Simulations of a ventrolateral medullary neural network for respiratory rhythmogenesis inferred from spike train cross-correlation

Connections among ventrolateral medullary respiratory neurons inferred from spike train analysis were incorporated into a model and simulated with the program SYSTM11 (MacGregor 1987). Inspiratory (I) and expiratory (E) neurons with augmenting (AUG) and decrementing (DEC) discharge patterns and rostral I-E/I neurons exhibited varying degrees of adaptation, but no endogenous bursting properties. Simulation parameters were adjusted so that respiratory phase durations, neuronal discharge patterns, and short-time scale correlations were similar to corresponding measurements from anesthetized, vagotomized, adult cats. Rhythmogenesis persisted when the strength of each set of connections was increased 100% over a smaller effective value. Changes in phase durations and discharge patterns caused by manipulation of connection strengths or population activity led to several predictions. (a) Excitation of the I-E/I population prolongs the inspiratory phase. (b) Rhythmic activity can be reestablished in the absence of I-E/I activity by unpatterned excitation of I-DEC and I-AUG neurons. (c) An increase in I-DEC neuron activity can cause an apneustic respiratory pattern. (d) A decrease in I-DEC neuron activity increases the slope of the inspiratory ramp and shortens inspiration. (e) Excitation of the E-DEC population prolongs the expiratory phase or produces apnea; inhibition of E-DEC neurons reduces expiratory time. (f) Excitation of E-AUG cells causes I-AUG neurons to exhibit a step rather than a ramp increase in firing rate at the onset of their active phase. The results suggest mechanisms by which the duration of each phase of breathing and neuronal discharge patterns may be regulated. Received: 24 February 1993/Accepted in revised form: 8 September 1993     

Medullary lateral tegmental field neurons influence the timing and pattern of phrenic nerve activity in cats.

In an effort to characterize the role of the medullary lateral tegmental field (LTF) in regulating respiration, we tested the effects of selective blockade of excitatory (EAA) and inhibitory amino acid (IAA) receptors in this region on phrenic nerve activity (PNA) of vagus-intact and vagotomized cats anesthetized with dial-urethane. We found distinct patterns of changes in central respiratory rate, duration of inspiratory and expiratory phases of PNA (Ti and Te, respectively), and I-burst amplitude after selective blockade of EAA and IAA receptors in the LTF. First, blockade of N-methyl-D-aspartate (NMDA) receptors significantly (P < 0.05) decreased central respiratory rate primarily by increasing Ti but did not alter I-burst amplitude. Second, blockade of non-NMDA receptors significantly reduced I-burst amplitude without affecting central respiratory rate. Third, blockade of GABAA receptors significantly decreased central respiratory rate by increasing Te and significantly reduced I-burst amplitude. Fourth, blockade of glycine receptors significantly decreased central respiratory rate by causing proportional increases in Ti and Te and significantly reduced I-burst amplitude. These changes in PNA were markedly different from those produced by blockade of EAA or IAA receptors in the pre-B?tzinger complex. We propose that a proper balance of excitatory and inhibitory inputs to several functionally distinct pools of LTF neurons is essential for maintaining the normal pattern of PNA in anesthetized cats.     

Species dependence of baroreceptor effects on ventilation in the cat and the dog

Reflex respiratory responses to brief carotid baroreceptor stimuli in vagotomized pentobarbital-anesthetized cats were characterized and compared with those reported previously for chloralose-anesthetized dogs. To eliminate effects due to the anesthetic choice, dogs were reexamined under pentobarbital. Stimuli were applied to the isolated carotid sinus (CS) of both animals within a single respiratory phase. The stimuli were either steps triggered after one of four delays (5, 25, 50, and 75% of the control phase duration) and terminated at the end of the phase or pulses lasting 300-500 ms. In cats, 80-mmHg steps during inspiration shortened inspiratory duration by 23.2*, 25.0*, 20.4*, and 4.1% (*P less than 0.01) at the above four delays, respectively; inspired volume decreased by 21.4*, 18.0*, 8.0*, and 2.2%. Steps during expiration lengthened expiration by 38.4*, 37.1*, 21.9*, and 3.4%; expired volume changed less than 4%. Qualitatively, similar responses were obtained with steps 40 mmHg in amplitude. In dogs, 40-mmHg stimuli lengthened both inspiration (by 12.8*, 8.9*, -1.2, and -2.5%) and expiration (by 75.2*, 57.9*, 54.0*, and 61.4*%) but tidal volume did not change. Similar differences were observed when pulses were used. Selective baroreceptor denervation in the cat and occipital arterial occlusion in the dog confirmed that the responses were not chemoreceptor mediated. We conclude that although CS baroreceptor activity inhibits ventilation in both cats and dogs, the pattern of the responses is strongly species dependent.     

