Connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons studied with cross-correlation in the decerebrate rat

Phrenic motor neuron survival below cervical spinal cord hemisection

Cervical spinal cord injury (cSCI) severs bulbospinal projections to respiratory motor neurons, paralyzing respiratory muscles below the injury. C2 spinal hemisection (C2Hx) is a model of cSCI often used to study spontaneous and induced plasticity and breathing recovery post-injury. One key assumption is that C2Hx dennervates motor neurons below the injury, but does not affect their survival. However, a recent study reported substantial bilateral motor neuron death caudal to C2Hx. Since phrenic motor neuron (PMN) death following C2Hx would have profound implications for therapeutic strategies designed to target spared neural circuits, we tested the hypothesis that C2Hx minimally impacts PMN survival. Using improved retrograde tracing methods, we observed no loss of PMNs at 2- or 8-weeks post-C2Hx. We also observed no injury-related differences in ChAT or NeuN immunolabeling within labelled PMNs. Although we found no evidence of PMN loss following C2Hx, we cannot rule out neuronal loss in other motor pools. These findings address an essential prerequisite for studies that utilize C2Hx as a model to explore strategies for inducing plasticity and/or regeneration within the phrenic motor system, as they provide important insights into the viability of phrenic motor neurons as therapeutic targets after high cervical injury.

Dynamic changes in arterial pressure following high cervical transection in the decerebrate rat

Objective: Spinal transection has variable effects on arterial pressure, with some investigators demonstrating a precipitous decline and others reporting only a minimal decrease below normal. Recovery of arterial pressure following spinalization occurs with varying time courses - in some cases over days and in others over weeks to months. Given these findings, we sought to systematically test the hypothesis that in the unanesthetized decerebrate rat, arterial pressure would recover to pre-transection values over an acute time course.Design: Experiments were performed on a total of six Sprague-Dawley unanesthetized decerebrate adult male rats. In four rats, we determined dynamic changes in arterial pressure and heart rate in response to C₁ transection.Results: Immediately following spinal cord injury, there were significant decreases in systolic blood (SBP) and mean arterial pressure (MAP), but not diastolic blood pressure (DBP). SBP, DBP, and MAP were significantly greater 170 min post-transection compared to immediate and 5 min-post transection values and were not statistically significantly different from pre-transection control. Heart rate decreased significantly following transection, but not immediately following the spinal cord injury. Lung inflation elicited depressor responses in all animals tested (n = 4 animals) and in three animals resulted in bradycardia. Hypercapnia tests effected a decrease in arterial pressure and heart rate (n = 3 animals).Conclusions: We demonstrate that in the unanesthetized decerebrate spinalized animal, arterial pressure is reduced by spinal transection and recovers over an acute time course to pre-transection values.

Disproportionate loss of excitatory inputs to smaller phrenic motor neurons following cervical spinal hemisection

KEY POINTS

Motor units, comprising a motor neuron and the muscle fibre it innervates, are activated in an orderly fashion to provide varying amounts of force. A unilateral C2 spinal hemisection (C2SH) disrupts predominant excitatory input from medulla, causing cessation of inspiratory-related diaphragm muscle activity, whereas higher force, non-ventilatory diaphragm activity persists. In this study, we show a disproportionately larger loss of excitatory glutamatergic innervation to small phrenic motor neurons (PhMNs) following C2SH, as compared with large PhMNs ipsilateral to injury. Our data suggest that there is a dichotomy in the distribution of inspiratory-related descending excitatory glutamatergic input to small vs. large PhMNs that reflects their differential recruitment.

ABSTRACT

Excitatory glutamatergic input mediating inspiratory drive to phrenic motor neurons (PhMNs) emanates primarily from the ipsilateral ventrolateral medulla. Unilateral C2 hemisection (C2SH) disrupts this excitatory input, resulting in cessation of inspiratory-related diaphragm muscle (DIAm) activity. In contrast, after C2SH, higher force, non-ventilatory DIAm activity persists. Inspiratory behaviours require recruitment of only smaller PhMNs, whereas with more forceful expulsive/straining behaviours, larger PhMNs are recruited. Accordingly, we hypothesize that C2SH primarily disrupts glutamatergic synaptic inputs to smaller PhMNs, whereas glutamatergic synaptic inputs to larger PhMNs are preserved. We examined changes in glutamatergic presynaptic input onto retrogradely labelled PhMNs using immunohistochemistry for VGLUT1 and VGLUT2. We found that 7 days after C2SH there was an ∼60% reduction in glutamatergic inputs to smaller PhMNs compared with an ∼35% reduction at larger PhMNs. These results are consistent with a more pronounced impact of C2SH on inspiratory behaviours of the DIAm, and the preservation of higher force behaviours after C2SH. These results indicate that the source of glutamatergic synaptic input to PhMNs varies depending on motor neuron size and reflects different functional control - perhaps separate central pattern generator and premotor circuits. For smaller PhMNs, the central pattern generator for inspiration is located in the pre-Bötzinger complex and premotor neurons in the ventrolateral medulla, sending predominantly ipsilateral projections via the dorsolateral funiculus. C2SH disrupts this glutamatergic input. For larger PhMNs, a large proportion of excitatory inputs appear to exist below the C2 level or from contralateral regions of the brainstem and spinal cord.

