Inspiratory Units Research Articles

Motor unit territories in human genioglossus estimated with multichannel intramuscular electrodes.

The discharge patterns of genioglossus motor units during breathing have been well-characterized in previous studies, but their localization and territories are not known. In this study, we used two newly developed intramuscular multichannel electrodes to estimate the territories of genioglossus motor units in the anterior and posterior regions of the muscle. Seven healthy men participated. Each electrode contained fifteen bipolar channels, separated by 1 mm, and was inserted percutaneously below the chin, perpendicular to the skin, to a depth of 36 mm. Single motor unit activity was recorded with subjects awake, supine, and breathing quietly through a nasal mask for 180 s. Motor unit territories were estimated from the spike-triggered averages of the electromyographic signal from each channel. A total of 30 motor units were identified: 22 expiratory tonic, 1 expiratory phasic, 2 tonic, 3 inspiratory tonic, and 2 inspiratory phasic. Motor units appeared to be clustered based on unit type, with peak activities for expiratory units predominantly located in the anterior and superficial fibers of genioglossus and inspiratory units in the posterior region. Of these motor unit types, expiratory tonic units had the largest estimated territory, a mean 11.3 mm (SD 1.9). Estimated territories of inspiratory motor units ranged from 3 to 6 mm. In accordance with the distribution of motor unit types, the estimated territory of genioglossus motor units varied along the sagittal plane, decreasing from anterior to posterior. Our findings suggest that genioglossus motor units have large territories relative to the cross-sectional size of the muscle. NEW & NOTEWORTHY In this study, we used a new multichannel intramuscular electrode to address a fundamental property of human genioglossus motor units. We describe the territory of genioglossus motor units in the anterior and posterior regions of the muscle and show a decrease in territory size from anterior to posterior and that expiratory-related motor units have larger estimated territories than inspiratory-related motor units.

A mechanism for upper airway stability during slow wave sleep.

The severity of obstructive sleep apnea is diminished (sometimes markedly) during slow wave sleep (SWS). We sought to understand why SWS stabilizes the upper airway. Increased single motor unit (SMU) activity of the major upper airway dilating muscle (genioglossus) should improve upper airway stability. Therefore, we hypothesized that genioglossus SMUs would increase their activity during SWS in comparison with Stage N2 sleep. The activity of genioglossus SMUs was studied on both sides of the transition between Stage N2 sleep and SWS. Sleep laboratory. Twenty-nine subjects (age 38 ± 13 yr, 17 males) were studied. SWS. Subjects slept overnight with fine-wire electrodes in their genioglossus muscles and with full polysomnographic and end tidal carbon dioxide monitors. Fifteen inspiratory phasic (IP) and 11 inspiratory tonic (IT) units were identified from seven subjects and these units exhibited significantly increased inspiratory discharge frequencies during SWS compared with Stage N2 sleep. The peak discharge frequency of the inspiratory units (IP and IT) was 22.7 ± 4.1 Hz in SWS versus 20.3 ± 4.5 Hz in Stage N2 (P < 0.001). The IP units also fired for a longer duration (expressed as a percentage of inspiratory time) during SWS (104.6 ± 39.5 %TI) versus Stage N2 sleep (82.6 ± 39.5 %TI, P < 0.001). The IT units fired faster during expiration in SWS (14.2 ± 1.8 Hz) versus Stage N2 sleep (12.6 ± 3.1 Hz, P = 0.035). There was minimal recruitment or derecruitment of units between SWS and Stage N2 sleep. Increased genioglossus SMU activity likely makes the airway more stable and resistant to collapse throughout the respiratory cycle during SWS.

Discharge patterns of human tensor palatini motor units during sleep onset.