Characterization of respiratory-modulated activities of hypoglossal motoneurons

In decerebrate, vagotomized, paralyzed, and ventilated cats, activities of the phrenic nerve and single hypoglossal nerve fibers were monitored. The great majority of hypoglossal neuronal activities were inspiratory (I), discharging during a period approximating that of phrenic. Many were not active at normocapnia but were recruited in hypercapnia or hypoxia. Once recruited, discharge frequencies, which rose quickly to near maximal levels in early to midinspiration, significantly increased with further augmentations of drive. Also, the onset of activities became progressively earlier, compared with phrenic discharge, in hypercapnia or hypoxia. Smaller numbers of hypoglossal fiber activities, having inspiratory-expiratory (I-E), expiratory (E), expiratory-inspiratory (E-I), or tonic discharge patterns, were also recorded. Activities of E, I-E, and those I fibers that became I-E in high drive may underlie the early burst of expiratory activity of the hypoglossal nerve. It is concluded that the firing and recruitment patterns of hypoglossal neurons differ from those of phrenic motoneurons. However, responses to chemoreceptor stimuli are similar among the two neuronal groups.     

Medullary raphe neuron activity is altered during fictive cough in the decerebrate cat.

Chemical lesions in the medullary raphe nuclei region influence cough. This study examined whether firing patterns of caudal medullary midline neurons were altered during cough. Extracellular neuron activity was recorded with microelectrode arrays in decerebrated, neuromuscular-blocked, ventilated cats. Cough-like motor patterns (fictive cough) in phrenic and lumbar nerves were elicited by mechanical stimulation of the intrathoracic trachea. Discharge patterns of respiratory and nonrespiratory-modulated neurons were altered during cough cycles (58/133); 45 increased and 13 decreased activity. Fourteen cells changed firing rate during the inspiratory and/or expiratory phases of cough. Altered patterns in 43 cells were associated with the duration of, or extended beyond, the cough episodes. The different response categories suggest that multiple factors influence the discharge patterns during coughing: e.g., respiratory-modulated and tonic inputs and intrinsic connections. These results suggest involvement of midline neurons (i.e., raphe nuclei) in the cough reflex.     

正常猫膈神经单纤维电活动特征

在麻醉猫和麻痹的切断迷走神经的清醒猫,观察了膈神经单纤维电活动特征。1.电活动类型:按膈神经单纤维放电与其总干放电的相位关系分为三种类型。(1)完全同步型,即单纤维放电与总干放电同时开始并同时停止,占76.9％。(2)部分同步型占15.4％,其中早期同步,即单纤维放电与总干放电同时开始,但提前终止,占1.9％,中期同步,即单纤维放电较总干放电开始晚,又提前终止,占5.8％,晚期同步,即单纤维放电较总干放电开始晚,但两者同时终止,占7.7％。(8)非同步型,即吸气相和呼气相都有放电,但呼气相时冲动频率较低,占7.7％。前两型为单纯的吸气性放电,共占92.3％。2.单纤维放电平均参数值:麻醉猫每次吸气发放11个冲动,其频率为21次/秒,清醒猫每次吸气发放18个冲动,其频率为34次/秒。结果表明:猫膈神经单纤维放电类型和文献上报导的直接记录膈神经运动神经元放电一致,即以单纯的吸气性放电为最多。     

