The role of the fusimotor system with respect to the contribution of the diaphragm and the intercostal muscles to the respiratory tidal volume

Summary The efferent electrical activity in the phrenic nerve can be quantified in such a way that it gives a good correlation to tidal volume. After administration of the drug benzoctamine this relationship changes: more phrenic nerve activity is needed for the same tidal volume. No changes were found in the neuro-muscular transmission from the phrenic nerve to the diaphragm. There was no alteration in dynamic compliance of the lungs or in airway resistance. The afferent phrenic nerve activity from proprioceptors in the diaphragm did not change. It seems unlikely that respiratory neurons in the brainstem were affected since the sensitivity of the respiratory system to CO₂ did not change.It is known that the tonic fusimotoneuron activity is suppressed at a supraspinal level by benzoctamine. Since intercostal muscles have muscle spindles and the diaphragm hardly has any, the intercostal muscle activity will be affected more than diaphragmatic activity by benzoctamine. This could actually be shown by quantifying the electromyogram of inspiratory external intercostal muscles.The tidal volume regulation is controlled by the vagal feedback loop. In order to reach a certain tidal volume after administration of benzoctamine, the contribution of the diaphragm has to increase because the activity of the intercostal muscles is diminished.     

Dependence of phrenic motoneurone output on the oscillatory component of arterial blood gas composition.

1. The hypothesis that respiratory oscillations of arterial blood gas composition influence ventilation has been examined. 2. Phrenic motoneurone output recorded in the C5 root of the left phrenic nerve and the respiratory oscillations of arterial pH in the right common carotid artery were measured in vagotomized anaesthetized dogs which had been paralysed and artificially ventilated. 3. The effect of a change in tidal volume for one or two breaths on phrenic motoneurone output was measured with the inspiratory pump set at a constant frequency similar to, and in phase with, the animal's own respiratory frequency. A reduction of tidal volume to zero or an increase by 30% led to a corresponding change of mean carotid artery pH level. The changes of carotid artery pH resulted in a change of phrenic motoneurone output, predominantly of expiratory time (Te) but to a lesser extent of inspiratory time (T1) and also peak amplitude of 'integrated' phrenic motoneurone output (Phr). Denervation of the carotid bifurcation blocked this response. 4. The onset of movement of the inspiratory pump was triggered by the onset of phrenic motoneurone output. When a time delay was interposed between them, the phase relationship between respiratory oscillations of arterial pH and phrenic motoneurone output altered. The dominant effect was to alter Te; smaller and less consistent changes of Phr and T1 were observed. 5. When the inspiratory pump was maintained at a constant frequency but independent of and slightly different from the animal's own respiratory frequency (as judged by phrenic motoneurone output), the phase relationship between phrenic motoneurone output and the respiratory oscillations of pH changed breath by breath over a sequence of 100-200 breaths, without change of the mean level of arterial blood gas composition. Te varied by up to 30% about its mean value depending on the phase relationship. Ti and Phr were also dependent on the phase relationship but varied to a lesser extent. The changes were comparable to the results obtained in paragraph 4. 6. It was concluded that phrenic motoneurone output is dependent in part on its relationship to the respiratory oscillations of arterial blood gas composition. 7. Information concerning a transient ventilatory disturbance is stored in the arterial blood in the form of an altered pattern of the respiratory oscillations of blood gas composition; this in turn can change breathing by an effect on the carotid bodies.     

The importance of timing on the respiratory effects of intermittent carotid body chemoreceptor stimulation

1. The respiratory response, measured directly as tidal volume or indirectly by using integrated peak phrenic activity, to brief intermittent chemical stimulation or depression of the carotid body was determined in anaesthetized cats. Recordings of carotid sinus nerve impulses allowed precise timing of the stimulus.2. Stimulation of the carotid body had a rapid effect on air flow, tidal volume and phrenic discharge rate only if given during inspiration. Increases in tidal volume and peak phrenic discharge occurred only if stimulation was applied during the last half of inspiration. Stimulation during expiration had no effect on the form or magnitude of subsequent breaths.3. Depression of the carotid body by NH(4)OH led to decreased tidal volume and phrenic discharge if it occurred during inspiration but had no effect if it occurred during expiration.4. Stimuli in expiration led to a prolongation of expiration. Stimuli in late inspiration caused prolongation of both inspiration and expiration.5. All of the effects noted were eliminated by bilateral carotid body denervation.6. The findings are similar to those following electrical stimulation of the carotid sinus nerve and are attributable to modulation of carotid body signals by the central respiratory neurones.     