Upper airway muscles such as genioglossus (GG) and tensor palatini (TP) reduce activity at sleep onset. In GG reduced muscle activity is primarily due to inspiratory modulated motor units becoming silent, suggesting reduced respiratory pattern generator (RPG) output. However, unlike GG, TP shows minimal respiratory modulation and presumably has few inspiratory modulated motor units and minimal input from the RPG. Thus, we investigated the mechanism by which TP reduces activity at sleep onset. The activity of TP motor units were studied during relaxed wakefulness and over the transition from wakefulness to sleep. Sleep laboratory. Nine young (21.4 ± 3.4 years) males were studied on a total of 11 nights. Sleep onset. Two TP EMGs (thin, hooked wire electrodes), and sleep and respiratory measures were recorded. One hundred twenty-one sleep onsets were identified (13.4 ± 7.2/subject), resulting in 128 motor units (14.3 ± 13.0/subject); 29% of units were tonic, 43% inspiratory modulated (inspiratory phasic 18%, inspiratory tonic 25%), and 28% expiratory modulated (expiratory phasic 21%, expiratory tonic 7%). There was a reduction in both expiratory and inspiratory modulated units, but not tonic units, at sleep onset. Reduced TP activity was almost entirely due to de-recruitment. TP showed a similar distribution of motor units as other airway muscles. However, a greater proportion of expiratory modulated motor units were active in TP and these expiratory units, along with inspiratory units, tended to become silent over sleep onset. The data suggest that both expiratory and inspiratory drive components from the RPG are reduced at sleep onset in TP.

Motor Unit Recruitment in Human Genioglossus Muscle in Response to Hypercapnia

single motor unit recordings of the genioglossus (GG) muscle indicate that GG motor units have a variety of discharge patterns, including units that have higher discharge rates during inspiration (inspiratory phasic and inspiratory tonic), or expiration (expiratory phasic and expiratory tonic), or do not modify their rate with respiration (tonic). Previous studies have shown that an increase in GG muscle activity is a consequence of increased activity in inspiratory units. However, there are differences between studies as to whether this increase is primarily due to recruitment of new motor units (motor unit recruitment) or to increased discharge rate of already active units (rate coding). Sleep-wake state studies in humans have suggested the former, while hypercapnia experiments in rats have suggested the latter. In this study, we investigated the effect of hypercapnia on GG motor unit activity in humans during wakefulness. sleep research laboratory. sixteen healthy men. each participant was administered at least 6 trials with P(et)CO(2) being elevated 8.4 (SD = 1.96) mm Hg over 2 min following a 30-s baseline. Subjects were instrumented for GG EMG and respiratory measurements with 4 fine wire electrodes inserted subcutaneously into the muscle. One hundred forty-one motor units were identified during the baseline: 47% were inspiratory modulated, 29% expiratory modulated, and 24% showed no respiratory related modulation. Sixty-two new units were recruited during hypercapnia. The distribution of recruited units was significantly different from the baseline distribution, with 84% being inspiratory modulated (P < 0.001). Neither units active during baseline, nor new units recruited during hypercapnia, increased their discharge rate as P(et)CO(2) increased (P > 0.05 for all comparisons). increased GG muscle activity in humans occurs because of recruitment of previously inactive inspiratory modulated units.

Recruitment and rate-coding strategies of the human genioglossus muscle

Single motor unit (SMU) analysis provides a means to examine the motor control of a muscle. SMUs in the genioglossus show considerable complexity, with several different firing patterns. Two of the primary stimuli that contribute to genioglossal activation are carbon dioxide (CO(2)) and negative pressure, which act through chemoreceptor and mechanoreceptor activation, respectively. We sought to determine how these stimuli affect the behavior of genioglossus SMUs. We quantified genioglossus SMU discharge activity during periods of quiet breathing, elevated CO(2) (facilitation), and continuous positive airway pressure (CPAP) administration (inhibition). CPAP was applied in 2-cmH(2)O increments until 10 cmH(2)O during hypercapnia. Five hundred ninety-one periods (each ∼ 3 breaths) of genioglossus SMU data were recorded using wire electrodes(n = 96 units) from 15 awake, supine subjects. Overall hypercapnic stimulation increased the discharge rate of genioglossus units (20.9 ± 1.0 vs. 22.7 ± 0.9 Hz). Inspiratory units were activated ∼ 13% earlier in the inspiratory cycle, and the units fired for a longer duration (80.6 ± 5.1 vs. 105.3 ± 4.2% inspiratory time; P < 0.05). Compared with baseline, an additional 32% of distinguishable SMUs within the selective electrode recording area were recruited with hypercapnia. CPAP led to progressive SMU inhibition; at ∼ 6 cmH(2)O, there were similar numbers of SMUs active compared with baseline, with peak frequencies of inspiratory units close to baseline, despite elevated CO(2) levels. At 10 cmH(2)O, the number of units was 36% less than baseline. Genioglossus inspiratory phasic SMUs respond to hypercapnic stimulation with changes in recruitment and rate coding. The SMUs respond to CPAP with derecruitment as a homogeneous population, and inspiratory phasic units show slower discharge rates. Understanding upper airway muscle recruitment/derecruitment may yield therapeutic targets for maintenance of pharyngeal patency.

