Influence of lung volume on activities of branches of the recurrent laryngeal nerve

To investigate the influence of inspiratory lung inflation on the respiratory activities of laryngeal motor nerves, vagally intact decerebrate paralyzed cats were ventilated by a servorespirator in accordance with their own phrenic nerve activity. Records were made of the activities of the phrenic nerve, the superior laryngeal nerve (SLN), the recurrent laryngeal nerve (RLN), and the intralaryngeal branches of the RLN serving the thyroarytenoid (TA) and posterior cricoarytenoid (PCA) muscles. Neural activities were assessed in the steady state at different end-tidal O2 and CO2 concentrations. Transient responses to withholding inspiratory lung inflation and to preventing expiratory lung emptying were also studied. Hypercapnia and hypoxia increased the inspiratory activities of the phrenic nerve, SLN, RLN, and its PCA branch. TA inspiratory activity was not changed. Expiratory activities of RLN, PCA, and TA were all increased in hypoxia. When lung inflation was withheld, neural inspiratory duration and the inspiratory activities of all nerves increased. The subsequent period of neural expiration was marked by an exaggerated burst of activity by the TA branch of the RLN. TA expiratory activity was also sharply increased after inspiratory efforts that were reflexly delayed by the prevention of lung emptying. TA activity in expiration was enhanced after vagotomy and was usually more prominent than when lung inflation was withheld before vagal section. The results demonstrate the importance and complexity of the influence of vagal afferents on laryngeal motor activity.     

Capsaicin administration inhibits the abducent branch but excites the thyroarytenoid branch of the recurrent laryngeal nerves in the rat.

Our recent study showed that both inspiratory and expiratory activities of the recurrent laryngeal nerve (RLN) were enhanced by capsaicin administration in the rat (Lu IJ, Ku LC, Lin JT, Lee KZ, and Hwang JC. Chin J Physiol 45: 143-154, 2002). There are two intralaryngeal branches of the RLN: one innervates the thyroarytenoid (TA) muscle and the other innervates the abductor (Abd) muscles. To examine whether these two intralaryngeal branches respond similarly to capsaicin administration, their discharges as well as activities of the phrenic nerve (PNA) and the superior laryngeal nerve (SLNA) were monitored in anesthetized and ventilated rats at normocapnia in hyperoxia. The low dose of capsaicin (0.625 microg/kg) produced a cardiopulmonary chemoreflex, showing apnea, a decrease in PNA, hypotension, and bradycardia, and significant decreases in SLNA and the activity of the Abd branch. Concurrently, there was an increase in the intralaryngeal TA activity during both apnea and the recovery from apnea. The high dose of capsaicin (1.25 microg/kg) evoked larger chemoreflexive responses and laryngeal nerve activities. In addition, both doses of capsaicin initiated a similar delay in the onset of Abd activity and SLNA but an earlier onset for the TA branch to commence during inspiration. A bilateral vagotomy abolished the laryngeal responses to capsaicin administration. However, PNA and blood pressure were enhanced with capsaicin administration after the vagotomy. These results suggest that laryngeal adduction in response to capsaicin administration is vagal afferent dependent and that it may also represent reflexive protection for the airway and lungs.     

Blood pressure changes alter tracheobronchial cough: computational model of the respiratory-cough network and in vivo experiments in anesthetized cats

We tested the hypothesis, motivated in part by a coordinated computational cough network model, that alterations of mean systemic arterial blood pressure (BP) influence the excitability and motor pattern of cough. Model simulations predicted suppression of coughing by stimulation of arterial baroreceptors. In vivo experiments were conducted on anesthetized spontaneously breathing cats. Cough was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) of inspiratory parasternal, expiratory abdominal, laryngeal posterior cricoarytenoid (PCA), and thyroarytenoid muscles along with esophageal pressure (EP) and BP were recorded. Transiently elevated BP significantly reduced cough number, cough-related inspiratory, and expiratory amplitudes of EP, peak parasternal and abdominal EMG, and maximum of PCA EMG during the expulsive phase of cough, and prolonged the cough inspiratory and expiratory phases as well as cough cycle duration compared with control coughs. Latencies from the beginning of stimulation to the onset of cough-related diaphragm and abdominal activities were increased. Increases in BP also elicited bradycardia and isocapnic bradypnea. Reductions in BP increased cough number; elevated inspiratory EP amplitude and parasternal, abdominal, and inspiratory PCA EMG amplitudes; decreased total cough cycle duration; shortened the durations of the cough expiratory phase and cough-related abdominal discharge; and shortened cough latency compared with control coughs. Reduced BP also produced tachycardia, tachypnea, and hypocapnic hyperventilation. These effects of BP on coughing likely originate from interactions between barosensitive and respiratory brainstem neuronal networks, particularly by modulation of respiratory neurons within multiple respiration/cough-related brainstem areas by baroreceptor input.     

