Influences on the respiratory rhythm originating from the lungs and the chest wall

Summary A sudden increase of the volume of the respiratory apparatus induced by the stimulation of the peripheral cut end of one phrenic nerve during expiration delays the onset of the expected spontaneous inspiration. The delay becomes longer as the stimulation is applied later in the expiration. This inhibitory influence on the inspiratory activity disappears after section of the vagi and should be due to the Hering-Breuer inflation reflex.After vagotomy the phrenic stimulation induces a shortening of the expiratory phase which disappears after excluding impulses from thoracic cage afferents by sectioning the spinal cord at T₁.At closed airway the downward intrathoracic pressure swing due to phrenic stimulation is seen to shorten the expiration; vagal influences are responsible for most of this facilitatory effect which to some extent is present also after vagotomy. The facilitatory vagal influence is tentatively identified with some receptors of the lungs and of the cardiovascular apparatus, known to have an excitatory influence on the respiratory center.The extravagal influence present either at open or closed airway might derive from various receptors of the rib cage.The vagal drive appears to play a major role in the control of the respiratory cycle.This research was supported by the Italian National Research Council (C. N. R.).     

Responses of ventral respiratory neurones in the rat to vagus stimulation and the functional division of expiration.

In anaesthetized rats, extracellular and intracellular recordings were taken from 106 respiratory neurones in the intermediate region of the nucleus ambiguus. We observed unprovoked shortening of expiratory time accompanied, in all classes of respiratory neurone, by the elimination of the changes in membrane potential that were characteristic of stage II expiration. The demonstration of the elimination of stage II expiration in both the rat and cat strongly supports the functional division of expiration into stage I expiration (post-inspiration) and stage II expiration. In order to identify the neurones in the rat that receive inputs from vagal afferents and modulate the central respiratory rhythm, we examined whether any respiratory neurones responded to stimulation of the vagus nerve. Some post-inspiratory and stage II expiratory neurones responded. The short latency (< 2 ms) of four of the responses indicates that some vagal afferents act on post-inspiratory neurones via a disynaptic pathway. While repetitive stimulation of the vagus nerve could inhibit the phrenic rhythm, it appears that most inspiratory neurones in the intermediate region of the nucleus ambiguous complex are not directly involved in integrating the information from vagal afferents with the central respiratory rhythm.     

Modulation of the Hering-Breuer reflex by raphe pallidus in rabbits

Activation of the pulmonary stretch receptors by lung inflation or vagal stimulation evokes Hering-Breuer (HB) reflex, which is characterized by inspiratory inhibition and expiratory prolongation. In this work, whether the HB reflex could be modulated by the serotonergic raphe pallidus (RP) was studied by comparing the strength of this reflex before and after electrical or chemical stimulation of the RP. Experiments were performed on urethane anesthetized adult rabbits. The HB reflex was simulated with electrical stimulation of the central end of cervical vagus nerve. The RP was stimulated electrically or chemically by microinjection of glutamate. We found that after either electrical stimulation or chemical stimulation of the RP, the inspiratory inhibition and expiratory prolongation of the HB reflex were significantly attenuated. This attenuation showed post-stimulation time dependency or short-term memory, as well as RP stimulation intensity dependency. Results of the present study suggested that the serotonergic RP could exert its respiratory effects by modulating the strength of HB reflex.     

Respiratory effects of stimulation of cell bodies of the A5 region in the anaesthetised rat