Discharge Patterns of Human Genioglossus Motor Units During Sleep Onset

Multiunit electromyogram recordings of genioglossus have demonstrated an abrupt reduction in the muscle's activity at sleep onset. Recent evidence from single motor unit recordings indicates that the human genioglossus muscle consists of motor units with a variety of discharge patterns. The aim of the present study was to determine the effect of sleep onset on the activity of individual motor units as a function of their particular discharge pattern. Genioglossus activity was assessed using intramuscular fine-wire electrodes via a percutaneous approach. Sleep onsets (alpha-to-theta transitions) were identified and the genioglossus electromyogram recordings analyzed for single motor unit activity. Sleep research laboratory. Sleep and respiratory data were collected in 8 healthy subjects (6 men). One hundred twenty-seven motor units were identified: 23% inspiratory phasic, 45% inspiratory tonic, 4% expiratory phasic, 9% expiratory tonic, 16% tonic, and 3% other. Approximately 50% of inspiratory units (phasic and tonic) ceased activity entirely at sleep onset, whereas those inspiratory units that continued to be active showed a reduction in the proportion of each breath over which they were active. However, the rate of discharge of inspiratory units during the period they did fire was not altered. In contrast, tonic and expiratory units were unaffected by sleep onset, maintaining their discharge pattern over the alpha-to-theta transition. Central control of inspiratory motoneuron output differs from that of tonic and expiratory units during sleep onset, suggesting that the maintenance of airway patency during sleep may become more reliant on the stiffening properties of tonic and expiratory modulated motor units.

Neural drive to human genioglossus in obstructive sleep apnoea

One postulated mechanism for obstructive sleep apnoea (OSA) is insufficient drive to the upper-airway musculature during sleep, with increased (compensatory) drive during wakefulness. This generates more electromyographic activity in upper airway muscles including genioglossus. To understand drives to upper airway muscles, we recorded single motor unit activity from genioglossus in male groups of control (n = 7, 7 +/- 2 events h(-1)) and severe OSA (n = 9, 54 +/- 4 events h(-1)) subjects. One hundred and seventy-eight genioglossus units were recorded using monopolar electrodes. Subjects were awake, supine and breathing through a nasal mask. The distribution of the six types of motor unit activity in genioglossus (Inspiratory Phasic, Inspiratory Tonic, Expiratory Phasic, Expiratory Tonic, Tonic and Tonic Other) was identical in both groups. Single unit action potentials in OSA were larger in area (by 34%, P < 0.05) and longer in duration (by 23%, P < 0.05). Inspiratory units were recruited earlier in OSA than control subjects. In control subjects, Inspiratory Tonic units peaked earlier than Inspiratory Phasic units, while in OSA subjects, Inspiratory Tonic and Phasic units peaked simultaneously. Onset frequencies did not differ between groups, but the peak discharge frequency for Inspiratory Phasic units was higher in OSA (22 +/- 1 Hz) than control subjects (19 +/- 1 Hz, P = 0.003), but conversely, the peak discharge frequency of Inspiratory Tonic units was higher in control subjects (28 +/- 1 Hz versus 25 +/- 1 Hz, P < 0.05). Increased motor unit action potential area indicates that neurogenic changes have occurred in OSA. In addition, the differences in the timing and firing frequency of the inspiratory classes of genioglossus motor units indicate that the output of the hypoglossal nucleus may have changed.

