Evaluation of role of upper cervical inspiratory neurons in respiration, emesis and cough

Neural substrates of cough control during coughing

Cough is known as a protective reflex to keep the airway free from harmful substances. Although brain activity during cough was previously examined mainly by functional magnetic resonance imaging (fMRI) with model analysis, this method does not capture real brain activity during cough. To obtain accurate measurements of brain activity during cough, we conducted whole-brain scans during different coughing tasks while correcting for head motion using a restraint-free positron emission tomography (PET) system. Twenty-four healthy right-handed males underwent multiple PET scans with [15O]H2O. Four tasks were performed during scans: "resting"; "voluntary cough (VC)", which simply repeated spontaneous coughing; "induced cough (IC)", where participants coughed in response to an acid stimulus in the cough-inducing method with tartaric acid (CiTA); and "suppressed cough (SC)", where coughing was suppressed against CiTA. The whole brain analyses of motion-corrected data revealed that VC chiefly activated the cerebellum extending to pons. In contrast, CiTA-related tasks (IC and SC) activated the higher sensory regions of the cerebral cortex and associated brain regions. The present results suggest that brain activity during simple cough is controlled chiefly by infratentorial areas, whereas manipulating cough predominantly requires the higher sensory brain regions to allow top-down control of information from the periphery.

Velocity storage: its multiple roles

Our research described in this article was motivated by the puzzling finding of the Skylab M131 experiments: head movements made while rotating that are nauseogenic and disorienting on Earth are innocuous in a weightless, 0-g environment. We describe a series of parabolic flight experiments that directly addressed this puzzle and discovered the gravity-dependent responses to semicircular canal stimulation, consistent with the principles of velocity storage. We describe a line of research that started in a different direction, investigating dynamic balancing, but ended up pointing to the gravity dependence of angular velocity-to-position integration of semicircular canal signals. Together, these lines of research and the theoretical framework of velocity storage provide an answer to at least part of the M131 puzzle. We also describe recently discovered neural circuits by which active, dynamic vestibular, multisensory, and motor signals are interpreted as either appropriate for action and orientation or as conflicts evoking motion sickness and disorientation.

Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

The nucleus retroambiguus as possible site for inspiratory rhythm generation caudal to obex

We investigated whether spinalized animals can produce inspiratory rhythm. We recorded spinal inspiratory phrenic (PNA) and cranial inspiratory hypoglossal (HNA) nerve activity in the perfused brainstem preparation of rat. Complete transverse transections were performed at 1.5 (pyramidal decussation) or 2mm (first cervical spinal segment) caudal to obex. Excitatory drive was enhanced by either extracellular potassium, hypercapnia or by stimulating arterial chemoreceptors. Caudal transections immediately eliminated descending network drive for PNA, while the cranial inspiratory HNA remained unaffected. After transection, PNA bursting remained sporadic even during enhanced excitatory drive. This implies, cervical spinal circuits lack intrinsic rhythmogenic capacity. Rostral transections also abolished PNA immediately. However, HNA also progressively lost its amplitude and rhythm. Chemoreceptor activation only triggered tonic, non-rhythmic HNA. Thus the integrity of ponto-medullary circuitry was maintained. Our results suggest that an area overlapping the caudal nucleus retroambiguus provides critical ascending input to the ponto-medullary respiratory network for inspiratory rhythm generation.

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.

Spinal circuitry and respiratory recovery following spinal cord injury

Numerous studies have demonstrated anatomical and functional neuroplasticity following spinal cord injury. One of the more notable examples is return of ipsilateral phrenic motoneuron and diaphragm activity which can be induced under terminal neurophysiological conditions after high cervical hemisection in the rat. More recently it has been shown that a protracted, spontaneous recovery also occurs in this model. While a candidate neural substrate has been identified for the former, the neuroanatomical basis underlying spontaneous recovery has not been explored. Demonstrations of spinal respiratory interneurons in other species suggest such cells may play a role; however, the presence of interneurons in the adult rat phrenic circuit - the primary animal model of respiratory plasticity - has not been extensively investigated. Emerging neuroanatomical and electrophysiological results raise the possibility of a more complex neural network underlying spontaneous recovery of phrenic function and compensatory respiratory neuroplasticity after C2 hemisection than has been previously considered.