To assess the importance of the pontine A5 region in modulating respiratory activity, electric current or microinjections of glutamate (10-30 nl, 1-3 nmol) were used to stimulate discrete zones within this region in the spontaneously breathing, anaesthetised rat. These stimuli evoked an expiratory facilitatory response, consisting of a decrease in respiratory rate (P < 0.01 electrical, P < 0.001 chemical) due to an increase of expiratory time (P < 0.01 in both cases) as measured from recordings of phrenic nerve activity. No changes were observed in inspiratory time. To avoid changes in PCO2, which could modulate the respiratory response, stimulation was also made during artificial ventilation. Under these conditions the expiratory facilitatory response elicited by glutamate was still present (P < 0.05), although its duration was reduced (P < 0.05), as was the magnitude of the phrenic burst (P < 0.05). At all the sites at which electrical stimulation and glutamate injection had evoked a respiratory response, electrical stimulation evoked a concomitant increase in both blood pressure and heart rate. Glutamate injection evoked a pressor response in 21 out of 30 animals. In eight animals the rise in blood pressure was followed by a fall in blood pressure and in one animal, a depressor response was observed. In all cases glutamate evoked an increase in heart rate. The expiratory facilitatory response was not evoked as a consequence of the increase of blood pressure since it was still present after the administration of guanethidine, which abolished the blood pressure changes. As glutamate is believed to excite perikarya rather than axons of passage these data indicate that expiratory facilitatory responses and the accompanying cardiovascular changes are the consequence of activating neurones located within the A5 region. The possible interactions between the A5 region and the medullary respiratory complex in eliciting these changes are discussed.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of a midsagittal lesion of the rabbit medulla. II. Vagal modulation of respiratory activity

Vagal reflexes were studied in 12 anaesthetized rabbits after a midsagittal lesion of the medulla oblongata. The rabbits were either breathing spontaneously or were ventilated by a phrenic nerve-controlled servorespirator without the use of muscle relaxants. After splitting the medulla, asynchronous respiratory activity of the left and right phrenic nerves was frequently seen. The vagal reflexes evoked by lung inflation and deflation, cigarette smoke and respiratory loading persisted after the lesion. After unilateral vagotomy, all reflexes were transmitted ipsilaterally to the applied stimulus. On the contralateral side, only reinforcement of inspiration was observed when both phrenic nerves fired in phase. In comparison with the nonlesioned rabbits, vagal input in the split-brainstem preparation has a stronger excitatory effect on inspiratory generation. The existence of self-excitatory neuronal connections in spinal motoneuronal pools is also suggested.     

Electrical stimulation of arterial and central chemosensory afferents at different times in the respiratory cycle of the cat: II. Responses of respiratory muscles and their motor nerves

The response patterns of the electrical activity of the respiratory motor nerves and muscles to brief electrical stimulation of the arterial and the intracranial chemosensory afferents were studied in anesthetized cats. Stimulation during inspiration increased the activity of phrenic nerve and the inspiratory muscles (intercostal, diaphragm) with a latency of 15–25 ms, whereas expiratory muscle activity in the following expiration remained almost unaltered. Stimulation during expiration increased the activity of expiratory nerves and muscles (intercostal, abdominal) after a delay of 80–120 ms. The later the stimulation occurred in the insor expiratory period the larger the increase in amplitude and in steepness of rise of the respective integrated activity in respiratory nerves and muscles. Stimulation in early inspiration shortened the discharge period of inspiratory muscles, whereas excitation in early expiration caused an earlier onset and prolonged the activity in the expiratory muscles. Stimulation in the late phase of ins- or expiration prolonged the discharge of the respective nerves and muscles. Both the arterial (carotid sinus nerve, CSN, and aortic nerve, AN) and intracranial chemosensory (VM) afferents stimuli were able to affect both the inspiratory and the expiratory mechanisms. The restriction of the effects to the phase of the stimulus suggests a mechanism by which these afferents, when activated during inspiration, effectively project only to inspiratory neurones, and vice versa for expiration.Supported by the Deutsche Forschungsgemeinschaft, SFB 114 Bionach     

Positive feedback facilitation of external intercostal and phrenic inspiratory activity by pulmonary stretch receptors

