Differential activation among five human inspiratory motoneuron pools during tidal breathing.

Neural drive to inspiratory pump muscles is increased under many pathological conditions. This study determined for the first time how neural drive is distributed to five different human inspiratory pump muscles during tidal breathing. The discharge of single motor units (n = 280) from five healthy subjects in the diaphragm, scalene, second parasternal intercostal, third dorsal external intercostal, and fifth dorsal external intercostal was recorded with needle electrodes. All units increased their discharge during inspiration, but 41 (15%) discharged tonically throughout expiration. Motor unit populations from each muscle differed in the timing of their activation and in the discharge rates of their motor units. Relative to the onset of inspiratory flow, the earliest recruited muscles were the diaphragm and third dorsal external intercostal (mean onset for the population after 26 and 29% of inspiratory time). The fifth dorsal external intercostal muscle was recruited later (43% of inspiratory time; P < 0.05). Compared with the other inspiratory muscles, units in the diaphragm and third dorsal external intercostal had the highest onset (7.7 and 7.1 Hz, respectively) and peak firing frequencies (12.6 and 11.9 Hz, respectively; both P < 0.05). There was a unimodal distribution of recruitment times of motor units in all muscles. Neural drive to human inspiratory pump muscles differs in timing, strength, and distribution, presumably to achieve efficient ventilation.     

Abdominal motor unit activity during respiratory and nonrespiratory tasks

Abdominalmuscles serve multiple roles, but the functional organization of theirmotoneurons remains unclear. To gain insight, we recorded single motorunit potentials from the internal oblique (IO) and transversusabdominis (TA) muscles of three standing subjects during quietbreathing, a leg lift, and an expiratory threshold load. Inspiratoryairflow, recorded from a pneumotachometer, provided tidal volumes andrespiratory cycle timing. Fine wires, implanted under ultrasonicimaging, detected single motor unit potentials that were visuallydistinguished by their spike morphology. From the number of spikes,firing profiles, times of occurrence in the respiratory cycle, andtheir onset, instantaneous, mean, and peak firing frequencies wededuced that 1) breathing patterns varied across tasks, 2) differentmotor units were recruited for each task with essentially no overlap,3) their firing displayed prominentexpiratory activity during each task, and4) the recruitment levels anddischarge patterns of IO and TA were different. We conclude that the IOand TA motor pools receive a strong central respiratory drive, yet eachpool receives its own distinct, task-dependent synapticinput.

     

Mechanical function of hyoid muscles during spontaneous breathing in cats

We assessed the mechanical behavior of the geniohyoid and sternohyoid muscles during spontaneous breathing using sonomicrometry in anesthetized cats. When the animals breathed O2, the hyoid muscles either became longer or did not change length (but never shortened) during inspiration. During progressive hyperoxic hypercapnia, transient increases in geniohyoid muscle inspiratory lengthening occurred in many animals; however, at high PCO2 the geniohyoid invariably shortened during inspiration (mean 4.9% of resting length at the end of CO2 rebreathing; P less than 0.001). The PCO2 at which geniohyoid inspiratory lengthening changed to inspiratory shortening was significantly higher than the CO2 threshold for the onset of geniohyoid electrical activity (P less than 0.01). For the sternohyoid muscle, hypercapnia caused inspiratory lengthening in 13 of 17 cats and inspiratory shortening in 4 of 17 cats; on average the sternohyoid lengthened by 1.6% of resting length at the end of CO2 rebreathing (P less than 0.01). Sternohyoid lengthening occurred in spite of this muscle being electrically active. These results suggest that the relationship between hyoid muscle electrical activity and respiratory changes in length is very complex, so that the presence of hyoid muscle electrical activity does not necessarily indicate muscle shortening, and among the geniohyoid and sternohyoid muscles, the geniohyoid has a primary role as a hypopharyngeal dilator in the spontaneously breathing cat, with the sternohyoid muscle acting in an accessory capacity.     