Targeted activation of spinal respiratory neural circuits

Spinal interneurons which discharge in phase with the respiratory cycle have been repeatedly described over the last 50 years. These spinal respiratory interneurons are part of a complex propriospinal network that is synaptically coupled with respiratory motoneurons. This article summarizes current knowledge regarding spinal respiratory interneurons and emphasizes chemical, electrical and physiological methods for activating spinal respiratory neural circuits. Collectively, the work reviewed here shows that activating spinal interneurons can have a powerful impact on spinal respiratory motor output, and can even drive rhythmic bursting in respiratory motoneuron pools under certain conditions. We propose that the primary functions of spinal respiratory neurons include 1) shaping the respiratory pattern into the final efferent motor output from the spinal respiratory nerves; 2) coordinating respiratory muscle activation across the spinal neuraxis; 3) coordinating postural, locomotor and respiratory movements, and 4) enabling plasticity of respiratory motor output in health and disease.

Heterogeneous glutamatergic receptor mRNA expression across phrenic motor neurons in rats

The diaphragm muscle comprises various types of motor units that are recruited in an orderly fashion governed by the intrinsic electrophysiological properties (membrane capacitance as a function of somal surface area) of phrenic motor neurons (PhMNs). Glutamate is the main excitatory neurotransmitter at PhMNs and acts primarily via fast acting AMPA and N-methyl-D-aspartic acid (NMDA) receptors. Differences in receptor expression may also contribute to motor unit recruitment order. We used single cell, multiplex fluorescence in situ hybridization to determine glutamatergic receptor mRNA expression across PhMNs based on their somal surface area. In adult male and female rats (n = 9) PhMNs were retrogradely labeled for analyses (n = 453 neurons). Differences in the total number and density of mRNA transcripts were evident across PhMNs grouped into tertiles according to somal surface area. A ~ 25% higher density of AMPA (Gria2) and NMDA (Grin1) mRNA expression was evident in PhMNs in the lower tertile compared to the upper tertile. These smaller PhMNs likely comprise type S motor units that are recruited first to accomplish lower force, ventilatory behaviors. In contrast, larger PhMNs with lower volume densities of AMPA and NMDA mRNA expression presumably comprise type FInt and FF motor units that are recruited during higher force, expulsive behaviors. Furthermore, there was a significantly higher cytosolic NMDA mRNA expression in small PhMNs suggesting a more important role for NMDA-mediated glutamatergic neurotransmission at smaller PhMNs. These results are consistent with the observed order of motor unit recruitment and suggest a role for glutamatergic receptors in support of this orderly recruitment. Cover Image for this issue: doi: 10.1111/jnc.14747.

Glutamatergic input varies with phrenic motor neuron size

Like all skeletal muscles, the diaphragm muscle accomplishes a range of motor behaviors by recruiting different motor unit types in an orderly fashion. Recruitment of phrenic motor neurons (PhMNs) is generally assumed to be based primarily on the intrinsic properties of PhMNs with an equal distribution of descending excitatory inputs to all PhMNs. However, differences in presynaptic excitatory input across PhMNs of varying sizes could also contribute to the orderly recruitment pattern. In the spinal cord of Sprague-Dawley rats, we retrogradely labeled PhMNs using cholera toxin B (CTB) and validated a robust confocal imaging-based technique that utilizes semiautomated processing to identify presynaptic glutamatergic (Glu) terminals within a defined distance around the somal membrane of PhMNs of varying size. Our results revealed an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. These smaller PhMNs are likely recruited first to accomplish lower force ventilatory behaviors of the diaphragm as compared with larger PhMNs in the upper tertile that are recruited to accomplish higher force expulsive behaviors. These results suggest that differences in excitatory synaptic input to PhMNs may also contribute to the orderly recruitment of diaphragm motor units.NEW & NOTEWORTHY The distribution of excitatory glutamatergic synaptic input to phrenic motor neurons differs across motor neurons of varying size. These findings support the size principle of motor unit recruitment that underlies graded force generation in a muscle, which is based on intrinsic electrophysiological properties of motor neurons resulting from differences in somal surface area. A higher density of glutamatergic inputs at smaller, more excitable motor neurons substantiates the earlier and more frequent recruitment of these units.

Coerced resilience in fire management

Mechanisms underlying the loss of ecological resilience and a shift to an alternate regime with lower ecosystem service provisioning continues to be a leading debate in ecology, particularly in cases where evidence points to human actions and decision-making as the primary drivers of resilience loss and regime change. In this paper, we introduce the concept of coerced resilience as a way to explore the interplay among social power, ecological resilience, and fire management, and to better understand the unintended and undesired regime changes that often surprise ecosystem managers and governing officials. Philosophically, coercion is the opposite of freedom, and uses influence or force to gain compliance among local actors. The coercive force imposed by societal laws and policies can either enhance or reduce the potential to manage for essential structures and functions of ecological systems and, therefore, can greatly alter resilience. Using a classical fire-dependent regime shift from North America (tallgrass prairie to juniper woodland), and given that coercion is widespread in fire management today, we quantify relative differences in resilience that emerge in a policy-coerced fire system compared to a theoretical, policy-free fire system. Social coercion caused large departures in the fire conditions associated with alternative grassland and juniper woodland states, and the potential for a grassland state to emerge to dominance became increasingly untenable with fire as juniper cover increased. In contrast, both a treeless, grassland regime and a co-dominated grass-tree regime emerged across a wide range of fire conditions in the absence of policy controls. The severe coercive forcing present in fire management in the Great Plains, and corresponding erosion of grassland resilience, points to the need for transformative environmental governance and the rethinking of social power structures in modern fire policies.