Stimulation of Phrenic nerve activity by an acetylcholine releasing drug: 4-aminopyridine

The effect of the acetylcholine releaser 4-aminopyridine on ventilation was studied by recording and quantifying the efferent phrenic nerve activity in 40 paralysed and vagotomized cats; with arterialP _{O 2},P _{CO 2} and pH kept constant.4-Aminopyridine, given intravenously or in the vertebral artery, stimulates the phrenic nerve activity in a dose dependent manner. The stimulatory effects of 4-aminopyridine on the phrenic nerve activity could be abolished completely by administration of high doses of atropine. We conclude that 4-aminopyridine, which is used clinically for the reversal of a neuromuscular block, stimulates the phrenic nerve activity. Since the role of cholinergic mechanisms in the central chemorecptionn has been well established, the effect on the phrenic nerve activity is most probably by an increased release of acetylcholine at the site of the central chemoreceptors.This study was carried out in the Department of Physiology University of Nijmegen. The Netherlands     

The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation

1. The respiratory response, measured directly as tidal volume or indirectly by using integrated peak phrenic activity, to intermittent electrical stimulation of the carotid sinus nerve was determined in anaesthetized cats.2. Stimulation at rates of 20-25 Hz for 0.5 sec had a rapid effect, increasing inspiratory airflow and phrenic discharge, but only if applied during inspiration. An increase in tidal volume or peak level of integrated phrenic discharge occurred only if the stimulus was exhibited during the second half of inspiration. Continuous stimulation had no greater effect on size or frequency of breathing than did intermittent inspiratory stimuli alone. Stimulation during expiration had no effect on the form or magnitude of subsequent breaths.3. Stimuli in expiration led to a prolongation of expiration. Stimuli in late inspiration caused a prolongation of both inspiration and expiration. Because of these effects, the respiratory rate could be changed by stimulation; in some instances entrainment of respiration by the intermittent carotid sinus nerve stimuli occurred.4. The findings are attributable to modulation of incoming carotid sinus nerve information by the central respiratory neurones, which use primarily that which arrives during inspiration. They show a possible mechanism by which oscillating signals may have a different effect than their mean level would indicate.     

Augmented breath provoked by lung inflation in cat

The rate of occurrence and magnitude of provoked augmented breath (PAB) were studied as the function of lung expansion applied at different intervals (15-180 s). Together with phrenic nerve activity (Phr.) the activities of recurrent laryngeal (RL) and hypoglossal (Hyp) nerves were investigated during PAB. The experiments were carried out in 10 cats anesthetised, paralysed and artificially ventilated by means of a phrenic nerve-driven respiratory. Lung expansion was performed by increasing the gain of the servorespirator for one breath. PAB could be elicited when the interval between subsequent inflations was longer than 30 s ("refractory time"). We did not find out any consistent relationship, common for all experiments, between the value of the interval (greater than 30 s) and the rate of occurrence of PAB as well as between the volume of lung inflation and the magnitude of PAB. During PAB registered on Phr., activities of RL and Hyp were usually inhibited. It is concluded that PAB depends upon the instantaneous balance of excitatory and inhibitory vagal influences centrally differentiated at various respiratory outputs. Its amplitude and occurrences are therefore difficult to predict. Thus, PAB can be hardly compared with spontaneous deep breath.     

Pre-inspiratory burst in the caudal medulla of rabbits

The activity of 48 respiratory units in the paraolivary region from the middle to the rostral end of the hypoglossal cranial nerve root, and the effect of electrical stimulation and L-glutamate applied to the region on phrenic nerve activity was investigated in 14 rabbits. Electrical stimulation (50 Hz, 0.2 ms current pulses at intensities 5-20 microA) and L-glutamate (30-100 ng) shortened the expiratory time and increased the respiratory rhythm with no change in tidal phrenic nerve activity. Rhythmic activity preceding the phrenic nerve activity (pre-inspiratory burst) was recorded in the paraolivary region. The temporal relationship between the pre-inspiratory (pre-I) burst and the phrenic activity remained constant even when the respiratory frequency was altered by passive lung inflation. These results suggest that structures in the paraolivary region may influence the respiratory rhythm in rabbits and that pre-I burst neurons may play a role in triggering periodic phrenic activity.     

