Comparison of the motor discharge to the crural and costal diaphragm in the rat

Midbrain control of breathing and blood pressure: The role of periaqueductal gray matter and mesencephalic collicular neuronal microcircuit oscillators

Neural circuitry residing within the medullary ventral respiratory column nuclei and dorsal respiratory group interact with the Kölliker-Fuse and medial parabrachial nuclei to generate the core breathing rhythm and pattern during resting conditions. Triphasic eupnea consists of inspiratory [I], post-inspiratory [post-I], and late-expiratory [E2] phases. Mesencephalic zones exert modulatory influences upon respiratory rhythm-generating circuitry, sympathetic oscillators, and parasympathetic nuclei. The earliest evidence supporting the existence of midbrain control of breathing derives from studies conducted by Martin and Booker in 1878. These authors demonstrated electrical stimulation of the deep layers of the mesencephalic colliculi in the rabbit augmented ventilation and sequentially elicited chest wall tremors and tetany. Investigations performed during the past several decades would demonstrate stimlation of distributed zones within the midbrain reticular formation elicits starkly disparate effects upon respiratory phase switching. Schmid, Böhmer, and Fallert demonstrated electrical stimulation of the nucleus rubre and emanating axon bundles alternately elicits or inhibits the activity of medullary expiratory- or inspiratory-related units and phrenic nerve discharge with differential latency. A series of studies would later indicate the red nucleus mediates hypoxic ventilatory depression. Periaqueductal gray matter neurons exhibit extensive afferent and efferent interconnectivity with suprabulbar, brainstem, and spinal cord zones aptly positioning these units to modulate breathing, autonomic outflow, nociception locomotion, micturtion, and sexual behavior. Experimental stimulatory activation of the tectal colliculi and periaqueductal gray matter via electrical current or glutamate, D,L-homocysteinic acid, or bicuculline microinjections coordinately modulates neuromotor inspiratory bursting frequency and amplitude and discharge of pre-Bötzinger complex, ventrolateral medullary late-I and post-I, and ventrolateral nucleus tractus solitarius decrementing early-I and augmenting and decrementing late-I neurons, elicits expiratory outflow and vocalization, and blunt the Hering-Breuer reflex in unanesthetzed decerebrate and anesthetized preprations of the cat and rat. Stimulation of the mesencephalic colliuli or dorsal divisions of the PAG potently amplifes renal sympathetic neural efferent activity, dynamic arterial pressure magnitude, and myocardial contraction frequency and elicits various behavioral defense responses. Elicited physiological effects exhibit extensive locoregional heterogeneity and variably enlist requisite contributions from the dorsomedial hypothalamus and/or lateral parabrachial nuclei. Stimulation of the dorsal mesencephalon occasionally elicits dynamic increases of arterial pressure magnitude exhibiting prominent oscillatory variability coherent with phrenic nerve discharge, perhaps by generating intra-neuraxial hysteresis, serving to intermittently deliver blood to organ vascular beds under high pressure in order to prevent organ edema, microcirculatory dysfunction, and downregulation of vascular smooth muscle alpha adrenergic receptors. Chemosensitive mesencephalic caudal raphé units and projections of hypoxia-sensitive units in the caudal hypothalamus to the periaqueductal gray matter may imply the existence of a diencephalo-smesencephalic chemosensitive network modulating breathing and sympathetic discharge.

