Biomedical subjects

T E Dick

Publications and source records attributed to T E Dick.

At least 37 records · Page 2Linked to original sources

PubMed1995

Paradoxical phase response at late expiration by superior laryngeal nerve stimulation.

Superior laryngeal nerve (SLN) stimulation during expiration prolongs the respiratory cycle in decerebrate, vagotomized and paralysed cats. In a few animals, however, the cycle can be terminated prematurely by the same stimulus. We developed a mathematical model of the respiratory neural network to stimulate these responses. The model contained inspiratory decrementing (I-DEC), and augmenting (I-AUG) and expiratory decrementing (E-DEC), and augmenting (E-AUG) neurones. Connections were based on published findings. SLN stimulation during late expiration prolonged the cycle when it was assumed to excite principally E-DEC neurones, whereas it terminated the cycle prematurely when it was assumed to excite both I-DEC and E-DEC neurones. Therefore, phase-resetting depends on the differential strength of afferent connections on the network's elements.

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PubMed1995

Swallowing in sleep and wakefulness in adult cats.

Clinical evidence indicates that swallowing, a vital function, may be impaired in sleep. To address this issue, we elicited swallows in awake and sleeping adult cats by injecting water through a nasopharyngeal tube. Our results indicate that swallowing occurs not only in non-rapid eye movement (NREM) sleep, but also in rapid eye movement (REM) sleep. In NREM sleep, the injections often caused arousal followed by swallowing, but, in the majority of cases, swallowing occurred in NREM sleep before arousal. These swallows in NREM sleep were entirely comparable to swallows in wakefulness. In contrast, the injections in REM sleep were less likely to cause arousal, and the swallows occurred as hypotonic events. Furthermore, apneas were sometimes elicited by the injections in REM sleep, and there was repetitive swallowing upon arousal. These results suggest that the hypotonic swallows of REM sleep were ineffective.

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PubMed1994

A 'pneumotaxic centre' in rats.

Electrical and chemical lesions in the ventrolateral pons produced apneustic breathing in anesthetized, vagotomized, paralyzed, ventilated adult rats (n = 13). Apneustic breathing did not develop if the vagi remained intact and was reversed partially with vagal (proximal end) stimulation. Physiologically, these data are similar to those obtained following dorsolateral pontine lesion in rat and other mammalian species and support the hypothesis that pontine neurons influence breathing similarly across mammalian species.

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PubMed1994

Pontine respiratory neurons in anesthetized cats.

The pontine respiratory neurons (PRG) in the 'pneumotaxic centre' have been hypothesized to contribute to phase-switching of neural respiratory activity, especially in terminating inspiration. To define the neural elements involved in phase-switching, we recorded respiratory neurons extra- and intracellularly in anesthetized cats with an intact central nervous system. In total, 54 neurons were recorded: 49 neurons with activity modulated by central respiratory rhythm (20 inspiratory, 17 postinspiratory and 12 expiratory) and 5 neurons with activity correlated to tracheal pressure. The recorded neurons were clustered in dorsolateral pontine tegmentum within the Kölliker-Fuse (KF) subnucleus of the parabrachial nuclei. Stable intracellular membrane potential was recorded in 11 of the 49 respiratory neurons (8 postinspiratory, 1 early inspiratory and 2 inspiratory). During continuous injection of chloride ions (n = 6), synaptic noise increased and IPSPs reversed, including a wave of IPSPs during stage-2 expiration in postinspiratory neurons. Further, relative input resistance varied through the respiratory cycle such that the least input resistance occurred during the neuron's (n = 5) quiescent period. No IPSPs nor EPSPs were evoked in pontine respiratory neurons by vagal stimulation. In conclusion, various types of respiratory neurons were recorded in the KF nucleus. Prominent excitatory and inhibitory postsynaptic activities were similar to those described for medullary neurons. These pontine respiratory neurons do not appear to receive a strong afferent input from the vagus. Rather, vagal afferent inputs seem to be directed towards non-respiratory neurons that are located more medially in the dorsal pons.