Respiratory oscillations of the arterialP_{{\text{O}}_{\text{2}} } and their effects on the ventilatory controlling system in the cat

The respiratory oscillations of the arterial were measured in paralyzed, artificially ventilated cats by a small (1.2 mm) fast-responding catheter oxygen electrode. The amplitude of these oscillations could be changed independently of the mean by a specially designed respirator circuit. was shown to increase with increasing tidal volume or decreasing frequency of the respirator, and with increasing mean . The amplitude of the oscillations was attenuated considerably from the left atrium to the aorta. No attenuation occurred from the aorta to the carotid artery, provided that the blood flow in the carotid artery was not impeded. The measured attenuation of the oscillations was compared to that calculated by Yokota and Kreuzer (1973) and found to be quite different.The output of the ventilatory controlling system of the cat was measured from the quantified phrenic nerve activity. When only was changed at a constant level mean , the quantified phrenic nerve activity did not change, indicating that the amplitude of the oscillations does not influence the ventilatory controlling system.In vagotomized animals, the periodicities of the oscillations and the phrenic nerve activity were completely dissociated. From the fact that no Cheyne-Stokes type of breathing occurred, it was concluded that the effect of timing is negligible.     

Increased extrajunctional acetylcholine sensitivity produced by chronic acetylcholine sensitivity produced by chronic post-synaptic neuromuscular blockade.

1. Anaesthetized rats were paralysed for periods of up to 3 days by chronic administration of D-tubocurarine (DTC), succinylcholine or alpha-bungarotoxin. 2. After 3 days of treatment with DTC, the phrenic nerve remained active. Neuromuscular transmission and spontaneous miniature end-plate potentials (m.e.p.p.s) were restored after removal of the DTC. Resting potentials and input resistances of muscle fibres that had been paralysed for 3 days were similar to those in denervated fibers. 3. Chronic neuromuscular blockade increased the binding of [125-I]-alpha-bungarotoxin by extrajunctional regions of muscle. The time course of the increase was similar to that seen after denervation. Binding to muscles from animals that were anaesthetized and respirated, but not paralysed, was not increased. 4. Three days of paralysis increased the sensitivity of the extrajunctional muscle membrane to acetylcholine (ACh) applied by iontophoresis. 5. Approximately the same proportion of muscle fibres from muscles paralysed for 3 days gave overshooting action potentials in the presence of tetrodotoxin 10-minus 6 g/ml. as did fibres form muscles denervated for 3 days. 6. Chronic paralysis did not change the accumulation of acetylcholinesterase above a ligation in the sciatic nerve. 7. These results are consistent with the idea that extrajunctional ACh sensitivity is normally controlled by muscle activity.     

On the regulation of depth and rate of breathing

1. The relationships between the depth of a breath and the durations of the inspiratory and expiratory phases have been studied in cat and in man during rebreathing, and in cat using artificial inflations of different magnitudes and timings.2. In the cat, the apparent volume threshold for termination of inspiration (Hering-Breuer threshold) decreased with time from the onset of the inspiratory phase.3. Both in rebreathing experiments and with artificially imposed inflations in the cat, the inspiratory duration T(I) was dependent upon tidal volume V(T), and this dependence could be expressed by a hyperbolic relationship of the form (V(T)-V(0)) T(I) = C where V(0) and C are constants.4. The time course of this ;Hering-Breuer' threshold was dependent on intact vagus nerves. After vagotomy the inspiratory duration remained essentially constant with changes in tidal volume produced either by artificial inflation or by the increased respiratory drive due to accumulation of CO(2) during rebreathing.5. In man during rebreathing, the relation between volume and inspiratory duration typically showed two different characteristics. 1, at tidal volumes up to 1.5-2 times eupnoeic values, inspiratory duration did not change as tidal volume increased in response to increased P(CO2). This range of operation has been designated range 1. 2, as tidal volume increased above this range 1 a second range designated range 2 was observed where inspiratory duration was volume dependent in the same manner as in the cat.6. In cat under pentobarbitone anaesthesia, a range 1 operation was not seen except after vagotomy. However, under urethane anaesthesia a range 1 plus a range 2 operation could be seen.7. The differences between cat and man appeared to be largely quantitative rather than qualitative.8. In both cat and man, expiratory duration was dependent on inspiratory duration, usually with a linear relationship.9. The experimental results were assembled in the form of an inspiratory characteristic and a timing relationship that serve as a model of the respiratory mechanisms controlling the depth and rate of breathing. The model predicts that the depth and duration of a breath are related in a definite manner fixed by the system characteristics and that ventilation is adjusted by setting the appropriate operating point on these characteristics. The operating point is determined primarily by how quickly lung volume increases, i.e. the rate of increase of inspiratory motor activity.     