Voluntary hyperventilation changes recruitment order of parasternal intercostal motor units

The order of recruitment of single-motor units in parasternal intercostal muscles during inspiration was studied in normal human subjects during quiet breathing and voluntary hyperventilation. Electromyograms were recorded from the second and third intercostal spaces by means of bipolar fine wire electrodes. Flow at the mouth, volume, end-expired CO2, and rib cage and abdominal anterior-posterior diameters were monitored. Single-motor units were identified using criteria of amplitude and shape, and the time of first appearance of each unit in each inspiration was noted. Hyperventilation was performed with visual feedback of the display of rib cage and abdomen excursions, keeping the ratio of rib cage to abdominal expansion. Subjects were normocapnic in quiet breathing and developed hypocapnia during hyperventilation. Recruitment order was stable in quiet breathing, but in some cases was altered during voluntary hyperventilation. Some low threshold units that fired early in the breath in quiet breathing fired earlier at the beginning of a period of voluntary hyperventilation but progressively later in the breath as hyperventilation went on, whereas later firing units moved progressively toward the early part of inspiration. This suggests that different groups of motoneurons in the pool supplying parasternal intercostal muscles receive different patterns of synaptic input.     

Geniohyoid muscle function in awake canines.

The geniohyoid (Genio) upper airway muscle shows phasic, inspiratory electrical activity in awake humans but no activity and lengthening in anesthetized cats. There is no information about the mechanical action of the Genio, including length and shortening, in any awake, nonanesthetized mammal during respiration (or swallowing). Therefore, we studied four canines, mean weight 28.8 kg, 1.5 days after Genio implantation with sonomicrometry transducers and bipolar electromyogram (EMG) electrodes. Awake recordings of breathing pattern, muscle length and shortening, and EMG activity were made with the animal in the right lateral decubitus position during quiet resting, CO2-stimulated breathing, inspiratory-resisted breathing (80 cmH2O. l-1. s), and airway occlusion. Genio length and activity were also measured during swallowing, when it shortened, showing a 9.31% change from resting length, and its EMG activity increased 6.44 V. During resting breathing, there was no phasic Genio EMG activity at all, and Genio showed virtually no movement during inspiration. During CO2-stimulated breathing, Genio showed minimal lengthening of only 0.07% change from resting length, whereas phasic EMG activity was still absent. During inspiratory-resisted breathing and airway occlusion, Genio showed phasic EMG activity but still lengthened. We conclude that the Genio in awake, nonanesthetized canines shows active contraction and EMG activity only during swallowing. During quiet or stimulated breathing, Genio is electrically inactive with passive lengthening. Even against resistance, Genio is electrically active but still lengthens during inspiration.     

Respiratory changes in parasternal intercostal length

In an attempt to understand the role of the parasternal intercostals in respiration, we measured the changes in length of these muscles during a variety of static and dynamic respiratory maneuvers. Studies were performed on 39 intercostal spaces from 10 anesthetized dogs, and changes in parasternal intercostal length were assessed with pairs of piezoelectric crystals (sonomicrometry). During static maneuvers (passive inflation-deflation, isovolume maneuvers, changes in body position), the parasternal intercostals shortened whenever the rib cage inflated, and they lengthened whenever the rib cage contracted. The changes in parasternal intercostal length, however, were much smaller than the changes in diaphragmatic length, averaging 9.2% of the resting length during inflation from residual volume to total lung capacity and 1.3% during tilting from supine to upright. During quiet breathing the parasternal intercostals always shortened during inspiration and lengthened during expiration. In the intact animals the inspiratory parasternal shortening was close to that seen for the same increase in lung volume during passive inflation and averaged 3.5%. After bilateral phrenicotomy, however, the parasternal intercostal shortening during inspiration markedly increased, whereas tidal volume diminished. These results indicate that 1) the parasternal intercostals in the dog are real agonists (as opposed to fixators) and actively contribute to expand the rib cage and the lung during quiet inspiration, 2) the relationship between lung volume and parasternal length is not unique but depends on the relative contribution of the various inspiratory muscles to tidal volume, and 3) the physiological range of operating length of the parasternal intercostals is considerably smaller than that of the diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)     

Firing properties and hypercapnic responses of single phrenic motor axons in the rat

Inspiratory phase activity was recorded from 33 phrenic motoneuron (PM) axonal fibers in anesthetized, vagotomized, artificially ventilated adult rats. During control conditions (no inspired CO2 added), the population of PM fibers could be separated into early and late onset types based on the time of firing onset relative to the onset of whole phrenic nerve activity. Mean discharge frequencies of both types were not significantly different. Compared with late PM's, early PM's had more spikes per inspiration, fired for a longer period, and the last spike occurred later and during the postinspiratory period. Further, the mean minimal interspike interval was shorter for early PM's than for late PM's. Increasing inspired CO2 to 0.03 and 0.05 resulted in earlier firing onsets and a greater number of spikes per inspiration, particularly for late PM's. Increases in mean firing frequency occurred for both PM types. Mean minimal interspike intervals for both types of PM's showed progressive reductions as CO2 rose. For almost all of the firing properties examined in this study, responses of rat PM axons were similar to those previously reported for the cat.     