Anesthesia↗

PubMed1993

Phase-dependent dynamic responses of respiratory motor activities following perturbation of the cycle in the cat.

1. Electroneurographical (ENG) activities of a phrenic nerve, a thyroarytenoid (TA) branch of a recurrent laryngeal nerve, and a triangularis sterni (TS) branch of an internal intercostal nerve were recorded in decerebrate, vagotomized and paralysed cats. A superior laryngeal nerve (SLN) was stimulated electrically. Our objective was to evaluate transient changes in motor activity following a brief perturbation of the respiratory cycle by SLN stimulation. 2. Each motor nerve recorded represents a separate phase of the respiratory cycle. We measured the duration of phrenic ENG activity for inspiratory phase duration (TI) and similarly the duration of TA and TS ENG activity for the duration of stages I and II of expiration, respectively. Changes in the duration of the total respiratory cycle (TTOT) were also measured. Therefore, the changes in TTOT were accounted for directly by changes in each phase of the respiratory cycle. 3. Perturbation during the inspiratory phase inhibited phrenic activity either reversibly or irreversibly (premature termination of inspiration) depending on the strength and timing of the stimulus. Reversible inhibition of inspiration was associated with a transient activation (< 100 ms) of the TA nerve followed by a reactivation of the phrenic nerve, but the duration of the subsequent stages I and II of expiration remained the same. Thus, the prolongation of TTOT was completely accounted for by the lengthening of TI. 4. Premature termination of inspiration was followed by either a shortening (the first half of inspiration) or a lengthening (the second half of inspiration) of the duration of stage I expiration and consistently by a shortening of the duration of stage II expiration. The magnitude of these changes in the durations of stages I and II of expiration was phase dependent. Changes in the duration of all three phases of motor activity contributed to the changes in TTOT. 5. Perturbation during stage I expiration prolonged this stage but did not affect the duration of the succeeding stage (stage II expiration). The increase in the duration of stage I expiration appeared constant and not dependent on the time when the perturbation was delivered in stage I expiration. Thus, the change in TTOT was less phase dependent during stage I expiration than during inspiration and stage II expiration and was accounted for by changes in the duration of TA activity alone. 6. Perturbation during stage II expiration inhibited TS activity and evoked TA activity transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1993

Interaction between central pattern generators for breathing and swallowing in the cat.

1. We examined the interaction between central pattern generators for respiration and deglutition in decerebrate, vagotomized, paralysed and ventilated cats (n = 10), by recording activity from the following nerves: hypoglossal, phrenic, thyroarytenoid and triangularis sterni. Fictive breathing was spontaneous with carbon dioxide above the apnoeic threshold (end-tidal PCO2, 32 +/- 4 mmHg) and fictive swallowing was induced by stimulating the internal branch of the left superior laryngeal nerve (SLN) continuously (0.2 ms pulse duration, 10 Hz). 2. In all ten animals, SLN stimulation evoked short bursts of thyroarytenoid and hypoglossal nerve activity indicative of fictive swallowing. In two of ten animals, respiration was inhibited completely during deglutition. In the other eight animals, fictive breathing and swallowing occurred simultaneously. 3. With SLN stimulation below threshold for eliciting swallowing, the respiratory rhythm decreased, the duration of inspiration did not change but the duration of expiration, especially stage II, increased. Integrated nerve activities indicated that the rate of rise and peak of phrenic nerve activity decreased, stage I expiratory activity of the thyroarytenoid and especially that of the hypoglossal nerve increased and stage II expiratory activity of the triangularis sterni nerve was suppressed completely. However, if inspired carbon dioxide was increased, i.e. hypercapnic ventilation, stage II expiratory activity remained partially during continuous SLN stimulation. 4. Fictive-swallowing bursts occurred only at respiratory phase transitions. At the minimal stimulus intensity that evoked repetitive swallowing bursts, the pattern of interaction between breathing and swallowing central pattern generators was consistent for each animal (n = 7) but was different across animals. In four animals, fictive swallows occurred at the phase transition between stage II expiration and inspiration, at the transition between inspiration and stage I expiration in one animal; and in two other animals, at the transition between stage I and II of expiration. 5. The response to SLN stimulation accommodated during the stimulus train. Accommodation was evident in both the interswallow interval (ISI) which lengthened, and the interaction pattern which had fewer swallows per breath as the stimulus period progressed. In contrast to the ISI, characteristics of the fictive swallow did not accommodate. For example, duration of the swallow was constant, distributed over a narrow range throughout the stimulus train. 6. We conclude that the central pattern generators for swallowing and breathing interact. The pattern of interaction supports the three-phase theory of respiratory pattern generation.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1992