Development of a decerebrate model for investigating mechanisms mediating viscero-sympathetic reflexes in the spinalized rat

Autonomic dysreflexia (AD) often occurs in individuals living with spinal cord injury (SCI) and is characterized by uncontrolled hypertension in response to otherwise innocuous stimuli originating below the level of the spinal lesion. Visceral stimulation is a predominant cause of AD in humans and effectively replicates the phenotype in rodent models of SCI. Direct assessment of sympathetic responses to viscerosensory stimulation in spinalized animals is challenging and requires invasive surgical procedures necessitating the use of anesthesia. However, administration of anesthesia markedly affects viscerosensory reactivity, and the effects are exacerbated following spinal cord injury (SCI). Therefore, the major goal of the present study was to develop a decerebrate rodent preparation to facilitate quantification of sympathetic responses to visceral stimulation in the spinalized rat. Such a preparation enables the confounding effect of anesthesia to be eliminated. Sprague-Dawley rats were subjected to SCI at the fourth thoracic segment. Four weeks later, renal sympathetic nerve activity (RSNA) responses to visceral stimuli were quantified in urethane/chloralose-anesthetized and decerebrate preparations. Visceral stimulation was elicited via colorectal distension (CRD) for 1 min. In the decerebrate preparation, CRD produced dose-dependent increases in mean arterial pressure (MAP) and RSNA and dose-dependent decreases in heart rate (HR). These responses were significantly greater in magnitude among decerebrate animals when compared with urethane/chloralose-anesthetized controls and were markedly attenuated by the administration of urethane/chloralose anesthesia after decerebration. We conclude that the decerebrate preparation enables high-fidelity quantification of neuronal reactivity to visceral stimulation in spinalized rats. NEW & NOTEWORTHY In animal models commonly used to study spinal cord injury, quantification of sympathetic responses is particularly challenging due to the increased susceptibility of spinal reflex circuits to the anesthetic agents generally required for experimentation. This constitutes a major limitation to understanding the mechanisms mediating regionally specific neuronal responses to visceral activation in chronically spinalized animals. In the present study, we describe a spinalized, decerebrate rodent preparation that facilitates quantification of sympathetic reactivity in response to visceral stimuli following spinal cord injury. This preparation enables reliable and reproducible quantification of viscero-sympathetic reflex responses resembling those elicited in conscious animals and may provide added utility for preclinical evaluation of neuropharmacological agents for the management of autonomic dysreflexia.

The Neuroplastic and Therapeutic Potential of Spinal Interneurons in the Injured Spinal Cord

The central nervous system is not a static, hard-wired organ. Examples of neuroplasticity, whether at the level of the synapse, the cell, or within and between circuits, can be found during development, throughout the progression of disease, or after injury. One essential component of the molecular, anatomical, and functional changes associated with neuroplasticity is the spinal interneuron (SpIN). Here, we draw on recent multidisciplinary studies to identify and interrogate subsets of SpINs and their roles in locomotor and respiratory circuits. We highlight some of the recent progress that elucidates the importance of SpINs in circuits affected by spinal cord injury (SCI), especially those within respiratory networks; we also discuss potential ways that spinal neuroplasticity can be therapeutically harnessed for recovery.

The bulbospinal network controlling the phrenic motor system: Laterality and course of descending projections

The respiratory rhythm is generated by the parafacial respiratory group, Bötzinger complex, and pre-Bötzinger complex and relayed to pre-motor neurons, which in turn project to and control respiratory motor outputs in the brainstem and spinal cord. The phrenic nucleus is one such target, containing phrenic motoneurons (PhMNs), which supply the diaphragm, the primary inspiratory muscle in mammals. While some investigators have demonstrated both ipsi- and contralateral bulbophrenic projections, there exists controversy regarding the relative physiological contribution of each to phasic and tonic drive to PhMNs and at which levels decussations occur. Following C1- or C2 spinal cord hemisection-induced silencing of the ipsilateral phrenic/diaphragm activity, respiratory stressor-induced, as well as spontaneous, recovery of crossed phrenic activity is observed, suggesting an important contribution of pathways crossing below the level of injury in driving phrenic motor output. The precise mechanisms underlying this recovery are debated. In this review, we seek to present a comprehensive discussion of the organization of the bulbospinal network controlling PhMNs, a thorough appreciation of which is necessary for understanding neural respiratory control, accurate interpretation of studies investigating respiratory recovery following spinal cord injury, and targeted development of therapies for respiratory neurorehabilitation in patients sustaining high cervical cord injury.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

The crossed phrenic phenomenon

The cervical spine is the most common site of traumatic vertebral column injuries. Respiratory insufficiency constitutes a significant proportion of the morbidity burden and is the most common cause of mortality in these patients. In seeking to enhance our capacity to treat specifically the respiratory dysfunction following spinal cord injury, investigators have studied the "crossed phrenic phenomenon", wherein contraction of a hemidiaphragm paralyzed by a complete hemisection of the ipsilateral cervical spinal cord above the phrenic nucleus can be induced by respiratory stressors and recovers spontaneously over time. Strengthening of latent contralateral projections to the phrenic nucleus and sprouting of new descending axons have been proposed as mechanisms contributing to the observed recovery. We have recently demonstrated recovery of spontaneous crossed phrenic activity occurring over minutes to hours in C₁-hemisected unanesthetized decerebrate rats. The specific neurochemical and molecular pathways underlying crossed phrenic activity following injury require further clarification. A thorough understanding of these is necessary in order to develop targeted therapies for respiratory neurorehabilitation following spinal trauma. Animal studies provide preliminary evidence for the utility of neuropharmacological manipulation of serotonergic and adenosinergic pathways, nerve grafts, olfactory ensheathing cells, intraspinal microstimulation and a possible role for dorsal rhizotomy in recovering phrenic activity following spinal cord injury.