Influence of motor units recruitment and firing rate on the soundmyogram and EMG characteristics in cat gastrocnemius

The separate contributions of the recruitment level and of the firing rate of the motor units on the soundmyogram and electromyogram time domain parameters were investigated during stimulation of the motor nerve of the cat gastrocnemius muscle. Upon orderly increase in the number of active motor units at a fixed firing rate, both the peak to peak amplitude (P-P_max) and the root mean square (RMS) of the sound myogram increased. At full recruitment the increase in firing rate from 2.5 to 50 Hz induced an exponential decline in the P-P_max. The RMS, however, followed this trend only from 15 to 50 Hz while showing an increase from 2.5 to 10 Hz. During simultaneous changes of recruitment and firing rate, the effect of increasing the number of motor units on the P-P_max and RMS is dampened by the increasing firing rate. The peak to peak amplitude of the EMG compound action potential increased with the number of active motor units. Moreover, its amplitude was not influenced by the firing rate. The EMG RMS, however, increases as a function of the firing rate. The results indicate that both the number and the firing rate of the active motor units contribute to the determination of the soundmyogram characteristics. Moreover, the peculiar changes of the soundmyogram time domain properties, compared to the ones of the EMG, allow one to differentiate the influence of the motor units number and firing rate on the electrical and mechanical performance of the muscle when stimulated.     

Motor unit firing rates and contractile properties in tibialis anterior of young and old men.

The effects of aging on motoneuron firing rates and muscle contractile properties were studied in tibialis anterior muscle by comparing results from six young (20.8 +/- 0.8 yr) and six old men (82.0 +/- 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal voluntary contraction (MVC) with twitch interpolation and stimulated twitch contractions. The old men had 26% lower MVC torque (P < 0.01) than did the young men, but percent activation was not different (99.1 and 99.3%, respectively). Twitch contraction durations were 23% longer (P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, motor unit firing rates were recorded. Results from approximately 950 motor unit trains in each subject group indicated that at all relative torque levels mean firing rates were 30-35% lower (P < 0.01) in the old subjects. Comparisons between young and old subjects' mean firing rates at each of 10%, 50%, and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations (r = -0.67 to -0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neuromuscular age-related remodeling principle.     

Respiratory and postural changes in intercostal muscle length in supine dogs

In an attempt to assess the physiological function(s) of the external (E) and internal interosseous (I) intercostal muscles, we measured the changes in intercostal muscle length during spontaneous breathing, during passive inflation, and during passive rotation of the trunk. Studies were performed on 46 muscles from 16 supine anesthetized dogs, and changes in muscle length were assessed by sonomicrometry. The changes were small during spontaneous breathing, whether before or after bilateral phrenicotomy, and the pattern was variable among animals and among interspaces. The E, however, particularly in the lower interspaces, often lengthened with inspiration, and the I, in particular in the upper interspaces, often shortened with inspiration. Only occasionally did the E and I in one interspace change in length in opposing directions. This was also true during passive inflation, where both E and I usually shortened in the upper interspaces and lengthened in the lower interspaces. By contrast, during passive rotation of the trunk, the E and I systematically changed in length in opposing directions, and either muscle could successively lengthen and shorten a substantial amount depending on the side of rotation. These results suggest that 1) the E and I in supine dogs do not behave as antagonistic muscles during moderate respiratory efforts; and 2) they do behave as antagonistic muscles during rotation of the trunk. A primary function of these muscles as rotators of the trunk, unlike breathing, may explain why two layers of intercostal muscles with different fiber orientation exist between the ribs.     