Effects of almitrine on genioglossal and diaphragmatic electromyograms

We previously demonstrated dose-dependent increases in both hypoglossal and phrenic electroneurograms after almitrine in anesthetized, paralyzed, and vagotomized cats. We have now investigated the effect of this peripheral chemoreceptor stimulant on diaphragmatic and genioglossal (GG, an upper airway-maintaining muscle) electromyograms in five unanesthetized, chronically instrumented, spontaneously breathing adult cats during slow-wave sleep. In 12 studies almitrine doses of 1.0-6.0 mg/kg increased inspired minute ventilation (VI), frequency (f), and tidal volume (VT) and decreased expiratory time (TE). However, almitrine doses as high as 6.0 mg/kg failed to augment phasic inspiratory GG activity. To determine why almitrine induced phasic inspiratory upper airway activity in anesthetized, vagotomized cats but not in sleeping cats, additional studies were performed. In four dose-response studies in three pentobarbital-anesthetized cats, almitrine, 1.0-6.0 mg/kg, did not produce phasic inspiratory GG activity. Almitrine did induce phasic inspiratory GG activity in two of three studies in three vagotomized, tracheostomized, alpha-chloralose-urethan-anesthetized cats. These results suggest that almitrine would not be useful in obstructive sleep apnea, yet because almitrine markedly increased VI, f, and VT and decreased TE in unanesthetized sleeping cats the drug may be effective in patients who lack normal central neural respiratory drive, such as the preterm infant.     

Synchronization of respiratory frequency by somatic afferent stimulation.

In cats anesthetized with chloralose-urethan, vagotomized, paralyzed, and artifically ventilated, superficial radial (cutaneous) and hamstring (muscle) nerve afferents were stimulated while phrenic nerve electrical activity was recorded. The results obtained with both types of nerves were similar. Stimulation in mid and late expiration advanced the onset of the next inspiration, shortening its duration. Stimulation in early inspiration advanced, while that in late inspiration delayed, the onset of the next expiration. These effects were often accompanied by changes in phrenic motoneuron firing patterns (earlier recruitment, increased discharge frequency, increased slope of integrated phrenic neurogram). Repetitive somatic afferent stimulation produced sustained increases in respiratory frequency in all cats and in half of them entrainment of respiratory frequency to the frequency of stimulation occurred at ratios such as 4:3, 4:5, 1:2, 1:3, 1:4, and 1:7. The lowest stimulus intensity required for evoking these phase shifts was between 5 and 10T (threshold of most excitable fibers) for muscle afferents and between 1 and 2T for cutaneous afferents. These results demonstrate the existence of a reflex mechanism capable of locking respiratory frequency to that of a periodic somatic afferent input. They also provide an experimental basis for the hypothesis that reflexes are resposible for the observed locking between step or pedal frequency and respiratory rate during exercise in man.     

Pharmacological and electrophysiological analysis of nicotine induced apnoea in the cat

Intravenous nicotine (20-60 micrograms/kg) produced an initial brief apnoea followed by hyperventilation in anaesthetized cats. The apneic response to nicotine remained uneffected by atropine, by phentolamine or propranolol. Hexamethonium and guanethedine sulphate antagonized the apneic response. In bilateral vagotomized cats, nicotine failed to produce respiratory apnoea. Veratridine and phenyldiguanide produced apnoea similar to that produced by nicotine within 2-3 sec. administered intraartrially. Nicotine failed to stimulate pulmonary stretch receptors as did veratridine in artificially ventilated cats. The alpha and gamma motoneurone activity of inspiratory and expiratory muscles and the phrenic efferent activity were inhibited during apnoea. These inhibitions were absent in vagotomized cats. In conclusion, these results suggest that the nicotine induced apneic response is mediated through pulmonary vagal afferents, probably through J-receptors, which in turn inhibit the motoneurone activity involving the respiratory muscles.     

