Behavioral inspiratory inhibition: inactivated and activated respiratory cells

Point Prevalence of the Biomechanical Dimension of Dysfunctional Breathing Patterns Among Competitive Athletes

ABSTRACT

Shimozawa, Y, Kurihara, T, Kusagawa, Y, Hori, M, Numasawa, S, Sugiyama, T, Tanaka, T, Suga, T, Terada, RS, Isaka, T, and Terada, M. Point prevalence of the biomechanical dimension of dysfunctional breathing patterns among competitive athletes. J Strength Cond Res 37(2): 270-276, 2023-There is growing evidence of associations between altered biomechanical breathing patterns and numerous musculoskeletal and psychological conditions. The prevalence of dysfunctional and diaphragmatic breathing patterns is unknown among athletic populations. The purpose of this study was to examine the prevalence of dysfunctional and diaphragmatic breathing patterns among athletic populations with a clinical measure to assess the biomechanical dimension of breathing patterns. Using a cross-sectional design, 1,933 athletes across multiple sports and ages were screened from 2017 to 2020. Breathing patterns were assessed using the Hi-Lo test in the standing position. Scores of the Hi-Lo test were determined based on the presence or absence of abdominal excursion, anterior-posterior chest expansion, superior rib cage migration, and shoulder elevation. The Hi-Lo test scores were used to categorize observational breathing mechanics as dysfunctional and diaphragmatic breathing patterns. The prevalence of athletes with dysfunctional breathing patterns was 90.6% (1,751 of 1,933). Athletes with diaphragmatic breathing patterns accounted for 9.4% of all athletes in our sample (182 of 1,933). There were no differences in the proportion of breathing patterns between male and female athletes ( p = 0.424). Breathing patterns observations were associated with sport-setting categories ( p = 0.002). The highest percentages of dysfunctional breathers were in middle school student athletes (93.7%), followed by elementary school student athletes (91.2%), high school student athletes (90.6%), professional/semiprofessional athletes (87.5%), and collegiate athletes (84.8%). The current study observed that dysfunctional breathing patterns (90.6%) in the biomechanical dimension were more prevalent than diaphragmatic breathing pattern (9.4%) among competitive athletes. These results suggest that clinicians may need to consider screening breathing patterns and implementing intervention programs aimed to improve the efficiency of biomechanical dimensions of breathing patterns in athletic populations. This study may help raise awareness of impacts of dysfunctional breathing patterns on athletes' health and performance.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

Respiratory muscle activity in voluntary breathing tracking tasks: Implications for the assessment of respiratory motor control

How the involuntary (bulbospinal) and voluntary (corticospinal) pathways interact in respiratory muscle control is not established. To determine the role of excitatory corticobulbar pathways in humans, studies typically compare electromyographic activity (EMG) or evoked responses in respiratory muscles during hypercapnic and voluntary tasks. Although ventilation is matched between tasks by having participants track signals of ventilation, these tasks may not result in matched respiratory muscle activity. The aim of this study was to describe respiratory muscle activity and ribcage and abdominal excursions during two different voluntary conditions, compared to hypercapnic hyperventilation. Ventilation was matched in the voluntary conditions via (i) a simple target of lung volume ('volume tracking') or (ii) targets of both ribcage and abdominal excursions, adjusted to end-expiratory lung volume in hypercapnic hyperventilation ('bands tracking'). Compared to hypercapnic hyperventilation, respiratory parameters such as tidal volume were similar, but the ratio of ribcage to abdominal excursion was higher for both voluntary tasks. Inspiratory scalene and parasternal intercostal muscle activity was higher in volume tracking, but diaphragm and abdominal muscle activity showed little to no change. There were no differences in muscle activity in bands tracking for any muscle, compared to hypercapnic hyperventilation. An elevated ratio of ribcage to abdominal excursion in the bands tracking task indicates that participants could not accurately match the targets in this condition. Inspiratory muscle activity is altered in some muscles in some voluntary tasks, compared to hypercapnia. Therefore, differences in muscle activity should be considered in interpretation of studies that use these protocols to investigate respiratory muscle control.