Neuroprotective and Neurorestorative Processes after Spinal Cord Injury: The Case of the Bulbospinal Respiratory Neurons

High cervical spinal cord injuries interrupt the bulbospinal respiratory pathways projecting to the cervical phrenic motoneurons resulting in important respiratory defects. In the case of a lateralized injury that maintains the respiratory drive on the opposite side, a partial recovery of the ipsilateral respiratory function occurs spontaneously over time, as observed in animal models. The rodent respiratory system is therefore a relevant model to investigate the neuroplastic and neuroprotective mechanisms that will trigger such phrenic motoneurons reactivation by supraspinal pathways. Since part of this recovery is dependent on the damaged side of the spinal cord, the present review highlights our current understanding of the anatomical neuroplasticity processes that are developed by the surviving damaged bulbospinal neurons, notably axonal sprouting and rerouting. Such anatomical neuroplasticity relies also on coordinated molecular mechanisms at the level of the axotomized bulbospinal neurons that will promote both neuroprotection and axon growth.

Intercostal muscle motor behavior during tracheal occlusion conditioning in conscious rats

A respiratory load compensation response is characterized by increases in activation of primary respiratory muscles and/or recruitment of accessory respiratory muscles. The contribution of the external intercostal (EI) muscles, which are a primary respiratory muscle group, during normal and loaded breathing remains poorly understood in conscious animals. Consciousness has a significant role on modulation of respiratory activity, as it is required for the integration of behavioral respiratory responses and voluntary control of breathing. Studies of respiratory load compensation have been predominantly focused in anesthetized animals, which make their comparison to conscious load compensation responses challenging. Using our established model of intrinsic transient tracheal occlusions (ITTO), our aim was to evaluate the motor behavior of EI muscles during normal and loaded breathing in conscious rats. We hypothesized that 1) conscious rats exposed to ITTO will recruit the EI muscles with an increased electromyogram (EMG) activation and 2) repeated ITTO for 10 days would potentiate the baseline EMG activity of this muscle in conscious rats. Our results demonstrate that conscious rats exposed to ITTO respond by recruiting the EI muscle with a significantly increased EMG activation. This response to occlusion remained consistent over the 10-day experimental period with little or no effect of repeated ITTO exposure on the baseline ∫EI EMG amplitude activity. The pattern of activation of the EI muscle in response to an ITTO is discussed in detail. The results from the present study demonstrate the importance of EI muscles during unloaded breathing and respiratory load compensation in conscious rats.

Injection of WGA-Alexa 488 into the ipsilateral hemidiaphragm of acutely and chronically C2 hemisected rats reveals activity-dependent synaptic plasticity in the respiratory motor pathways

WGA-Alexa 488 is a fluorescent neuronal tracer that demonstrates transsynaptic transport in the central nervous system. The transsynaptic transport occurs over physiologically active synaptic connections rather than less active or silent connections. Immediately following C2 spinal cord hemisection (C2Hx), when WGA-Alexa 488 is injected into the ipsilateral hemidiaphragm, the tracer diffuses across the midline of the diaphragm and retrogradely labels the phrenic nuclei (PN) bilaterally in the spinal cord. Subsequently, the tracer is transsynaptically transported bilaterally to the rostral Ventral Respiratory Groups (rVRGs) in the medulla over physiologically active connections. No other neurons are labeled in the acute C2Hx model at the level of the phrenic nuclei or in the medulla. However, with a recovery period of at least 7weeks (chronic C2Hx), the pattern of WGA-Alexa 488 labeling is notably changed. In addition to the bilateral PN and rVRG labeling, the chronic C2Hx model reveals fluorescence in the ipsilateral ventral and dorsal spinocerebellar tracts, and the ipsilateral reticulospinal tract. Furthermore, interneurons are labeled bilaterally in laminae VII and VIII of the spinal cord as well as neurons in the motor nuclei bilaterally of the intercostal and forelimb muscles. Moreover, in the chronic C2Hx model, there is bilateral labeling of additional medullary centers including raphe, hypoglossal, spinal trigeminal, parvicellular reticular, gigantocellular reticular, and intermediate reticular nuclei. The selective WGA-Alexa 488 labeling of additional locations in the chronic C2Hx model is presumably due to a hyperactive state of the synaptic pathways and nuclei previously shown to connect with the respiratory centers in a non-injured model. The present study suggests that hyperactivity not only occurs in neuronal centers and pathways caudal to spinal cord injury, but in supraspinal centers as well. The significance of such injury-induced plasticity is that hyperactivity may be a mechanism to re-establish lost function by compensatory routes which were initially physiologically inactive.