Respiratory-related Discharge of Genioglossus Muscle Motor Units

Little is known about the respiratory-related discharge properties of motor units driving any of the eight muscles that control the movement, shape, and stiffness of the mammalian tongue. To characterize the respiratory-related discharge of genioglossus motor units as synaptic drive to the hypoglossal motoneuron pool is increased with hypercapnia. We recorded airflow, genioglossus muscle EMG activity, and the respiratory-related discharge of 30 genioglossus muscle motor units in spontaneously breathing, urethane-anesthetized rats under control conditions and in hypercapnia (inspired CO2: 3, 6, 9, and 12%, 3-5 min at each level). All motor units were active throughout all or most of inspiration. Nine of 30 units showed "preinspiratory" activity (discharge onset within the last 20% of expiration), with continued discharge into inspiration. Six inspiratory units transitioned to a preinspiratory pattern when inspired CO2 exceeded 6%. For the majority of units (23/30), discharge rate increased with hypercapnia, with the maximum increase averaging about 50%. The average variability of interspike intervals within a spike train increased from 33% under baseline conditions to 50% with maximal hypercapnia. (1) The discharge pattern of genioglossus muscle motor units can be altered by hypercapnia; (2) most, but not all, genioglossus motor units receive synaptic input from CO2-sensitive chemoreceptors; (3) individual motor units have a wide range of CO2 sensitivities; and (4) hypercapnia significantly increases the variability of motor unit discharge, which may enhance muscle force output.

Firing patterns of pre-Bötzinger and Bötzinger neurons during hypocapnia in the adult rat

Controversy exists about how a coordinated respiratory rhythm is generated in the brainstem. Some authors suggest that neurons in the pre-Bötzinger complex are key to initiation of all types of breathing. While, on the other hand, it has been reported that some pre-Bötzinger neurons fail to maintain a rhythmic discharge in phase with phrenic nerve discharge during mechanical hyperventilation. Extracellular recordings were made from respiratory units in the pre-Bötzinger and Bötzinger complexes of 13 anaesthetised, paralysed and vagotomised rats. Central respiratory activity was monitored from the C5 phrenic nerve. During mechanical hyperventilation, several changes were observed in the phrenic neurogram. Firstly, the frequency and amplitude of integrated phrenic nerve discharge were reduced and reversibly stopped. Secondly, the patterned discharges changed from an augmenting to a variety of non-augmenting patterns in 53 of 60 cases. In some cases ( n=9) we observed that the pattern appeared to have two components, an early short duration discharge followed by a longer duration discharge. Respiratory units also started to show different firing patterns during mechanical hyperventilation. In general, they were divided into those units that fired tonically ( n=28) and units that became silent ( n=32), before phrenic nerve discharge ceased coincidently with complete apnoea. Of particular interest were those expiratory–inspiratory units in the pre-Bötzinger complex ( n=8) that narrowed their firing period towards late expiration and early inspiration during mechanical hyperventilation. Given their firing features, it is possible that these expiratory–inspiratory units may participate in generation of the early inspiratory component of phrenic nerve discharge.

The pre-Bötzinger complex and phase-spanning neurons in the adult rat

Recent in vitro studies indicate that neurons in the pre-Botzinger (pre-Bot) complex of neonatal rats play an essential role in respiratory rhythm generation. In the adult rat, however, the location and physiology of pre-Bot neurons is less clearly understood. The present study aims to investigate the firing patterns of neurons that are located between Botzinger and rVRG area, and the precise location of this transition zone in relation to other medullary nuclei. Sprague-Dawley rats (weighted between 400–550 g) were anaesthetised with 72 mg/kg sodium pentobarbitone and 0.4 mg/kg atropine (i.p.), and paralysed with 1 mg/kg pancuronium dibromide (i.v.), followed by additional doses as required. Extracellular recordings were made from 302 respiratory units located between 0 and 1.6 mm caudal to the facial nucleus and ventral to the nucleus ambiguus. As expected, expiratory units were mostly recorded from the rostral medulla (80%, 125/157) and inspiratory units were concentrated in the more caudal area (80%, 36/45). However, we report here that, between the Botzinger expiratory and rVRG inspiratory units, there exists a transition zone containing a mixture of phase-spanning units (41%, 41/100) as well as inspiratory (37%) and expiratory units (22%). The phase-spanning units are active across the expiratory-inspiratory phase, or vice versa. The rostrocaudal extension of this transition zone is about 400 μm, with the caudal end at the level of the caudal pole of the ambigual compact formation. Our preliminary data suggest that this transition zone could be the locus of pre-Bot neurons in the adult rat.

