A method for the assessment of phasic vagal influence on tidal volume

Modulation of upper airway muscle activities by bronchopulmonary afferents

Here we review the influence of bronchopulmonary receptors (slowly and rapidly adapting pulmonary stretch receptors, and pulmonary/bronchial C-fiber receptors) on respiratory-related motor output to upper airway muscles acting on the larynx, tongue, and hyoid arch. Review of the literature shows that all muscles in all three regions are profoundly inhibited by lung inflation, which excites slowly adapting pulmonary stretch receptors. This widespread coactivation includes the recruitment of muscles that have opposing mechanical actions, suggesting that the stiffness of upper airway muscles is highly regulated. A profound lack of information on the modulation of upper airway muscles by rapidly adapting receptors and bronchopulmonary C-fiber receptors prohibits formulation of a conclusive opinion as to their actions and underscores an urgent need for new studies in this area. The preponderance of the data support the view that discharge arising in slowly adapting pulmonary stretch receptors plays an important role in the initiation of the widespread and highly coordinated recruitment of laryngeal, tongue, and hyoid muscles during airway obstruction.

Coordination of intrinsic and extrinsic tongue muscles during spontaneous breathing in the rat

The muscular-hydrostat model of tongue function proposes a constant interaction of extrinsic (external bony attachment, insertion into base of tongue) and intrinsic (origin and insertion within the tongue) tongue muscles in all tongue movements (Kier WM and Smith KK. Zool J Linn Soc 83: 207-324, 1985). Yet, research that examines the respiratory-related effects of tongue function in mammals continues to focus almost exclusively on the respiratory control and function of the extrinsic tongue protrusor muscle, the genioglossus muscle. The respiratory control and function of the intrinsic tongue muscles are unknown. Our purpose was to determine whether intrinsic tongue muscles have a respiration-related activity pattern and whether intrinsic tongue muscles are coactivated with extrinsic tongue muscles in response to respiratory-related sensory stimuli. Esophageal pressure and electromyographic (EMG) activity of an extrinsic tongue muscle (hyoglossus), an intrinsic tongue muscle (superior longitudinal), and an external intercostal muscle were studied in anesthetized, tracheotomized, spontaneously breathing rats. Mean inspiratory EMG activity was compared at five levels of inspired CO2. Intrinsic tongue muscles were often quiescent during eupnea but active during hypercapnia, whereas extrinsic tongue muscles were active in both eupnea and hypercapnia. During hypercapnia, the activities of the airway muscles were largely coincident, although the onset of extrinsic muscle activity generally preceded the onset of intrinsic muscle activation. Our findings provide evidence, in an in vivo rodent preparation, of respiratory modulation of motoneurons supplying intrinsic tongue muscles. Distinctions noted between intrinsic and extrinsic activities could be due to differences in motoneuron properties or the central, respiration-related control of each motoneuron population.

Mechanical effect of muscle spindles in the canine external intercostal muscles

High-frequency mechanical vibration of the ribcage increases afferent activity from external intercostal muscle spindles, but the effect of this procedure on the mechanical behaviour of the respiratory system is unknown. In the present study, we have measured the changes in external intercostal muscle length and the craniocaudal displacement of the ribs during ribcage vibration (40 Hz) in anaesthetized dogs. With vibration, external intercostal inspiratory activity increased by approximately 50 %, but the respiratory changes in muscle length and rib displacement were unaltered. A similar response was obtained after the muscles in the caudal segments of the ribcage were sectioned and the caudally oriented force exerted by these muscles on the rib was removed, thus suggesting that activation of external intercostal muscle spindles by vibration generates little tension. Prompted by this observation, we also examined the role played by the external intercostal muscle spindles in determining the respiratory displacement of the ribs during breathing against high inspiratory airflow resistances. Although resistances consistently elicited prominent reflex increases in external intercostal inspiratory activity, the normal inspiratory cranial displacement of the ribs was reversed into an inspiratory caudal displacement. Also, this caudal rib displacement was essentially unchanged after section of the external intercostal muscles, whereas it was clearly enhanced after denervation of the parasternal intercostals. These findings indicate that stretch reflexes in external intercostal muscles confer insufficient tension on the muscles to significantly modify the mechanical behaviour of the respiratory system.