Effects on respiratory pattern of focal cooling in the medulla of the dog

Studies in cats have shown that, in addition to respiratory neuron groups in the dorsomedial (DRG) and ventrolateral (VRG) medulla, neural structures in the most ventral medullary regions are important for the maintenance of respiratory rhythm. The purpose of this study was to determine whether a similar superficially located ventral region was present in the dog and to assess the role of each of the other regions in the canine medulla important in the control of breathing, in 20 anesthetized, vagotomized, and artificially ventilated dogs, a cryoprobe was used to cool selected regions of the medulla to 15-20 degrees C. Respiratory output was determined from phrenic nerve or diaphragm electrical activity. Cooling in or near the nucleus of the solitary tract altered timing and produced little change in the amplitude or rate of rise of inspiratory activity; lengthening of inspiratory time was the most common timing effect observed. Cooling in ventrolateral regions affected the amplitude and rate of rise of respiratory activity. Depression of neural tidal volume and apnea could be produced by unilateral cooling in two ventrolateral regions: 1) near the nucleus ambiguus and nucleus para-ambiguus and 2) just beneath the ventral medullary surface. These findings indicate that in the dog dorsomedial neural structures influence respiratory timing, whereas more ventral structures are important to respiratory drive.     

Intracellular recordings of respiratory neurons in the lateral medulla of piglets

Membrane potentials of respiratory neurons in the ventral respiratory group were recorded using intracellular techniques in the medulla of newborn piglets. Three types of neurons were demonstrated: inspiratory neurons with an augmenting pattern of spike activity during inspiration; postinspiratory neurons with a short decrementing firing pattern that started immediately after inspiration ended; and stage II expiratory neurons with an augmenting spiking pattern that began shortly after inspiratory termination and ended before onset of the next inspiration. When not firing, the membrane potential trajectories of each cell type revealed two complementary patterns of relative inhibition. This latter finding suggests arrival of inhibitory synaptic potentials during these periods. These findings suggest that the respiratory control mechanisms of the newborn piglet are organized in a three-phased manner similar to that of adult cats.     

Central inspiratory influence on abdominal expiratory nerve activity

Our purpose was to determine whether the intensity of abdominal expiratory nerve discharge is conditioned by the intensity of the preceding inspiratory phrenic discharge, independent of mechanical and chemical afferent influences. In decerebrate, paralyzed, vagotomized cats with bilateral pneumothoraxes, we recorded phrenic and abdominal (cranial iliohypogastric nerve, L1) nerve activities at hyperoxic normocapnia. We reduced the duration and intensity (i.e., integrated peak height) of phrenic nerve discharge for single cycles by stimulating the cut central end of the superior laryngeal nerve (SLN) during the central inspiratory phase (75 microA, 20-50 Hz, 0.2-ms pulse). Premature termination of inspiration consistently reduced expiratory duration (TE) and abdominal expiratory nerve activity (area of integrated neurogram), but the average reduction in TE was much less than the reduction in abdominal nerve activity (14 vs. 51%). Stimulation of the cut central end of the vagus nerve yielded similar results, as did spontaneous premature terminations of inspiration, which we observed in one cat. SLN stimulation during hyperoxic hypercapnia resulted in more variable responses, and higher stimulation frequencies were usually required to abort inspiration. SLN (or vagal) stimulation during expiration consistently increased abdominal expiratory nerve activity. We speculate that this facilitatory response is gated during inspiration, thereby allowing the inspiratory conditioning effect on the subsequent expiration to be expressed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Opiate slowing of feline respiratory rhythm and effects on putative medullary phase-regulating neurons