Costal and crural diaphragm function during sustained hypoxia in awake canines

In humans and other mammals, isocapnic hypoxia sustained for 20-60 min exhibits a biphasic ventilation pattern: initial increase followed by a significant ventilatory decline ("roll-off") to a lesser intermediate plateau. During sustained hypoxia, the mechanical action and activity of the diaphragm have not been studied; thus we assessed diaphragm function in response to hypoxic breathing. Thirteen spontaneously breathing awake canines were exposed to moderate levels of sustained isocapnic hypoxia lasting 20-25 min (80 ± 2% pulse oximeter oxygen saturation). Breathing pattern and changes in muscle length and electromyogram (EMG) activity of the costal and crural diaphragm were continuously recorded. Mean tidal shortening and EMG activity of the costal and crural diaphragm exhibited an overall biphasic pattern, with initial brisk increase followed by a significant decline (P < 0.01). Although costal and crural shortening did not differ significantly with sustained hypoxia, this equivalence in segmental shortening occurred despite distinct and differing EMG activities of the costal and crural segments. Specifically, initial hypoxia elicited a greater costal EMG activity compared with crural (P < 0.05), whereas sustained hypoxia resulted in a lesser crural EMG decline/attenuation than costal (P < 0.05). We conclude that sustained isocapnic hypoxia elicits a biphasic response in both ventilation and diaphragmatic function and there is clear differential activation and contribution of the two diaphragmatic segments. This different diaphragm segmental action is consistent with greater neural activation of costal diaphragm during initial hypoxia, then preferential sparing of crural activation as hypoxia is sustained. NEW & NOTEWORTHY In humans and other mammals, during isocapnic hypoxia sustained for 20-60 min ventilation exhibits a biphasic pattern: initial increase followed by significant ventilatory decline ("roll-off"). During sustained hypoxia, the function of the diaphragm is unknown. This study demonstrates that the diaphragm reveals a biphasic action during the time-dependent hypoxic "roll-off" in ventilation. These results also highlight that the two diaphragm segments, costal and crural, show differing, distinctive contributions to diaphragm function during sustained hypoxia.

Central mechanisms regulating coordinated cardiovascular and respiratory function during stress and arousal

Actual or potentially threatening stimuli in the external environment (i.e., psychological stressors) trigger highly coordinated defensive behavioral responses that are accompanied by appropriate autonomic and respiratory changes. As discussed in this review, several brain regions and pathways have major roles in subserving the cardiovascular and respiratory responses to threatening stimuli, which may vary from relatively mild acute arousing stimuli to more prolonged life-threatening stimuli. One key region is the dorsomedial hypothalamus, which receives inputs from the cortex, amygdala, and other forebrain regions and which is critical for generating autonomic, respiratory, and neuroendocrine responses to psychological stressors. Recent studies suggest that the dorsomedial hypothalamus also receives an input from the dorsolateral column in the midbrain periaqueductal gray, which is another key region involved in the integration of stress-evoked cardiorespiratory responses. In addition, it has recently been shown that neurons in the midbrain colliculi can generate highly synchronized autonomic, respiratory, and somatomotor responses to visual, auditory, and somatosensory inputs. These collicular neurons may be part of a subcortical defense system that also includes the basal ganglia and which is well adapted to responding to threats that require an immediate stereotyped response that does not involve the cortex. The basal ganglia/colliculi system is phylogenetically ancient. In contrast, the defense system that includes the dorsomedial hypothalamus and cortex evolved at a later time, and appears to be better adapted to generating appropriate responses to more sustained threatening stimuli that involve cognitive appraisal.

Disinhibition of the midbrain colliculi unmasks coordinated autonomic, respiratory, and somatomotor responses to auditory and visual stimuli

The midbrain superior and inferior colliculi have critical roles in generating coordinated orienting or defensive behavioral responses to environmental stimuli, and it has been proposed that neurons within the colliculi can also generate appropriate cardiovascular and respiratory responses to support such behavioral responses. We have previously shown that activation of neurons within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus can evoke a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity and phrenic nerve activity. In this study, we tested the hypothesis that, under conditions in which collicular neurons are disinhibited, coordinated cardiovascular, somatomotor, and respiratory responses can be evoked by natural environmental stimuli. In response to natural auditory, visual, or somatosensory stimuli, powerful synchronized increases in sympathetic, respiratory, and somatomotor activity were generated following blockade of GABAA receptors in a specific region in the midbrain colliculi of anesthetized rats, but not under control conditions. Such responses still occurred after removal of most of the forebrain, including the amygdala and hypothalamus, indicating that the essential pathways mediating these coordinated responses were located within the brain stem. The temporal relationships between the different outputs suggest that they are driven by a common population of “command neurons” within the colliculi.