Effect of pulmonary stretch receptor feedback and CO(2) on upper airway and respiratory pump muscle activity in the rat

1. Our purpose was to examine the effects of chemoreceptor stimulation and lung inflation on neural drive to tongue protrudor and retractor muscles in the rat. 2. Inspiratory flow, tidal volume, transpulmonary pressure, compliance and electromyographic (EMG) activity of genioglossus (GG), hyoglossus (HG) and inspiratory intercostal (IIC) muscles were studied in 11 anaesthetized, tracheotomized and spontaneously breathing rats. Mean EMG activity during inspiration was compared with mean EMG activity during an occluded inspiration, at each of five levels of inspired CO(2) (0, 3, 6, 9 and 12 %). 3. Lung inflation suppressed EMG activity in all muscles, with the effect on both tongue muscles exceeding that of the intercostal muscles. Static elevations of end-expiratory lung volume evoked by 2 cmH(2)O positive end-expiratory pressure (PEEP) had no effect on tongue muscle activity. 4. Despite increasing inspiratory flow, tidal volume and transpulmonary pressure, the inhibition of tongue muscle activity by lung inflation diminished as arterial PCO2 (P(a),CO(2)) increased. 5. The onset of tongue muscle activity relative to the onset of IIC muscle activity advanced with increases in P(a),CO(2) but was unaffected by lung inflation. This suggests that hypoglossal and external intercostal motoneuron pools are controlled by different circuits or have different sensitivities to CO(2), lung inflation and/or anaesthetic agents. 6. We conclude that hypoglossal motoneuronal activity is more strongly influenced by chemoreceptor-mediated facilitation than by lung volume-mediated inhibition. Hypoglossal motoneurons driving tongue protrudor and retractor muscles respond identically to these stimuli.

Effects of inspiratory flow on diaphragmatic motor output in normal subjects

Increasing inspiratory flow (V) has been shown to shorten neural inspiratory time (TI(n)) in normal subjects breathing on a mechanical ventilator, but the effect of V on respiratory motor output before inspiratory termination has not previously been studied in humans. While breathing spontaneously on a mechanical ventilator, eight normal subjects were intermittently exposed to 200-ms-duration positive pressure pulses of different amplitudes at the onset of inspiration. Based on the increase in V above control breaths (DeltaV), trials were grouped into small, medium, and large groups (mean DeltaV: 0.51, 1.11, and 1.65 l/s, respectively). We measured TI(n), transdiaphragmatic pressure (Pdi), and electrical activity (electromyogram) of the diaphragm (EMGdi). Transient increases in V caused shortening of TI(n) from 1.34 to 1.10 (not significant), 1.55 to 1.11 (P < 0.005), and 1.58 to 1.17 s (P < 0. 005) in the small, medium, and large DeltaV groups, respectively. EMGdi measured at end TI(n) of the pulse breaths was 131 (P < 0.05), 142, and 155% (P < 0.05) of the EMGdi of the control breaths at an identical time point in the small, medium, and large trials, respectively. The latency of the excitation was 126 +/- 42 (SD) ms, consistent with a reflex effect. Increasing V had two countervailing effects on Pdi: 1) a depressant mechanical effect due primarily to the force-length (11.2 cmH(2)O/l) relation of the diaphragm, and 2) an increase in diaphragm activation. For the eight subjects, mean peak Pdi did not change significantly, but there was significant intersubject variability, reflecting variability in the strength of the excitation reflex. We conclude that increasing inspiratory V causes a graded facilitation of EMGdi, which serves to counteract the negative effect of the force-length relation on Pdi.

