Localization of respiratory bulbospinal and propriobulbar neurons in the region of the nucleus ambiguus of the rat

The location of neurons within the ventral respiratory group (VRG) of rat was mapped following injections of 3 different fluorochrome tracers into different sites known to receive projections from VRG neurons. Injection sites included muscles innervated by the vagus (X) and glossopharyngeal (IX) nerves, and the sites of expiratory activity in the caudal medulla and of inspiratory activity in the spinal cord at the C4 level. Labeling of vagal motoneurons resulting from fluorochrome injections into muscles innervated by X and IX nerves was always ipsilateral to the site of injection. Both propriobulbar and bulbospinal neurons had primarily ipsilateral projections. No double-labeled cell bodies were observed. The cell bodies of the 3 types of neurons, propriobulbar, bulbospinal and vagal/glossopharyngeal, were unevenly distributed along the rostrocaudal axis of the VRG, suggesting a complex mosaic of neurons which regulate respiratory-related functions such as swallowing and vocalization.     

Possible roles for GABAergic inhibition in the vocal control system of the zebra finch

The final common output from the telencephalic vocal control system in songbirds is the projection from nucleus RA, which drives respiratory and syringeal muscles via medullary nuclei. We examined the possible role of GABAergic inhibition in RA of adult male zebra finches by micro-injecting bicuculline, an antagonist of inhibitory GABA(A) receptors, while recording simultaneously with multiple microelectrodes. Following bicuculline injection, the normally high spontaneous activity of RA neurons exhibited a pattern of rhythmic bursting lasting up to 30 min. The bursts were often accompanied by involuntary vocalizations: monosyllabic notes resembling calls. Other experiments used microinjections that were below threshold for involuntary vocalization. When the bird sang to a female during a period after the injection, song structure was degraded: song duration was lengthened, noisiness increased, and novel syllables appeared. The results suggest that GABA normally contributes to regulating excitability in RA. When this regulation is blocked, activity increases sufficiently to engage the respiratory and vocal musculature. Synaptic inputs that affect GABAergic interneurons in RA could thus play a role in initiation and control of vocalization. The abnormal vocalizations produced in the presence of bicuculline suggest that GABAergic inhibition may normally help to shape the pattern of learned vocalizations, as well as to regulate overall RA activity.     

Sympathetic skin response evoked by respiratory stimulation as a measure of sympathetic function.

OBJECTIVES: To compare respiratory and electrical methods of evoking a sympathetic skin response (SSR). METHODS: SSRs evoked by both electrical and respiratory stimulation were recorded from the palms of 47 healthy volunteers. Expiration and inspiration were used as separate stimuli. The correlation coefficients between the amplitude and latency of the SSR from the palm electrodes and the various components of heart rate variability were calculated. RESULTS: Waveform patterns of the SSRs obtained from electrical stimulation showed varied responses to and habituation to this type of stimulation. On the other hand, no subjects showed a phase change in SSR waveform patterns between the first and last expiratory stimuli. The potentials recorded after expiratory stimulation had significantly greater amplitudes than those recorded after electrical stimuli. The low frequency component of heart rate variability induced by expiratory stimulation was significantly greater than that induced by electrical stimulation. The SSR may also correlate strongly with the change of respiratory rate since a more rapid pressure change occurs during expiratory movement than during inspiratory movements. CONCLUSIONS: The SSR evoked by expiratory stimulation is more reliable than either electrical stimulation or inspiratory stimulation for determining sympathetic function.     

Split medulla preparation in the cat: arterial chemoreceptor reflex and respiratory modulation of the renal sympathetic nerve activity

The study was undertaken in order to assess the changes in sympathetic output in a split medulla preparation of the cat which, as shown earlier, has impaired respiratory rhythm generation. The effects of medullary midsagittal sections on renal sympathetic nerve firing were investigated in chloralose anesthetized, paralyzed and artificially ventilated cats. Recordings of phrenic and recurrent laryngeal nerve activity served as indices of central respiratory rhythm generation. Sections, 5 mm deep from the dorsal medullary surface and extending 6 mm rostrally and 3 mm caudally to the obex, did not produce any significant changes in heart rate, blood pressure or tonic renal sympathetic nerve firing levels. They decreased or abolished, however, the respiratory rhythmicity in renal sympathetic nerve which paralleled the reduction of inspiratory discharges in phrenic and recurrent laryngeal nerves, and abolished the carotid body chemoreceptor-sympathetic reflex. The inspiratory activity remaining after the sections could still be enhanced by chemoreceptor stimulation. The inhibitory baroreceptor and pulmonary stretch receptor sympathetic reflexes, and the central excitatory effect of CO2 on renal sympathetic nerve firing were not altered. The effects of electrical stimulation within the midsagittal plane of the medulla have shown that descending pathways from the medullary inspiratory neurons (or their medullary collaterals) do not participate in the facilitation of spinal preganglionic neurons during inspiration and in relaying the pulmonary stretch receptor inhibitory sympathetic reflex. A region located close to the obex was identified from which excitatory responses in renal sympathetic nerves, compatible with the response obtained by carotid sinus nerve stimulation, could be evoked. It is concluded that a lesion in the midline of the lower medulla at the level of the obex selectively destroys cells or pathways which relay the carotid body chemoreceptor-sympathetic reflex.     

