Behavior of expiratory neurons in response to mechanical and chemical loading

Expiratory load compensation is associated with electroencephalographic premotor potentials in humans

In normal humans during quiet breathing, expiration is mostly driven by elastic recoil of the lungs. Expiration becomes active when ventilation must be increased to meet augmented metabolic demands, or in response to expiratory loading, be it experimental or disease-related. The response to expiratory loading is considered to be mediated by both reflex and cortical mechanisms, but the latter phenomenon have not been neurophysiologically characterized. We recorded the EEG in 20 healthy volunteers (9 men, 11 women, age: 22 to 50 yr) during unloaded breathing, voluntary expirations, and in response to 50 cmH2O·l(-1)·s expiratory resistive load (ERL), 20 cmH2O expiratory threshold load (high ETL), and 10 cmH2O expiratory threshold load (low ETL). EEGs were processed by ensemble averaging expiratory time-locked segments and examined for pre-expiratory potentials, defined as a slow negative shift from the baseline signal preceding expiration, and suggestive of cortical preparation of expiration involving the supplementary motor area. Four subjects were excluded because of technical EEG problems. Pre-expiratory potentials were present in one subject at baseline and in all subjects during voluntary expirations. They were present in eight subjects during low ETL, in 15 subjects during high ETL, and in 13 subjets during ERL (control vs. low ETL, P = 0.008; control vs. high ETL, P < 0.001; and control vs. ERL, P < 0.001). Respiratory discomfort was more intense in the presence of pre-expiratory potentials (P < 0.001). These results provide a neurophysiological substrate to a cortical component of the physiological response to experimental expiratory loads in humans.

Respiratory-related activation of human abdominal muscles during exercise

We tested the hypothesis that abdominal muscles are active during the expiratory phase of the respiratory cycle during exercise. Electromyographic (EMG) activities of external oblique and rectus abdominis muscles were recorded during incremental exercise to exhaustion and during 30 min of constant work rate exercise at an intensity of 85 % of the peak oxygen consumption rate (V(O(2))). High amplitude intramuscular EMG activities of both abdominal muscles could be evoked with postural manoeuvres in all subjects. During cycling, respiratory-related activity of the external obliques was evoked in four of seven subjects, whereas rectus abdominis activity was observed in six of the seven subjects. We measured only the activity that was confined exclusively to the expiratory phase of the respiratory cycle. Expiratory activity of both muscles increased with exercise intensity, although peak values averaged only 10-20 or 20-40 % of the peak activity (obtained during maximal, voluntary expiratory efforts) for the external oblique and rectus abdominis muscles, respectively. To estimate how much of the recorded abdominal muscle activity was supporting leg movements during exercise, we compared the activity at the very end of incremental exercise to that recorded during the first five respiratory cycles after the abrupt cessation of exercise, when ventilation was still very high. Although external oblique activity was reduced after exercise stopped, clear expiratory activity remained. Rectus abdominis activity remained high after exercise cessation, showing a gradual decline that approximated the decline in ventilation. During constant work rate exercise, EMG activities increased to 40-50 and 5-10 % of peak in rectus and external oblique muscles, respectively, and then plateaued for the remainder of the bout in spite of a continual upward drift in (V(O(2))) and pulmonary ventilation. Linear regression analysis showed that the rise in respiratory-related expiratory muscle activity during progressive intensity exercise was significantly correlated with ventilation, although weakly. In constant work rate exercise, expiratory EMG activities increased, but the changes were highly variable and did not change as a function of exercise time, even though ventilation drifted significantly with time. These experiments suggest that abdominal muscles play a role in regulating the ventilatory response to progressive intensity bicycle exercise, although some of the observed activity may support postural adjustments or limb movements. The contribution of abdominal muscles to ventilation during constant work rate exercise is variable, and expiratory activity does not 'drift' significantly with time.