Specificity in monosynaptic and disynaptic bulbospinal connections to thoracic motoneurones in the rat

The respiratory activity in the intercostal nerves of the rat is unusual, in that motoneurones of both branches of the intercostal nerves, internal and external, are activated during expiration. Here, the pathways involved in that activation were investigated in anaesthetised and in decerebrate rats by cross-correlation and by intracellular spike-triggered averaging from expiratory bulbospinal neurones (EBSNs), with a view to revealing specific connections that could be used in studies of experimental spinal cord injury. Decerebrate preparations, which showed the strongest expiratory activity, were found to be the most suitable for these measurements. Cross-correlations in these preparations showed monosynaptic connections from 16/19 (84%) of EBSNs, but only to internal intercostal nerve motoneurones (24/37, 65% of EBSN/nerve pairs), whereas disynaptic connections were seen for external intercostal nerve motoneurones (4/19, 21% of EBSNs or 7/25, 28% of EBSN/nerve pairs). There was evidence for additional disynaptic connections to internal intercostal nerve motoneurones. Intracellular spike-triggered averaging revealed excitatory postsynaptic potentials, which confirmed these connections. This is believed to be the first report of single descending fibres that participate in two different pathways to two different groups of motoneurones. It is of interest compared with the cat, where only one group of motoneurones is activated during expiration and only one of the pathways has been detected. The specificity of the connections could be valuable in studies of plasticity in pathological situations, but care will be needed in studying connections in such situations, because their strength was found here to be relatively weak.

A detailed surgical method for mechanical decerebration of the rat

Surgical methods currently employed for the mechanical decerebration of a rat are only briefly described in the literature; hence, an information void exists for researchers wishing to adopt this technique successfully. Decerebration can lead to a high rate of mortality owing to cranial bleeding, particularly if the operator is inexperienced and uninformed. The illustrated methodology presented here describes, in detail, steps in the decerebration process and indicates effective approaches, such as reversible occlusion of a carotid artery, the combined use of tissue adhesive and haemostatic sponge, and perisurgical monitoring of blood pressure, by which to control blood loss and thus maintain mean postsurgical blood pressure within acceptable physiological parameters. By using this methodology, animal losses can be minimized, particularly in the early stages of adoption of the technique, ultimately reducing the numbers required for a study of this nature.

The phrenic motor nucleus in the adult mouse

The present study was performed to establish an anatomical context for studies of phrenic motor function in mouse models of central nervous system trauma and disease. Application of cholera toxin β-subunit to the diaphragm of adult C57BL/6 mice revealed a columnar organization of phrenic motoneurons (PhMNs) which extended from rostral C3 to C6. Injection of Miniruby into the ventrolateral medulla revealed decussating, anterogradely labeled axons in the cervical spinal cord. In addition, application of the transneuronal tracer pseudorabies virus (PRV) to the right hemidiaphragm demonstrated a population of putative pre-phrenic interneurons at the level of the infected PhMN pool. These neuroanatomical features of the mouse phrenic nucleus are consistent with those described in other species and provide a foundation for studies of neuroplasticity and repair in relation to a functionally and anatomically identified spinal network.

Spontaneous respiratory rhythm generation in in vitro upper cervical slice preparations of neonatal mice

Isolated upper cervical slice preparations were prepared from neonatal mice to examine whether spontaneous respiratory activity could be generated in the preparations. By using brainstem-spinal cord preparations, we first recorded from the cervical C1-C2 and C4 ventral roots rhythmic bursts which were synchronized with respiratory burst activity of the hypoglossal (XIIth) nerve. Following transection just above the C1 segment, smaller and slower rhythmic bursts still persisted in the C1/C2 ventral roots and these were synchronized with those in the C4 ventral root. The present result, that a bursting rhythm remained in the C1/C2 slices, suggests that the spinal neuronal circuit for generating respiratory rhythm is localized in the upper cervical segments which contain upper cervical inspiratory neurons.

Patterns of expiratory and inspiratory activation for thoracic motoneurones in the anaesthetized and the decerebrate rat

The nervous control of expiratory muscles is less well understood than that of the inspiratory muscles, particularly in the rat. The patterns of respiratory discharges in adult rats were therefore investigated for the muscles of the caudal intercostal spaces, with hypercapnia and under either anaesthesia or decerebration. With neuromuscular blockade and artificial ventilation, efferent discharges were present for both inspiration and expiration in both external and internal intercostal nerves. This was also the case for proximal internal intercostal nerve branches that innervate only internal intercostal and subcostalis muscles. If active, this region of muscle in other species is always expiratory. Here, inspiratory bursts were almost always present. The expiratory activity appeared only gradually and intermittently, when the anaesthesia was allowed to lighten or as the pre-decerebration anaesthesia wore off. The intermittent appearance is interpreted as the coupling of a slow medullary expiratory oscillator with a faster inspiratory one. The patterns of nerve discharges, in particular the inspiratory or biphasic activation of the internal and subcostalis layers, were confirmed by observations of equivalent patterns of EMG discharges in spontaneously breathing preparations, using denervation procedures to identify which muscles generated the signals. Some motor units were recruited in both inspiratory and expiratory bursts. These patterns of activity have not previously been described and have implications both for the functional role of multiple respiratory oscillators in the adult and for the mechanical actions of the muscles of the caudal intercostal spaces, including subcostalis, which is a partly bisegmental muscle.