Respiratory neuronal activity during apnea and other breathing patterns induced by laryngeal stimulation

Respiration cycles through three distinct phases (inspiration, postinspiration, and expiration) each having corresponding medullary cells that are excited during one phase and inhibited during the other two. Laryngeal stimulation is known to induce apnea in newborn animals, but the cellular mechanisms underlying this effect are not known. Intracellular recording of ventral respiratory group neurons was accomplished in intact anesthetized, paralyzed, and mechanically ventilated piglets. Apnea was induced by insufflation of the larynx with ammonia-saturated air, smoke, or water. Laryngeal insufflation induced phrenic nerve apnea, stimulation of postinspiratory neurons, and stable membrane potentials in inspiratory and expiratory cells consistent with postinspiratory inhibition. Usually the membrane potential of each neuronal type cycled through an expiratory level before onset of the first recovery breath. Variants of the apnea response, probably reflecting the aspiration reflex or sniffing, sneezing, coughing, and swallowing, were also observed. These latter patterns showed oscillation between inspiration and postinspiration without an apparent intervening stage II expiratory phase. However, stage II expiratory activity always preceded onset of the first ramp inspiration after such a pattern. These findings suggest that activation of postinspiratory mechanisms causes profound alterations in the respiratory pattern and that stage II expiration importantly modulates recovery of ramp inspiratory activity. The mechanism of this latter effect may be inhibition of early inspiratory neurons with consequent postinhibitory rebound.     

Posterior cricoarytenoid activity in normal adults during involuntary and voluntary hyperventilation

The effect of isocapnic hypoxia and hyperoxic hypercapnia on the electrical activity of the posterior cricoarytenoid (PCA) muscle was determined in eight normal adult humans by use of standard rebreathing techniques and was compared with PCA activity during voluntary hyperventilation performed under isocapnic and hypocapnic conditions. PCA activity was recorded with intramuscular hooked-wire electrodes implanted through a fiberoptic nasopharyngoscope. During quiet breathing in all subjects, the PCA was phasically active on inspiration and tonically active throughout the respiratory cycle. At comparable increments in respiratory output, hypercapnia, hypoxia, and voluntary hyperventilation appeared to be associated with similar increases in phasic or tonic PCA activity. During quiet breathing, the onset of phasic PCA activity usually occurred before inspiratory airflow and extended beyond the start of expiratory airflow. The duration of phasic PCA preactivation and postinspiratory phasic PCA activity remained unchanged during progressive hypercapnia and progressive hypoxia. The results, in combination with recent findings for vocal cord adductors, suggest that vocal cord position throughout the respiratory cycle during hyperpnea is actively controlled by simultaneously acting and antagonistic intrinsic laryngeal muscles.     

Differential effects of clonidine on upper airway abductor and adductor muscle activity in awake goats.

The purpose of this study was to determine the extent to which alpha(2)-adrenoceptor (alpha(2)-AR) pathways affect the central motor output to upper airway muscles that regulate airflow. Electromyogram (EMG) measurements were made from posterior cricoarytenoid (PCA), cricothyroid (CT), thyroarytenoid (TA), and middle (MPC) and inferior (IPC) pharyngeal constrictor muscles in awake standing goats. Systemic administration of the alpha(2)-AR agonist clonidine induced a highly dysrhythmic pattern of ventilation in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns, including prolonged and variable expiratory time intervals. Periods of apnea were commonly observed. Dysrhythmic ventilatory patterns induced by clonidine were associated with differential recruitment of upper airway muscles. alpha(2)-AR stimulation preferentially decreased the activity of the PCA, CT, and IPC muscles while increasing TA and MPC EMG activities. Clonidine-induced apneas were associated with continuous tonic activation of laryngeal (TA) and pharyngeal (MPC) adductors, leading to airway closure and arterial oxygen desaturation. Tonic activation of the TA and MPC muscles was interrupted only during the first inspiratory efforts after central apnea. Laryngeal abductor, diaphragm, and transversus abdominis EMG activities were completely silenced during apneic events. Ventilatory and EMG effects were reversed by selective alpha(2)-AR blockade with SKF-86466. The results demonstrate that alpha(2)-AR pathways are important modulators of central respiratory motor outputs to the upper airway muscles.     

