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Departments of 1 Physical Therapy and Exercise Science and of 2 Physiology, State University of New York at Buffalo, Buffalo, New York 14214
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Abdominal muscles serve multiple roles, but the functional organization of their motoneurons remains unclear. To gain insight, we recorded single motor unit potentials from the internal oblique (IO) and transversus abdominis (TA) muscles of three standing subjects during quiet breathing, a leg lift, and an expiratory threshold load. Inspiratory airflow, recorded from a pneumotachometer, provided tidal volumes and respiratory cycle timing. Fine wires, implanted under ultrasonic imaging, detected single motor unit potentials that were visually distinguished by their spike morphology. From the number of spikes, firing profiles, times of occurrence in the respiratory cycle, and their onset, instantaneous, mean, and peak firing frequencies we deduced that 1) breathing patterns varied across tasks, 2) different motor units were recruited for each task with essentially no overlap, 3) their firing displayed prominent expiratory activity during each task, and 4) the recruitment levels and discharge patterns of IO and TA were different. We conclude that the IO and TA motor pools receive a strong central respiratory drive, yet each pool receives its own distinct, task-dependent synaptic input.
single motor units; abdominal motor control; motor unit recruitment; firing profiles; task groups; functional organization; motor pools
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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THE ABDOMINAL MUSCLES serve many functions. Yet the functional organization and neural control of their spinal motoneurons is poorly understood. Many electromyographic (EMG) studies have analyzed the output of the different muscle layers comprising the human abdominal wall during a variety of respiratory and nonrespiratory activities including reflex responses (7), speech (6, 10), postural adjustments and trunk movements (20, 21), voluntary activation (4), vomiting (19), and other expulsive functions (3, 16) (for a recent review see chapt. 4 in Ref. 3). Results from studies using both surface (27) and indwelling (5) electrodes have suggested that recruitment of the abdominal muscles is task dependent. How specific abdominal motor units are activated during different tasks has not been studied. Therefore, in this study, we have analyzed and compared the firing patterns of motor units in the internal oblique (IO) and transversus abdominis (TA) muscles recorded simultaneously from implanted wires during quiet breathing (QB), during a sustained leg lift (LL), and during an expiratory threshold load (ETL) in three standing adults.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Subjects
Three healthy men (23-48 yr of age) with no history of neuromuscular or cardiorespiratory disease performed each task described below at least twice. The study was approved by the Human Subjects Review Committee of the School of Health-Related Professions of the State University of New York at Buffalo. The subjects signed an approved consent form before participating in the study.Tasks
The standing subjects were asked to 1) breathe quietly (QB), 2) extend and hold the right leg 10 in. (~stride length) forward in a sustained LL while keeping the knee straight, and 3) expire against a column of water 7.5 cm deep. This latter task, called ETL, was imposed by inserting a rigid tube from the expiratory side of a two-way breathing valve into the water column. These three tasks were chosen because each resulted in low levels of abdominal motor unit activity, making feasible recognition of single unit potentials and their isolation from the muscle's interference pattern. Each task was performed twice for at least 2 min. Once the respiratory pattern had stabilized after initiation of a task, data were analyzed over five consecutive breaths.Acquisition of Breathing Data
The subject wore a nose clip and breathed through a mouthpiece attached to a low-resistance, two-way breathing valve (Hans Rudolph no. 2700). Inspiratory flow was recorded with a pneumotachometer (Fleisch, model 2) attached to the inspiratory port. The durations of inspiration (TI), expiration (TE), and total cycle duration (Ttot) were measured from this inspiratory flow signal. Tidal volume (VT) and mean inspiratory flow (VT/ TI) were determined on a breath-by-breath basis from the calibrated recording of the integrated pneumotachometer signal.Detection of Single Motor Unit Activity
The upper right side of the abdomen was imaged ultrasonically (Scanner 400/450, Pie Medical) to identify the depths from the skin's surface and thicknesses of the underlying external oblique (EO), IO, and TA muscles. The location of the thickest part of each muscle, considered the optimal site for implantation of the fine-wire recording electrodes, was marked on the skin, and the depth of each muscle at these sites was recorded in millimeters. Means for the three subjects are shown in Table 1. The upper abdominal region used for insertion for the IO and TA electrodes is indicated in the top of Fig. 1.