Influence of phasic volume feedback on abdominal expiratory nerve activity

Our purpose was to examine the influence of phasic lung volume feedback on the activities of motor nerves innervating the diaphragm and transversus abdominis muscles during hypercapnia and hypoxia. We studied seventeen decerebrate cats that were paralyzed and ventilated with a servo-respirator controlled by the integrated phrenic neurogram. The effects of phasic lung volume feedback were assessed by withholding pulmonary inflation during the central inspiratory period. Withholding lung inflation for a single respiratory cycle under hyperoxic, normocapnic conditions consistently prolonged the durations of the inspiratory and expiratory periods, and caused marked increases in the peak electrical activities of both phrenic and abdominal nerves. Hyperoxic hypercapnia (PaCO2 50-80 mmHg) and isocapnic hypoxia (PaO2 60-35 mmHg) increased peak phrenic and abdominal neural activities, and withholding pulmonary inflation under these conditions caused even greater augmentations of inspiratory and expiratory motor output. The augmentation of expiratory activity by withholding lung inflation was proportionately greater than the concomitant prolongation of the central expiratory period. All responses to non-inflation maneuvers were abolished following bilateral cervical vagotomy. The results indicate that vagally mediated volume feedback during inspiration can attenuate the output of abdominal motoneurons in the subsequent expiratory period. Moreover, hypoxia, which attenuates abdominal motor activity in vagotomized animals, enhances this activity when the vagi are intact.

Central respiratory oscillator: phase-response analysis

To investigate properties of the central respiratory oscillator, phrenic nerve activity, perturbed by electrical stimulation of the middle external intercostal nerve, was analyzed in rabbit by using a phase-response curve (PRC). During inspiration, the stimuli (4-8 pulses) caused all-or-none responses, i.e. a phase advance or no phase shift, and strong stimuli (10 pulses) induced only phase advances. During expiration only graded phase delays were observed. The overall slope of PRC was 0 for 2 pulses and 1 for 10 pulses. At the transition from expiration (E) to inspiration (I), the PRC was discontinuous. This discontinuity corresponds to a phase singularity. In contrast, at the transition from I to E, the PRC was continuous. Therefore, our findings indicate that E-I switching may differ from I-E switching in nature. The respiratory rhythm could not be stopped by perturbation at the phase singularity as predicted from the PRCs. Similarities between the reported PRCs, obtained by inhibitory stimulation of an endogenous bursting neuron and the PRCs in the present study, suggest a possibility that endogenous bursting neurons take part in the function of a mammalian central respiratory oscillator.

Effect of expiratory glottic constriction on lung volume and pattern of breathing in adult dogs

We examined the effect of laryngeal constriction on the pattern of breathing in 4 anaesthetized adult mongrel dogs. By means of a T-shaped tracheostomy tube the larynx could be repeatedly excluded or included in the breathing circuit. Marked expiratory activity of the thyroarytenoid muscle (TA), the main glottic constrictor, was induced by injection of 100-400 ml of air into the pleural space or by inhalation of histamine aerosol (2 dogs) which resulted in rapid shallow breathing. In 21 pairs of runs in the 4 dogs switching from tracheostomy breathing to oral breathing decreased mid-expiratory flow and frequency by 85 +/- 2% (P less than 0.001) and 48 +/- 4% (P less than 0.001), respectively. Although expiratory duration (TE) increased, end-expiratory lung volume also increased by 40 +/- 8 ml or 21 +/- 4% of the tidal volume (VT) during tracheostomy breathing (P less than 0.001). In contrast, VT remained unchanged (P = 0.9). Instantaneous ventilation decreased due to both the prolongation of TE and an increase in inspiratory duration. Our results indicate that laryngeal braking can be recruited in adult dogs and interacts with reflex mechanisms that modulate respiratory timing, thereby significantly influencing end-expiratory lung volume, ventilation and the pattern of breathing. Simulation of the laryngeal mechanism by expiratory resistive loading at the tracheostomy below the larynx points to a non-reflex mechanical effect of the larynx as a resistance in series.

