Vol. 84, Issue 5, 1707-1715, May 1998
Abdominal motor unit activity during respiratory and
nonrespiratory tasks
Threethambal
Puckree1,
Frank
Cerny1, and
Beverly
Bishop2
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 |
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 |
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 |
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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Table 1.
Mean depth, muscle thickness, and implantation site from costal angle
for the three subjects, as measured during ultrasonic imaging of
the surface of the abdominal wall
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Fig. 1.
Diagram showing area of electrode insertion
(top). Digitized single motor unit
potentials (SMUPs; SMUP no. indicated by a subscripted no.) recorded
from the same internal oblique
(IO4)
(left) from
subject
(S)
1 and transversus abdominis
(TA2)
(right) muscles in
S3 over the
course of  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.
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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.
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
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| Interspike interval (ISI) |
Time between 2 consecutive spikes fired by the same motor unit, ms
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| Instantaneous frequency |
ISI, impulses/s
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| Doublets |
Two spikes, the ISI of which is <20 ms (13)
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| Firing profile |
In a motor unit, plot of the time of occurrence of each spike
throughout a breathing cycle
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| Mean frequency |
Total number of spikes occurring during a breathing cycle, impulses/s
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| Peak frequency |
Shortest ISI within a respiratory cycle, excluding doublets, impulses/s
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| 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 |
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.
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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Fig. 2.
Top traces: compressed recording of
SMUPs from IO and TA during QB, LL, and ETL. Bottom
trace: inspiratory flow with inspiration down.
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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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Fig. 3.
A: mean tidal volume
(VT; in liters) and durations of
inspiration (I) and expiration (E); B:
firing profiles of most active units detected by IO recording
electrode. Arbitary nos. identify motor units and are displayed in an
ascending sequence in accordance with the time the unit became active
during the task. Left panel: during a sustained leg lift
(LL); right panel: during a
7.5-cmH2O ETL. (Activity during QB
is omitted because so few IO units were active, and these fired only
sporadically in occasional breaths.)
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During ETL, when expiration was prolonged, 12 of the 34 analyzable IO
units discharged at least 10 spikes/breath (Fig. 3). This activity
occurred predominantly during expiration, as shown by their firing
patterns in Fig. 3, right.
IO4 was among the units that fired
heavily during expiration. In fact, Fig. 4
shows that the number of spikes per unit time occurring in expiration
(TE) was significantly greater
than during inspiration (TI)
in all three tasks.
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Fig. 4.
Total no. of IO4 spikes (±SE)
per inspiratory (TI) or
expiratory duration (TE) for 5 consecutive inspirations (open bars) and expirations (solid bars)
during each task. * Significant differences
(P < 0.05) between inspiration and
expiration.
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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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Fig. 5.
A:
IO4 firing profiles and
B: instantaneous frequencies (Freq)
during QB, LL, and ETL. Vertical dotted lines on
x-axis indicate beginnings and
terminations of inspiration and expiration. Horizontal dotted lines
indicate mean firing frequencies for the unit during each task. imps,
Impulses.
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During QB and LL, the IO4 spikes
occurred in bursts during late inspiration and throughout expiration,
with marked suppression of firing during early inspiration. During ETL,
IO4 firing again accelerated
during expiration, with very short pauses during inspiration, a
discharge pattern markedly different from that during QB or LL. Figure
5B shows the firing pattern of
IO4 as instantaneous frequencies
during the three tasks. Expiratory enhancement occurred whether the
task served a respiratory or nonrespiratory function. Rate coding in
IO4 during each task is shown by
the data in Fig. 6. During the expiratory
phase of QB, the firing rate of
IO4 increased from a mean onset
frequency of 1 impulse/s to a peak frequency of 12.5 impulses/s, during
LL from 3.6 to 9 impulses/s, and during ETL from 3.5 to 14 impulses/s.
(Doublets were excluded when peak frequencies were calculated.) Mean
firing frequencies of IO4
(horizontal dotted lines in Fig. 5B
and bars in Fig. 6) were not different across tasks.
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Fig. 6.
Means of onset, mean, and peak firing frequencies of
IO4 during QB, LL, and ETL over 5 consecutive breaths.
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Figure 7 shows three frequency
distributions, one for each task, with the number of ISIs for
IO4 on the
y-axis and the duration of the ISIs
and the instantaneous firing frequencies (impulses/s) on the
x-axis. Regardless of the task, the
ranges of these three frequency distributions are nearly coincident,
and their means and modes are identical and are locked at an interpulse
interval of 150 ms (or an instantaneous frequency of 7.5 impulses/s). In other words, a change in task did not change the rate
coding of IO4. The differences in
ISIs among tasks, of course, reflect the changes in the
timing of events in the respiratory cycle. For example,
compared with the breathing pattern during QB and LL, the pattern
during ETL was deep and slow, with expiration significantly prolonged
(Table 2), accounting for the greater number of ISIs during ETL (Fig.
6). Despite the prolongation of
TE and
Ttot and the increased number of
ISIs during ETL, the mode of the durations of the ISIs did not change
from the 150 ms seen in the other tasks.
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Fig. 7.
Frequency distributions showing no. of
IO4 interspike intervals (ISIs)
during each expiration as a function of durations of ISIs in ms and of
instantaneous frequencies (IF) in impulses/s. Modes of distributions
are indicated by vertical line.
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In summary, the firing profiles of IO units displayed a respiratory
rhythm. The number of IO units recruited varied across tasks, with the
greatest number of units recruited units during ETL. Very few IO units
(one in S1 and
S2 and two in
S3) were
recruited in all three tasks, and in those few a change in task did not affect rate coding.
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.
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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Fig. 9.
A: firing profiles;
B: instantaneous frequencies during
QB, LL, and ETL for TA7. Dotted
lines are as defined in Fig. 5.
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Fig. 10.
Total no. of TA7 spikes (±SE)
per TI or
TE for 5 consecutive
inspirations (open bars) and expirations (solid bars) during each task.
* Significant differences (P < 0.05) between inspiration and expiration.
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The firing rate of TA7 increased
during QB from an onset frequency of 1.7 impulses/s to a peak frequency
of 5.6 impulses/s, during LL from 4.6 to 9.4 impulses/s, and during ETL
from 2.8 to 40.0 impulses/s (Fig. 11).
This steep rise in firing rate from onset frequency to peak frequency
during ETL shows that rate coding within a breathing cycle was more
dramatic than that during LL or QB.
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Fig. 11.
Means of onset, mean, and peak frequencies of
TA7 during QB, LL, and ETL over 5 consecutive breaths.
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The three frequency distributions in Fig.
12 show the number of
TA7 spike intervals
(y-axis) vs. the durations of ISIs or
instantaneous firing rates (x-axis).
The modes of these three frequency distributions for QB, LL, and ETL
were 240, 160, and 100 ms, respectively, showing their task dependency.
In other words, during ETL, the
TA7 unit was more likely to fire
at 15 impulses/s than at any other frequency, whereas during QB it was
likely to fire much more slowly at 3-5 impulses/s, or during LL at
5-7.5 impulses/s.
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Fig. 12.
Frequency distributions for TA7
data derived and plotted in a way analogous to that for
IO4 in Fig. 7. Vertical dotted
lines indicate modes of each distribution.
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In summary, the firing profiles of all TA units displayed a marked
respiratory rhythm. The number of TA units recruited was task
dependent, with more units recruited during ETL than during QB or LL.
Very few TA units (e.g., TA7)
discharged during all three tasks, and in those few rate coding was
task dependent.
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DISCUSSION |
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.
| |
FOOTNOTES |
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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