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

Reflex response to airway occlusion in human inspiratory muscles when recruited for breathing and posture

Briefly occluding the airway during inspiration produces a short-latency reflex inhibition in human inspiratory muscles. This occlusion reflex seems specific to respiratory muscles; however, it is not known whether the reflex inhibition has a uniform effect across a motoneuron pool when a muscle is recruited concurrently for breathing and posture. In this study, participants were seated and breathed through a mouthpiece that occluded inspiratory airflow for 250 ms at a volume threshold of 0.2 liters. The reflex response was measured in the scalene and sternocleidomastoid muscles during 1) a control condition with the head supported in space and the muscles recruited for breathing only, 2) a postural condition with the head unsupported and the neck flexors recruited for both breathing and to maintain head posture, and 3) a large-breath condition with the head supported and the volume threshold raised to between 0.8 and 1.0 liters to increase inspiratory muscle activity. When normalized to its preocclusion mean, the reflex response in the scalene muscles was not significantly different between the large-breath and control conditions, whereas concomitant recruitment of these muscles for posture control reduced the reflex response by half compared with the control condition. A reflex response occurred in sternocleidomastoid when it contracted phasically as an accessory muscle for inspiration during the large-breath condition. These results indicate that the occlusion reflex does not produce a uniform effect across the motoneuron pool and that afferent inputs for this reflex most likely act via intersegmental networks of premotoneurons rather than at a motoneuronal level.

NEW & NOTEWORTHY In this study, we investigated the effect of nonrespiratory activity on the reflex response to brief sudden airway occlusions in human inspiratory muscles. We show that the reflex inhibition in the scalene muscles was not uniform across the motoneuron pool when the muscle was recruited concurrently for breathing and postural control. The reflex had a larger effect on respiratory-driven motoneurons than those recruited to maintain head posture.

Reduced Phrenic Motoneuron Recruitment during Sustained Inspiratory Threshold Loading Compared to Single-Breath Loading: A Twitch Interpolation Study

In humans, inspiratory constraints engage cortical networks involving the supplementary motor area. Functional magnetic resonance imaging (fMRI) shows that the spread and intensity of the corresponding respiratory-related cortical activation dramatically decrease when a discrete load becomes sustained. This has been interpreted as reflecting motor cortical reorganization and automatisation, but could proceed from sensory and/or affective habituation. To corroborate the existence of motor reorganization between single-breath and sustained inspiratory loading (namely changes in motor neurones recruitment), we conducted a diaphragm twitch interpolation study based on the hypothesis that motor reorganization should result in changes in the twitch interpolation slope. Fourteen healthy subjects (age: 21–40 years) were studied. Bilateral phrenic stimulation was delivered at rest, upon prepared and targeted voluntary inspiratory efforts (“vol”), upon unprepared inspiratory efforts against a single-breath inspiratory threshold load (“single-breath”), and upon sustained inspiratory efforts against the same type of load (“continuous”). The slope of the relationship between diaphragm twitch transdiaphragmatic pressure and the underlying transdiaphragmatic pressure was −1.1 ± 0.2 during “vol,” −1.5 ± 0.7 during “single-breath,” and −0.6 ± 0.4 during “continuous” (all slopes expressed in percent of baseline.percent of baseline⁻¹) all comparisons significant at the 5% level. The contribution of the diaphragm to inspiration, as assessed by the gastric pressure to transdiaphragmatic pressure ratio, was 31 ± 17% during “vol,” 22 ± 16% during “single-breath” (p = 0.13), and 19 ± 9% during “continuous” (p = 0.0015 vs. “vol”). This study shows that the relationship between the amplitude of the transdiaphragmatic pressure produced by a diaphragm twitch and its counterpart produced by the underlying diaphragm contraction is not unequivocal. If twitch interpolation is interpreted as reflecting motoneuron recruitment, this study supports motor reorganization compatible with “diaphragm sparing” when an inspiratory threshold load becomes sustained.

Afferent neural pathways mediating cough in animals and humans

The airways and lungs are densely innervated by sensory nerves, which subserve multiple roles in both the normal physiological control of respiratory functions and in pulmonary defense. These sensory nerves are therefore not homogeneous in nature, but rather have physiological, molecular and anatomical phenotypes that reflect their purpose. All sensory neuron subtypes provide input to the central nervous system and drive reflex changes in respiratory and airway functions. But less appreciated is that ascending projections from these brainstem inputs to higher brain regions can also induce behavioural changes in respiration. In this brief review we provide an overview of the current understanding of airway sensory pathways, with specific reference to those involved in reflex and behavioural cough responses following airways irritation.