The pattern of sympathetic neurone activity during expiration in the cat

The properties of sympathetic preganglionic neurone activity during expiration were studied in pentobarbitone-anaesthetized (n = 26) and in non-anaesthetized, mid-collicular decerebrate (n = 5), paralysed, artificially ventilated cats in which the electrical activity of the phrenic nerve and of the cervical sympathetic trunk was recorded. In control conditions (end-tidal PCO2 between 35 and 40 mmHg, zero end-expiratory pressure) sympathetic activity during expiration was either steady at a low level (n = 11) or showed a modest progressive increase from a low level in early expiration (n = 17). Very infrequently (n = 3), it showed a transient increase during the second half of expiration. Artificial ventilation with positive end-expiratory pressures in the range from 2.1 +/- 0.4 (mean +/- S.D.) to 6.7 +/- 0.6 cmH2O caused, in cats with intact vagus nerves, an increase in sympathetic neurone activity during the second half of expiration. Within this range of pressures, the magnitude of the increase was related to the magnitude of the positive end-expiratory pressure. This effect reversed at higher positive end-expiratory pressures. Pressures in excess of 10.2 +/- 1.8 cmH2O caused inhibition of sympathetic activity. The sympatho-excitatory effect of positive end-expiratory pressure disappeared after bilateral cervical vagotomy. With intact vagus nerves, it also disappeared at levels of systemic hypocapnia (end-tidal PCO2 less than or equal to 15 mmHg) which abolished phrenic nerve activity. In hypocapnia, artificial ventilation with peak tracheal pressures greater than 7.2 +/- 1.1 cmH2O caused inhibition of sympathetic activity, while ventilation with lower end-expiratory pressures had no effect on sympathetic activity. It may be concluded that the sympatho-excitatory effect of positive end-expiratory pressure is mediated by vagal afferents and requires a certain level of brain-stem respiratory neurone activity. Sympatho-excitation during expiration was also observed, in normocapnic conditions, during short-duration static lung inflation with tracheal pressures in the range from 2.5 +/- 0.3 to 7.0 +/- 0.8 cmH2O as well as during artificial ventilation with zero end-expiratory pressure when lung inflation occurred in expiration. These responses were abolished by bilateral cervical vagotomy and during systemic hypocapnia. Sympatho-excitation during expiration was also observed when systemic hypercapnia was produced in vagotomized cats by artificial ventilation with gas mixtures containing 5 or 10% CO2.(ABSTRACT TRUNCATED AT 400 WORDS)

Response of abdominal muscle to graded mechanical loads

Abdominal muscle electromyograms were monitored in response to graded mechanical loads added to either inspiration or expiration in anesthetized cats. The surgical preparation, the apparatus, and the levels of loads applied (resistive, tracheal occlusion, and continuous positive pressure) were matched with those employed in our previous study on medullary expiratory neurons [Baker et al, 1979]. Although expiratory neuron firing was significantly increased by each of three graded levels of expiratory resistive loads, abdominal muscles were activated in only about 50% of the animals exposed to the highest-level resistive load. The smaller resistive loads failed to elicit any discernible abdominal muscle activity. These findings suggest that the lower motor neurons have a higher threshold for activation than the medullary neurons. Recruitment of medullary expiratory neurons and integration of synaptic input at the spinal level must play important roles in the response to expiratory loading. Abdominal muscles did not respond to mechanical loading during inspiration. Bilateral cervical vagotomy eliminated the abdominal muscle responses to expiratory loads.

Effects of carotid chemoreceptor excitation on medullary expiratory neurons in cats

A previous report (Lipski et al., 1977, J. Physiol. (London) 269, 797-810) demonstrated an inhibition of the medullary dorsal inspiratory neurons when a phasic chemoreceptor stimulus was applied during expiration. The present study tested the response of expiratory neurons which might mediate this inhibition. Recordings were made in cats anaesthetized with chloralose-urethane from the C5 phrenic rootlet and mainly from the rostral (Bötzinger) and caudal (nucleus retroambigualis, NRA) groups of medullary expiratory neurons. Carotid chemoreceptors were excited by close arterial injections of CO2-equilibrated saline. The stimuli were applied automatically with a preset delay within the respiratory cycle. The stimuli applied in inspiration excited both the phrenic activity and ventral inspiratory neurons within less than 0.5 sec. The stimuli applied in expiration excited 17 out of 25 NRA units, none of which projected to the contralateral dorsal (NTS) respiratory group. Four out of 9 units within be responsible for the expiratory inhibition of NTS inspiratory cells produced by chemoreceptor stimulation, while the caudal expiratory neurons are involved in the mediation of the chemoreceptor-induced effects upon the spinal neurons.