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Preparation and Insertion of Recording Electrodes
Four fine insulated wires (diameter 0.002 mm, Stablohn 800A-H-poly-nylon green, California Fine Wire) were threaded through each 26-gauge hypodermic needle. About 2 mm of the protruding tips of the four wires were folded over the needle's bevel, making "hooks" to secure the wires in the muscle, once implanted. Each of the two recording electrodes was examined under the microscope to ensure that the only surfaces devoid of insulation were the cross-sectional areas at the perpendicular cut at each wire's tip. The other ends of the wires were flamed to remove the insulation for the connection to the amplifier. The needle-wire assemblies were dry autoclaved at 120°C for 1 h. After sterilization, each electrode was tested to guarantee that the wires were free to move freely within its needle's shaft.The sterile needle was inserted to the appropriate depth after the predetermined abdominal insertion site was cleansed with 10% Clinidine. Before the needle was withdrawn, the muscle was again ultrasonically imaged to determine that the needle's tip was in the intended muscle layer. Slight movement of the needle produced sufficient distortions of the muscle tissue to verify that the tip of the needle was in the desired location. One pair of wires from each insertion site was connected to a small-head amplifier (built in house), a signal conditioner, an audioamplifier, and an oscilloscope (Gould, model 1604). The subject was asked to contract his abdominal muscles slightly. If single motor unit potentials (SMUPs) were not distinguishable on the speaker and oscilloscope, a second pair of wires was selected and tested. This procedure was repeated until "clean," sharp SMUPs were detected. The pair of wires that detected these clean signals was used throughout the entire experiment while the other two wires were just left dangling in space. The needle was withdrawn, leaving the wires in place within the muscle. To avoid their inadvertent dislodgement, sterile tape was placed on the skin over the emergence site of the wires. For the remainder of the experiment, all recordings from this muscle were made from this same pair of wires. The same procedure was followed for implanting the second recording electrode in the other abdominal muscle. A surface electrode taped over the right iliac crest served as ground.
Data Acquisition and Analysis
The SMUPs detected from the selected pair of implanted wires and the pneumotach signal were recorded on FM tape for off-line digitization and analysis. The analog EMGs and the flow signal were played back at 2.38 in./s, sampled at 20 kHz, and digitized at 12-bit resolution.Isolation of SMUPs. The Enhanced Graphics and Acquisition and Analysis System (EGAA, version 4.1, 1994, developed by RC Electronics) was applied to the first subject's digitized signals from the implanted wires, and Spike 2 software (Cambridge Electronic Design, Chicago, IL) was applied to the recordings from the other two subjects. Both programs generated a template of a spike based on its rise time, peak amplitude, decay time, duration, number of phases, and spike area. Neither software program automatically sorted potentials generated by a single motor unit satisfactorily. Therefore, final selection, isolation, and sorting of SMUPs throughout an experiment was done by visually matching each spike by overlaying it on the computer-generated template. This procedure resulted in the isolation of many individual spikes of small amplitude, which led to a large total number of waveforms that could not be grouped with others as a motor unit (Table 2). Many of these waveforms fired infrequently and randomly, suggesting they were of little physiological significance. Analysis was performed only on those units that fired more than ten times during the five consecutive breaths examined during a stable breathing pattern following the initiation of each task.
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The selected waveforms of the potentials remained stable over hours, as shown by the examples in Fig. 1. Variations in a waveform's amplitude up to 28% were considered within an acceptable limit (26). All SMUPs for a given unit were assigned a number (e.g., IO1, IO2, etc.) based on that unit's order of appearance in the experiment.
Analysis of SMUPs. The time of occurrence of the discharges of each motor unit during each task was examined in relation to the TI and TE determined from the inspiratory flow trace. The time at which each SMUP occurred was determined by placing a cursor at the peak of the SMUP and manually recording the displayed time of occurrence. These values were entered into a spreadsheet for subsequent calculation of several variables.