Both in lightly pentobarbitone anesthetized and decerebrate cats increments in lung volume (V) during inspiration caused facilitation of inspiratory activity both in phrenic (Phr) and external intercostal (EI) motoneurons. This effect had low volume threshold, well below eupnoeic tidai volumes. It was readily reduced or abolished by small additional doses of pentobarbitone. This facilitatory effect appeared with considerably greater magnitude in El than in Phr. The response magnitude was linearly related to the corresponding increments in V? but not to increments in airflow (V?). Sustained elevation of V at zero V caused sustained facilitation of EI and Phr. This positive feedback facilitation which was similarly obtained in spontaneously breathing and paralysed cats occurred continuously with great regularity in every breath. It was abolished by bilateral vagotomy but could then be elicited by electrical stimulation of the central end of the vagus nerve at the same threshold strengths required to elicit a just detectable shortening of inspiratory duration. The results indicate that the slowly adapting pulmonary stretch receptors are responsible for this positive feedback facilitation prior to the negative feedback effect on the inspiratory ‘off-switch’ elicited by the same receptors. Clear distinctions are described between the reflex characteristics of this ‘low-threshold’ volume dependent facilitatory reflex and the ‘high-threshold’ transient excitatory reflex effects provoked by large and rapid inflations.     

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

Summary The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 m thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 A, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.     

Characterization of hindlimb muscle afferents involved in ventilatory effects observed in decerebrate and spinal preparations

Summary Neurogenic changes of phrenic activity have previously been observed during periodic passive motions of one hindlimb in decorticate, unanaesthetized and curarized rabbit preparations before and after high spinal transection (Palisses et al. 1988). In decerebrate and spinal preparations, we aimed to determine, through rhythmic electrical stimulation of hindlimb muscle nerves, which muscle afferents are involved in these effects. In decerebrate preparations, these electrical stimulations (trains of shocks at 80 Hz for 300 ms every second for 20 s) produced ventilatory effects when group I+II afferent fibres of either flexor or extensor nerves were stimulated together and more powerful changes as soon as group III fibres were recruited. Stimulation of group I fibres alone induced no such effects. When present, these changes in respiratory activity consisted of a maintained decrease of the respiratory period due to both inspiratory and expiratory time shortening; in addition, the amplitude of the phrenic bursts greatly increased at the onset of electrical stimulation. After spinal transection at C2 level and pharmacological activation by nialamide and DOPA, only short-lasting phrenic bursts developed spontaneously; the electrical stimulation of group II and mainly group III flexor afferent fibres induced large amplitude phrenic activity whereas the stimulation of the same extensor afferents was relatively ineffective. The activation of phrenic motoneurones during group III flexor afferent stimulation was closely linked to each 300 ms period of stimulation. While the phrenic effects obtained in the spinal preparations by natural and by electrical periodic stimulation are quite similar to each other, those produced in decerebrate preparations differ substantially. It is concluded that the regulation of phrenic activity in decerebrate and spinal rabbit preparations by hindlimb proprioceptive afferents involves different muscle receptors; perhaps joint proprioceptors for the medullary resetting and muscle receptors connected to group III afferent fibres for the spinal reflex activation of phrenic motoneurones.     

Effects of prestimulus respiratory levels on inhibitory respiratory response by nociceptive muscular afferents

We have previously shown that the inhibitory respiratory response, which we call post-stimulus suppression, is induced by nociceptive muscular afferents. This phenomenon is thought to be caused by a negative feedback induced by excessive afferent inputs. In the present study, we investigated whether augmented levels of prestimulus respiration would influence the magnitude of poststimulus suppression by recording the phrenic nerve discharges in chloralose-urethane anesthetized, vagotomized, paralyzed and artificially ventilated cats. The respiratory level was augmented by means of either hypercapnia, hypoxia or naloxone administration, all of which markedly facilitated the peak amplitude (PK) of integrated phrenic discharges, neural tidal volume. When the electrical stimulation of thin-fiber muscular afferents was performed at these augmented PK levels, the magnitude of poststimulus suppression in the PK was markedly attenuated without consistently altering the facilitatory response during the stimulation period. It seems that the facilitatory component of the augmented level of resting respiration may reduce the inhibitory component of poststimulus suppression. The results indicate that prestimulus respiratory activity is an important factor in determining the magnitude of poststimulus suppression.     

