Influence of lung volume on respiratory responses to spontaneous bladder contractions

Coordination of breathing with nonrespiratory activities

Many articles in this section of Comprehensive Physiology are concerned with the development and function of a central pattern generator (CPG) for the control of breathing in vertebrate animals. The action of the respiratory CPG is extensively modified by cortical and other descending influences as well as by feedback from peripheral sensory systems. The central nervous system also incorporates other CPGs, which orchestrate a wide variety of discrete and repetitive, voluntary and involuntary movements. The coordination of breathing with these other activities requires interaction and coordination between the respiratory CPG and those governing the nonrespiratory activities. Most of these interactions are complex and poorly understood. They seem to involve both conventional synaptic crosstalk between groups of neurons and fluid identity of neurons as belonging to one CPG or another: neurons that normally participate in breathing may be temporarily borrowed or hijacked by a competing or interrupting activity. This review explores the control of breathing as it is influenced by many activities that are generally considered to be nonrespiratory. The mechanistic detail varies greatly among topics, reflecting the wide variety of pertinent experiments.

Influence of hypercapnia and hypocapnia on bladder contractions and their respiratory consequences

Rhythmic contractions of the detrusor muscle, induced by gradual filling of the urinary bladder in decerebrate or anesthetized cats, are accompanied by decreased inspiratory activity in motor nerves to respiratory muscles, particularly those of the upper airway. We have examined the influence of hypercapnia and hypocapnia on these contractions and the accompanying activities of the phrenic and hypoglossal nerves in decerebrate, vagotomized, paralyzed and ventilated cats, some of which had denervated carotid chemoreceptors. Hypercapnia slowed, and then reversibly abolished bladder contractions in most animals, regardless of the state of the carotid chemoreceptors. Bladder contractions were well maintained in progressive hypocapnia, even at end-tidal CO(2) levels below the 'apneic' thresholds of the hypoglossal and phrenic activities. The reductions of the nerve activities in response to bladder contractions were not significantly altered by hypercapnia or hypocapnia. The abolition of bladder contractions by hypercapnia is unlikely to reflect a direct effect of CO(2) or H+ ion on the contractile mechanism of the detrusor muscle, but may be based on inhibition of stretch receptors in the bladder wall and/or an effect of CO(2) or H+ in or near the micturition centers in the brain stem.

Respiratory response to baroreceptor stimulation and spontaneous contractions of the urinary bladder

Spontaneous contractions of the urinary bladder (SBCs) and experimental elevations of carotid sinus pressure (CSP) have been shown to result in respiratory inhibition with preferential reduction in hypoglossal (HG) nerve activity as compared with that of phrenic nerve discharge. We assessed the interaction between these respiratory inhibitory stimuli in decerebrate, vagotomized, paralyzed and artificially ventilated cats. We denervated the right carotid sinus and pressurized the isolated left carotid sinus region within the linear range of the baroreflex, while maintaining systemic arterial pressure at approximately 100 mmHg. We monitored the HG and phrenic nerve responses to SBCs, to elevations in CSP between SBCs, and to elevations in CSP during SBCs. Our results show that superimposing these stimuli results in respiratory inhibition, especially of HG activity, that exceeds that resultant from either stimulus alone. We speculate that the combined presence of SBCs and episodic hypertension may contribute to the development of periodic breathing or obstructive apnea, particularly during sleep.

Acute changes in bladder volume produce minimal cardio-respiratory responses in lightly anesthetised humans

PURPOSE

To examine whether changes in bladder volume elicit reflex cardiovascular and respiratory responses in humans under general anesthesia with sevoflurane and nitrous oxide.