Glossary
The following terminology is used throughout the remaining text.
| Spike | Compound action potential of a single motor unit |
| Interspike interval (ISI) | Time between 2 consecutive spikes fired by the same motor unit, ms |
| Instantaneous frequency | ISI, impulses/s |
| Doublets | Two spikes, the ISI of which is <20 ms (13) |
| Firing profile | In a motor unit, plot of the time of occurrence of each spike throughout a breathing cycle |
| Mean frequency | Total number of spikes occurring during a breathing cycle, impulses/s |
| Peak frequency | Shortest ISI within a respiratory cycle, excluding doublets, impulses/s |
| Onset frequency | ISI between the 1st and 2nd spikes of a rhythmical burst (i.e., ISI at the reinitiation of a unit's firing after an inhibition), impulses/s |
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Breathing Patterns
Data in Table 3 show that the breathing pattern differed among tasks. Breathing was significantly deeper and slower during ETL than during QB or LL. A lengthening of TE during ETL accounted for the slow breathing frequency. The large VT with no significant change in TI during ETL resulted in a higher VT/ TI than that during either QB or LL.
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Muscle Activity
During QB, the interference patterns of both IO and TA displayed intermittent firing of several units with little or no respiratory rhythm (Fig. 2). However, during LL and ETL, both IO and TA showed considerable motor unit recruitment with the greatest enhancement occurring in expiration. For ease of presentation, data from one subject (S1) are presented in full detail; data from the other two subjects (S2 and S3) were qualitatively similar and, therefore, are only summarized as appropriate. The reported observations for individual units (e.g., IO4 and TA7) were repeatable in six of six trials for S1. Observations in S2 and S3 were similarly repeatable, where such data were available, in four of six and five of six trials, respectively.
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Recruitment in S1's IO. During QB, 14 IO units were identified in S1's records. Thirteen of these units discharged only occasionally, with spikes occurring sporadically in either phase of respiration. Only one of the detected units, namely IO4, fired in each of the five breaths, and these discharges occurred predominantly in expiration, with a short pause, total silence, or an occasional spike occurring in inspiration.
During the sustained LL, SMUPs of ten IO units were analyzed as shown in Fig. 3, left. Most units fired quite sporadically at well-spaced intervals but predominantly during expiration. IO4 (bottom trace), the only unit showing any significant number of spikes, fired during each expiration and was essentially silenced in inspiration.
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Rate coding of IO4. Of the 34 SMUPs identified in IO, only IO4 was recruited for each of the three tasks. The firing profiles and instantaneous frequencies of IO4 during each task are shown in Fig. 5, A and B, respectively.
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Recruitment of TA in S1. During quiet breathing, only one TA (TA7) unit was identified in S1's records. This unit discharged only occasionally, with spikes occurring sporadically in either phase of respiration. TA7 also fired during LL and ETL and was the only TA unit to be recruited for all three tasks. Its firing profile and that of the other 22 TA units recruited during LL and ETL are shown in Fig. 8. Each of these units displayed a respiratory rhythm with acceleration of firing during expiration and a slowing or pause during inspiration. During LL, TA7 fired one doublet with an ISI of 4.4 ms during the third expiration. During ETL, 19 doublets with ISIs ranging from 1 to 18 ms occurred in the firing profiles of three TA units, namely, TA7, TA48, and TA55.
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Rate coding of TA7. Of the 23 SMUPs detected in TA in S1, only TA7 was recruited for each of the three tasks. Its firing profiles and instantaneous frequencies during each task are shown in Fig. 9. The number of spikes fired per second during TI and TE during each task is shown in Fig. 10. During ETL, the number of spikes per second during TE was six times higher than during LL or QB, reflecting a very strong drive from central expiratory neurons.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The major findings of this study are that 1) different sets of IO and TA motor units were used for the different tasks, with overlap between sets in only one or two motor units in each muscle in each subject; 2) even though the breathing pattern varied from task to task (Table 2), motor units in both muscles displayed a respiratory rhythm in their firing profiles in both respiratory (QB and ETL) and nonrespiratory (LL) tasks; and 3) rate coding was similar across tasks in some units (e.g., IO4), whereas rate coding was task-dependent in others (e.g., TA7).
Recruitment Levels Are Task Specific
The number of motor units recruited varied between muscles during the same task and differed dramatically in the same muscle across tasks. During QB, activity was detected in more IO than TA units (Table 2), a fact recently reported by Abe et al. (1). However, most of the active units fired only sporadically. Therefore, it is unlikely that either of these muscles was making much of a physiological contribution to QB.During the dynamic act of raising the leg, recruitment is massive, making isolation of SMUPs from the dense interference pattern impossible and revealing that a significant fraction of the IO and TA motoneurons are phasic for limb movements. Of course, the behavior of these units is not revealed in the present analysis. While the LL was sustained, however, the IO and TA motor units recruited, with few exceptions (e.g., IO4 and TA7 in S1), were different from those units recruited during QB.