Peripheral chemoreceptors tune inspiratory drive via tonic expiratory neuron hubs in the medullary ventral respiratory column network

Models of brain stem ventral respiratory column (VRC) circuits typically emphasize populations of neurons, each active during a particular phase of the respiratory cycle. We have proposed that "tonic" pericolumnar expiratory (t-E) neurons tune breathing during baroreceptor-evoked reductions and central chemoreceptor-evoked enhancements of inspiratory (I) drive. The aims of this study were to further characterize the coordinated activity of t-E neurons and test the hypothesis that peripheral chemoreceptors also modulate drive via inhibition of t-E neurons and disinhibition of their inspiratory neuron targets. Spike trains of 828 VRC neurons were acquired by multielectrode arrays along with phrenic nerve signals from 22 decerebrate, vagotomized, neuromuscularly blocked, artificially ventilated adult cats. Forty-eight of 191 t-E neurons fired synchronously with another t-E neuron as indicated by cross-correlogram central peaks; 32 of the 39 synchronous pairs were elements of groups with mutual pairwise correlations. Gravitational clustering identified fluctuations in t-E neuron synchrony. A network model supported the prediction that inhibitory populations with spike synchrony reduce target neuron firing probabilities, resulting in offset or central correlogram troughs. In five animals, stimulation of carotid chemoreceptors evoked changes in the firing rates of 179 of 240 neurons. Thirty-two neuron pairs had correlogram troughs consistent with convergent and divergent t-E inhibition of I cells and disinhibitory enhancement of drive. Four of 10 t-E neurons that responded to sequential stimulation of peripheral and central chemoreceptors triggered 25 cross-correlograms with offset features. The results support the hypothesis that multiple afferent systems dynamically tune inspiratory drive in part via coordinated t-E neurons.

Tonic and phasic drive to medullary respiratory neurons during periodic breathing

It is unknown how central neural activity produces the repetitive termination and restart of periodic breathing (PB). We hypothesized that inspiratory and expiratory neural activities would be greatest during the waxing phase and least during the waning phase. We analyzed diaphragmatic and medullary respiratory neural activities during PB in intact unanesthetized adult cats. Diaphragmatic activity was increased and phasic during the waxing phase and was decreased and tonic during the waning phase. Activity of expiratory (n=21) and inspiratory (n=40) neurons was generally increased and phasic during the waxing phase and was decreased and more tonic during the waning phase. During apneas associated with PB, diaphragmatic activity was silent and most, but not all, inspiratory cells were inactive whereas most expiratory cells decreased activity but remained tonically active. We suggest that reduced strength of reciprocal inhibition, secondary to reduced respiratory drive, allows for simultaneous tonic activity of inspiratory and expiratory neurons of the central pattern generator, ultimately resulting in central apnea.

Voluntary and involuntary ventilation do not alter the human inspiratory muscle loading reflex

The reflex mechanism of the short-latency inhibitory reflex to transient loading of human inspiratory muscles is unresolved. Muscle afferents mediate this reflex, but they may act via pontomedullary inspiratory centers, other bulbar networks, or spinal circuits. We hypothesized that altered chemical drive to breathe would alter the initial inhibitory reflex if the neural pathways involve inspiratory medullary centers. Inspiration was transiently loaded in 11 subjects during spontaneous hypercapnic hyperpnea and matched voluntary hyperventilation. Electromyographic activity was recorded bilaterally from scalene muscles with surface electrodes. The latencies of the initial inhibitory response (IR) onset (32 ± 0.7 and 38 ± 1 ms for spontaneous and voluntary conditions respectively, P < 0.001) and subsequent excitatory response (ER) onset (80 ± 2.9 and 78 ± 2.6 ms, respectively, P = 0.46) and the normalized sizes of IR (65 ± 2 and 67 ± 3%, respectively, P = 0.50) and ER (51 ± 8 and 69 ± 6%, respectively, P = 0.005) were measured. Mean end-tidal Pco2 was 43 ± 1.5 Torr with dead space ventilation and was 14 ± 0.6 Torr with matched voluntary hyperventilation ( P < 0.001). A mean minute volume >30 liters was achieved in both conditions. The absence of significant difference in the size of the IR suggested that the IR reflex arc does not transit the brain stem inspiratory centers and that the reflex may be integrated at a spinal level. In voluntary hyperventilation, an initial excitation occurred more frequently and, consequently, the IR onset latency was significantly longer. The size of the later ER was also greater during voluntary hyperventilation, which is consistent with it being mediated via longer, presumably cortical, pathways, which are influenced by voluntary drive.