Patterns of spontaneous and reflexly-induced activity in phrenic and intercostal motoneurons

Summary The discharge frequencies of 35 single phrenic and 13 inspiratory intercostal motoneurons were recorded in anaesthetised paralysed cats. Chemical stimulation by asphyxia or hypercapnia increased the discharge frequency and number of motoneurons active within each inspiratory discharge without altering the general pattern of respiratory activity, but mechanical stimulation of the epipharynx and electrical stimulation of the pharyngeal branch of the glossopharyngeal nerve caused repetitive bursts of very high frequency (up to 400 impulses/ sec) in inspiratory motoneurons, with disruption of their normal phasic activity. The latency of the motoneuron response to electrical stimulation of the glossopharyngeal nerve ranged from 15–30 msec and varied with respiratory phase, being shorter during spontaneous inspiratory activity.Phrenic motoneurons were divided according to their order of recruitment during inspiratory activity, and the later recruited (high-threshold) units had significantly larger spike amplitudes than motoneurons which discharged throughout inspiration. High-threshold motoneurons also achieved higher maximum discharge frequencies in response to electrical stimulation of the glossopharyngeal nerve, and it is suggested that these properties are important in increasing the tension developed by respiratory muscles near the end of inspiration when there is greater elastic resistance to lung inflation.     

NMDA and non-NMDA receptors may play distinct roles in timing mechanisms and transmission in the feline respiratory network.

1. Activation of N-methyl-D-aspartate (NMDA) glutamate receptors in the brainstem network of respiratory neurones is required to terminate inspiration in the absence of lung afferents, but it is not required in the inspiratory motor act of lung inflation. In the present study we examined the involvement of non-NMDA ionotropic glutamate receptors in these two mechanisms in the adult mammal. 2. Adult cats were either decerebrated or anaesthetized with sodium pentobarbitone, paralysed and ventilated. Inspiratory motor output was recorded from the phrenic nerve and central respiratory activity from neurones in the bulbar ventral respiratory group. 3. In decerebrate vagotomized cats, ionophoretic application of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX) onto single respiratory neurones decreased their spontaneous discharge rate and abolished the excitatory effect of exogenously applied (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) but not NMDA. 4. In these animals, intravenous infusion (12 mg kg-1) of the non-NMDA receptor blockers GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylene-dioxy-5-H-2,3-benzodi aze pine) or NBQX: (1) decreased (in 10/15 cats) or abolished (in 5/15 cats) the inspiratory-related discharge of the phrenic nerve; (2) did not prolong the inspiratory phase; (3) reduced or abolished the spontaneous discharge of respiratory neurones; and (4) profoundly decreased the excitatory effects of AMPA but not NMDA ionophoresed onto these neurones. When both the phrenic nerve and the recorded respiratory neurone were silenced, neuronal excitation by ionophoretic application of NMDA first revealed a subthreshold respiratory modulation without lengthening of the inspiratory phase, then respiratory modulation became undetectable. 5. Additional blockade of NMDA receptors by a small dose (0.15 mg kg-1) of dizocilpine (MK-801), abolished the phrenic nerve activity which persisted after NBQX (apnoea), but the discharge or the subthreshold modulation of the bulbar respiratory neurones showed a lengthening of the inspiratory phase (apneusis). 6. Elevation of FA,CO2 increased or re-established phrenic nerve discharges after blockade of non-NMDA receptors or of both NMDA and non-NMDA receptors. 7. Small doses of NBQX or GYKI 52466 induced apnoea in five of five cats anaesthetized with sodium pentobarbitone. 8. In decerebrate animals with intact vagi, GYKI 52466 and NBQX depressed the Hering-Breuer expiratory-lengthening reflex. 9. The results suggest that: (1) there is a specialization of different classes of glutamate receptors participating in timing mechanisms and transmission within the mammalian respiratory network. Neural transmission predominantly involves activation of non-NMDA receptors, acting in synergy with NMDA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Augmentation of muscle nociceptive respiratory reflex facilitation by vagal afferents