METHODS

Fourteen patients free of autonomic disorders were anesthetized with sevoflurane 0.5% and nitrous oxide 60% in oxygen that were approximately equivalent to 0.9 MAC. Warmed saline (6 ml x kg(-1), 37 degrees C) was instilled into the pre-emptied urinary bladder, and then the bladder was kept distended for five minutes. Following the distension, the instilled saline was drained to the pre-instilled volume of the bladder Arterial blood pressure, respiratory flow, and intra-vesicle pressure were continuously measured, and mean arterial pressure, pulse rate, respiratory rate, tidal volume, and minute ventilation were estimated offline from these signals.

RESULTS

Bladder emptying produced small decreases in mean blood pressure (from 83.4+/-4.3 to 80.0+/-4.4 mm Hg, mean +/- SE, P = 0.017) and pulse rate (from 72.2+/-2.9 to 69.4+/-2.7 bpm, mean +/- SEM, P = 0.004). Only minimal respiratory reflexes were invoked by the bladder volume changes.

CONCLUSION

In lightly anesthetized humans, the acute changes in bladder volume produce only mild cardiovascular and respiratory responses.

Respiratory response to spontaneous contractions of the urinary bladder in awake and decerebrate rats

Spontaneous contractions of the urinary bladder (SBCs) have been shown to decrease the frequency and depth of respiration in anesthetized or unanesthetized, decerebrate cats. The respiratory responses to bladder voiding reflexes in the awake state have not been previously addressed. Because a chronic rat model for the study of bladder function has been established and breathing measurements can be made in the awake rat, we chose the rat as an experimental model to assess whether SBCs would alter breathing in the conscious, intact animal. Respiratory frequency increased during bladder contractions but tidal volume remained unaffected. To assess whether the respiratory response to bladder reflexes in rats differed from that previously observed in cats, we also studied decerebrate, vagotomized, paralyzed and artificially ventilated rats. Contrary to the respiratory inhibition observed in decerebrate cats, phrenic and hypoglossal nerve activities remained unaffected during SBCs in decerebrate rats. These results indicate a species difference in the coupling between respiration and bladder voiding reflexes.

Bladder contractions alter inspiratory termination by superior laryngeal and intercostal nerve stimulation

Gradual distension of the urinary bladder evokes spontaneous bladder contractions (SBCs), which are associated with reduced inspiratory activity in the phrenic and other inspiratory motor nerves. We examined the influence of isovolumetric SBCs on the threshold for termination of phrenic inspiration by electrical stimulation of superior laryngeal and/or mid-thoracic intercostal nerves (ICN) in decerebrate, vagotomized, paralyzed, ventilated cats. Although SBCs reduced phrenic inspiratory activity, the threshold for inspiratory termination by nerve stimulation was increased. The results emphasize the complexity of the synaptic connections among brain stem neurons governing micturition and breathing.

Roles of the pontine pneumotaxic and micturition centers in respiratory inhibition during bladder contractions

In decerebrate or anesthetized cats with moderately distended urinary bladders, spontaneous bladder contractions (SBCs) have been shown to decrease phrenic and hypoglossal nerve activities. To determine the involvement of both the pontine micturition center (PMC) and the pneumotaxic center in the respiratory response to SBCs, we recorded phrenic and hypoglossal nerve activities in decerebrate, paralyzed, vagotomized, artificially ventilated cats. Electrical stimulation of the PMC in cats with subthreshold bladder volumes below the threshold for SBCs elicited both increases in intravesical pressure (IVP) and attenuation of respiratory motor nerve activities. Respiration was not altered after PMC lesions, which abolished SBCs, contractions in response to PMC stimulation, and respiratory inhibition due to passive bladder distension. Electrical stimulation of the pneumotaxic center altered respiratory motor nerve activities and increased IVP in cats with subthreshold bladder volumes. Pneumotaxic center lesions caused apneusis, but did not abolish the SBCs, which continued to attenuate the apneustic respiratory motor nerve activity. These results indicate that the PMC is an important component of the reflex pathway from urinary bladder distension to respiratory inhibition, whereas the pneumotaxic center does not appear to be an essential part of this pathway.