The electro-mechanical response of canine inspiratory intercostal muscles to increased resistance: the cranial rib-cage

1. The effect of graded increases in inspiratory airflow resistance on the electrical activity and the mechanical behaviour of the three groups of inspiratory intercostal muscles (parasternal intercostal, external intercostal, levator costae) situated in the cranial portion of the rib-cage has been studied in ten anaesthetized, spontaneously breathing dogs. The mechanical behaviour of the muscles was determined by measuring the respiratory changes in muscle length and the displacements of the rib. 2. During unloaded inspiration, the three muscles were active, the rib moved in the cranial direction, and the parasternal intercostal and levator costae muscles shortened; in most animals, the external intercostals shortened as well. 3. Graded increases in inspiratory airflow resistance elicited a progressive inhibition of parasternal intercostal activity and a gradual facilitation of external intercostal and levator costae activities. Concomitantly, the parasternal intercostals continued to shorten during inspiration. However, both the external intercostals and the levator costae progressively lengthened, and the rib was gradually displaced in the caudal direction. This pattern persisted after increases in chemical respiratory drive had developed. 4. Sectioning the phrenic nerve roots did not alter the electrical or the mechanical response of the parasternal intercostal muscles to loading, but it markedly affected the response of the external intercostals and levator costae. After phrenicotomy, the external intercostals and levator costae continued to shorten during loaded breaths, the rib continued to be displaced in the cranial direction, and although the rate of inspiratory muscle shortening and of rib motion decreased, the facilitation of external intercostal and levator costae activities was markedly reduced or abolished. 5. Lengthening of the external intercostals and caudal displacement of the rib was reproduced by isolated stimulation of the phrenic nerves. 6. The reflex facilitation of external intercostal and levator costae activities that takes place during inspiratory resistive loading thus results primarily from the collapsing action of the diaphragm on the cranial portion of the rib-cage and the consequent lengthening of these muscles. The mechanical effectiveness of this reflex facilitation, however, appears to be relatively small.

Differential control of the inspiratory intercostal muscles during airway occlusion in the dog

1. The effect of airway occlusion on the electrical activity of the three groups of inspiratory intercostal muscles (external intercostal, levator costae, parasternal intercostal) situated in the cranial portion of the rib-cage has been studied in thirty anaesthetized, spontaneously breathing dogs. 2. The three muscles were active during normal inspiration, and their activity was prolonged similarly during airway occlusion. However, a comparison of activity during occluded and unoccluded inspirations indicated that airway occlusion caused a facilitation of external intercostal and levator costae activities but an inhibition of parasternal intercostal activity. 3. The facilitation of external intercostal and levator costae activities was markedly reduced after section of the phrenic nerves and completely suppressed after section of the appropriate thoracic dorsal roots. 4. The inhibition of parasternal intercostal activity was not affected by section of the phrenic nerves or by section of the thoracic dorsal roots. This phenomenon, however, was abolished after bilateral cervical vagotomy. 5. Activation of the external intercostals and levator costae during inspiratory efforts are thus highly dependent on segmental reflexes arising in these muscles. In contrast, activation of the parasternal intercostals resembles that of the diaphragm in the sense that it depends primarily on the central respiratory drive.

Inspiratory facilitation and inhibition from pulmonary stretch receptors in rabbits

In anesthetized rabbits firing rate (FR) of single fibers of diaphragm and of parasternal intercostal muscles (PIM) was determined at 30, 50 and 70% of control inspiratory time. During first inspiratory effort after airway occlusion at end expiration it increased on the average by 12.1 +/- 0.6 and 43.0 +/- 2.2% relative to control. Under pulmonary stretch receptor (PSR) block with SO2 this increase disappeared in diaphragm and fell to 28.3 +/- 1.8% in PIM. During first inspiratory efforts under PSR block FR decreased by 11.3 +/- 2.2% in diaphragm and 18.3 +/- 1.6 in PIM relative to unblocked efforts. In open inspirations FR under block did not decrease significantly either in diaphragm or PIM relative to unblocked inspirations. Moving average electromyography of diaphragm and of PIM showed similar trends. These results suggest that PSR discharge at FRC activates a mechanism facilitating inspiratory activity while during inspiration it also activates a mechanism inhibiting this activity since early inspiration. Both effects are greater on PIM.