Efferent activity in the phrenic nerve during the startle reflex in chloralose-anesthetized cats

Reflex activity in the phrenic nerve was studied in chloralose anesthetized cats during development of somatic startle reflexes in limb and lower intercostal nerves. It was shown that the main component of this activity during low-threshold reflexes evoked by acoustic, tactile and low-threshold somatic afferent stimulation was depression of phrenic inspiratory activity. The following reflex discharges were prevalent components of phrenic responses to high-threshold afferent stimulation: early, propriospinal (intercostal-to-phrenic reflex) and late, suprasegmental ones. The latter were of two types: inspiratory (observed mainly during inspiration in about 75% of experiments) and expiratory (observed during expiration in 25% of experiments) which could be classified as "phrenic startle reflexes". Modulation of all responses during the respiratory cycle was described. Structural characteristics of reflex responses evoked in the phrenic nerve by stimulation of various respiratory and nonrespiratory bulbar sites as well as their respiratory modulation have been analyzed. Organization of possible neurophysiological mechanisms of phrenic responses during startle reflexes is discussed.     

Respiratory interneurons in the C5 segment of the spinal cord of the cat

Extracellular recordings were made in the C5 segment of the spinal cord of anaesthetised cats from 129 units which showed respiratory phased discharge. The majority of recordings (88%) were thought to arise from the somata of respiratory spinal interneurons. Inspiratory units and expiratory units comprised 42% and 52% of all recorded units. A small number of postinspiratory units were also found (n = 5). Most units did not respond to electrical stimulation of the ipsilateral superior laryngeal (SLN) and phrenic nerves (PN), but a few expiratory (n = 2) and postinspiratory units (n = 1) were excited by SLN stimulation, while 6 inspiratory units had their discharge suppressed by the same stimulus. PN stimulation evoked a long latency (2-7 ms) burst of firing in 2 inspiratory and 1 expiratory interneurons. It is concluded that these respiratory interneurons may provide a segmental input to phrenic motoneurons, in addition to synaptic drives mediated by bulbospinal pathways.     

Identification and connections of inspiratory premotor neurons in songbirds and budgerigar

Recordings of extracellular unit activity in the ventrolateral medulla and of electyromyographic activity in either the M. scalenus, a principal inspiratory muscle, or the abdominal expiratory muscles, were used to identify inspiratory related (IR) neurons. IR neurons extended from levels caudal to the obex through the caudal level of the descending vestibular nucleus. This distribution was found to correspond to that of a subset of cells retrogradely labeled from injections of neuronal tracers into the upper thoracic spinal cord, where motoneurons innervating the M. scalenus were located by retrograde transport. Injections of biotinylated dextran amine at the recording sites resulted in projections to the spinal cord and brainstem. Bulbospinal axons traveled in the lateral funiculus, predominantly contralaterally, and terminated in relation to the dendrites and cell bodies of motoneurons innervating the M. scalenus. Brainstem nuclei receiving projections from injections at IR loci included the retroambigualis, tracheosyringeal motor nucleus, ventrolateral nucleus of the rostral medulla, infra-olivaris superior, ventrolateral parabrachial nucleus, and the dorsomedial nucleus of the intercollicular complex. In the finches, there were also bilateral projections to nucleus uvaeformis of the posterior thalamus. The spinal and brainstem projections are similar to those found in pigeon (Reinke and Wild, [1997] J. Comp. Neurol. 379:347–362), and probably mediate the intricate coordination of the vocal (syringeal) and respiratory systems for the control of vocalization. The distribution of IR neurons in birds is similar to that of the rostral ventral respiratory group (rVRG) in mammals. J. Comp. Neurol. 391:147–163, 1998. © 1998 Wiley-Liss, Inc.     