Opiates have effects on respiratory neurons that depress tidal volume and air exchange, reduce chest wall compliance, and slow rhythm. The most dose-sensitive opioid effect is slowing of the respiratory rhythm through mechanisms that have not been thoroughly investigated. An in vivo dose-response analysis was performed on medullary respiratory neurons of adult cats to investigate two untested hypotheses related to mechanisms of opioid-mediated rhythm slowing: 1) Opiates suppress intrinsic conductances that limit discharge duration in medullary inspiratory and expiratory neurons, and 2) opiates delay the onset and lengthen the duration of discharges postsynaptically in phase-regulating postinspiratory and late-inspiratory neurons. In anesthetized and unanesthetized decerebrate cats, a threshold dose (3 microg/kg) of the mu-opioid receptor agonist fentanyl slowed respiratory rhythm by prolonging discharges of inspiratory and expiratory bulbospinal neurons. Additional doses (2-4 microg/kg) of fentanyl also lengthened the interburst silent periods in each type of neuron and delayed the rate of membrane depolarization to firing threshold without altering synaptic drive potential amplitude, input resistance, peak action potential frequency, action potential shape, or afterhyperpolarization. Fentanyl also prolonged discharges of postinspiratory and late-inspiratory neurons in doses that slowed the rhythm of inspiratory and expiratory neurons without altering peak membrane depolarization and hyperpolarization, input resistance, or action potential properties. The temporal changes evoked in the tested neurons can explain the slowing of network respiratory rhythm, but the lack of significant, direct opioid-mediated membrane effects suggests that actions emanating from other types of upstream bulbar respiratory neurons account for rhythm slowing.     

面神经核腹内侧区微量注射三种递质的呼吸效应

实验用兔,在乌拉坦静脉麻醉、切断双侧颈迷走神经、自主呼吸条件下进行,以膈神经放电作呼吸指标。观察了面神经核腹内侧区(VMNF)微量注射三种递质对呼吸节律的影响。结果如下:(1)VMNF 区微量注射肾上腺素呼吸频率增加,膈神经吸气性放电的递增速度加快,积分幅度升高,VMNF 区微量注射妥拉苏林,呼吸频率下降且妥拉苏林可阻断肾上腺素的呼吸效应。(2)VMNF 区微量注射γ-氨基丁酸、甘氨酸导致呼吸频率下降,吸气时程、呼气时程延长。提示肾上腺素、γ-氨基丁酸、甘氨酸可能作为递质作用于 VMNF 区的神经元而发挥呼吸调节作用。     

Phase resetting of the respiratory cycle before and after unilateral pontine lesion in cat.

The pontine respiratory group (PRG) facilitates the mechanism for terminating the inspiratory phase but may influence other phases in the respiratory cycle as well. We determined the effects of PRG lesions on the response of the respiratory cycle to superior laryngeal nerve stimulation delivered in each phase of the cycle in decerebrate, vagotomized, paralyzed, and ventilated cats (n = 6). We measured the duration of inspiration (TI) and expiration (TE) for three breaths before and in the perturbed breath and TI for three breaths after the perturbation. The delay to next inspiration was plotted against the phase at which the stimulus was delivered. Before lesioning, premature inspiratory termination was followed by phase-dependent shortening of TE. After lesioning, premature inspiratory termination did not systematically change the following TE. Breath-by-breath variability (measured 50 breaths) increased and stimulus after-effects (prolonged TI in the subsequent cycle) were augmented following lesions. These data indicate that the PRG plays an important role in the control of TE after perturbation and in the stability of the respiratory central pattern generator.     

Inspiratory neuron activity in the ventrolateral medulla of the dog

The purpose of this study was to describe the distribution and activity pattern of respiratory neurons located in the ventrolateral medulla (VLM) of the dog. Spike activity of 129 respiratory neurons was recorded in 23 ketamine-anesthetized spontaneously breathing dogs. Pontamine blue dye was used to mark the location of each neuron. Most VLM neurons displaying respiratory related spike patterns were located in a column related closely to ambigual and retroambigual nuclei. Both inspiratory and expiratory neurons were present with inspiratory units being grouped more rostrally. The predominant inspiratory neuron firing pattern was "late" inspiratory, although eight "early" types were located. All expiratory firing patterns were the late expiratory variety. Each neuron burst pattern was characterized by determining burst duration (BD), spikes per burst (S/B), peak frequency (PF), time to peak frequency (TPF), rate of rise to peak frequency (PF/TPF), and mean frequency. CO2-induced minute ventilation increases were associated with decreases in BD and TPF and increases in PF, S/B, and PF/TPF. In 11 experiments the relative influences of vagotomy and tracheal occlusion on late inspiratory units were compared. Tracheal occlusion increased late inspiratory BD and S/B but did not alter PF/TPF. Vagotomy increased BD and S/B beyond those obtained by tracheal occlusion and, in some neurons, decreased the PF/TPF. We conclude that the location of respiratory units in the VLM of the dog is similar to that in other species, the discharge pattern of VLM respiratory units is similar to those in cat VLM, and vagotomy and tracheal occlusion affect discharge patterns differently.     