Interplay Between the Inspiratory and Postural Functions of the Human Parasternal Intercostal Muscles

The parasternal intercostal muscles are obligatory inspiratory muscles. To test the hypothesis that they are also involved in trunk rotation and to assess the effect of any postural role on inspiratory drive to the muscles, intramuscular electromyographic (EMG) recordings were made from the parasternal intercostals on the right side in six healthy subjects during resting breathing in a neutral posture (“neutral breaths”), during an isometric axial rotation effort of the trunk to the right (“ipsilateral rotation”) or left (“contralateral rotation”), and during resting breathing with the trunk rotated. The parasternal intercostals were commonly active during ipsilateral rotation but were consistently silent during contralateral rotation. In addition, with ipsilateral rotation, peak parasternal inspiratory activity was 201 ± 19% (mean ± SE) of the peak inspiratory activity in neutral breaths ( P < 0.001), and activity commenced earlier relative to the onset of inspiratory flow. These changes resulted from an increase in the discharge frequency of motor units (14.3 ± 0.3 vs. 11.0 ± 0.3 Hz; P < 0.001) and the recruitment of new motor units. The majority of units that discharged during ipsilateral rotation were also active in inspiration. However, with contralateral rotation, parasternal inspiratory activity was delayed relative to the onset of inspiratory flow, and peak activity was reduced to 72 ± 4% of that in neutral breaths ( P < 0.001). This decrease resulted from a decrease in the inspiratory discharge frequency of units (10.5 ± 0.2 vs. 12.0 ± 0.2 Hz; P < 0.001) and the derecruitment of units. These observations confirm that in addition to an inspiratory function, the parasternal intercostal muscles have a postural function. Furthermore the postural and inspiratory drives depolarize the same motoneurons, and the postural contraction of the muscles alters their output during inspiration in a direction-dependent manner.

Control of breathing and volitional respiratory rhythm in humans

When breathing frequency (f) is imperceptibly increased during a volitionally paced respiratory rhythm imposed by an auditory signal, tidal volume (Vt) decreases such that minute ventilation (V̇e) rises according to f-induced dead-space ventilation changes ( 18 ). As a result, significant change in alveolar ventilation and Pco2 are prevented as f varies. The present study was performed to determine what regulatory properties are retained by the respiratory control system, wherein the spontaneous automatic rhythmic activity is replaced by a volitionally paced rhythm. Six volunteers were asked to trigger each breath cycle on hearing a brief auditory signal. The time interval between subsequent auditory signals was imperceptibly changed for 10–15 min, during 1) air breathing ( condition 1), 2) the addition of a 300 ml of instrumental dead space ( condition 2), 3) an increase in the inspired level of CO2 ( condition 3), and 4) light exercise ( condition 4). We found that as f was slowly increased the elaborated Vt decreased in accordance to the background level of CO2 and metabolic rate. Indeed, for any given breath duration, Vt was shifted upward in condition 2 vs. 1, whereas the slope of Vt changes according to the volitionally rhythm was much steeper in conditions 3 and 4 vs. 1. The resulting changes in V̇e offset any f-induced changes in dead-space ventilation in all conditions. We conclude that there is an inherent, fundamental mechanism that elaborates Vt based on f when imposed by the premotor cortex in humans. The chemoreflex and exercise drive to breath interacts with this cortically mediated rhythm maintaining alveolar rather than V̇e constant as f changes. The implications of our findings are discussed in the context of our current understanding of the central generation of breathing rhythm.

Breathing rhythms and emotions

Respiration is primarily regulated for metabolic and homeostatic purposes in the brainstem. However, breathing can also change in response to changes in emotions, such as sadness, happiness, anxiety or fear. Final respiratory output is influenced by a complex interaction between the brainstem and higher centres, including the limbic system and cortical structures. Respiration is important in maintaining physiological homeostasis and co-exists with emotions. In this review, we focus on the relationship between respiration and emotions by discussing previous animal and human studies, including studies of olfactory function in relation to respiration and the piriform-amygdala in relation to respiration. In particular, we discuss oscillations of piriform-amygdala complex activity and respiratory rhythm.