Vagal influence on the facilitation of phrenic neural activity during respiratory phase-locked, gastrocnemius muscle nerve nociceptive electrical stimulation was examined in anesthetized, glomectomized, paralyzed, and artificially ventilated cats. (1) In the vagi-intact state, respiratory reflex facilitation was characterized by a sharp rise in peak amplitude, maximum rate of rise or slope, and mean rate of rise of integrated phrenic nerve activity. This was greater during inspiratory phase-locked (T1-locked) muscle nerve electrical stimulation than during expiratory phase-locked (TE-locked) muscle nerve electrical stimulation. "Evoked post-inspiratory phrenic activity" during the early expiratory phase was also observed during TE-locked muscle nerve electrical stimulation. (2) Bilateral vagotomy significantly attenuated the respiratory facilitation during both T1- and TE-locked muscle nerve electrical stimulation. In particular, the "evoked post-inspiratory phrenic activity" during TE-locked muscle nerve electrical stimulation was also attenuated or almost completely abolished. (3) Conditioning electrical stimulation of the vagus nerve revealed facilitatory reflexes which co-exist with inspiratory inhibitory reflexes. (4) The "evoked post-inspiratory phrenic activity" during TE-locked muscle nerve electrical stimulation, which was attenuated or abolished after vagotomy, was restored after vagal T1-locked conditioning stimuli combined with TE-locked muscle nerve electrical stimulation. The results suggest that vagal facilitatory reflexes augment the respiratory reflex facilitation during muscle nociceptive stimulation.     

Interaction of pulmonary afferents and pneumotaxic center in control of respiratory pattern in cats.

The interaction between the pulmonary afferents (PA) and the pneumotaxic center (PC) in control of respiratory pattern was studied in lightly anesthetized paralyzed cats before and after bivagotomy or lesions of the PC using inflations controlled by the onset or cessation of phrenic nerve discharge, i.e., cycle-triggered inflations. This interaction was also studied using electrical stimulation of the central stumps of cut vagi. Introduction of a delay between inspiratory onset and the commencement of an inflation at constant flow and duration resulted in increases of the durations of inspiration (T1) and expiration (TE) and amplitude of the integrated phrenic nerve discharge (A). The lung volume at inspiratory cutoff, i.e., the volume threshold, increased markedly as T1 increased. There were linear relationships between T1 and TE and between T1 and A. At constant alveolar CO2 and tidal volume, the quantitative effects of delay were dependent on the rate of inflation; i.e., when the flow increased, the volume threshold for a given T1 decreased. Bilateral vagotomy abolished the effects of delay and flow. PC lesions, which resulted in apneusis when the cycle-triggered inflations were stopped, produced the following changes compared to the delay effects seen in intact cats: a) the volume threshold for zero delay doubled and its rate of decrease with increased T1 was significantly smaller, and b) the change in TE for a given change in T1 was reduced markedly. Introduction of a delay between inspiratory onset and the start of electrical stimulation of the afferent vagi resulted in effects similar to those seen for delays in cycle-triggered inflations. The T1-TE relationship remained linear when the stimulus trains ended with inspiratory cessation. These results suggest that: a) the inspiratory cutoff mechanism is responsive to the rate, as well as the level, of lung inflation; b) all of the lung volume information affecting inspiratory cutoff in paralyzed cats is carried via the vagi; c) an intact PC is necessary for the generation of a normal time dependence of the volume threshold for inspiratory cutoff; d) the PC plays an important role in matching TE to T1 when the latter changes. For inflations and vagal stimulations applied during expiration, with introduction of a delay between inspiratory cessation and the start of cycle-triggered inflation or vagal stimulation, the results indicated that the expiratory cutoff mechanism has an irrevocable phase of 300-450 ms.     