Salivary secretion elicited by activation of the parabrachial nuclei in the cat.

The lateral pontine tegmentum contains the parabrachial nuclei (NPB) which have been identified as a relay nucleus for cardiovascular, respiratory and gustatory systems, but their role in the regulation of these systems is not well understood. We examined the effects of electrical and chemical stimulation of the NPB on blood pressure, phrenic and hypoglossal nerve activity and salivary secretion. These variables were measured in eight anesthetized (alpha-chloralose/urethane, 30/150 mg/kg, n = 5) or decerebrate (n = 3) cats before, during, and after trains of electrical stimulation (1 ms pulse duration, 10 Hz 5 min train duration, currents as low as 10 microA) delivered unilaterally to NPB. Stimulation of the NPB elicited copious salivary secretion (1100 +/- 270 mg, mean +/- S.D.; P less than 0.001). Secretion was blocked completely by prior administration of atropine. The effects of the stimulus train on the respiratory and cardiovascular systems were variable and inconsistent even though short-latency responses of phrenic and hypoglossal nerve activities to single pulses were consistent. The short-latency response of phrenic nerve activity was biphasic, a decrease followed by an increase in activity; the response of hypoglossal nerve activity was monophasic, a transient increase in activity. Effects of electrical stimulation were replicated by the injection of an excitatory amino acid agonist (kainic acid) into the dorsolateral pons. Injection of 50 nl of 10 mM kainic acid into the NPB evoked salivary secretion, indicating that this response was elicited by stimulation of cell bodies in the region. In addition, chemical excitation increased breathing frequency, peak phrenic nerve activity, and blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

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PubMed1992

Breath-to-breath variability in hypoglossal motor unit firing.

Instability in the magnitude and timing of motor output to pharyngeal dilator muscles occurs during breathing. This contributes to alterations in upper airway resistance, and is one of several factors that play a role in the pathophysiology of obstructive apneas. To define the motor unit mechanisms accounting for such variability, geniohyoid motor unit activity was recorded simultaneously with diaphragm EMG in anesthetized cats spontaneously breathing 7% CO2 in O2. Variability was quantified with the coefficient of variation [CV = (SD/mean) x 100%]. In this preparation, we confirmed greater breath-to-breath variability of geniohyoid compared to diaphragm peak moving average EMGs. During recordings of geniohyoid motor unit activity, average CV of other respiratory parameters were as follows: peak diaphragm EMG 5.8%, inspiratory time 3.5%, expiratory time 3.8%. The average CV for geniohyoid motor unit activity patterns were substantially higher: spikes per breath 15.6%, mean firing frequency 13.3%, peak firing frequency 19.0%, minimal firing frequency 26.3%, onset time 40.9%, offset time 10.0% and duration of firing 12.8%. Values differed considerably among motor units, even when activity was recorded simultaneously. These findings suggest that variability is present in both intensity and timing of geniohyoid motor unit firing during breathing, and that different geniohyoid motor units appear to have varying degrees of stability during breathing.

Action Potentials↗

PubMed1992

Phase resetting of the respiratory cycle before and after unilateral pontine lesion in cat.