During ETL, many motor units in both muscles were recruited. In fact, the sum of units recruited in IO and TA during ETL was greater than the number participating in QB and LL combined. This large-scale recruitment during ETL was expected, since the abdominal muscles are the major expiratory muscles and ETL is a reliable stimulus for evoking the abdominal expiratory reflex (2).
Of the total motor units isolated for analysis in the IO and TA, only one unit in each of the two muscles in S1 and S2, and two units in S3, participated in all three tasks. In other words, the recruitment of most units was task specific. This finding supports the concept that the abdominal motor pools are organized on the basis of task groups (14). This fractionation does not occur in most limb muscles, the motoneurons of which are recruited in multiple motor tasks. The almost total absence of overlap in the participation of IO and TA motor units in the three tasks suggests that at least the low-threshold IO and TA motoneurons are fractionated into subpopulations (14). Whether these low-threshold motoneurons are differentially responsive to segmental and descending inputs or whether the inputs are inhomogeneously distributed to these motoneurons cannot be discerned from our results.
Interpretation of our findings must be made recognizing that many factors affect recruitment order. Among these factors are the intrinsic properties of the motoneurons such as size, threshold, organization, and efficiency of synaptic inputs. In addition, many anatomic factors influence feedback from sensory structures in the muscles, including the projections of their segmental and ascending fibers. Functional factors affecting recruitment order are conditions such as force level, changing muscle lengths due to changes in lung volume, and distribution of the descending signals. None of these factors were controlled or measured in the present study but they most likely varied across tasks. Hence, the sources and mechanisms accounting for the differences in the IO and TA recruitment levels during the three tasks remain to be elucidated.
The absence of overlap in the behavior of most of the IO and TA motor units supports the concept that their motor pools are functionally organized into subpopulations or task groups. A "task group" comprises all the motoneurons contributing to a specific muscle contraction. In some muscles, a task group may completely overlap with an anatomically defined motor pool (e.g., extensor carpi radialis) or form a subpopulation of the motor pool (e.g., extensor digitorum communis) (22). Additional studies are required to determine the detailed functional organization of the IO and TA motor pools.
Influence of Respiration
Anatomically the IO and TA motor pools lie in close positional relationship within the ventral horns (19, 25, 28), but anatomical proximity does not necessarily imply functional uniformity. The various motor pools innervating the abdominal muscles span multiple spinal segments within the ventral horns. Descending axons from central expiratory neurons provide presynaptic inputs via massive terminal arborizations along the length of the column of motoneurons (11, 18). On the basis of this well-distributed common input, it might be expected that neighboring motoneuron pools would be concurrently activated to similar levels and with similar firing patterns. In fact, most early surface EMGs were interpreted as if the entire abdominal wall behaved as a functional unit, particularly during respiratory tasks (8). Subsequent reports, however, have revealed that this is not the case (1, 2). Regionalization of functions among the abdominal muscles is now a well-accepted fact, with TA being recognized as a major contributor to expiration and rectus abdominis (RA) as a major hip and spine flexor (27).The firing profiles of IO and TA units tended to remain in phase with respiration, despite changes in breathing pattern across both respiratory and nonrespiratory tasks (Table 2). This observation reflects the common input to both motor pools from central respiratory neurons. Any change in the depth and timing of a breath alters feedback about the instanteous changes in lung volume and trunk wall configuration. Such pulmonary and somatic proprioceptive feedback powerfully modulates the output of central inspiratory and expiratory neurons (9) and the spinal motoneurons (12). The resulting compensatory responses serve to optimize the length of the diaphragm and the pattern of breathing (15) and to minimize the work and cost of breathing.