Control of breathing during cortical substitution of the spontaneous automatic respiratory rhythm

This study addresses the following question: does the ventilatory control system adjust total ventilation in accord with the regulatory demands of the physiological dead space ventilation (VD) when the breathing frequency changes, and if so, how? A simple proportionality between the amplitude of the respiratory motor output (VT) and the respiratory period (TTOT) during such changes will not provide for regulation of arterial (PaCO2); the relationship requires a positive intercept of magnitude VD, i.e. VT=VATTOT+VD. We therefore determined the relationship between VT and TTOT when breathing frequency was changed in a ramp-like manner (from 6 to 20 cycles/min), in an imperceptible manner, during a paced-breathing protocol in which the subjects voluntarily triggered the breath onset, thereby imposing a rhythm different from the one spontaneously generated by the automatic central pattern generators (CPGs). While the resulting breath magnitude was strongly correlated to the breath duration (slope: 6.50+/-2.91 l/min) there was, in all cases, a statistically significant positive intercept on the VT axis (238+/-112 ml) leading to a relationship of the form: VT=VATTOT+VD. Consequently, the ventilatory output changed as a function of the breathing frequency-induced dead space ventilation changes, maintaining end-tidal PCO2 (PETCO2) constant. These results are consistent with a centrally set program for generating regulatory combinations of respiratory cycle durations and magnitudes that "take into account" the f-induced variation of dead space ventilation. This appears not to be dependent on the structures producing the respiratory rhythm (cortex versus central pattern generators). It is suggested that, during volitional control of breathing rhythm, the signal used for adjusting the magnitude to the timing of the ventilatory output is derived from information contained in the duration of preceding expiration.

Drive to the human respiratory muscles

The motor control of the respiratory muscles differs in some ways from that of the limb muscles. Effectively, the respiratory muscles are controlled by at least two descending pathways: from the medulla during normal quiet breathing and from the motor cortex during behavioural or voluntary breathing. Neurophysiological studies of single motor unit activity in human subjects during normal and voluntary breathing indicate that the neural drive is not uniform to all muscles. The distribution of neural drive depends on a principle of neuromechanical matching. Those motoneurones that innervate intercostal muscles with greater mechanical advantage are active earlier in the breath and to a greater extent. Inspiratory drive is also distributed differently across different inspiratory muscles, possibly also according to their mechanical effectiveness in developing airway negative pressure. Genioglossus, a muscle of the upper airway, receives various types of neural drive (inspiratory, expiratory and tonic) distributed differentially across the hypoglossal motoneurone pool. The integration of the different inputs results in the overall activity in the muscle to keep the upper airway patent throughout respiration. Integration of respiratory and non-respiratory postural drive can be demonstrated in respiratory muscles, and respiratory drive can even be observed in limb muscles under certain circumstances. Recordings of motor unit activity from the human diaphragm during voluntary respiratory tasks have shown that depending on the task there can be large changes in recruitment threshold and recruitment order of motor units. This suggests that descending drive across the phrenic motoneurone pool is not necessarily consistent. Understanding the integration and distribution of drive to respiratory muscles in automatic breathing and voluntary tasks may have implications for limb motor control.

Interactions between volitional and automatic breathing during respiratory apraxia

The sites and forms of interactions between voluntary breathing acts and automatic respiratory rhythm generation are the subject of considerable research interest. We report here observations of the control of breathing in a patient suffering from an advanced form of progressive supranuclear palsy (Steele-Richardson-Olszewski syndrome). This patient demonstrated a severely compromised ability to perform volitional respiratory acts upon command, despite exacerbated behavioural and automatic control of respiration. The presence of residual volitional control of breathing in this patient provided interesting insights concerning the interaction between the automatic and the voluntary control of respiration. We observed that (1) when the subjects was asked to inspire voluntarily he could at best mobilize a volume similar to spontaneous VT and only very slowly; (2) automatic breathing movements persisted, superimposed onto the active voluntary movements, with an amplitude that decreased when the inspiratory activity, albeit weak, reached its maximal level; (3) during breath holding both the amplitude and the frequency of the basal spontaneous rhythmic activity were depressed. This observation therefore supports the idea of a strong interaction between volitional and automatic breathing in the form of an inhibition of automatic activity during voluntary breathing. Although, the site of interaction (spinal versus supraspinal) could not be determined during volitional inspiration, the effect of breath holding on the frequency of the spontaneous breathing activity supports the view that a volitional breathing arrest has some inhibitory effects on the respiratory oscillator at the medullary level. Finally, in an attempt to reconcile the persistence of a rhythmic activity during voluntary inspiration and expiration with previous data from the literature, it is proposed that the normal suppression of the automatic activity during voluntary inspiration relies on cortical and sub-cortical structures involved in the planning, i.e. the praxic component, of a respiratory task rather than on projections originating from the primary motor cortex.