家兔单侧、双侧B(o)tzinger复合体内微量注射甘氨酸及士的宁对膈神经放电的影响

实验在３４只氨基甲酸乙酯麻醉,断双侧瞳神经,肌松,人工通气的家兔上进行。单侧和双侧Ｂｏｔｚｉｎｇｅｒ复合体内微量注射抑制性神经递质甘氨酸及其受体阻断剂 宁,观察膈神经放电的变化。     

Central adaptation to inspiratory-inhibiting expiratory-prolonging vagal input

We reported earlier on the changes in excitability of central respiratory switching mechanisms in the course of a brief inspiratory-inhibiting vagal stimulus (J. Appl. Physiol. 50: 1183-1192, 1981). To further define the dynamics of central processing of such input we studied the changes in the excitability of timing mechanisms in the immediate (less than 1.0 s) and late (1-20 s) periods after stimulus removal. We also examined the changes in respiratory timing in the course of protracted (greater than 20 s) stimulation. Studies were done using pentobarbital-anesthetized cats. For studies involving long-term stimulation or late off responses, cats were paralyzed, vagotomized, carotid denervated, and artificially ventilated. We found that the inspiratory inhibitory influence of a brief stimulus continues, in a declining fashion, for 0.3-10 s after removal of the stimulus. This was followed by a paradoxical response, inspirations were prolonged and expirations were shortened, which was maximal 1-2 s after stimulus removal and which declined gradually over a period of 6-16 s. There was progressive decline in inspiratory-shortening expiratory-prolonging influence in the course of sustained stimuli. These results indicate substantial adaptation in the course of even brief stimuli and provide an explanation for inspiratory-expiratory duration and expiratory-inspiratory duration linkages.     

兔孤束核腹外侧区微量注射谷氨酸钠及甘氨酸对膈神经放电的影响

实验在４０只麻醉、制动、断双侧颈迷走神经和人工通气的家兔上进行。在孤束核腹外侧区微量注射神经元胞体兴奋剂谷氨酸钠和抑制剂甘氨酸,探讨膈神经放电的变化。结果：微量注射谷氨酸钠,可使膈神经放电脉冲数明显增加,吸气时程延长,呼气时程缩短,呼吸频率变化不明显;微量注射甘氨酸,则膈神经放电脉冲数显著减少,甚至停止,吸气时程缩短,呼气时程不规则延长,呼吸频率降低。上述结果提示：孤束核腹外侧区对呼吸节律的形成具     

Some effects of superior laryngeal nerve stimulation on sympathetic preganglionic neuron firing

Repetitive electrical stimulation of afferent fibers in the superior laryngeal nerve (SLN) evoked depressant or excitatory effects on sympathetic preganglionic neurons of the cervical trunk in Nembutal-anesthetized, paralyzed, artifically ventilated cats. The depressant effect, which consisted of suppression of the inspiration-synchronous discharge of units with such firing pattern, was obtained at low strength and frequency of stimulation (e.g. 600 mV, 30 Hz) and was absent at end-tidal CO2 values below threshold for phrenic nerve activity. The excitatory effect required higher intensity and frequency of stimulation and was CO2 independent. The depressant effect on sympathetic preganglionic neurons with inspiratory firing pattern seemed a replica of the inspiration-inhibitory effect observed on phrenic motoneurons. Hence, it could be attributed to the known inhibition by the SLN of central inspiratory activity, if it is assumed that this is a common driver for phrenic motoneurons and some sympathetic preganglionic neurons. The excitatory effect, on the other hand, appears to be due to connections of SLN afferents with sympathetic preganglionic neurons, independent of the respiratory center.