Spinal projections of premotor expiratory neurons in the caudal ventral respiratory group (VRG-E) are the major common source of expiratory control of the abdominal motoneurons (24). In the cat, abdominal motoneurons receive only sparse monosynaptic excitation from these VRG-E premotoneurons (17, 18), with spinal interneurons being the recipients and integrators of the descending VRG-E signals destined for the spinal abdominal motoneurons. Whether monosynaptic projections are more common in humans than in cats is not known. Nonetheless, in this study, most of the IO and TA units that discharged in each of the five monitored breaths during each of the three tasks displayed a rhythmic firing pattern, with its peak frequency occurring in expiration and a minimum frequency, or pause, occurring in inspiration. This tendency for expiratory enhancement of firing and inspiratory inhibition was prominent during both the respiratory (QB or ETL) and the nonrespiratory (LL) tasks. The fact that the differences in breathing pattern across tasks were reflected in IO and TA firing profiles supports the concept that human abdominal motoneurons are recipients of breath-by-breath inputs from central respiratory neurons.
Rate Coding
Both IO4 and TA7 showed major increases in their instantaneous firing rates from onset frequency at the beginning of an expiration to the peak frequency reached during the expiratory burst (Figs. 6 and 10). These increases in firing frequency were qualitatively similar for the two motor units. However, IO4 and TA7 differed in their patterns of rate coding across tasks. The modes of the frequency distributions for IO4 in Fig. 7 were locked at 7.5 impulses/s (150 ms ISI), regardless of the task or breathing pattern. This constancy of rate coding across tasks in IO4 is a reflection of the strength of the VRG-E excitatory input to the motoneuron innervating the IO4 motor unit. In contrast, the modes of analogous frequency distributions for TA7 (Fig. 12) differed with each task. During ETL, the mode was at a relatively high instantaneous firing frequency (20 impulses/s; or an ISI of 50 ms), during LL at a lower frequency (6.25 impulses/s), and at an even lower frequency (4.25 impulses/s) during QB. Thus, during ETL when expiratory flow was impeded, the expiratory drive dominated.Mean firing frequency, an index of the excitatory drive to a motoneuron, of IO4 was very similar from task to task (Figs. 5B and 7), suggesting that the expiratory drive to IO4 remained similar across tasks. In contrast, the mean firing frequency of TA7 varied across tasks. The mean frequency during ETL (10.5 impulses/s) was considerably above that during QB (4.5 impulses/s) or LL (5.8 impulses/s), as shown by the dashed horizontal lines in Figs. 9B and 11. Hence, during ETL, the TA7 motoneuron presumably received a significantly stronger presynaptic drive from the VRG-E neurons than during QB or LL. This supports the long-held belief that TA is the major contributor to the expiratory function of the abdominal wall (5).
In the only other known study on the behavior of single motor units in abdominal muscles, Sant'Ambrogio et al. (23) reported on the behavior of units in the RA and EO while a subject voluntarily increased his abdominal pressure. When generating a 10-mmHg increase in abdominal pressure, the RA and EO motor units fired at 15 impulses/s, their peak firing frequency. A small initial increase in discharge frequency from onset to peak frequency always preceded the recruitment of new units. Because the peak frequency was only 50% of that during a maximal voluntary contraction, the authors concluded that recruitment was the major mechanism used during this voluntary control of RA and EO. Because of the differences in tasks, it is not possible to compare our results with those of Sant'Ambrogio et al. In fact, the present study is the first report about recruitment and rate coding in IO and TA motor units during the performance of respiratory as well as nonrespiratory tasks. The results suggest that the pattern of rate coding may be muscle specific.
In summary, in this study, abdominal motor unit behavior has been quantitatively analyzed during three natural behaviors in the absence of any artificial constraints, other than a necessary restraint on the level of activity. The results have confirmed that most IO and TA motoneurons receive strong drives from central expiratory neurons during both respiratory and nonrespiratory tasks. The results have revealed that different groups of IO and TA motor units are recruited for different tasks, with little overlap among groups. In those few IO and TA units that participated in each task, distinct recruitment and discharge patterns were seen.
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FOOTNOTES |
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Address for reprint requests: F. Cerny, Dept. of Physical Therapy, Exercise, and Nutrition Sciences, 410 Kimball Tower, SUNY at Buffalo, Buffalo, NY 14214 (E-mail: cerny{at}acsu.buffalo.edu).
Received 30 July 1996; accepted in final form 23 December 1997.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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3 h during indicated experimental conditions of quiet
breathing (QB), a sustained leg lift (LL), and a
7.5-cmH2O expiratory threshold
load (ETL). Stability of SMUPs over time, despite changes in posture or
respiratory load, suggests an absence of electrode movement. Grd,
ground.