Breath-holding and its breakpoint

This article reviews the basic properties of breath-holding in humans and the possible causes of the breath at breakpoint. The simplest objective measure of breath-holding is its duration, but even this is highly variable. Breath-holding is a voluntary act, but normal subjects appear unable to breath-hold to unconsciousness. A powerful involuntary mechanism normally overrides voluntary breath-holding and causes the breath that defines the breakpoint. The occurrence of the breakpoint breath does not appear to be caused solely by a mechanism involving lung or chest shrinkage, partial pressures of blood gases or the carotid arterial chemoreceptors. This is despite the well-known properties of breath-hold duration being prolonged by large lung inflations, hyperoxia and hypocapnia and being shortened by the converse manoeuvres and by increased metabolic rate. Breath-holding has, however, two much less well-known but important properties. First, the central respiratory rhythm appears to continue throughout breath-holding. Humans cannot therefore stop their central respiratory rhythm voluntarily. Instead, they merely suppress expression of their central respiratory rhythm and voluntarily 'hold' the chest at a chosen volume, possibly assisted by some tonic diaphragm activity. Second, breath-hold duration is prolonged by bilateral paralysis of the phrenic or vagus nerves. Possibly the contribution to the breakpoint from stimulation of diaphragm muscle chemoreceptors is greater than has previously been considered. At present there is no simple explanation for the breakpoint that encompasses all these properties.

Invited review: Neural network plasticity in respiratory control

Respiratory network plasticity is a modification in respiratory control that persists longer than the stimuli that evoke it or that changes the behavior produced by the network. Different durations and patterns of hypoxia can induce different types of respiratory memories. Lateral pontine neurons are required for decreases in respiratory frequency that follow brief hypoxia. Changes in synchrony and firing rates of ventrolateral and midline medullary neurons may contribute to the long-term facilitation of breathing after brief intermittent hypoxia. Long-term changes in central respiratory motor control may occur after spinal cord injury, and the brain stem network implicated in the production of the respiratory rhythm could be reconfigured to produce the cough motor pattern. Preliminary analysis suggests that elements of brain stem respiratory neural networks respond differently to hypoxia and hypercapnia and interact with areas involved in cardiovascular control. Plasticity or alterations in these networks may contribute to the chronic upregulation of sympathetic nerve activity and hypertension in sleep apnea syndrome and may also be involved in sudden infant death syndrome.

Gain modulation of respiratory neurons

A possible mechanism underlying adaptive control of the respiratory system is gain modulation of the discharge frequency (F(n)) patterns of medullary respiratory neurons mediated by GABA(A) receptors. Antagonism of GABA(A) receptors with bicuculline results in an F(n) pattern that is an amplified replica of the underlying control pattern. The contours of F(n) patterns remain proportional to one another. Studies suggest that a tonic GABA(A)ergic input constrains the control- and reflexly-induced activities of these neurons to about 35-50% of the discharge rate without this inhibitory input. The pharmacology of this mechanism is unusual in that picrotoxin, a noncompetitive GABA(A) receptor antagonist, does not produce gain modulation, but is able to block the silent phase inhibition (e.g. E phase of an I neuron). Alterations in the amplitude of spike afterhyperpolarizations mediated by Ca(2+) activated K(+) channels also produces gain modulation. This mechanism modulates exogenously- and endogenously-induced neuronal activities, whereas the bicuculline-sensitive GABAergic mechanism modulates only the respiratory-related activities. Thus, these two forms of gain modulation, acting in cascade manner, may provide robust mechanisms for the optimal control of respiratory, as well as other behavioral functions (e.g. coughing, sneezing, vomiting) mediated by respiratory premotor neurons.

Concurrent inhibition and excitation of phrenic motoneurons during inspiration: phase-specific control of excitability