Excitability changes of the inspiratory "off-switch" mechanism tested by electrical stimulation in nucleus parabrachialis in the cat

The time course of the excitability of the inspiratory "off-switch" mechanism with and without phasic vagal stretch receptor feedback has been studied in cats under light pentobarbitone anesthesia by electrical stimulation in the rostral pons using brief tetanic stimulation (300 Hz for 0.2 s). The threshold strength required just to elicit inspiratory "off-switch" was high early in inspiration and fell steeply with time. The threshold curves were steeper with than without phasic vagal feedback, and the difference reflects the phasic vagal contribution to the excitability of the inspiratory "off-switch" the absence of phasic vagal vagal feedback the time course of this threshold curve usually corresponded closely to that of the "integrated" phrenic activity at all PCO2 levels and body temperatures tested indicating that the "integrated" phrenic activity can be used as an index of the centrally generated inspiratory activity. In response to a rise in PCO2 both the rate of change of excitability of the inspiratory "off-switch" mechanism and its initial threshold level was increased. Changes in body temperature caused no change in the initial threshold but produced marked changes in the rate of rise of the "off-switch" excitability; Following an "augmented breath" the inspiratory "off-switch" threshold was markedly reduced

Recruitment and discharge frequency of phrenic motoneurones during inspiration

The discharges of 107 phrenic motor axons were recorded from cats under chloralose-urethane anaesthesia with spinal cords transected at T1 or with intact neuraxis. During inspiratory occlusions in spinal cats, each motoneurone was recruited at a mouth pressure constant at a given end tidal CO2; no motoneurone was recruited at a pressure greater than 70% of maximum. In eupnoea (32.3 torr CO2) 73% of motoneurones were recruited during the first 30% of inspiration; during CO2 rebreathing (60.8 torr CO2), 89% were recruited in the first 30% of inspiration. Neurones recruited earlier in inspiration had a lower onset frequency than later recruited units; all increased instantaneous frequency in a linear relation to pressure. Early recruited units showed a smaller increase in frequency per unit change in pressure than did later recruited units. During CO2 rebreathing, mean and peak frequencies increased on average 0.92 and 1.78 spikes.sec(-1) (%CO2)(-1), respectively, these increases being significantly less for early than for late recruited neurones. The data show that a stable order of recruitment of phrenic motoneurones exists during inspiration, the excitability of each motoneurone likely determining its time of recruitment. Above threshold, later recruited motoneurones are more 'sensitive' to a change in input. Recruitment of motoneurones is responsible for pressure generation at the start of inspiration and increase in discharge frequency (rate coding) is the dominant mechanism in the second half of inspiration.

Immediate response to expiratory threshold load

In anesthetized rabbits, cats and dogs the end-inspiratory volume referred to normal FRC (deltaVe.i.) of the first breath during expiratory threshold load (ETL) was greater than control deltaVe.i. The increase of deltaVe.i. was roughly proportional to the increase of end-expiratory volume relative to normal FRC (deltaVe.e.) caused by ETL. During squeezing of the rib cage or of the abdomen or after cordotomy at T1 the phenomenon occurred nearly to the same extent. This suggests that afferent impulses from the chest wall are not essential in determining the phenomenon. The first inspiration during ETL occurred after a delay that was roughly proportional to the increase of deltaVe.e: the arterial Pco2, however, was not significantly increased at the time of the first inspiration during ETL. After bilateral vagotomy the first inspiration during ETL was no longer increased. An explantation of the increased deltaVe.i. of the first inspiration during ETL is provided on the basis of the vagal dependent tidal volume-inspiratory duration characteristic of the respiratory center.