Multiple phases of excitation and inhibition in central respiratory drive potentials of thoracic motoneurones in the rat

Intracellular recordings were made from motoneurones with axons in the intercostal nerves of T9 or T10 in adult rats, with neuromuscular blockade and artificial ventilation, under hypercapnia and under either anaesthesia or decerebration. In nearly all motoneurones, central respiratory drive potentials (CRDPs) were seen, which included an excitatory wave in inspiration, in expiration, or in both of these. This was the case both for motoneurones with axons in the internal intercostal nerve (n = 81) and for those with axons in the external intercostal nerve (n = 5). In the decerebrates, motoneurones with purely inspiratory CRDPs were rare (1/44), but those excited in both phases (showing biphasic CRDPs) were common (22/44). For about one-third of biphasic CRDPs (11/30), the inspiratory depolarization was seen to reverse to a hyperpolarization when the motoneurone was depolarized, which was interpreted as indicating concurrent inhibition and excitation during this phase. A few motoneurones were seen where depolarization revealed signs of inhibition in both phases. The results confirm the novel observations of biphasic excitation in individual intercostal nerve branches, EMG sites and motor units reported in a companion paper. They also provide new insights into the functional roles of inhibition in motoneurones physiologically activated in natural rhythmic behaviours.

Are rodents an appropriate pre-clinical model for treating spinal cord injury? Examples from the respiratory system

Because most studies of the effects of spinal cord injury (SCI) and resulting repair and treatments use rodent models, it is important to determine if these models are relevant to humans. In this review, we focus on alterations in respiratory function as a result of SCI. Several injury paradigms have been used in the rat to examine restoration of post-lesion respiratory function and potential benefits from repair strategies designed for humans. Unlike the corticospinal locomotor system, respiratory neural organization is well preserved between rodents and humans, and resembles the general organization of motor pathways in primates. These similarities justify the use of the rodent respiratory system as a model to analyze SCI and putative repair strategies.

Intercostal and abdominal respiratory motoneurons in the neonatal rat spinal cord: spatiotemporal organization and responses to limb afferent stimulation

Respiration requires the coordinated rhythmic contractions of diverse muscles to produce ventilatory movements adapted to organismal requirements. During fast locomotion, locomotory and respiratory movements are coordinated to reduce mechanical conflict between these functions. Using semi-isolated and isolated in vitro brain stem-spinal cord preparations from neonatal rats, we have characterized for the first time the respiratory patterns of all spinal intercostal and abdominal motoneurons and explored their functional relationship with limb sensory inputs. Neuroanatomical and electrophysiological procedures were initially used to locate intercostal and abdominal motoneurons in the cord. Intercostal motoneuron somata are distributed rostrocaudally from C(7)-T(13) segments. Abdominal motoneuron somata lie between T(8) and L(2). In accordance with their soma distributions, inspiratory intercostal motoneurons are recruited in a rostrocaudal sequence during each respiratory cycle. Abdominal motoneurons express expiratory-related discharge that alternates with inspiration. Lesioning experiments confirmed the pontine origin of this expiratory activity, which was abolished by a brain stem transection at the rostral boundary of the VII nucleus, a critical area for respiratory rhythmogenesis. Entrainment of fictive respiratory rhythmicity in intercostal and abdominal motoneurons was elicited by periodic low-threshold dorsal root stimulation at lumbar (L(2)) or cervical (C(7)) levels. These effects are mediated by direct ascending fibers to the respiratory centers and a combination of long-projection and polysynaptic descending pathways. Therefore the isolated brain stem-spinal cord in vitro generates a complex pattern of respiratory activity in which alternating inspiratory and expiratory discharge occurs in functionally identified spinal motoneuron pools that are in turn targeted by both forelimb and hindlimb somatic afferents to promote locomotor-respiratory coupling.

Postnatal developmental changes in activation profiles of the respiratory neuronal network in the rat ventral medulla

Two putative respiratory rhythm generators (RRGs), the para-facial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC), have been identified in the neonatal rodent brainstem. To elucidate their functional roles during the neonatal period, we evaluated developmental changes of these RRGs by optical imaging using a voltage-sensitive dye. Optical signals, recorded from the ventral medulla of brainstem-spinal cord preparations of neonatal (P0-P4) rats (n = 44), were analysed by a cross correlation method. With development during the first few postnatal days, the respiratory-related activity in the pFRG reduced and shifted from a preinspiratory (P0-P1) to an inspiratory (P2-P4) pattern, whereas preBötC activity remained unchanged. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) augmented preinspiratory activity in the pFRG, while the mu-opioid antagonist naloxone induced changes in spatiotemporal activation profiles that closely mimicked the developmental changes. These results are consistent with the recently proposed hypothesis by Janczewski and Feldman that the pFRG is activated to compensate for the depression of the preBötC by perinatal opiate surge. We conclude that significant reorganization of the respiratory neuronal network, characterized by a reduction of preinspiratory activity in the pFRG, occurs at P1-P2 in rats. The changes in spatiotemporal activation profiles of the pFRG neurones may reflect changes in the mode of coupling of the two respiratory rhythm generators.