The movements that define behavior are controlled by motoneuron output, which depends on the excitability of motoneurons and the synaptic inputs they receive. Modulation of motoneuron excitability takes place over many time scales. To determine whether motoneuron excitability is specifically modulated during the active versus the quiescent phase of rhythmic behavior, we compared the input-output properties of phrenic motoneurons (PMNs) during inspiratory and expiratory phases of respiration. In neonatal rat brainstem-spinal cord preparations that generate rhythmic respiratory motor outflow, we blocked excitatory inspiratory synaptic drive to PMNs and then examined their phase-dependent responses to superthreshold current pulses. Pulses during inspiration elicited fewer action potentials compared with identical pulses during expiration. This reduced excitability arose from an inspiratory-phase inhibitory input that hyperpolarized PMNs in the absence of excitatory inspiratory inputs. Local application of bicuculline blocked this inhibition as well as the difference between inspiratory and expiratory firing. Correspondingly, bicuculline locally applied to the midcervical spinal cord enhanced fourth cervical nerve (C4) inspiratory burst amplitude. Strychnine had no effect on C4 output. Nicotinic receptor antagonists neither potentiated C4 output nor blocked its potentiation by bicuculline, further indicating that the inhibition is not from recurrent inhibitory pathways. We conclude that it is bulbospinal in origin. These data demonstrate that rapid changes in motoneuron excitability occur during behavior and suggest that integration of overlapping, opposing synaptic inputs to motoneurons is important in controlling motor outflow. Modulation of phasic inhibition may represent a means for regulating the transfer function of PMNs to suit behavioral demands.

Activity of medullary respiratory neurons during ventilator-induced apnea in sleep and wakefulness

Mechanical ventilation of cats in sleep and wakefulness causes apnea, often within two to three cycles of the ventilator. We recorded 137 medullary respiratory neurons in four adult cats during eupnea and during apnea caused by mechanical ventilation. We hypothesized that the residual activity of respiratory neurons during apnea might reveal its cause(s). The results showed that residual activity depended on 1) the amount of nonrespiratory inputs to the cell (cells with more nonrespiratory inputs had greater amounts of residual activity); 2) the cell type (expiratory cells had more residual activity than inspiratory cells); and 3) the state of consciousness (more residual activity in wakefulness and rapid-eye-movement sleep than in non-rapid-eye-movement sleep). None of the cells showed an activation during ventilation that could explain the apnea. Residual activity of approximately one-half of the cells was modulated in phase with the ventilator. The strength of this modulation was quantified by using an effect-size statistic and was found to be weak. The patterns of modulation did not support the idea that mechanoreceptors excite some respiratory cells that, in turn, inhibit others. Indeed, most cells, inspiratory and expiratory, discharged during the deflation-inflation transition of ventilation. Residual activity failed to reveal the cause of apnea but showed that during apnea respiratory neurons act as if they were disinhibited and disfacilitated.

Anxiety, antisocial behavior, and heart rate regulation in adolescent males

We explored relationships between anxiety and antisocial behavior and autonomic heart rate regulation in a homogenous sample (N = 175) of 15-year-old males. Measures of anxiety and antisocial behavior were obtained at yearly intervals over a period of 4-6 years. Components of heart rate variability associated with postural (sympathetic) and respiratory (vagal) change and transfer of respiratory to heart rate variability were estimated at age 15 using spectral analytic techniques. Anxiety and antisocial behavior were predictably related to enhanced and diminished levels of mean heart rate, respectively. Anxiety was also predictably related to enhanced sympathetic mediation of phasic postural effects on heart rate. Antisocial behavior was unexpectedly related to disruption of vagally mediated, phasic respiratory effects on heart rate. Anxiety and antisocial behavior showed distinct relationships to heart rate, and to the autonomically mediated components of heart rate variability from postural and respiratory sources. Spectral analytic techniques helped elucidate these unique regulatory patterns, suggesting utility for future research in this area.

Ventilatory conditioning by self-stimulation in rats: a pilot study

This article describes an experimental attempt to condition breathing pattern in rats. In this experiment, a freely moving rat was first rewarded by an electrical stimulation of the medial forebrain bundle whenever inspiratory duration (TI) exceeded 300 ms. A bidirectional control was then used: TIs longer than 400 ms were rewarded, and then TIs shorter than 300 ms were rewarded. The frequency of TIs longer than 300 ms increased when this event was rewarded, further increased when TIs above 400 ms were rewarded, and decreased during reversal conditioning (TI < 300 ms). At the beginning of the experiment, stimulation caused increased arousal and motor activity, but after prolonged conditioning, the brain stimulation was associated with quiet wakefulness. Although the general procedure appears to be well-suited to the experimental study of voluntary breathing, some possible improvements are suggested for further, more extensive investigations.

Performance and learning during voluntary control of breath patterns

Fourteen subjects learned to adjust their breath pattern to two target breaths displayed on a video screen, by using visual feedback, during two sessions 24 h apart. These two targets were respectively the smallest and the largest breaths of a ten-breath sample previously recorded from each subject's resting spontaneous breathing. Performances were significantly better for the large than for the small target breath. This cannot be directly inferred from current knowledge related to the control of movement time and amplitude, but rather it may be inferred from the periodic character of breathing, to the higher mental load during the small breath task, or to the presumably different frequencies of target breaths in the whole span of spontaneous breathing. In the second session, performance on the two targets levelled out as a result of learning.