The pontine respiratory group (PRG) facilitates the mechanism for terminating the inspiratory phase but may influence other phases in the respiratory cycle as well. We determined the effects of PRG lesions on the response of the respiratory cycle to superior laryngeal nerve stimulation delivered in each phase of the cycle in decerebrate, vagotomized, paralyzed, and ventilated cats (n = 6). We measured the duration of inspiration (TI) and expiration (TE) for three breaths before and in the perturbed breath and TI for three breaths after the perturbation. The delay to next inspiration was plotted against the phase at which the stimulus was delivered. Before lesioning, premature inspiratory termination was followed by phase-dependent shortening of TE. After lesioning, premature inspiratory termination did not systematically change the following TE. Breath-by-breath variability (measured 50 breaths) increased and stimulus after-effects (prolonged TI in the subsequent cycle) were augmented following lesions. These data indicate that the PRG plays an important role in the control of TE after perturbation and in the stability of the respiratory central pattern generator.

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PubMed1992

Intrinsic properties of pharyngeal and diaphragmatic respiratory motoneurons and muscles.

Breathing is a complex act requiring the coordinated activity of multiple groups of muscles. Thoracic and abdominal respiratory muscles expand and contract the lungs, whereas pharyngeal and laryngeal respiratory muscles maintain upper airway patency and regulate upper airway resistance. An appreciation of the importance of the latter muscle group in maintaining ventilatory homeostasis and in the pathophysiology of sleep apnea has led to extensive studies examining the neural regulation of pharyngeal dilator muscles. The present review examines the role of heterogeneity in motoneuron and muscle properties in determining the diversity in the electrical and mechanical behaviors of thoracic compared with pharyngeal muscle groups. Specifically, phrenic and hypoglossal motoneuron electrophysiological properties influence whether and the extent to which these neurons will fire in response to a given synaptic input arising from chemo- and mechanoreceptors and from respiratory and nonrespiratory pattern generators. Furthermore, thoracic and pharyngeal muscle properties determine the mechanical response to motoneuronal activity, including the speed of contraction, relationships between motoneuron firing frequency and force production, and whether force is maintained during repetitive activation. Heterogeneity in the functional capabilities of these motoneurons and muscles is in turn determined by diversity of their structural and biochemical properties. Thus, intrinsic properties of respiratory motoneurons and muscles act in concert with neuronal drives in defining the complex electrical and mechanical behavior of pharyngeal and thoracic respiratory motor systems.

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PubMed1990

Artifactual averaged 'twitch tension' waveforms resulting from synchronized activity: recording from feline diaphragmatic motor units.

Averaging techniques have been used to measure contractile properties of spontaneously active motor units (MUs). This study examined the potential for artifactual results due to synchronization between the triggering, single-MU action potentials, and activity of other MUs within the muscle. A muscle strip was formed in situ from feline diaphragm. Single MUs were recorded from the strip and from the contralateral diaphragm. The diaphragm including the muscle strip continue to contract rhythmically in this preparation and a high-gain, AC-coupled recording of force was averaged using MUs recorded in either hemidiaphragm to trigger the averager. Twitch-tension waveforms occurred in 42 of 49 cases triggering from spikes of MUs contained within the strip and in 13 of 19 averages triggered from contralateral MUs. The waveforms generated using contralateral MUs as triggers could only arise from synchronization with MUs contained within the diaphragmatic strip. Although twitch waveforms that were generated from external and internal triggers could appear similar qualitatively, contraction times were significantly (P less than 0.05) longer for averages using contralateral MUs. This study demonstrates that synchronization of triggering events is a major source for error in determining mechanical properties of MUs.

Action Potentials↗

PubMed1990

Fiber subtype distribution of pharyngeal dilator muscles and diaphragm in the cat.

