state phonation, and it is this group of units that will be considered in detail (average and range of units/subject were 2.4 and 1-7, respectively). It should be realized, however, that this group encompasses a considerable variety of phonations. Approximately one-half were observed during "normal pitch and loudness" (n = 36), but the remainder were associated with loud, soft, or high pitch phonations. Although there is obviously different control of these muscles to produce this variety of voices, the ISIs did not differ significantly when the different phonation tasks were compared within the young, older, and IPD groups (P = 0.51, P = 0.84, and P = 0.41, respectively). Therefore units associated with these different types of steady phonation were simply classed as bring in the "steady phonation " group. The other 52 unit records, classed as the "other behavior" group, were obtained during speaking tasks, quiet resting respiration, the attempt on the part of the subject to perform a valsalva maneuver, or between periods of phonation when the subject was ready to speak. These latter units often fired regularly (definitely not showing a respiratory pattern), but because there was no phonation we do not feel that they can be included in the steady phonation group.
STEADY PHONATION GROUP. Figure 3, top bar graph, shows the means and SDs of the ISI's for the three groups of subjects. Units in each group were pooled across muscles. Because a statistical analysis (MANOVA) revealed a significant effect of gender (P = 0.01) for the mean ISI variable, males and females were analyzed separately. In all of the following statistical analyses the calculated P values were multiplied by the number of posthoc comparisons that were tested for each dependent variable (method of Bonferroni), and these adjusted P values were compared with P < 0.05 to test significance. There are three significant differences among the four comparisons made of the ISI values:
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1. | Older TA was greater than older CT (P = 0.030). |
2. | Older TA was greater than young TA (P = 0.003) |
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1. | Older TA was greater than older CT (P = 0.04). |
OTHER BEHAVIOR.
Thus far, our analysis of firing rates of LMUs was during steady
phonation. It could be reasonably argued that such rates are not
necessarily representative of LMU activity during other activities of
laryngeal muscles. To evaluate this possibility, we characterized the
typical firing rates of units observed:
1. | During quiet breathing (respiration). |
2. | When no phonation was present but the subject was prepared to phonate (no behavior). |
3. | During the valsalva maneuver (valsalva). |
4. | During periods of speaking. |
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DISCUSSION |
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Comparison of different groups of subjects
When the mean ISIs of units from older and IPD males were pooled
across muscles, they were significantly longer than those of young
males. If the ISIs from the male population were further fractionated
into units recorded from the TA and CT muscles, there was evidence of
an age effect on the mean ISI and CV of TA but not CT motor units. The
ISIs of IPD male TA units were longer than those of young subjects, but
this difference was not significant. Although there are cautionary
factors to be considered when generalizing the results of our analysis,
we feel the data are sufficient to suggest that the firing rate of TA
motor units is decreased in older and IPD men. No such effect is seen
in females or in the CT of either males or females. These results are
similar in certain respects to two prior studies in which it was found
that single motor units in the abductor digit minimi of older subjects
had lower firing rates and more variability than units recorded in younger subjects (Nelson et al. 1984; Soderberg
et al. 1991
). Howard and colleagues (1988)
also noted that
motor units of the brachial biceps decreased their firing rate as a
function of age. These authors did not find a gender effect, however. A
study of single motor units firing in the first dorsal interosseus of
Parkinson's disease subjects and matched controls during steady
contractions of 10% of the maximal voluntary contraction found that
mean firing rates were about the same (10/s) in the two groups but that
units from the Parkinson's patients were twice as variable in their firing intervals as were those from control subjects (Dengler et
al. 1986
). Although ours and all of these earlier studies
differ in the specifics of their results, they all support the general idea that motor unit rate and variability are affected in some degree
by age and Parkinson's disease.
Our finding of age-related motor unit firing characteristics in men but
not women appears to be unique compared with studies of motor units in
limb muscles. It is noteworthy, however, that previous studies of
age-related changes of the voice found clear gender differences. The
Fo of the voice of men tends to increase with age, whereas
it decreases in women (Higgins and Saxman 1991; Honjo and Isshiki 1980
). Higgins and Saxman (1991)
also
noted that during vowel production in older men but not in older women there was greater glottal flow amplitude than there was in young men, a
fact that might be related to decreased activation of the TA. Sapienza
and Dutka (1996)
observed no aging effect on phonation variables in
their study of women. Thus we think that our observed gender effect,
with men but not women showing the effects of age, may have functional correlates.
Comparing LMUs with motor units of skeletal muscles
Are laryngeal motor units in the human similar in most respects to
motor units in human skeletal muscles, or do they have special
properties? Given the short contraction times of laryngeal muscles of
cats and dogs (Hirose et al. 1969; Mårtensson
and Skoglund 1964
), one might have expected that spontaneous
motor unit firing rates of human laryngeal muscles would be
substantially higher than those of human skeletal muscles. It appears,
from our data, that this is not the case. We observed a range of firing
rates of 5-6 to 50 s/s and a typical rate during phonation of ~24
s/s. These values are not obviously higher than the values reported in
previous studies of motor unit firing rate and variability (Table
3). Therefore we suggest that human
laryngeal muscles are probably no "faster" than other muscles of
the body, although laryngeal muscles are clearly faster than limb
muscles in cats and dogs. It should be noted that this conclusion makes
the assumption that the contraction time of a muscle is directly
related to the ISI of spontaneous firing during behavior. It should
also be noted that the firing rates that appear in Table 3 represent in
some cases our "best guess" in trying to provide a simple
indication of the firing rates observed in these studies. These data
are reported in a wide variety of ways. In some cases, we based our estimates on information provided in graphs and scatter plots. The
description, in Table 3, of the behavior as "steady" refers to
tasks where the directly measured force of contraction of the studied
muscle was not changing over time, i.e., did not gradually increase or
decrease.