Mechanisms and clinicophysiological implications of the sniff- and gasp-like aspiration reflex

Mechanical stimulation of the pharyngeal mucosa in cats and some other mammals evokes the ‘aspiration reflex’ (AR) characterized by rapid and strong inspiratory efforts not followed by active expirations. It resembles other spasmodic inspiratory acts such as the sniff, the gasp, and the sigh in several aspects, e.g. reflex or semireflex triggering from the upper airways, the sudden onset and termination of such inspirations, the massive recruitment, steep rise and high-peak amplitude of inspiratory unit activity, analogous ventilatory pattern, and contribution to arousal. The similarity of these spasmodic acts is manifested mainly in enhanced speed and volume of inhalation, although of different intensity, which is determined by the varying degree of forced inspiratory activity and a concomitant inhibition of expiratory activity. The extent of the inspiratory dilation of the glottis and the timing and range of late-inspiratory and/or postinspiratory glottal narrowing modulate the depth of aspiration. Thus, the inhalation can be moderate as in sniffing, which provides a transfer of odorants to the olfactory mucosa. In AR the airstream is presumably strong enough to tear off the mechanical particles from the naso- and oropharynx and to convey them into the hypopharynx to allow their subsequent elimination by reflex swallowing or coughing. Prolonged glottal opening allows either the transfer of some additional air to the bronchi by sighing to prevent the development of atelectasis, or redistribution of a larger amount of fresh air into the lungs by gasping to support autoresuscitation. Should aspiration be a common effective component in these spasmodic processes, then the easily elicitable AR could be beneficial as a simple model for studying their properties in health and disease.

Respiratory Rhythm In The Isolated Central Nervous System Of Newborn Opossum

1. Respiratory activity was recorded from spinal ventral roots in the isolated intact central nervous system (CNS) of newborn opossum, Monodelphis domestica. These signals occurred in synchrony with movements of the ribs and the electromyogram (EMG) recorded from the intercostal muscles during inspiration. Rhythmical activity could be recorded for more than 6 h in acute preparations. 2. The rhythm-generating region was shown to be located in the lower brain stem by perfusing different CNS regions with medium containing 20 mmoll-1 Mg2+, which blocks synaptic transmission reversibly in the opossum CNS. The conclusion that respiration was generated by neurones in the lower brain stem was further confirmed by selective ablation of part of the CNS. 3. Recordings were made from 128 neurones in the respiratory region of the lower brain stem with activity related to the respiratory rhythm. They consisted of two inspiratory groups and two expiratory groups. In the groups of inspiratory units, recordings were made from 69 early inspiratory and 38 inspiratory units. In the groups of expiratory units, recordings were made from 17 post-inspiratory and 4 expiratory units. The sites of 22 respiratory neurones were marked in 4-day-old animals by injecting Pontamine Sky Blue. These neurones were distributed from 175 microns anterior to 525 microns posterior to the obex, from 225 to 450 microns lateral to the midline and from 175 to 425 microns deep to the ventral surface of the brain stem. 4. The respiratory rhythm recorded in the isolated CNS was influenced by pH and neurotransmitters. The respiratory rate decreased by about 26% at high pH (7.7) and increased by about 33% at low pH (7.1). Bath application of noradrenaline (30-100 mumol l-1) decreased the respiratory rate and increased the amplitude of the rhythmic bursts significantly. All these effects were reversible. 5. The results presented here indicate that the isolated intact CNS of newborn opossum offers advantages for exploring mechanisms responsible for generating the respiratory rhythm.

P-61 - Effect of morphine on the correlation between inspiratory unit activity in the brain stem and phrenic high-frequency oscillation in rabbits.

P-61 - Effect of morphine on the correlation between inspiratory unit activity in the brain stem and phrenic high-frequency oscillation in rabbits.

Responses of respiratory modulated and tonic units in the retrotrapezoid nucleus to CO 2