Modification of medullary respiratory-related discharge patterns by behaviors and states of arousal

The modulatory influences of behaviors and states of arousal on bulbar respiratory-related unit (RRU) discharge patterns were studied in an unanesthetized, freely behaving guinea pig respiratory model system. When fully instrumented, this model system permits concurrent monitoring and recording of (i) single units from either Bötzinger complex or nucleus para-ambiguus; (ii) electrocorticogram; and, (iii) diaphragmatic EMG. In addition to being used in surveys of RRU discharge patterns in freely behaving states, the model system also offered a unique opportunity in investigating the effects of pentobarbital on RRU discharge patterns before, throughout the course of, and during recovery from anesthesia. In anesthetized preparations, a particular RRU discharge pattern (such as tonic, incrementing or decrementing) typically displayed little, if any notable variation. The most striking development following pentobarbital was a state of progressive bradypnea attributable to a significantly augmented RRU cycle duration, burst duration and an increase in the RRU spike frequencies during anesthesia. In freely behaving states, medullary RRU activities rarely adhered to a fixed, immutable discharge pattern. More specifically, the temporal organization (such as burst duration, cycle duration, and the extent of modulation of within-burst spike frequencies) of RRU discharge patterns regularly showed complex and striking variations, not only with states of arousal (sleep/wakefulness, anesthesia) but also with discrete alterations in electrocorticogram (ECoG) activities and a multitude of on-going behavioral repertoires such as volitional movement, postural modification, phonation, mastication, deglutition, sniffing/exploratory behavior, alerting/startle reflexes. Only during sleep, and on occasions when the animal assumed a motionless, resting posture, could burst patterns of relatively invariable periodicity and uniform temporal attributes be observed. RRU activities during sniffing reflex is worthy of further note in that, based on power spectrum analyses of concurrently recorded ECoG activities, this particular discharge pattern was clearly associated with the activation of a 6-10 Hz theta rhythm. These findings indicated that bulbar RRU activity patterns are subject to change by not only behaviors and sleep/wakefulness cycles, but also a variety of modulatory influences and feedback/feedforward biases from other central and peripheral physiological control mechanisms.

The cortical drive to human respiratory muscles in the awake state assessed by premotor cerebral potentials

1. We investigated the possibility of a cortical contribution to human respiration by recording from the scalp of awake subjects the premotor cerebral potentials that are known to precede voluntary limb movements. 2. Electroencephalographic activity (EEG) was recorded from scalp electrodes and averaged for 1.8-2.0 s before the time at which airway pressure exceeded an inspiratory or expiratory threshold. Clear premotor cerebral potentials were recorded during brisk, self-paced nasal inhalations or exhalations. In ten subjects, a slow cortical negativity (Bereitschaftspotential) was apparent in the averaged EEG, commencing 1.2 +/- 0.3 s before the onset of inspiratory (scalene) or expiratory (abdominal) muscle activity (EMG). It was maximal at the vertex, with a mean slope of 12.3 +/- 5.8 microV/s, and was followed by a post-movement positivity. 3. In four subjects the inspiratory premotor potential culminated in a large negativity, the motor potential, which began 24 +/- 15 ms before the onset of scalene EMG. It is argued that such a short latency is consistent with a volitionally generated respiratory command which travels relatively directly to the respiratory muscles, having a total central delay which is no longer than that for voluntary finger movements. 4. That the respiratory premotor and motor potentials did not originate in subcortical structures was supported by their absence in a patient suffering from chronic reflexogenic hiccups, in whom cerebral activity was back-averaged from each brisk hiccup. 5. During quiet breathing, in which subjects were relaxed and distracted from thinking about their respiration, no premotor cerebral potentials preceding inspiration could be detected. This failure was not due to the slow rate of rise of inspiratory activity during quiet breathing as compared with a brisk sniff, because premotor potentials were detected when subjects intermittently generated slow active expiratory efforts. 6. These observations suggest that during quiet breathing the cerebral cortex does not contribute to respiratory drive on a breath-by-breath basis. Conversely, the presence of clear premotor cerebral potentials when subjects performed self-paced inspiratory or expiratory manoeuvres illustrates the powerful cortical projection to human respiratory muscles.