Chronic intrinsic transient tracheal occlusion elicits diaphragmatic muscle fiber remodeling in conscious rodents

BACKGROUND

Although the prevalence of inspiratory muscle strength training has increased in clinical medicine, its effect on diaphragm fiber remodeling is not well-understood and no relevant animal respiratory muscle strength training-rehabilitation experimental models exist. We tested the postulate that intrinsic transient tracheal occlusion (ITTO) conditioning in conscious animals would provide a novel experimental model of respiratory muscle strength training, and used significant increases in diaphragmatic fiber cross-sectional area (CSA) as the primary outcome measure. We hypothesized that ITTO would increase costal diaphragm fiber CSA and further hypothesized a greater duration and magnitude of occlusions would amplify remodeling.

METHODOLOGY/PRINCIPAL FINDINGS

Sprague-Dawley rats underwent surgical placement of a tracheal cuff and were randomly assigned to receive daily either 10-minute sessions of ITTO, extended-duration, 20-minute ITTO (ITTO-20), partial obstruction with 50% of cuff inflation pressure (ITTO-PAR) or observation (SHAM) over two weeks. After the interventions, fiber morphology, myosin heavy chain composition and CSA were examined in the crural and ventral, medial, and dorsal costal regions. In the medial costal diaphragm, with ITTO, type IIx/b fibers were 26% larger in the medial costal diaphragm (p<0.01) and 24% larger in the crural diaphragm (p<0.05). No significant changes in fiber composition or morphology were detected. ITTO-20 sessions also yielded significant increases in medial costal fiber cross-sectional area, but the effects were not greater than those elicited by 10-minute sessions. On the other hand, ITTO-PAR resulted in partial airway obstruction and did not generate fiber hypertrophy.

CONCLUSIONS/SIGNIFICANCE

The results suggest that the magnitude of the load was more influential in altering fiber cross-sectional area than extended-duration conditioning sessions. The results also indicated that ITTO was associated with type II fiber hypertrophy in the medial costal region of the diaphragm and may be an advantageous experimental model of clinical respiratory muscle strength training.

Synchronized activation of sympathetic vasomotor, cardiac, and respiratory outputs by neurons in the midbrain colliculi

The superior and inferior colliculi are believed to generate immediate and highly coordinated defensive behavioral responses to threatening visual and auditory stimuli. Activation of neurons in the superior and inferior colliculi have been shown to evoke increases in cardiovascular and respiratory activity, which may be components of more generalized stereotyped behavioral responses. In this study, we examined the possibility that there are “command neurons” within the colliculi that can simultaneously drive sympathetic and respiratory outputs. In anesthetized rats, microinjections of bicuculline (a GABAA receptor antagonist) into sites within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus evoked a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA). Each burst of RSNA had a duration of ∼300–400 ms and occurred slightly later (peak to peak latency of 41 ± 8 ms) than the corresponding burst of PNA. The bursts of RSNA and PNA were also accompanied by transient increases in arterial pressure and, in most cases, heart rate. Synchronized bursts of RSNA and PNA were also evoked after neuromuscular blockade, artificial ventilation, and vagotomy and so were not dependent on afferent feedback from the lungs. We propose that the synchronized sympathetic-respiratory responses are driven by a common population of neurons, which may normally be activated by an acute threatening stimulus.