A μ-opioid receptor agonist DAMGO induces rapid breathing in the arterially perfused in situ preparation of rat

Ventilatory responses to opioids are complex and not yet fully understood. We evaluated concentration-dependent effects of a selective μ-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) on respiratory output in the arterially perfused in situ rat preparation, which preserves the integrity of the ponto-medullary respiratory network. DAMGO (300-3400 nM) was added accumulatively to the perfusate. DAMGO increased inspiratory time and diminished central vagal post-inspiratory activity. At 300-500 nM DAMGO caused rapid breathing with shortening of expiratory time. The change of breathing pattern occurred within a single breath. Bilateral vagotomy did not affect the change in respiratory pattern, suggesting that it was of central origin. Additional DAMGO up to 1800 nM did not affect the rapid breathing pattern, and further elevated concentrations (up to 3400 nM) caused inconsistent results. Since the rapid breathing pattern was associated with the obliteration of vagal post-inspiratory activity, we conclude that DAMGO reconfigures the respiratory output to an inspiratory phase-dominant, rapidly alternating inspiration-expiration pattern.     

Synaptic interactions between respiratory neurons during inspiratory on-switching evoked by vagal stimulation in decerebrate cats

To elucidate neuronal mechanisms underlying phase-switching from expiration to inspiration, or inspiratory on-switching (IonS), postsynaptic potentials (PSPs) of bulbar respiratory neurons together with phrenic nerve discharges were recorded during IonS evoked by vagal stimulation in decerebrate and vagotomized cats. A single shock stimulation of the vagus nerve applied at late-expiration developed an inspiratory discharge in the phrenic neurogram after a latency of 79+/-11 ms (n = 11). Preceding this evoked inspiratory discharge, a triphasic response was induced, consisting of an early silence (phase 1 silence), a transient burst discharge (phase 2 discharge) and a late pause (phase 3 pause). During phase 1 silence, IPSPs occurred in augmenting inspiratory (aug-I) and expiratory (E2) neurons, and EPSPs in postinspiratory (PI) neurons. During phase 2 discharge, EPSPs arose in aug-I neurons and IPSPs in PI and E2 neurons. These initial biphasic PSPs were comparable with those during inspiratory off-switching evoked by the same stimulation applied at late-inspiration. In both on- and off-switching, phase-transition in respiratory neuronal activities started to arise concomitantly with the phrenic phase 3 pause. These results suggest that vagal inputs initially produce a non-specific, biphasic response in bulbar respiratory neurons, which consecutively activates a more specific process connected to IonS.     

Influence of rubrospinal tract and the adjacent mesencephalic reticular formation on the activity of medullary respiratory neurons and the phrenic nerve discharge in the rabbit

Suprapontine brain sites acting on the central respiratory system have been demonstrated to give rise to inspiratory as well as expiratory facilitatory effects. In the present study the inspiratory inhibitory effect which has been reported in the cat to be elicited consistently by electrical stimulation of the rubrospinal tract and the adjacent mesencephalic reticular formation was examined in the urethane-anaesthetized rabbit. Stimulation of these sites with single electrical shocks of moderate intensity induced a short latency (onset after 3.0 ms) transient (duration: 29 ms) inhibition of the phrenic nerve activity (PHR). Short volleys of stimuli applied in mid- to late-inspiration led to a premature off-switch of inspiration. The extracellularly recorded discharge activity of the different types of medullary respiration-related units (RRU) reflected these alterations, accordingly. Axonal connections of RRU with mesencephalic structures were evaluated. Examination of orthodromic responses of medullary RRU to stimulation of this pathway revealed that most bulbospinal inspiratory neurons (10 out of 13) were paucisynaptically inhibited after short latency (at least 1.2 ms). The conduction time from bulbospinal inspiratory neurons to the recording site of PHR was 1.6 ms. Thus, a disynaptic pathway — including bulbospinal inspiratory neurons — is suggested inducing inspiratory inhibition 3.0 ms after single shock midbrain stimulation. This inhibition results in disfacilitation of phrenic motoneurons. The fact that extensive electrolytic lesions of the pneumotaxic center in rostral pons did not abolish the observed inspiratory inhibitions excludes these structures from being involved. A direct pathway from the red nucleus and the adjacent reticular formation to phrenic nuclei of the spinal cord, however, can not be excluded from being involved in the demonstrated inspiratory inhibition. The described effects may play a role in behavioral or voluntary control of respiration.     