In previous studies differences were frequently found between the pharyngeal dilator muscles and the thoracic respiratory muscles in their patterns of electrical and mechanical activity during the respiratory cycle, with both resting and stimulated breathing. However, little is known about the intrinsic properties of the pharyngeal muscles and how they relate to the intrinsic properties of the diaphragm. In the present study, the fiber subtype distributions of two pharyngeal dilator muscles, the geniohyoid and the sternohyoid, were ascertained histochemically in the cat. The geniohyoid and the sternohyoid muscles had a preponderance of fast glycolytic (FG) fibers (mean 48 and 55%, respectively), a smaller number of fast oxidative-glycolytic (FOG) fibers (mean 36 and 31%, respectively), and few slow oxidative (SO) fibers (mean 16 and 14%, respectively). The percentages of SO fibers of both hyoid muscles were significantly (P less than 0.01) lower than that of the costal diaphragm, and the percentages of FOG and FG fibers were significantly higher than that of the diaphragm. In conclusion, the geniohyoid and sternohyoid muscles have histochemical characteristics usually associated with fast contraction and intermediate endurance properties.

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PubMed1990

Contractile and endurance properties of geniohyoid and diaphragm muscles.

Despite the wealth of information about the neural control of pharyngeal dilator muscles, little is known about their intrinsic physiological properties. In the present study the in situ isometric contractility and endurance of a pharyngeal dilator, the geniohyoid muscle, were compared with properties of the diaphragm in 12 anesthetized artificially ventilated cats. The contraction time (means +/- SE) of the geniohyoid (27 +/- 2 ms) was shorter than that of the diaphragm (36 +/- 3 ms; P less than 0.0005), as was the half-relaxation time (29 +/- 2 vs. 45 +/- 4 ms; P less than 0.002). The faster contraction and relaxation of the geniohyoid compared with the diaphragm were appropriately reflected in the shape of the force-frequency curves for the two muscles, with that of the geniohyoid located to the right of the diaphragm force-frequency curve. The endurance properties of the two muscles were assessed using repetitive stimulation at 40 Hz in trains lasting 0.33 s, with one train repeated every second. The ratio of force at the end of 2 min of repetitive stimulation to initial force was 0.67 +/- 0.06 for the geniohyoid and 0.15 +/- 0.03 for the diaphragm (P less than 0.00001). After the repetitive stimulation, the muscle force generated in response to a range of stimulus frequencies was reduced to a greater extent for the diaphragm than for the geniohyoid muscle. These results indicate that the geniohyoid muscle has a faster physiological profile than does the diaphragm yet is relatively resistant to fatigue when driven at high rates.

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PubMed1989

Relationship between diaphragmatic activation and twitch tension to superimposed electrical stimulation in the cat.

The purpose of these experiments was to evaluate the validity of the 'twitch-occlusion' method as an index of the extent of diaphragmatic activation and to assess the extent of diaphragmatic activation during inspiration. Studies were performed in situ on innervated, perfused muscle strips from the costal region of the diaphragm in ten anesthetized cats. We measured isometric tension generated by the diaphragm during inspiration and following an interpolated electrical stimulation (2 Hz, 100 microseconds, 3.0 x threshold) of the nerve. The extent of MU activation was assessed by comparing twitch amplitudes during electrical stimulation applied in expiration and in inspiration. Spontaneous inspiratory activity was induced by adding CO2 to the inspired oxygen. Within an animal, the relationship between diaphragmatic activation and twitch occlusion was linear (range of r values was from -0.88 to -0.94). The extent of spontaneous diaphragmatic activation was normalized by dividing tension at end inspiration by the average twitch tension caused by stimuli applied during expiration. Across animals, twitch amplitude was inversely related to diaphragmatic activation (y = -0.36x + 1.13, r = -0.94). At a respiratory drive with end-tidal PCO2 approximately 1% above apneic threshold (end-tidal PCO2 between 5 and 6%), twitch occlusion was less than 5.0%. Increasing end-tidal PCO2 to at least 5% above apneic threshold (end-tidal PCO2 between 9 and 11%), twitch occlusion was still less than 50%. These results from a preparation that allows direct measurement of isometric tension of the diaphragm show that the interpolated twitch is linearly related to the extent of muscle activation through a broad range of muscle activity. In addition, these data indicate that a higher respiratory drives there exists a large reserve in the phrenic motor pool.