Respiratory activities of intralaryngeal branches of the recurrent laryngeal nerve

To distinguish experimentally between motor nerve activity destined for vocal cord abductor muscles and that bound for muscles that adduct the cords, we recorded efferent activities of intralaryngeal branches of the recurrent laryngeal nerve (RLN) in decerebrate, vagotomized, paralyzed, ventilated cats. Activities of the whole RLN and phrenic nerve were also recorded. Nerve activities were assessed at several steady-state end-tidal O2 and CO2 concentrations. The nerve to the thyroarytenoid (TA) muscle, a vocal cord adductor, was only slightly active under base-line (normocapnic, hyperoxic) conditions but in most cats developed strong activity during expiration in hypocapnia or hypoxia. In severe hypocapnia, phasic expiratory TA activity persisted even during phrenic apnea, indicating continuing activity of the respiratory rhythm generator. The nerve to the posterior cricoarytenoid (PCA) muscle, the vocal cord abductor, was always active in inspiration but often showed expiratory activity as well. This expiratory activity was usually enhanced by hypercapnia and often inhibited by hypoxia. The results are consistent with previous electromyographic findings and emphasize the importance of distinguishing abductor from adductor activity in studies of laryngeal control.     

Preliminary results on the splanchnico-laryngeal reflex in the cat]

Electrical stimulation of low threshold splanchnic afferent nerves in lightly anesthetized cat results in phrenic and recurrent laryngeal nerve responses. Both phrenic and recurrent laryngeal inspiratory nerve activities are inhibited, whereas expiratory recurrent nerve activity is triggered and even increased. The significance of this reflex is discussed in relation to laryngeal adductor muscle contractions and the abdominal pressure increase.     

Respiratory activity of posterior cricoarytenoid muscle and vocal cords in humans

We examined the respiratory activity of the posterior cricoarytenoid muscle (PCA) simultaneously with the movements of the vocal cords during tidal breathing and panting in four normal seated subjects. A bipolar electrode was constructed to record the surface electromyogram (EMG) of the PCA. The glottis was visualized with a fiberoptic bronchoscope, and the glottic image was recorded simultaneously with tidal volume and a digital time marker on video tape. During quiet breathing the integrated EMG signal (EPCA) showed consistent phasic variations in each subject. The inspiratory onset of EPCA in the four subjects preceded inspiratory flow by 170 +/- 80, 650 +/- 310, 130 +/- 80, and 130 +/- 90 ms (mean +/- SD), respectively. This lead time of the PCA was similar to that between the onset of glottic widening and inspiration in each subject. The proportion of each cycle during which EPCA increased (the duty cycle) was 31 +/- 3% (mean +/- SE), whereas the inspiratory portion of the respiratory cycle constituted 37 +/- 2% (mean +/- SE), respectively. The duty cycle of the PCA remained relatively constant in the same subject on different days. During panting at functional residual capacity, the EPCA increased to 142 +/- 11% of the peak activity recorded during the preceding control breaths. This was accompanied by a sustained increase in the glottic width to 91 +/- 9% of the peak value in the preceding breaths. These results confirm the role of the PCA as a principal abductor of the vocal cords and indicate a temporal relationship between PCA activation and the inspiratory phase of the respiratory cycle during tidal breathing in humans.     

Upper airway negative-pressure effects on respiratory activity of upper airway muscles

Influence of upper airway negative-pressure change on the respiratory activity of various upper airway muscles was investigated in 13 anesthetized rabbits. Phasic inspiratory activity increased or appeared during virtually all negative-pressure trials in nasolabial, cricothyroid, and posterior cricoarytenoid muscles. No phasic inspiratory activity was seen in the sternothyroid (ST) and sternohyoid (SH) muscles before negative-pressure applications but appeared during 80% of trials in ST and 62% of trials in SH. During maintained negative pressure, a gradual decline in activity was often observed in the nasolabial and laryngeal muscles, whereas a rapid decline in activity was seen in the cervical strap muscles. Reflex effects of negative pressure was markedly reduced or abolished by sectioning the internal branch of the superior laryngeal nerve bilaterally. Reflex augmentation of upper airway muscle activity reported here may have functional significance in the maintenance of upper airway patency. It could prevent upper airway collapse when negative pressure swings in the upper airway increase or facilitate recovery when large negative pressure swings are produced by obstructed inspiratory efforts.     