Influence of hypercapnic acidosis and hypoxia on abdominal expiratory nerve activity in the rat

We studied the influence of hypercapnic acidosis and hypoxia on the neural drive to abdominal muscles in anesthetized and decerebrate rats; this information is unavailable despite widespread use of the rat as an experimental model in respiratory physiology and neurobiology. To minimize confounding influences from receptors in the lungs and chest wall, the animals were vagotomized, paralyzed and mechanically ventilated, and electrical activity was recorded from abdominal muscle nerves. In anesthetized and decerebrate rats, both stimuli evoked steady, low amplitude expiratory discharge that persisted throughout the expiratory phase (E-all activity), but was inhibited during inspiration. We also observed late expiratory, high-amplitude bursts (E2 activity) superimposed on this steady activity, but only at the highest levels of respiratory drive. Hypoxia enhanced abdominal motor activity transiently, whereas hypercapnic acidosis caused a sustained increase in activity. Thus, both hypercapnic acidosis and hypoxia activate abdominal muscle motoneurons in the absence of phasic afferent inputs.

Respiratory action of the intercostal muscles

The mechanical advantages of the external and internal intercostals depend partly on the orientation of the muscle but mostly on interspace number and the position of the muscle within each interspace. Thus the external intercostals in the dorsal portion of the rostral interspaces have a large inspiratory mechanical advantage, but this advantage decreases ventrally and caudally such that in the ventral portion of the caudal interspaces, it is reversed into an expiratory mechanical advantage. The internal interosseous intercostals in the caudal interspaces also have a large expiratory mechanical advantage, but this advantage decreases cranially and, for the upper interspaces, ventrally as well. The intercartilaginous portion of the internal intercostals (the so-called parasternal intercostals), therefore, has an inspiratory mechanical advantage, whereas the triangularis sterni has a large expiratory mechanical advantage. These rostrocaudal gradients result from the nonuniform coupling between rib displacement and lung expansion, and the dorsoventral gradients result from the three-dimensional configuration of the rib cage. Such topographic differences in mechanical advantage imply that the functions of the muscles during breathing are largely determined by the topographic distributions of neural drive. The distributions of inspiratory and expiratory activity among the muscles are strikingly similar to the distributions of inspiratory and expiratory mechanical advantages, respectively. As a result, the external intercostals and the parasternal intercostals have an inspiratory function during breathing, whereas the internal interosseous intercostals and the triangularis sterni have an expiratory function.

Descending respiratory polysynaptic inputs to cervical and thoracic motoneurons diminish during early postnatal maturation in rat spinal cord

Isolated brainstem-spinal cord preparations were used to explore the coexistence of a direct and an indirect descending drive from the brainstem respiratory centre to cervical and thoracic respiratory motoneurons in the neonatal Sprague-Dawley rat. Polysynaptic spinal relay pathways from the respiratory centre were suppressed by selectively perfusing the cord with mephenesin (1 mM) or a solution enriched with Ca2+ and Mg2+. At birth, both direct and spinally relayed pathways are functional and contribute equally to the global descending respiratory drive. However, during the first postnatal week, significant maturational changes appear in the way the respiratory centre controls its target respiratory motoneurons in the cervical and thoracic spinal cord, with the direct respiratory drive becoming progressively predominant with maturation (from 50% to around 75% of the global descending command). The relative contributions of the monosynaptic and the polysynaptic spinal pathways may therefore have important implications for effective respiratory control during early postnatal development.

Chemical activation of C1-C2 spinal neurons modulates intercostal and phrenic nerve activity in rats

Chemical activation of upper cervical spinal neurons modulates activity of thoracic respiratory interneurons in rats. The aim of the present study was to examine the effects of chemical activation of C1-C2 spinal neurons on thoracic spinal respiratory motor outflows. Electroneurograms of left phrenic ( n = 23) and intercostal nerves (ICNs, n = 93) between T3 and T8 spinal segments were recorded from 36 decerebrated, vagotomized, paralyzed, and ventilated male rats. To activate upper cervical spinal neurons, glutamate pledgets (1 M, 1 min) were placed on the dorsal surface of the C1-C2 spinal cord. Glutamate on C1-C2 increased ICN tonic activity in 56/59 (95%) ICNs. The average maximal tonic activity of ICN was increased by 174% ( n = 59). After spinal transection at rostral C1, glutamate on C1-C2 still increased ICN tonic activity in 33/35 ICNs. However, the effects of C1-C2 glutamate on ICN phasic activity were highly variable, with observations of augmentation or suppression of both inspiratory and expiratory discharge. C1-C2 glutamate augmented the average amplitude of phrenic burst by 20%, whereas the increases in amplitude of ICN inspiratory activity, when they occurred, averaged 120%. The burst rate of phrenic nerve discharge was decreased from 34.2 ± 1.6 to 26.3 ± 2.0 (mean ± SE) breaths/min during C1-C2 glutamate. These data suggested that upper cervical propriospinal neurons might play a role in descending modulation of thoracic respiratory and nonrespiratory motor activity.

A commentary on eupnoea and gasping

This commentary discusses the differences between patterns of bursting activity recorded from the phrenic nerves of different species, in several experimental preparations and under differing conditions. The spectrum of bursting activity patterns varies from that termed eupnoic to that termed gasping. Taking the pattern of activity recorded in the least reduced preparation as a standard for normality, i.e. eupnoea, consideration is given to the possible factors affecting the pattern of bursting activity in progressively reduced preparations. An examination of the conditions of these preparations leads to the conclusion that tissue gas exchange is a major determinant of bursting pattern, and consideration is given to the possible differences in respiratory rhythm generation that can be inferred from these different patterns.