Vestibular inputs to propriospinal interneurons in the feline C1-C2 spinal cord projecting to the C5-C6 ventral horn

The resting length of respiratory muscles must be altered during changes in posture in order to maintain stable ventilation. Prior studies showed that although the vestibular system contributes to these adjustments in respiratory muscle activity, the medullary respiratory groups receive little vestibular input. Additionally, previous transneuronal tracing studies demonstrated that propriospinal interneurons in the C(1)-C(2) spinal cord send projections to the ipsilateral diaphragm motor pool. The present study tested the hypothesis that C(1)-C(2) interneurons mediate vestibular influences on diaphragm activity. Recordings were made from 145 C(1)-C(2) neurons that could be antidromically activated from the ipsilateral C(5)-C(6 )ventral horn, 60 of which had spontaneous activity, during stimulation of vestibular receptors using electric current pulses or whole-body rotations in vertical planes. The firing of 19 of 31 spontaneously active neurons was modulated by vertical vestibular stimulation; the response vector orientations of many of these cells were closer to the pitch plane than the roll plane, and their response gains remained relatively constant across stimulus frequencies. Virtually all spontaneously active neurons responded robustly to electrical vestibular stimulation, and their response latencies were typically shorter than those for diaphragm motoneurons. Nonetheless, respiratory muscle responses to vestibular stimulation were still present after inactivation of the C(1)-C(2) cord using large injections of either muscimol or ibotenic acid. These data suggest that C(1)-C(2) propriospinal interneurons contribute to regulating posturally related responses of the diaphragm, although additional pathways are also involved in generating this activity.

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.

Cough induced by ossification of the ligamentum flavum in the high cervical spine

✓ The authors report a very rare case of high cervical ossification of the ligamentum flavum (OLF) in a 40-year-old woman who developed an intractable cough after a traffic accident. The patient's symptoms subsided immediately after decompressive laminectomy and removal of the lesion. To the authors' knowledge, this is the first reported case of high cervical OLF in a patient who presented with a cough. The pathophysiological mechanism underlying the cough was determined to be symptomatic of high cervical spine OLF.

Central nervous mechanisms of cough

Cough is an airway defensive reflex substantially consisting in a modified respiratory act. Transection experiments have shown that the fundamental structures responsible for this reflex are located within the medulla oblongata. Electrical stimulation applied to the medulla failed to provide convincing evidence of a cough centre distinct from the brainstem respiratory network. In fact, electrical stimuli affect not only neuronal somata, but also intramedullary cough-related pathways. Studies on the behaviour of medullary respiratory neurones have led to the conclusion that the same respiratory neurones involved in the generation of the eupnoeic pattern of breathing also participate in the production of the cough motor pattern. These findings support the existence of multifunctional neural networks in the mammal brainstem. Bötzinger complex expiratory neurones with augmenting discharge patterns have been suggested to convey an excitatory drive to the expiratory bulbospinal neurones of the caudal ventral respiratory group and, hence, to expiratory motoneurones. The excitatory drive to caudal medullary expiratory neurones is mediated by ionotropic glutamate receptors. Recent lines of evidence indicate that the Bötzinger complex and the caudal ventral respiratory group have a crucial role in determining both the inspiratory and the expiratory components of the cough motor pattern.

Role of the medial medullary reticular formation in relaying vestibular signals to the diaphragm and abdominal muscles

Changes in posture can affect the resting length of respiratory muscles, requiring alterations in the activity of these muscles if ventilation is to be unaffected. Recent studies have shown that the vestibular system contributes to altering respiratory muscle activity during movement and changes in posture. Furthermore, anatomical studies have demonstrated that many bulbospinal neurons in the medial medullary reticular formation (MRF) provide inputs to phrenic and abdominal motoneurons; because this region of the reticular formation receives substantial vestibular and other movement-related input, it seems likely that medial medullary reticulospinal neurons could adjust the activity of respiratory motoneurons during postural alterations. The objective of the present study was to determine whether functional lesions of the MRF affect inspiratory and expiratory muscle responses to activation of the vestibular system. Lidocaine or muscimol injections into the MRF produced a large increase in diaphragm and abdominal muscle responses to vestibular stimulation. These vestibulo-respiratory responses were eliminated following subsequent chemical blockade of descending pathways in the lateral medulla. However, inactivation of pathways coursing through the lateral medulla eliminated excitatory, but not inhibitory, components of vestibulo-respiratory responses. The simplest explanation for these data is that MRF neurons that receive input from the vestibular nuclei make inhibitory connections with diaphragm and abdominal motoneurons, whereas a pathway that courses laterally in the caudal medulla provides excitatory vestibular inputs to these motoneurons.