Laryngeal influences on breathing pattern and posterior cricoarytenoid muscle activity

Receptors responding to transmural pressure, airflow, and contraction of laryngeal muscles have been previously identified in the larynx. To assess the relative contribution of these three types of receptors to the reflex changes in breathing pattern and upper airway patency, we studied diaphragmatic (DIA) and posterior cricoarytenoid muscle (PCA) activity in anesthetized dogs during spontaneous breathing and occluded efforts with and without bypassing the larynx. Inspiratory duration (TI) was longer, mean inspiratory slope (peak DIA/TI) was lower, and PCA activity was greater with upper airway occlusion than with tracheal occlusion (larynx bypassed). Bilateral section of the superior laryngeal nerves eliminated these differences. When respiratory airflow was diverted from the tracheostomy to the upper airway the only change attributable to laryngeal afferents was an increase in PCA activity. These results confirm the importance of the superior laryngeal nerves in the regulation of breathing pattern and upper airway patency and suggest a prevalent role for laryngeal negative pressure receptors.     

Association of Anemia with Reduced Central Respiratory Drive in the Piglet

Central respiratory drive was studied in 13 piglets of both sexes varying in age from 19 to 67 days. The distal trachea was cannulated and the maximum rate of isometric inspiratory pressure change (dP/dt)_max, was measured at the airway. Curves were constructed relating this measurement to changes in arterial PCO₂ during carbon dioxide rebreathing. Data were obtained at intervals corresponding to stepwise reductions in central respiratory drive produced by added chloralose anaesthesia. Laryngeal reflex activation was achieved by electrical stimulation of the superior laryngeal nerves (SLN). This caused permanent respiratory arrest at a critical level of central respiratory depression expressed as the slope of the curve relating (dP/dt)_max to arterial PCO₂. Severely anemic piglets showed markedly decreased central respiratory drive at a given dose of anesthesia compared to controls. This was consistent with the observed greater sensitivity to laryngeal nerve stimulation in these animals. It is concluded that anemia may be associated with impaired functional maturation of central respiratory mechanisms and consequent susceptibility to laryngeal reflex apnea and asphyxial death. These observations may pertain to factors associated with the sudden infant death syndrome.     

Recovery of breathing pattern after 15 min of cerebral ischemia in rabbits

The study was undertaken to ascertain the neural control of breathing and vagal reflexes during and after cerebral ischemia. The experiments were performed on anesthetized, paralyzed, and artificially ventilated rabbits. Cerebral ischemia was induced by reversible intrathoracic occlusion of the brachiocephalic trunk and the left subclavian and both internal thoracic arteries for 15 min. The effect of cerebral ischemia on breathing pattern was assessed by monitoring the integrated activities of phrenic and recurrent laryngeal nerves. Ischemia produced enhancement of breathing followed by apnea and gasping. During enhanced breathing as well as during gasping, the inspiratory-inhibiting effect of lung inflation (Breuer-Hering reflex) was abolished. When brain circulation was restored, respiratory activity started with gasps, which later were intermingled with eupneic type of inspirations. During the onset of a eupneic breath, lung inflation produced inspiratory facilitation but never an inhibition. However, after 30 min of recovery from cerebral ischemia, the Breuer-Hering reflex was restored. Results show that precise analysis of vagal reflexes and respiratory pattern during ischemia and resuscitation may be used as an indicator of resumption of autonomic activity in the brain stem.     

Thyroarytenoid muscle activity during loaded and nonloaded breathing in adult humans