Coordination of swallowing and respiration in unconscious subjects

We investigated the coordination of swallowing and breathing in 11 unconscious patients with an endotracheal tube in place during the recovery period from general anesthesia. Swallows occurred during both the inspiratory and expiratory phases with no preponderant occurrence during either phase. When a swallow occurred during inspiration, the inspiration was interrupted immediately and was followed by expiration, but the durations of both inspiration and expiration were progressively increased as the time from the onset of inspiration to the onset of swallowing was progressively delayed. A swallow coinciding with the expiratory phase progressively prolonged the duration of the expiration that had been interrupted as the timing of swallowing was progressively delayed. Repeated swallows invariably and in a predictable manner caused changes in the breathing pattern. Thus when the frequency of regularly repeated swallows was relatively high, the breathing pattern was characterized by regular, shallow, and rapid breaths. When the frequency of regularly repeated swallows was relatively low, the breathing pattern was characterized by regular, deep, and slow breaths. When the frequency of repeated swallows was irregular, the breathing patterns were characterized by inconsistent changes in tidal volume and respiratory frequency. Our results indicate that, in unconscious subjects, some mechanisms integrating respiration and swallowing are operative and responsible for changes in breathing patterns during swallowing.     

Recruitment and rate-coding strategies of the human genioglossus muscle

Single motor unit (SMU) analysis provides a means to examine the motor control of a muscle. SMUs in the genioglossus show considerable complexity, with several different firing patterns. Two of the primary stimuli that contribute to genioglossal activation are carbon dioxide (CO(2)) and negative pressure, which act through chemoreceptor and mechanoreceptor activation, respectively. We sought to determine how these stimuli affect the behavior of genioglossus SMUs. We quantified genioglossus SMU discharge activity during periods of quiet breathing, elevated CO(2) (facilitation), and continuous positive airway pressure (CPAP) administration (inhibition). CPAP was applied in 2-cmH(2)O increments until 10 cmH(2)O during hypercapnia. Five hundred ninety-one periods (each ～ 3 breaths) of genioglossus SMU data were recorded using wire electrodes(n = 96 units) from 15 awake, supine subjects. Overall hypercapnic stimulation increased the discharge rate of genioglossus units (20.9 ± 1.0 vs. 22.7 ± 0.9 Hz). Inspiratory units were activated ～ 13% earlier in the inspiratory cycle, and the units fired for a longer duration (80.6 ± 5.1 vs. 105.3 ± 4.2% inspiratory time; P < 0.05). Compared with baseline, an additional 32% of distinguishable SMUs within the selective electrode recording area were recruited with hypercapnia. CPAP led to progressive SMU inhibition; at ～ 6 cmH(2)O, there were similar numbers of SMUs active compared with baseline, with peak frequencies of inspiratory units close to baseline, despite elevated CO(2) levels. At 10 cmH(2)O, the number of units was 36% less than baseline. Genioglossus inspiratory phasic SMUs respond to hypercapnic stimulation with changes in recruitment and rate coding. The SMUs respond to CPAP with derecruitment as a homogeneous population, and inspiratory phasic units show slower discharge rates. Understanding upper airway muscle recruitment/derecruitment may yield therapeutic targets for maintenance of pharyngeal patency.     

Inspiratory and expiratory muscle function during continuous positive airway pressure in dogs

Continuous positive airway pressure (CPAP) is known to produce activation of the expiratory muscles. Several factors may determine whether this activation can assist inspiration. In this study we asked how and to what extent expiratory muscle contraction can assist inspiration during CPAP. Respiratory muscle response to CPAP was studied in eight supine anesthetized dogs. Lung volume and diaphragmatic initial length were defended by recruitment of the expiratory muscles. At the maximum CPAP of 18 cmH2O, diaphragmatic initial lengths were longer than predicted by the passive relationship by 52 and 46% in the costal and crural diaphragmatic segments, respectively. During tidal breathing after cessation of expiratory muscle activity, a component of passive inspiration occurred before the onset of inspiratory diaphragmatic electromyogram (EMG). At CPAP of 18 cmH2O, passive inspiration represented 24% of the tidal volume (VT) and tidal breathing was within the relaxation characteristic. Diaphragmatic EMG decreased at CPAP of 18 cmH2O; however, VT and tidal shortening were unchanged. We identified passive and active components of inspiration. Passive inspiration was limited by the time between the cessation of expiratory activity and the onset of inspiratory activity. We conclude that increased expiratory activity during CPAP defends diaphragmatic initial length, assists inspiration, and preserves VT. Even though breathing appeared to be an expiratory act, there remained a significant component of active inspiratory diaphragmatic shortening, and the major portion of VT was produced during active inspiration.     