Respiratory depression caused by either morphine microinjection or repetitive electrical stimulation in the region of the nucleus parabrachialis of cats

In chloralose-urethane anesthetized, vagotomized, paralyzed and artificially ventilated cats, respiratory response to either repetitive electrical stimulation or micro-injection of morphine in the rostral pons was studied by recording the phrenic nerve discharges. In the region of the nucleus parabrachialis (PBN) and its ventral reticular formation, electrical stimulation delivered in 20 successive expiratory periods caused the respiratory depression to last long after the termination of stimulation. This respiratory-depressant effect could be reversed by naloxone. By a single electrical stimulation delivered in most of these effective sites, a phasic phrenic excitation was consistently elicited in the period of both expiration and inspiration, and the reduction in expiratory duration could be observed when the stimulation was delivered in expiratory period. In the microinjection study of 2.66 nmol morphine in 0.1 l in the localized area of the dorsolateral portion of the PBN, a significant reduction in both respiratory outputs and the rate of increase in inspiratory activity could be induced within 1 min after the application. The respiratory depression thus caused by both methods was quite similar in several respiratory variables. Thus an involvement of the PBN region in long-lasting respiratory modulation mediated by endogenous opioid system is suggested.     

Roles of vagal afferents on discharge patterns and CO2-responsiveness of efferent superior laryngeal, hypoglossal, and phrenic respiratory activities in anesthetized rats

Discharge patterns and CO2-responsiveness of the efferent respiratory activities in the superior laryngeal (Xsl), hypoglossal (XII), and phrenic (Phr) nerves were compared between vagi-intact and -denervated rats. Bilateral cervical vagotomy decreased the respiratory frequency (f), minute Phr activity (peak integrated Phr activity X f) and consequently elevated end-tidal PCO2 (PETCO2). Augmentation of the peak inspiratory activity following vagotomy was much larger in the Xsl and XII nerves than that in the Phr nerve. After vagotomy, the time delay from the onset of inspiratory activities in the Xsl or XII nerve to the onset of the Phr bursts was greatly prolonged. While in the vagi-intact rats the peak inspiratory activities of these cranial nerves were not increased in response to elevated PETCO2, the activities, in particular of the XII nerve, were augmented by high PETCO2 in the absence of vagal afferents. These results suggested that the vagal afferents inhibit not only phasic Phr discharges but also inspiratory output neurons in the Xsl or XII motor nuclei being activated in normo- and hypercapnia and that they facilitate temporally the onset and progress of inspiratory activities in various groups of respiratory output neurons.     

Inspiratory on-switch evoked by stimulation of the mesencephalon: Activity of phrenic and laryngeal motoneurones

Summary In anaesthetized cats (chloralose-urethan) the effects of brief tetanic electrical stimulation (50 to 100 ms) of the mesencephalic central gray matter and reticular formation on the inspiratory on-switch were studied during the expiratory (E) phase on the gross and unitary activities of phrenic, laryngeal inspiratory and laryngeal expiratory nerves. On the inspiratory laryngeal and phrenic nerves, stimulation elicited a short latency gross response concomitant with the train: the inspiratory Primary Response (Prim.R.) which is followed by an inspiratory Patterned Response (Patt.R.) of longer duration which corresponded to the inspiratory on-switch. The Patt.R. generally appeared from the Prim.R. within a latent period (Silent Phase: Sil.P.) as long as 100 ms. On the expiratory laryngeal nerve, stimulation elicited a brief activation (expiratory Prim.R.) concomitant with the beginning of the inspiratory laryngeal Prim.R. and which rapidly stopped as the latter continued during the stimulus train. The inspiratory Prim.R. corresponded to a simultaneous activation of both early and late (so defined during their spontaneous discharge) inspiratory motoneurones. The laryngeal motoneurones were more strongly activated than the phrenic ones. During the inspiratory Patt.R. all the phrenic motoneurones presented a recruitment delay earlier, compared with the spontaneous one, whereas the recruitment drastically changed from an inspiratory laryngeal motoneurone to another. Thus, the two pools of motoneurones presented different properties of activation. During the inspiratory Sil.P. no concomitant expiratory laryngeal activation was observed when most of the inspiratory motoneurones were inactive. As some inspiratory laryngeal motoneurones did not stop firing, the existence of some central respiratory neurones exhibiting a similar persistent activity and subserving the inspiratory on-switch mechanisms may be hypothesized.Supported by CNRS (LA 205 and ATP n 4188) and Fondation pour la Recherche Médicale     