Previous fiber-optic studies in humans have demonstrated narrowing of the glottic aperture in expiration during application of expiratory resistive loads. Nine healthy subjects were studied to determine the effect of expiratory resistive loads on the electromyographic activity of the thyroarytenoid (TA) muscle, a vocal cord adductor. Four of the nine subjects also underwent the application of inspiratory resistive loads and voluntary prolongation of either inspiratory (TI) or expiratory (TE) time. TA activity was recorded by intramuscular hooked-wire electrodes. During quiet breathing in all subjects, the TA was phasically active on expiration and often tonically active throughout the respiratory cycle. TA expiratory activity progressively increased with increasing levels of expiratory load. Inspiratory loads resulted in increased TA "inspiratory" activity. Voluntary prolongation of TE to times similar to those reached during loaded breathing induced increases in TA expiratory activity similar to those reached during the loaded state. Voluntary prolongation of TI was associated with an increase in TA inspiratory activity. Similar increases in TI during inspiratory loading or voluntary conditions were associated with comparable increases in TA inspiratory activity in three of the four subjects. In conclusion, increased activation of TA during the application of expiratory resistive loads implies that the reported narrowing of glottic aperture during expiratory loading is an active phenomenon. Changes in activation of the TA with resistive loads appear to be related to changes in respiratory pattern.     

Medullary neurons mediating the inhibition of inspiration by intercostal muscle tendon organs?

Studies were conducted to test the hypothesis that nonrespiratory-modulated units are last-order interneurons mediating the effects of intercostal muscle tendon organs on medullary inspiratory neuron activity. Vagotomized, anesthetized, or decerebrate cats were used. Results show the following. 1) Afferents from different receptor types (i.e., intercostal tendon organs and chest wall cutaneous receptors) that inhibit medullary inspiratory neuron activities evoke the same units. 2) Gastrocnemius muscle group I afferent fibers evoke some of the same units as intercostal afferents but do not alter respiratory activity. 3) The "pneumotaxic center" and laryngeal nerve afferents, which inhibit medullary inspiratory activity, evoke different medullary units than intercostal afferents. 4) Evoked units are not active in spontaneously breathing cats. Additional results suggest that a few respiratory neurons near the retrofacial nucleus may be involved in the mediation of the inspiratory inhibitory effects of intercostal tendon organs. These results do not establish the mechanism by which intercostal muscle tendon organs reduces medullary inspiratory activity.     

Posterior cricoarytenoid and diaphragm activities during tidal breathing in neonates

To investigate airflow regulation in newborn infants, we recorded airflow, volume, diaphragm (Di), and laryngeal electromyogram (EMG) during spontaneous breathing in eight supine unsedated sleeping full-term neonates. Using an esophageal catheter electrode, we recorded phasic respiratory activity consistent with that of the principal laryngeal abductors, the posterior cricoarytenoids (PCA). Sequential activation of PCA and Di preceded inspiration. PCA activity typically peaked early in inspiration followed by either a decrescendo or tonic EMG activity of variable amplitude during expiration. Expiratory airflow retardation, or braking, accompanied by expiratory prolongation and reduced ventilation, was commonly observed. In some subjects we observed a time interval between PCA onset and a sudden increase in expiratory airflow just before inspiration, suggesting that release of the brake involved an abrupt loss of antagonistic adductor activity. Our findings suggest that airflow in newborn infants is controlled throughout the breathing cycle by the coordinated action of the Di and the reciprocal action of PCA and laryngeal adductor activities. We conclude that braking mechanisms in infants interact with vagal reflex mechanisms that modulate respiratory cycle timing to influence both the dynamic maintenance of end-expiratory lung volume and ventilation.     

Uncoupling of upper airway motor activity from phrenic bursting by positive end-expired pressure in the rat.

Phasic bursting in the hypoglossal nerve can be uncoupled from phrenic bursting by application of positive end-expired pressure (PEEP). We wished to determine whether similar uncoupling can also be induced in other respiratory-modulated upper airway (UAW) motor outputs. Discharge of the facial, hypoglossal, superior laryngeal, recurrent laryngeal, and phrenic nerves was recorded in anesthetized, ventilated rats during stepwise changes in PEEP with a normocapnic, hyperoxic background. Application of 3- to 6-cmH(2)O PEEP caused the onset inspiratory (I) UAW nerve bursting to precede the phrenic burst but did not uncouple bursting. In contrast, application of 9- to 12-cmH(2)O PEEP uncoupled UAW neurograms such that rhythmic bursting occurred during periods of phrenic quiescence. Single-fiber recording experiments were conducted to determine whether a specific population of UAW motoneurons is recruited during uncoupled bursting. The data indicate that expiratory-inspiratory (EI) motoneurons remained active, while I motoneurons did not fire during uncoupled UAW bursting. Finally, we examined the relationship between motoneuron discharge rate and PEEP during coupled UAW and phrenic bursting. EI discharge rate was linearly related to PEEP during preinspiration, but showed no relationship to PEEP during inspiration. Our results demonstrate that multiple UAW motor outputs can be uncoupled from phrenic bursting, and this response is associated with bursting of EI nerve fibers. The relationship between PEEP and EI motoneuron discharge rate differs during preinspiratory and I periods; this may indicate that bursting during these phases of the respiratory cycle is controlled by distinct neuronal outputs.     