Chemical activation of C(1)-C(2) spinal neurons modulates activity of thoracic respiratory interneurons in rats

Discharge patterns of thoracic dorsal horn neurons are influenced by chemical activation of cell bodies in cervical spinal segments C(1)-C(2). The present aim was to examine whether such activation would specifically affect thoracic respiratory interneurons (TRINs) of the deep dorsal horn and intermediate zone in pentobarbital sodium-anesthetized, paralyzed, artificially ventilated rats. We also characterized discharge patterns and pathways of TRIN activation in rats. A total of 77 cells were classified as TRINs by location, continued burst activity related to phrenic discharge when the respirator was stopped, and lack of antidromic response from selected pathways. A variety of respiration-phased discharge patterns was documented whose pathways were interrupted by ipsilateral C(1) transection. Glutamate pledgets (1 M, 1 min) on the dorsal surface of the spinal cord inhibited 22/49, excited 15/49, or excited/inhibited 3/49 tested cells. Incidence of responses did not depend on whether the phase of TRIN discharge was inspiratory, expiratory, or biphasic. Phrenic nerve activity was unaffected by chemical activation of C(1)-C(2) in this preparation. Besides supraspinal input, TRIN activity may be influenced by upper cervical modulatory pathways.

Respiratory pre-motor control of hypoglossal motoneurons in the rat

The goal of this study was to determine the origin and transmission pathway of respiratory drive to hypoglossal motoneurons. First we recorded intracellularly from 28 antidromically activated inspiratory hypoglossal motoneurons (resting membrane potential, -50+/-3 mV), and found that injection of chloride ions had no discernible effect on the shape of their membrane potential trajectories. We concluded that the membrane potential trajectories of these hypoglossal motoneurons were determined primarily by inspiratory excitation. To determine the origin of this excitation we cross-correlated the extracellular discharge of medullary inspiratory neurons, including those in the hypoglossal motor nucleus, with the hypoglossal nerve discharge. We found 27 inspiratory neurons within the hypoglossal motor nucleus that were not antidromically activated from the ipsilateral hypoglossal nerve; their cross-correlograms featured either central peaks (1.7+/-0.2 ms) alone (n=14; 39%), or central peaks (1.3+/-0.2 ms) followed by troughs (1.3+/-0.1 ms) at short latencies (1.1+/-0.4 ms) (n=13; 36%), and suggest that these neurons are hypoglossal interneurons. We recorded from 238 inspiratory neurons throughout the rest of the medulla; the cross-correlograms of 19 neurons (8%), located mostly in the lateral tegmental field, displayed narrow half-amplitude peaks (1.0+/-0.1 ms) at short latencies (0.9+/-0.1 ms), which we interpreted as evidence for monosynaptic excitation of hypoglossal motoneurons.We conclude that the respiratory control of hypoglossal motoneurons originates from inspiratory premotor neurons scattered throughout the lateral tegmental field and interneurons within the hypoglossal motor nucleus.

Effects of abdominal or cardiopulmonary sympathetic afferents on upper cervical inspiratory neurons

Responses of upper cervical inspiratory neurons (UCINs) to abdominal visceral or cardiopulmonary sympathetic stimulation were studied using extracellular recordings from 213 UCINs in 54 pentobarbital sodium-anesthetized and paralyzed rats. Phrenic nerve activity was used to assess inspiration. The UCINs discharging during inspiration only were mainly in the C(1) segment, whereas phase-spanning UCINs were mostly in the C(2) segment. Phase-spanning activity was typically retained after overventilation or vagotomy. When greater splanchnic nerve (GSN) or cardiopulmonary sympathetic afferent (CPSA) fibers were electrically stimulated, augmented UCIN activity was observed in 65% of cells responding to CPSA stimulation but in only 17% of cells responding to GSN. Response latencies were 10.7 +/- 0.5 and 20.6 +/- 1.5 (SE) ms, respectively. Many augmented responses to CPSA stimulation (64%) and all augmented responses to GSN stimulation were followed by suppression of UCIN discharge (biphasic response). Phrenic nerve activity was suppressed by both GSN and CPSA stimulation, but with shorter latency for the latter (29 +/- 0.7 vs. 14.0 +/- 0.7 ms). Excitation of UCINs using CPSA stimulation occurs more often and by a more direct pathway than for GSN input.

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.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Synaptic connections to phrenic motoneurons in the decerebrate rat

Phrenic motoneuron membrane potential trajectories in decerebrate rats exhibit three stages; depolarisation during inspiration, a decreased depolarisation during early expiration and hyperpolarization during late expiration. These trajectories are a result of excitation by ventral-group medullary inspiratory neurons and upper-cervical inspiratory neurons during inspiration and the early part of expiration, and inhibition from Bötzinger-complex expiratory neurons during the late part of expiration.

Bilaterally independent respiratory rhythms in the decerebrate rat

In rats, respiratory neurons in the medulla oblongata are arranged in longitudinally distributed groups that are duplicated on each side of the neuraxis. Our aim was to determine whether respiratory rhythm is generated independently by each side. We made a complete mid-sagittal section of the medulla oblongata, 3.5 mm rostral and 3.5 mm caudal to the obex, in decerebrate, vagotomized, and paralysed adult rats. Respiratory rhythm, monitored by recording the activity of both left and right phrenic nerves, was maintained and became asynchronous between the left and right sides. We concluded that in the adult rat each half of the medulla oblongata is capable of generating respiratory rhythm independently.