Midbrain and medullary control of postinspiratory activity of the crural and costal diaphragm in vivo

Studies on brain stem respiratory neurons suggest that eupnea consists of three phases: inspiration, postinspiration, and expiration. However, it is not well understood how postinspiration is organized in the diaphragm, i.e., whether postinspiration differs in the crural and costal segments of the diaphragm and what the influence is of postinspiratory neurons on diaphragm function during eupnea. In this in vivo study we investigated the postinspiratory activity of the two diaphragm segments during eupnea and the changes in diaphragm function following modulation of eupnea. Postinspiratory neurons in the medulla were stereotaxically localized extracellularly and neurochemically stimulated. We used three types of preparations: precollicularly decerebrated unanesthetized cats and rats and anesthetized rats. In all preparations, during eupnea, postinspiratory activity was found in the crural but not in the costal diaphragm. When eupnea was discontinued in decerebrate cats in which stimulation in the nucleus retroambiguus induced activation of laryngeal or abdominal muscles, all postinspiratory activity in the crural diaphragm was abolished. In decerebrate rats, stimulation of the midbrain periaqueductal gray abolished postinspiration in the crural diaphragm but induced activation in the costal diaphragm. In anesthetized rats, stimulation of medullary postinspiratory neurons abolished the postinspiratory activity of the crural diaphragm. Vagal nerve stimulation in these rats increased the intensity of postinspiratory neuronal discharge in the solitary nucleus, leading to decreased activity of the crural diaphragm. These data demonstrate that three-phase breathing in the crural diaphragm during eupnea exists in vivo and that postinspiratory neurons have an inhibitory effect on crural diaphragm function.

Motor control of the diaphragm in anesthetized rabbits

Diaphragmatic regions are recruited in a specialized manner either as part of a central motor program during non-respiratory maneuvers, e.g. vomiting, or because of reflex responses, e.g. esophageal distension. Some studies in cats and dogs suggest that crural and costal diaphragm may be differentially activated also in response to respiratory stimuli from chemoreceptors or lung and chest wall mechanoreceptors. To verify whether this could occur also in other species, the EMG activity from the sternal, costoventral, costodorsal, and crural diaphragm was recorded in 42 anesthetized rabbits in response to various respiratory maneuvers, such as chemical stimulation, mechanical loading, lung volume and postural changes before and after vagotomy, or a non-respiratory maneuver such as esophageal distension. Regional activity was evaluated from timing of the raw EMG signal, and amplitude and shape of the moving average EMG. In all animals esophageal distension caused greater inhibition of the crural than sternal and costal diaphragm, whereas under all the other conditions differential diaphragmatic activation never occurred. These results indicate that in response to respiratory stimuli the rabbit diaphragm behaves as a single unit under the command of the central respiratory control system.

Phrenic motoneuron discharge patterns during hypoxia-induced short-term potentiation in rats

Hypoxia-induced short-term potentiation (STP) of respiratory motor output is manifested by a progressive increase in activity after the acute hypoxic response and a gradual decrease in activity on termination of hypoxia. We hypothesized that STP would be differentially expressed between physiologically defined phrenic motoneurons (PhrMNs). Phrenic nerve "single fiber" recordings were used to characterize PhrMN discharge in anesthetized, vagotomized and ventilated rats. PhrMNs were classified as early (Early-I) or late inspiratory (Late-I) according to burst onset relative to the contralateral phrenic neurogram during normocapnic baseline conditions. During hypoxia (F(I)O(2) = 0.12-0.14, 3 min), both Early-I and Late-I PhrMNs abruptly increased discharge frequency. Both cell types also showed a progressive increase in frequency over the remainder of hypoxia. However, Early-I PhrMNs showed reduced overall discharge duration and total spikes/breath during hypoxia, whereas Late-I PhrMNs maintained constant discharge duration and therefore increased the number of spikes/breath. A population of previously inactive (i.e., silent) PhrMNs was recruited 48 +/- 8 s after hypoxia onset. These PhrMNs had a Late-I onset, and the majority (8/9) ceased bursting promptly on termination of hypoxia. In contrast, both Early-I and Late-I PhrMNs showed post-hypoxia STP as reflected by greater discharge frequencies and spikes/breath during the post-hypoxic period (P < 0.01 vs. baseline). We conclude that the expression of phrenic STP during hypoxia reflects increased activity in previously active Early-I and Late-I PhrMNs and recruitment of silent PhrMNs. post-hypoxia STP primarily reflects persistent increases in the discharge of PhrMNs, which were active before hypoxia.