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Summary Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the presynaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and offswitch neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of off-switch, postinspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration. The variability of inspiratory duration increased five-fold whereas the variability of expiration was unchanged. We conclude that in the apneustic state, (1) inspiratory on-switching and the successive activation of the different inspiratory neuronal types are preserved; (2) near the end of the inspiratory ramp, the reversible phase of inspiratory off-switching is prolonged, producing the apneustic plateau, and (3) the irreversible phase of offswitching is impaired by a reduced activity of postinspiratory neurons. These results support the 3-phase model of respiratory rhythm generation, in which key roles are played by early-inspiratory and post-inspiratory neurons.     

Facilitation of the human nociceptive reflex by stimulation of Aβ-fibres in a secondary hyperalgesic area sustained by nociceptive input from the primary hyperalgesic area

Hyperalgesia was induced in healthy volunteers by topical capsaicin applied on the dorsum of the foot within the receptive field of the sural nerve. Under presence of hyperalgesia different normally non-noxious conditioning stimuli were applied to the hyperalgesic area and the polysynaptic nociceptive spinal reflex and pain ratings were used to assess central excitability. The nociceptive reflex was measured in the knee extensor and flexor muscles evoked by electrical stimulation of the sural nerve trunk at an intensity of 1.5 times the initial reflex threshold (an intensity above the pain threshold). Thermal stimulation of the primary hyperalgesic area (re)established both on-going spontaneous pain and secondary hyperalgesia. Thus, increased nociceptive reflexes were recorded and increased pain intensity reported when Aβ-fibres in the secondary hyperalgesic area were activated concurrently with the reflex testing after a non-noxious thermal stimulation of the primary hyperalgesic area. The Aβ-fibre activation was achieved by continuous low-intensity electrical stimulation (40 Hz) that was initiated after on-going pain produced by the thermal stimulation had waned. The same measurement without prior thermal conditioning stimulation of the primary area resulted in no reflex facilitation, indicating rapid changes in the central excitability with existence of on-going nociceptive activity. This indicates that the development and maintenance of secondary hyperalgesia are dependent on sustained peripheral nociceptive activity. The study also shows that a central summation of nociceptive and non-nociceptive afferent activity can occur once secondary hyperalgesia is present.     

Restoration of respiratory muscle function following spinal cord injury. Review of electrical and magnetic stimulation techniques

Respiratory complications are a leading cause of morbidity and mortality in patients with spinal cord injury. Several techniques, currently available or in development, have the capacity to restore respiratory muscle function allowing these patients to live more normal lives and hopefully reduce the incidence of respiratory complications. Bilateral phrenic nerve pacing, a clinically accepted technique to restore inspiratory muscle function, allows patients with ventilator dependent tetraplegia complete freedom from mechanical ventilation. Compared to mechanical ventilation, phrenic nerve pacing provides patients with increased mobility, improved speech, improved comfort level and reduction in health care costs. The results of clinical trials of laparoscopically placed intramuscular diaphragm electrodes suggest that diaphragm pacing can also be achieved without the need for a thoracotomy and associated long hospital stay, and without manipulation of the phrenic nerve which carries a risk of phrenic nerve injury. Other clinical trials are being performed to restore inspiratory intercostal function. In patients with only unilateral phrenic nerve function who are not candidates for phrenic nerve pacing, combined intercostal and unilateral diaphragm pacing appears to provide benefits similar to that of bilateral diaphragm pacing. Clinical trials are also underway to restore expiratory muscle function. Magnetic stimulation, surface stimulation and spinal cord stimulation of the expiratory muscles are promising techniques to restore an effective cough mechanism in this patient population. These techniques hold promise to reduce the incidence of respiratory tract infections, atelectasis and respiratory failure in patients with spinal cord injury and reduce the morbidity and mortality associated with these complications.