Effect of anatoxin-a(s) from Anabaena flos-aquae NRC-525-17 on blood pressure, heart rate, respiratory rate, tidal volume, minute volume, and phrenic nerve activity in rats

The effects of anatoxin-a(s) [antx-a(s)] from the cyanobacterium Anabaena flos-aquae NRC-525-17 on mean arterial blood pressure, heart rate, respiratory rate, tidal volume, minute volume, and phrenic nerve activity were evaluated in anesthetized Sprague-Dawley rats. Anatoxin-a(s) was administered by continuous intravenous infusion. The initial effect of the toxin was to slow the heart rate and reduce arterial blood pressure, followed by much more pronounced reductions in these parameters. The marked decline in heart rate and blood pressure frequently occurred before there was a large decrease in respiratory minute volume [reduced by only 15.4 +/- 3% (mean +/- S.E.) compared to the predose period], suggesting that antx-a(s) has an important muscarinic action on the cardiovascular system in vivo. Phrenic nerve amplitude increased, but, nevertheless, tidal and minute volumes decreased progressively, indicating that antx-a(s), unlike most low-molecular-weight organophosphorus cholinesterase inhibitors, does not have any remarkable inhibitory action on central mediation of respiration.     

Denervation causes changes in electrophysiological properties in rat phrenic motoneurons

Thirty-nine male adult rats were divided into a control group and a denervation group that had been subjected to phrenicotomy 4 weeks earlier. Electrophysiological membrane properties (input resistance and rheobase) of phrenic motoneurons were measured from intracellular recordings made with glass microelectrodes. Under anesthetized and artificially ventilated conditions, the recorded motoneurons were divided into recruited (spike discharge) and non-recruited (depolarization only) types. There was a significant inverse relationship between the rheobase and input resistance in the control rats, but not in the denervated rats. In the control rats, the mean value of rheobase in the non-recruited motoneurons was significantly higher than that in the recruited motoneurons. In denervated rats, however, the mean value of rheobase in the recruited motoneurons was identical to that in the non-recruited motoneurons. The results indicated that phrenicotomy induced a de-differentiation of electrophysiological properties of the phrenic motoneurons, and that these changes might be restricted to the motoneurons innervating fast-twitch, low fatigue resistance muscle fibers.     

Chemical activation of caudal medullary expiratory neurones alters the pattern of breathing in the cat.

1. The purpose of this work was to ascertain whether the activation of caudal expiratory neurones located in the caudal part of the ventral respiratory group (VRG) may affect the pattern of breathing via medullary axon collaterals. 2. We used microinjections of DL-homocysteic acid (DLH) to activate this population of neurones in pentobarbitone-anaesthetized, vagotomized, paralysed and artificially ventilated cats. Both phrenic and abdominal nerve activities were monitored; extracellular recordings from medullary and upper cervical cord respiratory neurones were performed. 3. DLH (160 mM) microinjected (10-30 nl for a total of 1.6-4.8 nmol) into the caudal VRG, into sites where expiratory activity was encountered, provoked an intense and sustained activation of the expiratory motor output associated with a corresponding period of silence in phrenic nerve activity. During the progressive decline of the activation of abdominal motoneurones, rhythmic inspiratory activity resumed, displaying a decrease in frequency and a marked reduction or the complete suppression of postinspiratory activity as its most consistent features. 4. Medullary and upper cervical cord inspiratory neurones exhibited inhibitory responses consistent with those observed in phrenic nerve activity, while expiratory neurones in the caudal VRG on the side contralateral to the injection showed excitation patterns similar to those of abdominal motoneurones. On the other hand, in correspondence to expiratory motor output activation, expiratory neurones of the Bötzinger complex displayed tonic discharges whose intensity was markedly lower than the peak level of control breaths. 5. Bilateral lignocaine blockades of neural transmission at C2-C3 affecting the expiratory and, to a varying extent, the inspiratory bulbospinal pathways as well as spinal cord transections at C2-C3 or C1-C2, did not suppress the inhibitory effect on inspiratory neurones of either the ipsi- or contralateral VRG in response to DLH microinjections into the caudal VRG. 6. The results show that neurones within the column of caudal VRG expiratory neurones promote inhibitory effects on phrenic nerve activity and resetting of the respiratory rhythm. We suggest that these effects are mediated by medullary bulbospinal expiratory neurones, which may, therefore, have a function in the control of breathing through medullary axon collaterals.     