alpha1-adrenergic receptor-induced slow rhythmicity in nonrespiratory cervical motoneurons of neonatal rat spinal cord

Previous studies have reported that the alpha1-adrenergic system can activate spinal rhythm generators belonging to the central respiratory network. In order to analyse alpha1-adrenergic effects on both cranial and spinal motoneuronal activity, phenylephrine (1-800 microM) was applied to in vitro preparations of neonatal rat brainstem-spinal cord. High concentration of phenylephrine superfusion exerted multiple effects on spinal cervical outputs (C2-C6), consisting of a lengthening of respiratory period and an increase in inspiratory burst duration. Furthermore, in 55% of cases a slow motor rhythm recorded from the same spinal outputs was superimposed on the inspiratory activity. However, this phenylephrine-induced slow motor rhythm generated at the spinal level was observed neither in inspiratory cranial nerves (glossopharyngeal, vagal and hypoglossal outputs) nor in phrenic nerves. Whole-cell patch-clamp recordings were carried out on cervical motoneurons (C4-C5), to determine first which motoneurons were involved in this slow rhythm, and secondly the cellular events underlying direct phenylephrine effects on motoneurons. In all types of motoneurons (inspiratory and nonrespiratory) phenylephrine induced a prolonged depolarization with an increase in neuronal excitability. However, only nonrespiratory motoneurons showed additional rhythmic membrane depolarizations (with spiking) occurring in phase with the slow motor rhythm recorded from the ventral root. Furthermore the tonic depolarization produced in all motoneurons results from an inward current [which persists in the presence of tetrodotoxin (TTX)] associated with a decrease in neuron input conductance, with a reversal potential varying as a Nernstian function of extracellular K+ concentration. Our results indicate that the alpha1-adrenoceptor activation: (i) affects both the central respiratory command (i.e. respiratory period and inspiratory burst duration) and spinal inspiratory outputs; (ii) induces slow spinal motor rhythmicity, which is unlikely to be related to the respiratory system; and (iii), increases motoneuronal excitability, probably through a decrease in postsynaptic leak K+ conductance.

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.

Reconfiguration of the respiratory network at the onset of locust flight

Reconfiguration of the respiratory network at the onset of locust flight. J. Neurophysiol. 80: 3137-3147, 1998. The respiratory interneurons 377, 378, 379 and 576 were identified within the suboesophageal ganglion (SOG) of the locust. Intracellular stimulation of these neurons excited the auxillary muscle 59 (M59), a muscle that is involved in the control of thoracic pumping in the locust. Like M59, these interneurons did not discharge during each respiratory cycle. However, the SOG interneurons were part of the respiratory rhythm generator because brief intracellular stimulation of these interneurons reset the respiratory rhythm and tonic stimulation increased the frequency of respiratory activity. At the onset of flight, the respiratory input into M59 and the SOG interneurons was suppressed, and these neurons discharged in phase with wing depression while abdominal pumping movements remained rhythmically active in phase with the slower respiratory rhythm (Fig. ). The suppression of the respiratory input during flight seems to be mediated by the SOG interneuron 388. This interneuron was tonically activated during flight, and intracellular current injection suppressed the respiratory rhythmic input into M59. We conclude that the respiratory rhythm generator is reconfigured at flight onset. As part of the rhythm-generating network, the interneurons in the SOG are uncoupled from the rest of the respiratory network and discharge in phase with the flight rhythm. Because these SOG interneurons have a strong influence on thoracic pumping, we propose that this neural reconfiguration leads to a behavioral reconfiguration. In the quiescent state, thoracic pumping is coupled to the abdominal pumping movements and has auxillary functions. During flight, thoracic pumping is coupled to the flight rhythm and provides the major ventilatory movements during this energy-demanding locomotor behavior.