We hypothesized that the retrotrapezoid nucleus (RTN) contains both respiratory modulated (RM) and non-respiratory modulated (NRM) neurons which participate in the ventilatory response to increased CO 2. We made extracellular recordings of the activity of 46 single units in the RTN of 9 decerebrate, paralyzed, ventilated cats (5 intact; 4 with carotid body and sinus ablation) under eucapnic (P CO2 = 34.2±3.5 mmHg; mean ± SD) and hypercapnic (P CO2 = 47.4 ± 3.4 mmHg) conditions. To define a RM unit, we used the η 2 statistic which is the ratio of the variance of the unit firing rate within respiratory cycles to that across respiratory cycles. We classified the units of RM ( N=17) if the η 2 values in eucapnia or hypercapnia were ≥0.25 and as NRM ( N=29) if the values were <0.25. Overall, 19/46 units (41%) increased their firing rate with increased CO 2, 5 decreased their firing rate, and 22 had no significant change in firing rate. Of 17 RM units, 8 (47%) increased their mean firing rate with hypercapnia from 7.6±3.9 to 23.2±6.8 spikes/sec. These included 5 inspiratory units, 2 inspiratory units that had an onset of firing in late expiration (Pre-I/I), and 1 expiratory unit. Seven of these also changed their discharge pattern (eucapnic η 2=0.02 to 0.12; hypercapnic η 2=0.34 to 0.79) of 29 NRM units 11 (38%) showed a significant increase in mean firing rate with CO 2 stimulation from 19.8±7.2 to 31.3±82. spikes/sec. The RTN has RM units which change their discharge pattern and firing rate in response to increased CO 2, as do units within the medulla and pons, and it has NRM units which are also responsive to increased CO 2. These data indicate that some neurons of the RTN are involved in the central chemoreceptor response but they provide no direct evidence that chemoreception resides within the RTN.

Upper cervical inspiratory neurons in the rat: an electrophysiological and morphological study.

Upper cervical inspiratory neurons from a distinct neuronal column located near the lateral edge of the intermediate grey matter in the rostral spinal segments. Previous studies conducted in cats have demonstrated synaptic inputs to these neurons from several respiratory-related regions of the medulla, and long descending axonal projections mainly towards the motoneurons supplying the intercostal muscles. The aim of this study was to examine the electrophysiological and morphological properties of this propriospinal system in the rat. Extracellular recordings were made from 127 cervical inspiratory units, mainly in the C1 and C2 segments. Eighty-two percent could be antidromically activated from the C7/C8 border. No evidence of monosynaptic connection was obtained by cross-correlating the activity of some of these units with the discharge of the phrenic nerve. Intracellular recordings were made from seven neurons, three of which were labelled with biotinamide (neurobiotin). Long "survival times" after intracellular injections (up to 23 h) resulted in staining of axons for long distances, at least to the C5 segment. Each of the three labelled axons issued only one short collateral which arborized in the region of the phrenic nucleus. These results demonstrate that upper cervical inspiratory neurons in the rat have features similar to those previously described in the cat, including only a limited projection to the phrenic nucleus. In addition, this study provides the first morphological identification of these neurons.

Increased correlation between bulbar inspiratory unit activity and phrenic high-frequency oscillation by fominoben, a respiratory analeptic

Increased correlation between bulbar inspiratory unit activity and phrenic high-frequency oscillation by fominoben, a respiratory analeptic

P-245 - Effect of fominoben and its metabolite, PB 88, on the correlation between inspiratory unit activity and phrenic high-frequency oscillation in the brain stem of rabbits.

P-245 - Effect of fominoben and its metabolite, PB 88, on the correlation between inspiratory unit activity and phrenic high-frequency oscillation in the brain stem of rabbits.

Correlation between inspiratory unit activity in the brain stem and phrenic high-frequency oscillation in rabbits

Correlation between inspiratory unit activity in the brain stem and phrenic high-frequency oscillation in rabbits

Respiratory interneurons in the C 5 segment of the spinal cord of the cat

Extracellular recordings were made in the C 5 segment of the spinal cord of anaesthetised cats from 129 units which showed respiratory phased discharge. The majority of recordings (88%) were thought to arise from the somata of respiratory spinal interneurons. Inspiratory units and expiratory units comprised 42% and 52% of all recorded units. A small number of postinspiratory units were also found ( n = 5). Most units did not respond to electrical stimulation of the ipsilateral superior laryngeal (SLN) and phrenic nerves (PN), but a few expiratory ( n = 2) and postinspiratory units ( n = 1) were excited by SLN stimulation, while 6 inspiratory units had their discharge suppresed by the same stimulus. PN stimulation evoked a long latency (2–7 ms) burst of firing in 2 inspiratory and 1 expiratory interneurons. It is concluded that these respiratory interneurons may provide a segmental input to phrenic motoneurons, in addition to synaptic drives mediated by bulbospinal pathways.

Effect of thiamylal on correlation of inspiratory unit activity to phrenic high-frequency oscillation in the brain stem of the rabbit

Effect of thiamylal on correlation of inspiratory unit activity to phrenic high-frequency oscillation in the brain stem of the rabbit