Spontaneous ventilation and respiratory motor output during carbachol-induced atonia of REM sleep in the decerebrate cat.

Microinjections of carbachol into the pons induce a state that resembles rapid eye movement (REM) sleep in intact cats and, in decerebrate, artificially ventilated cats, produce postural atonia accompanied by a powerful depression of the respiratory motor output. In this study, pontine carbachol was used in decerebrate, spontaneously breathing cats to assess the effects of mechanical and chemical respiratory reflexes on the magnitude and pattern of the carbachol-induced depression of breathing, and to determine whether the depression is altered in those animals in which rapid eye movements are present. Phrenic nerve activity and tidal volume were only transiently depressed at the onset of the carbachol-induced postural atonia, whereas the decrease in respiratory rate and the depressions of hypoglossal and intercostal activities persisted until the response was reversed by a pontine microinjection of atropine 15-101 minutes after the onset of carbachol response. Ventilation was reduced to 70% of control during the steady-state conditions. The irregularity of breathing, characterized by the inter-quartile ranges of the distributions of the peak phrenic nerve activity and respiratory timing, did not increase following pontine carbachol. Neither vagotomy nor vigorous eye movements were associated with increased breathing irregularity. This contrasts with the irregular breathing (with minor average changes in ventilation) typical of natural REM sleep. We propose that the carbachol-injected decerebrate cat provides a useful model of the depressant effects that neural events associated with REM sleep may have on breathing.     

Respiratory responses to ionotropic glutamate receptor antagonists in the ventral respiratory group of the rabbit

Selective excitatory amino acid receptor antagonists acting on either N-methyl-D-aspartic acid (NMDA) or non-NMDA receptors were microinjected (30-50 nl) bilaterally into different subregions of the ventral respiratory group (VRG) of alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1 or 10 mM) within the inspiratory portion of the VRG (iVRG) dose-dependently decreased the peak amplitude and rate of rise of phrenic nerve activity, without significant changes in respiratory timing. Decreases in respiratory frequency and peak phrenic amplitude up to apnoea were evoked by 20 mM D-AP5; phrenic nerve activity was restored transiently by hypoxic or hypercapnic stimulation during D-AP5-induced apnoea. Microinjections of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 or 20 mM) into the iVRG provoked less intense depressant respiratory effects. No significant respiratory responses were evoked by microinjections of these antagonists into more caudal VRG subregions. The results suggest that ionotropic glutamate receptors within the iVRG are involved mainly in the control of the intensity of inspiratory activity, with a major role played by NMDA receptors. Glutamate receptor antagonism in the iVRG does not seem to impair the basic mechanisms underlying respiratory rhythm generation.     

Modulation of expiratory motor output evoked by chemical activation of pre-Bötzinger complex in vivo

We have previously demonstrated that chemical stimulation of the pre-B?tzinger complex (pre-B?tC) in the anesthetized cat produces either phasic or tonic excitation of phrenic nerve discharge. This region is characterized by a mixture of inspiratory-modulated, expiratory-modulated, and phase-spanning (including pre-inspiratory (pre-I)) neurons; however, its influence on expiratory motor output is unknown. We, therefore, examined the effects of chemical stimulation of the pre-B?tC on expiratory motor output recorded from the caudal iliohypogastric (lumbar, L(2)) nerve. We found that unilateral microinjection of DL-homocysteic acid (DLH; 10 mM; 10-20 nl) into 16 sites in the pre-B?tC enhanced lumbar nerve discharge, including changes in timing and patterning similar to those previously reported for phrenic motor output. Both increased peak amplitude and frequency of phasic lumbar bursts as well as tonic excitation of lumbar motor activity were observed. In some cases, evoked phasic lumbar nerve activity was synchronized in phase with phrenic nerve discharge. These findings demonstrate that chemical stimulation of the pre-B?tC not only excites inspiratory motor activity but also excites expiratory motor output, suggesting a role for the pre-B?tC in generation and modulation of inspiratory and expiratory rhythm and pattern.