A Simulation Study to Examine the Effect of Common Motoneuron Inputs on Correlated Patterns of Motor Unit Discharge

The influence of common oscillatory inputs to the motoneuron pool on correlated patterns of motor unit discharge was examined using model simulations. Motor unit synchronization, in-phase fluctuations in mean firing rates known as ‘common drive’, and the coefficient of variation of the muscle force were examined as the frequency and amplitude of common oscillatory inputs to the motoneuron pool were varied. The amount of synchronization, the peak correlation between mean firing rates and the coefficient of variation of the force varied with both the frequency and amplitude of the common input signal. Values for ‘common drive’ and the force coefficient of variation were highest for oscillatory inputs at frequencies less than 5 Hz, while synchronization reached a maximum when the frequency of the common input was close to the average motor unit firing rate. The frequency of the common input signal for which the highest levels of synchronization were observed increased as motoneuron firing rates increased in response to higher target force levels. The simulation results suggest that common low-frequency oscillations in motor unit firing rates and short-term synchronization result from oscillatory activity in different bands of the frequency spectrum of shared motoneuron inputs. The results also indicate that the amount of synchronization between motor unit discharges depends not only on the amplitude of the shared input signal, but also on its frequency in relation to the present firing rates of the individual motor units.     

Relationship between diaphragm length and abdominal dimensions

We examined the relationship between changes in abdominal cross-sectional area, measured by respiratory inductive plethysmography, and changes in length in the costal and crural parts of the diaphragm, measured by sonomicrometry, in nine supine, anesthetized dogs. During passive inflation, both parts of the diaphragm shortened and abdominal cross-sectional area increased. During passive deflation, both parts of the diaphragm lengthened and abdominal cross-sectional area decreased. We subsequently used the relationship between costal and crural diaphragmatic length, respectively, and abdominal cross-sectional area during passive inflation-deflation to predict the length changes in the costal and crural diaphragm during quiet breathing before and after bilateral phrenicotomy. In the intact animal the inspiratory shortening in the crural diaphragm was almost invariably greater than predicted from the relationship during passive inflation. During inspiration after phrenicotomy the crural diaphragm invariably lengthened, whereas the costal diaphragm often shortened. In general there was a good correlation between the measured and predicted length change for the crural diaphragm (r = 0.72 before and 0.79 after phrenicotomy) and a poor one for the costal diaphragm (r = 0.05 before and 0.19 after phrenicotomy).     

Transverse abdominis length changes during eupnea, hypercapnia, and airway occlusion

The abdominal muscles accelerate airflow during expiration and may also influence the end-expiratory volume and configuration of the thorax. Although much is known about their electrical activity, the degree to which they change length during the respiratory cycle has not been previously assessed. In the present study we measured respiratory changes in transverse abdominis length using sonomicrometry in 14 pentobarbital sodium-anesthetized supine dogs and compared length changes to simultaneously recorded tidal volume and transverse abdominis electromyograms (EMG). To determine muscle resting length at passive functional residual capacity (LFRC), the animals were hyperventilated to apnea. The transverse abdominis was electrically active in all animals during resting O2 breathing (eupnea). During inspiration the transverse abdominis lengthened above resting length in all 14 dogs by a mean of 3.7 +/- 1.1% LFRC; during expiration the transverse abdominis shortened below resting length in 13 of 14 dogs by a mean of 4.2 +/- 0.9% LFRC. Increasing hyperoxic hypercapnia (produced in 9 animals) progressively heightened transverse abdominis EMG and progressively increased the extent of muscle shortening below resting length (to 12.6 +/- 3.2% LFRC at a PCO2 of 90 Torr). During single-breath airway occlusion substantial inspiratory lengthening of the transverse abdominis occurred, both during O2 breathing and during CO2 rebreathing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of timing of inspiratory occlusion on cerebral evoked potentials in humans

Previous studies from these laboratories have shown that airway occlusion applied from the onset of inspiration or during midinspiration is associated with cerebral evoked potentials in human subjects. The hypothesis tested in the present study was that the more abrupt decrease in mouth pressure produced by midinspiratory occlusion will be associated with evoked potentials that have shorter peak latencies and greater peak amplitudes than those produced by occlusions from the onset of inspiration. The second objective of the present study was to determine whether there is bilateral projection of inputs from the respiratory system to the somatosensory cortex. Random presentation of 64 midinspiratory occlusions and 64 occlusions from the onset of inspiration was performed in eight subjects. The inspirations preceding the occlusions served as control. Evoked potentials were recorded from the scalp with electrode pairs Cz-C3 and Cz-C4. Reaction time to each type of occlusion was measured from the burst in electromyogram activity produced by contraction of the muscles encircling the eye. Each type of inspiratory occlusion was associated with evoked potentials that could be recorded bilaterally. The peak amplitudes of the evoked potentials recorded over the right cerebral hemisphere were significantly greater than those recorded from the left side. The peak amplitude was greater and the peak latency shorter for the evoked potentials produced by the midinspiratory occlusions. The results are consistent with the hypothesis that afferents mediating these potentials are stimulated by added loads to breathing and project bilaterally to the somatosensory cortex in humans.     