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PubMed1989

Motor unit regulation of mammalian pharyngeal dilator muscle activity.

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Anesthesia↗

PubMed1988

Electrophysiological determination of the axonal projections of single dorsal respiratory group neurons to the cervical spinal cord of cat.

Antidromic microstimulation and orthodromic extracellular spike-triggered averaging were used to determine the axonal positions, divergence and terminations of 16 axons arising from bulbospinal, inspiratory (I) neurons. Activity from these neurons was recorded in the dorsal respiratory groups (DRG) of 12 cats. Systematic tracking was done both transversely and longitudinally in the contralateral fifth and sixth cervical segments of the spinal cord. Axonal position was determined by antidromically activating axons and by recording axonal field potentials. Thirteen axons were located in the lateral funiculus, two in the ventrolateral funiculus and one in the ventral funiculus. Axonal conduction velocity (CV) was calculated from conduction distance and conduction time, the latter defined as the interval of time from the recorded action potential in the medulla to the recorded averaged axonal potential in the spinal cord. Average (+/- S.D.) axonal CV was 52 +/- 15 m/s. Terminal potentials were recorded for 13 of these axons and were coincident with the location of evoked field potentials resulting from antidromic stimulation of phrenic motoneurons. In addition, terminal potentials from single I cells were recorded at multiple sites along the longitudinal axis of the phrenic motor column. These data indicate that axons of spontaneously active, DRG bulbospinal I cells descend predominantly in the lateral columns and diverge and terminate extensively within the phrenic motor column.

Action Potentials↗

PubMed1988

Projections and terminations of single respiratory axons in the cervical spinal cord of cat.

Position, divergence, branching, and termination patterns of single, respiratory axons were studied in cat cervical spinal cord by injecting horseradish peroxidase (HRP) intra-axonally. We stained 12 axons which were characterized by their firing patterns and by electrical stimulation. Five axons discharged during inspiration (I); the remaining 7 discharged during expiration (E). No injected axon was evoked by stimulating ipsilateral phrenic nerve roots while 7 (4 I, 3 E) of 12 were excited at a short latency from stimulating at a medullary site (on the midline, 1-2 mm rostral to the obex, approximately 3 mm below the dorsal medullary surface) where many bulbospinal respiratory axons decussate. All injected stem axons were located in the ventral and ventrolateral funiculi, traversed in a rostrocaudal direction, and were stained for lengths ranging from 3.6 to 12.4 mm. Mean axonal diameter was 2.9 microns. In 6 axons (4 I, 2 E), 14 collaterals were stained: 1 on each E axon, 2 on one I axon, 3 each on 2 others and 4 on another I axon. Collaterals emerged perpendicularly from the descending stem axon and projected directly to the ventral horn. The average distance between neighboring collaterals was 1.0 mm (n = 7). Collaterals did not arborize until they were near or within the ventral horn. Both en passant and terminaux types of presynaptic boutons were found primarily within the rostrocaudal cylinder that defined the phrenic motor column. In addition, some boutons were located dorsomedial to the phrenic motor column. We conclude that I axons, presumably of medullary origin, have multiple collaterals which terminate primarily in the phrenic motor column. However, the same axon can have terminals in different regions of the ventral horn, which are known to contain dendrites of phrenic motoneurons.

Animals↗

PubMed1988

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats.

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recorded from eight pentobarbital anesthetized cats. Augmented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 +/- 5 to 12 +/- 2 arbitrary units (P less than 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths (P less than 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During resting breathing the motor units had a mean relative expiratory onset time of 46 +/- 4% of expiration, and a mean firing frequency of 19 +/- 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency (P less than 0.05) and number of impulses per breath (P less than 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced during augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.

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