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One might wonder whether the motor units we reported are in the
"just-recruited" portion of their range of firing rates and that
the rates were therefore considerably lower than they would have been
if the motoneurons were excited well beyond their recruitment threshold. We think this is unlikely for three reasons. First, we were
able in a few instances (e.g., Fig. 1) to follow the firing rate of a
motor unit during an ascending pitch phonation, where we observed a
large increase beyond the rate at recruitment. Further the rate
appeared to become stable, although the muscle as a whole continued to
be recruited to higher levels of activity. The second reason is based
on an earlier study (Hsiao et al. 1994), which reported
on the firing rates of a very unusual CT single motor unit in the
larynx of a trained singer. This unit could be clearly identified
during all vocal behaviors, including a loud voice at the top of the
singer's vocal range. During this "maximal phonation" behavior,
the unit reached a firing rate of 30 s/s, only slightly higher than the
rate of 24 s/s observed during phonations at normal pitch and loudness.
The third reason is based on a model recently presented by Matthews
(1996)
. Matthews argued that some features of the ISI distributions of
motor units can be related to the time course of motoneuron
afterhyperpolarization. In effect, the model suggests that, if a motor
unit is just recruited and firing near the lower end of its range of
firing rates, the ISI distributions for this unit will have a
rightward-skewed "tail" (long intervals), whereas this tail would
not be present if it were firing in the middle or upper portion of its
range of rates. Thus the presence or absence of a tail on ISI
distributions could potentially answer the question of whether our
units were firing at or well above their minimal firing rates. Although
we did not have sufficient data to compute ISI histograms for all our
motor units, such a histogram could be computed for four units (Fig.
4). None of these four units exhibited a rightward scatter toward long
intervals. Thus we think our rates reflect the activity of motoneurons
driven well into the middle or upper regions of their range of firing rates. In making this suggestion, however, we also feel that LMUs may
become excited to higher rates, at ~50 s/s, during "maximal" contractions such as those occurring during cough (Fig. 2).
The ISI variability of LMUs appears to be considerably lower than the
variability of motor units of skeletal muscles (Table 3).
Bigland-Richie and colleagues (1983) explicitly estimated the CV of the
units they recorded during maximal contractions as being ~30%. It
should be noted that their method of recording single motor units, by
advancing a metal microelectrode through maximally contracting muscles,
provided them with excellent (though brief) records of the potentials;
the 30% CV was therefore not a product of extra nor missing spikes.
Although the variability of firing of motor units in laryngeal muscles
is low, the oculomotor system makes it clear that the laryngeal system
is not unique in this regard. Fuchs and Luschei (1970)
reported that
the ISI CV of abducens motoneurons of the monkey during eye gaze
fixation was ~10%. In addition, Matthews (1996)
has shown that at
least some low-threshold soleus motor units produce ISI CVs of ~10% when firing at their highest rates. Although the entire range of
reported ISI CVs in the human should therefore be regarded as 10-30%,
it is noteworthy that the modal ISI CV in our population of motor units
was ~10%, and 43% of the motor units studied had CVs of <10%.
Thus we would suggest the laryngeal muscles are "special" with
regard to low ISI variability, although not unique in this regard among
the motor systems of the body.
In Matthews model (1996), ISI variability is viewed as the effect of
summing synaptic "noise" with the afterhyperpolarization of the
motoneuron membrane voltage after an action potential. One source of
such synaptic noise would be the result of small but continuous input
from local reflex interneurons. It is possible that laryngeal
motoneurons are subject to less synaptic noise during phonation
compared with spinal motoneurons during steady contractions. This seems
plausible. Laryngeal muscles have muscle spindles, but they are few in
number (Larson et al. 1974
; Rossi and Cortesina
1965
), and there does not appear to be a stretch reflex
(Testerman 1970
) in this system. Of course, there are
powerful reflex effects exerted on the laryngeal muscles by
mechanoreceptors and chemoreceptors in the laryngeal mucosa and
adjoining tissues (Davis et al. 1993
), which are
responsible for coughing and rapid glottal closure. Given the
sensitivity of these afferents (Davis and Nail 1987
) and
their reflex connections it is difficult to explain why these reflexes
do not normally interfere with phonation. Phonation produces vibration
of all the tissues in the vicinity of the larynx, as anyone may
ascertain by feeling the thyroid cartilage with the tip of their finger
during phonation. To explain the lack of reflexes during phonation, it
was suggested, as one possibility (Davis et al. 1993
),
that descending inputs to the laryngeal motor system suppress the
reflex response to what is in fact a large afferent inflow from the
laryngeal tissues. Were these afferent signals to produce synaptic
effects on the laryngeal motoneurons during phonation, one would expect
their ISI variability to be very high. In fact, it appears to be among
the lowest reported for the human body, an observation that could be
explained by suppression of afferent input during phonation. In this
regard, it is noteworthy that the ventilatory response to breathing
high CO2 is suppressed during speech (Phillipson et
al. 1978
).
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ACKNOWLEDGMENTS |
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We thank B. A. Recker for help in marking records and all of the subjects who participated in this study for their time, effort, and good cheer in the face of trying circumstances.
This work was supported by National Institute of Deafness and Other Communications Disorders Grant R01-DC01150 to L. O. Ramig, and by National Center for Voice and Speech Grant P60 DC-00976.
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FOOTNOTES |
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Present address and address for reprint requests: E. S. Luschei, 14719 N. E. 65th, Redmond, WA 98052.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 7 July 1998; accepted in final form 19 January 1999.
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REFERENCES |
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