Synchronization of motor units and its simulation in parallel feedback system

Comparison of the discharge patterns of soleus motor units with associated changes in force exerted by foot during quiet stance have already demonstrated the following facts. In the initial stage of standing, the motor units exhibited stationary and asynchronous discharges. The force showed a sporadic presence of the high frequency oscillation in the 8–10 Hz band. After five to ten minutes of standing, the firing rate of individual motor unit discharges increased to about 10 spikes/sec and discharges of each motor unit were synchronized and phase-locked to each of the accompanying force oscillation. During this transitional stage, the discharges of motor units were characterized by spike dropouts from an otherwise regular spike train. To simulate the changes in the discharge characteristics, we have proposed a parallel feedback model of the stretch reflex arc. This was made of multiple -motoneurons, motor units and muscle spindles. And motor units interact each other through group Ia afferent signals. As a result of simulation, motor units were found to exhibit stationary and asynchronous discharges when feedback gain was kept small. With an increase of feedback gain, the firing rate of individual motor units increased and finally the discharges of them were synchronized. During this transitional stage, the spike dropouts were observed in accordance with the experimental results. The neuronal mechanism of synchronization may partly be explained by the interactions of motor neurons through the above stated parallel feedback system.     

Motor unit activation patterns during concentric wrist flexion in humans with different muscle fibre composition

Muscle activity was recorded from the flexor carpi radialis muscle during static and dynamic-concentric wrist flexion in six subjects, who had exhibited large differences in histochemically identified muscle fibre composition. Motor unit recruitment patterns were identified by sampling 310 motor units and counting firing rates in pulses per second (pps). During concentric wrist flexion at 30% of maximal exercise intensity the mean firing rate was 27 (SD 13) pps. This was around twice the value of 12 (SD 5) pps recorded during sustained static contraction at 30% of maximal voluntary contraction, despite a larger absolute force level during the static contraction. A similar pattern of higher firing rates during dynamic exercise was seen when concentric wrist flexion at 60% of maximal exercise intensity [30 (SD 14) pps] was compared with sustained static contraction at 60% of maximal voluntary contraction [19 (SD 8) pps]. The increase in dynamic exercise intensity was accomplished by recruitment of additional motor units rather than by increasing the firing rate as during static contractions. No difference in mean firing rates was found among subjects with different muscle fibre composition, who had previously exhibited marked differences in metabolic response during corresponding dynamic contractions. It was concluded that during submaximal dynamic contractions motor unit firing rate cannot be deduced from observations during static contractions and that muscle fibre composition may play a minor role. Accepted: 5 May 1998     

Influence of nasal airflow temperature and pressure on alae nasi electrical activity.

The influence of nasal airflow, temperature, and pressure on upper airway muscle electromyogram (EMG) was studied during steady-state exercise in five normal subjects. Alae nasi (AN) and genioglossus EMG activity was recorded together with nasal and oral airflows and pressures measured simultaneously by use of a partitioned face mask. At constant ventilations between 30 and 50 l/min, peak inspiratory AN activity during nasal breathing (7.2 +/- 1.4 arbitrary units) was greater than that during oral breathing (1.0 +/- 0.3 arbitrary units; P less than 0.005). In addition, the onset of AN EMG activity preceded inspiratory flow by 0.38 +/- 0.03 s during nasal breathing but by only 0.17 +/- 0.04 s during oral breathing (P less than 0.04). When the subject changed from nasal to oral breathing, both these differences were apparent on the first breath. However, peak AN activity during nasal breathing was uninfluenced by inspiration of hot saturated air (greater than 40 degrees C), by external inspiratory nasal resistance, or by changes in the expiratory route. The genioglossus activity did not differ between nasal and oral breathing (n = 2). Our findings do not support reflex control of AN activity sensitive to nasal flow, temperature, or surface pressure. We propose a centrally controlled feedforward modulation of phasic inspiratory AN activity linked with the tonic drive to the muscles determining upper airway breathing route.