Multifunctional ventral respiratory group: bulbospinal expiratory neurons play a role in pudendal discharge during vomiting

Pudendal motoneurons are activated in phasic bursts during the retching and expulsion phases of vomiting. The resulting contraction of the anal and urethral sphincters serves to maintain continence during the large increase in abdominal pressure that occurs during vomiting. We evaluated the contribution of bulbospinal expiratory neurons located in the portion of the ventral respiratory group (VRG) caudal to the obex (nucleus retroambigualis) to the control of pudendal motoneurons during fictive vomiting in decerebrate, paralyzed cats. Pudendal nerve discharge is abolished by cutting the axons of caudal VRG expiratory neurons as they cross the midline between the obex and C1 before descending in the spinal cord. All caudal VRG expiratory neurons that were antidromically activated from the sacral spinal cord, where the pudendal motor pool (nucleus of Onuf) is located, discharged strongly during the end of the expulsion phase of vomiting. However, only a small proportion of these neurons was active in phase with pudendal discharge during the retching phase. The apparent involvement of caudal VRG expiratory neurons in the control of pudendal motoneurons during vomiting is another example of the multifunctional role that can be played by respiratory-related neurons in the mammalian nervous system.

Synaptic excitation in the thoracic spinal cord from expiratory bulbospinal neurones in the cat

1. Synaptic actions in the thoracic spinal cord of individual expiratory bulbospinal neurones were studied in anaesthetized cats by the use of two techniques: (i) the monosynaptic connections to motoneurones were assessed by cross-correlations between the discharges of the neurones and efferent discharges in the internal intercostal nerves of several segments bilaterally; and (ii) distributions of terminal and focal synaptic potentials were measured by extracellular spike-triggered averaging in the thoracic ventral horn. 2. Monosynaptic connections were identified by both the durations and timings of observed cross-correlation peaks, taking into account accurate conduction velocity measurements derived from collision tests and from spike-triggered averaging. Discrimination was made against peaks resulting from presynaptic synchronization. 3. Monosynaptic connections to motoneurones were identified for twenty-three out of twenty-seven neurones. The connections to nerves on the side ipsilateral to the cell somata were, on average, about 36% of the strength of those on the contralateral side. The overall strength of the connections was about twice as strong as previous estimates for similar connections from inspiratory bulbospinal neurones to phrenic motoneurones. The monosynaptic pathway was calculated to be able to provide most of the depolarization for the motoneurones concerned and therefore was likely to be the main determinant of their firing patterns under the conditions of these experiments. 4. However, taking into account previous measurements it is considered possible that these connections may only involve a minority of motoneurones, perhaps only 10% of the expiratory population. Thus, in general, the control of the whole pool of expiratory motoneurones, despite the strong monosynaptic connections measured here, is suggested to be mainly dependent on spinal interneurones, as has been concluded previously for inspiratory motoneurones. 5. Spike-triggered averaging revealed that nearly all neurones gave signs of collaterals in each of the segments investigated (T7, T8 or T9), as shown by the presence of terminal potentials or focal synaptic potentials, but the projection within a given thoracic segment was non-uniform, in that large-amplitude potentials were more common in the rostral than the caudal part of the segment. This non-uniformity could be a factor involved in the apparently non-heterogeneous connections to the motoneurones.

Brainstem neurons with projecting axons to both phrenic and abdominal motor nuclei: a double fluorescent labeling study in the cat

The distribution of retrogradely doubly labeled brainstem neurons were analyzed in the cat after injection of two different fluorescent markers into the phrenic and abdominal motor nuclei. Diamidino Yellow (DY) was first injected either ipsilaterally or bilaterally into the ventral horn of lumbar spinal cord, and then Fast Blue (FB) into the right ventral horn of cervical spinal cord. Doubly labeled neurons were mainly found in the caudal ventrolateral medulla (retroambiguus region), in the dorsomedial and dorsolateral regions of the nucleus of the tractus solitarius (NTS) and in the raphe nuclei. In addition, doubly labeled neurons were found in the parabrachial and Kölliker-Fuse nuclei. Our results give anatomical evidence that pontine and medullary neurons are the source of a common pathway to both phrenic and abdominal motor nuclei. These neurons might be involved in strain efforts for expulsion such as vomiting or defecation.