Effects of L-glutamate and GABA on the response of expiratory neurons to mechanical loads

Expiratory neurons in the area of the nucleus retroambigualis were studied in anesthetized cats to determine their responsiveness to the iontophoretic application of the putative neurotransmitters, glutamate and gamma-aminobutyric acid (GABA). Previous studies with glutamate and GABA revealed that these two substances were very effective in modulating the spontaneous activity of phasic medullary respiratory neurons. Mechanical loading of expiration, both resistive and elastic, was employed to test whether the presence of these transmitter substances altered the sensitivity of the expiratory cell to its volume related vagal input. Expiratory unit activity analysis included: spikes/burst, burst duration, and average firing rate. Addition of mechanical loads on expiration caused consistent increases in all parameters monitored. Iontophoretically applied glutamate (means = 65 nA) resulted in modest increases in all parameters. When mechanical loads were applied in the presence of a sustained level of glutamate the effects were additive. The general shape of the firing profile observed with loading remained essentially unchanged. Application of GABA (means = 46 nA) resulted in a significant decrease in the parameters monitored. However, as long as phasic activity remained, loads applied in the presence of GABA produced approximately the same absolute change as they did during control. Some cells exposed to high concentrations of GABA lost their phasic activity. This study suggests that either the synaptically activated receptors are not affected by glutamate or that these particular sites are not accessible via iontophoretic application. GABA depressed the activity of the cells in a graded fashion, but in modest concentrations did not interfere with the overall effectiveness of the vagally mediated input.

Nucleus retroambigualis respiratory neurons: responses to intercostal and abdominal muscle afferents

Studies were performed on anesthetized (Dial), paralyzed, vagotomized, artificially ventilated cats. Phrenic efferent activity, ventral respiratory group neuron activity in the region of the nucleus retroambigualis and, in some instances, thoracic dorsal root compound action potentials were recorded during electrical stimulation of intercostal nerve afferents (INS). Phrenic activity and inspiratory (I) neurons were inhibited by stimulating external, internal and lateral intercostal nerve afferents. Some expiratory (E)-neurons were also inhibited by these afferents. No I or E-neurons were facilitated with INS. Changes in I and E activity were correlated with muscle proprioceptor and cutaneous receptor afferent fibers. It is concluded that the dominant effect of intercostal and abdominal muscle proprioceptive afferent information on medullary respiratory activity in inhibition of inspiratory activity.

The effect of the resistive loading of inspiration and expiration on pulmonary stretch receptor discharge

Anesthetized, spontaneously breathing cats were used to examine the hypothesized role of slowly adapting pulmonary stretch receptors (PSR) in the control of breath duration. Initially, graded inspiratory and expiratory resistive loads were added to elucidate the inspiratory and expiratory volume-time relationship with both vagi intact. Unilateral vagotomy increased the slope of the VI--TI relationship indicating a reduction of the volume related modulation of TI. PSR frequency (fPSR) at end-inspiration also progressively decreased resulting in a fPSR--TI relationship qualitatively similar to the VI--TI curve. Expiratory resistive loading also produced an increased slope for the VE--TE relationship when the right vagus nerve was severed. The prolongation of TE was associated with a progressive increase in the number of PSR discharges during the loaded expiration. These results support the hypothesized role of PSR in the vagally mediated prolongation of TI and TE during resistive loading. In a subsequent series of experiments, the changes in fPSR were correlated with the tidal volume and transpulmonary pressure (PTP) changes. The fPSR was linearly related to PTP during both eupnic and loaded breathing. When fPSR was plotted against volume, a clockwise hysteresis was observed. These results suggest that in the spontaneously breathing cat, intrathoracic PSR frequency varies as a function of the transmural pressure across the airways.