Involvement of bulbar respiratory and non-respiratory neurons in onset of expiration reflex

It was shown that excitation of high- and low-threshold superior laryngeal afferents triggers reflexes of varying complexity in a considerable proportion of non-respiratory neurons during experiments on cats anesthetized by Nembutal involving stimulation-induced expiration reflex. Systemic alterations in background firing activity were noted during this reflex in "respiratory" neurons; reflex reaction setting in as a result of low-threshold laryngeal afferent activation was also recorded in 22.4% of this group. Oligo- and polysynaptic excitatory connections were found between low-threshold laryngeal afferents and inspiratory beta neurons, P-cells, and laryngeal muscle motoneurons as opposed to inhibitory connections with inspiratory gammaneurons. This article discusses involvement of the neurons investigated in mechanisms of inspiratory inhibition, closure of the vocal chords, and adaptive decline in breathing rate occurring during expiration reflex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 5, pp. 670–680, September–October, 1990.     

Mu-opioid receptor agonist effects on medullary respiratory neurons in the cat: evidence for involvement in certain types of ventilatory disturbances

Mu-opioid receptor agonists depress tidal volume, decrease chest wall compliance, and increase upper airway resistance. In this study, potential neuronal sites and mechanisms responsible for the disturbances were investigated, dose-response relationships were established, and it was determined whether general anesthesia plays a role. Effects of micro-opioid agonists on membrane properties and discharges of respiratory bulbospinal, vagal, and propriobulbar neurons and phrenic nerve activity were measured in pentobarbital-anesthetized and unanesthetized decerebrate cats. In all types of respiratory neurons tested, threshold intravenous doses of the micro-opioid agonist fentanyl slowed discharge frequency and prolonged duration without altering peak discharge intensity. Larger doses postsynaptically depressed discharges of inspiratory bulbospinal and inspiratory propriobulbar neurons that might account for depression of tidal volume. Iontophoresis of the micro-opioid agonist DAMGO also depressed the intensity of inspiratory bulbospinal neuron discharges. Fentanyl given intravenously prolonged discharges leading to tonic firing of bulbospinal expiratory neurons in association with reduced hyperpolarizing synaptic drive potentials, perhaps explaining decreased inspiratory phase chest wall compliance. Lowest effective doses of fentanyl had similar effects on vagal postinspiratory (laryngeal adductor) motoneurons, whereas in vagal laryngeal abductor and pharyngeal constrictor motoneurons, depression of depolarizing synaptic drive potentials led to sparse, very-low-frequency discharges. Such effects on three types of vagal motoneurons might explain tonic vocal fold closure and pharyngeal obstruction of airflow. Measurements of membrane potential and input resistance suggest the effects on bulbospinal Aug-E neurons and vagal motoneurons are mediated presynaptically. Opioid effects on the respiratory neurons were similar in anesthetized and decerebrate preparations.     

Rapid and transient excitation of respiration mediated by central chemoreceptor

We evaluated rapid and transient changes in phrenic nerve (PN) and internal intercostal (IIC) activities when 0.2-0.5 ml of saline saturated with 100% CO2 was injected into the vertebral artery during various respiratory phases in decerebrated spontaneously breathing cats. The injections evoked an initial transient inhibition of ongoing PN or IIC activity with a mean onset latency of 0.17 s, followed by excitation of subsequent respiratory activities with an onset latency ranging from 0.4 to 2.7 s; the average onset latency of expiratory excitation (1.49 s) was significantly longer than that of inspiratory facilitation (0.89 s). The initial inhibitory responses were analogous to reflex effects of injections of phenyl biguanide, indicating that the initial inhibition was due to activation of vascular nociceptors and the subsequent excitation was due to stimulation of the central chemoreceptors. In addition, CO2-saline injections during hypocapnic apnea developed a quick reappearance of respiratory rhythm, and the first facilitatory effect appeared in tonic IIC activity, which became more active before rhythm started. In summary, the present study, by use of a technique of vertebral arterial injections of 100% CO2-saline, revealed dynamic properties of respiratory control system mediated by central chemoreceptors and vascular nociceptors.