Spontaneous and reflexly evoked laryngeal abductor and adductor muscle activity of cat

A study of spontaneous and reflexly evoked activity of laryngeal abductor (posterior cricoarytenoid) and adductor (thyroarytenoid and lateral cricoarytenoid) muscles was carried out in cats anesthetized with chloraloseurethane or made decerebrate and supplemented with ketamine HCl. The posterior cricoarytenoid muscle was active largely during inspiration but showed tonic activity throughout the respiratory cycle; the thyroarytenoid and lateral cricoarytenoid muscles were rhythmically active during expiration. Anesthetic amounts of pentobarbital abolished adductor rhythmicity and enhanced cyclic inspiratory activity of the abductor muscle. Hyperventilation increased the tonic adductor muscle activity while diminishing abductor muscle activity prior to resolution of apnea. Glossopharyngeal (epipharyngeal branch) and superior laryngeal nerve stimulation evoked chiefly excitatory effects on adductors and largely and attenuating effect on the abductor during inspiration. Stimulation of caudal intercostal nerves caused similar effects but to a lesser degree. Peripheral phrenic nerve stimulation during inspiration facilitated reflex abductor muscle activity whereas such stimulation during expiration facilitated reflex adductor muscle activity. The collective evidence further supports the conclusion that the larynx has a dual function: that of a respiratory organ (widening of the glottis during inspiration and its narrowing during expiration) and of a guardian of the lower respiratory tract from invading foreign matter (reflex sphincter action of contracting adductor muscles with relaxation of the abductor).     

Types and locations of respiratory-related neurons in lateral tegmental field of cat medulla oblongata

Extracellular microelectrode recordings were made from a total of 868 neurons in the medullas of cats in regions known to contain high densities of respiratory-related neurons (solitary tract complex, nucleus ambiguus/retroambigualis, lateral tegmental field). Both the discharge patterns and the locations of units were noted and correlated with a recently described substructure of the tegmental field of the cat medulla in which neuronal cell bodies are found associated with sheets of blood vessels supplying the brainstem. The majority of cells were phasically firing (59%) with activity confined to either the inspiratory or the expiratory phase, 21% were tonically firing cells with no discernible respiratory modulation and 20% were silent neurons, responsive to electrical stimulation of the vagus nerves or the dorsolateral pons in the vicinity of nucleus parabrachialis, but not to various respiratory stimuli. Within the solitary tract complex inspiratory discharge patterns were predominant (94%), while in nucleus ambiguus/retroambigualis 26% of the neurons had expiratory patterns with the rest being inspiratory (68%) or tonic (6%). Within the lateral tegmental field, the percentages of inspiratory, expiratory and tonic patterns were 51, 9 and 40%. Thus, inspiratory type patterns were found throughout the medulla, but expiratory patterns were most common in the ambiguus/retroambigualis nuclei. Found within all 3 major regions, but primarily within the lateral tegmental field of the rostral medulla were neurons that discharged with a brief burst at the inspiratory to expiratory phase transition. These cells had properties consistent with the off-switch mechanism: extreme late-inspiratory onset of discharge with the onset time being delayed by lung inflation, peak discharge at or slightly after the peak activity of the diaphragmatic EMG and a discharge rate which was insensitive to lung inflation. Within the lateral tegmental field, where longitudinal sheets of blood vessels running radially with respect to the IVth ventricle have been described, it was found that 85% of the tonically active units and 93% of the respiratory modulated cells were located less than 200 microns from the planes of these sheets. In addition, 87% of the neurons that could be antidromically or synaptically activated from the dorsolateral rostral pons were similarly located.     

Distribution and connections of inspiratory premotor neurons in the brainstem of the pigeon (Columba livia)

We have recorded extracellular, inspiratory-related (IR) unit activity in the medulla at locations corresponding to those of neurons retrogradely labeled by injections of retrograde tracers in the lower brachial and upper thoracic spinal cord, injections that covered cell bodies and dendrites of motoneurons innervating inspiratory muscles. Bulbospinal neurons were distributed throughout the dorsomedial and ventrolateral medulla, from the spinomedullary junction through about 0.8 mm rostral to the obex. Almost all of the 104 IR units recorded were located in corresponding parts of the ventrolateral medulla, rostral to nucleus retroambigualis, where expiratory related units are found. Injections of biotinylated dextran amine at the recording sites labeled projections both to the spinal cord and to the brainstem. In the lower brachial and upper thoracic spinal cord, bulbospinal axons traveled predominantly in the contralateral dorsolateral funiculus and terminated in close relation to the dendrites of inspiratory motoneurons retrogradely labeled with cholera toxin B-chain. In the brainstem, there were predominantly ipsilateral projections to the nucleus retroambigualis, tracheosyringeal motor nucleus (XIIts), ventrolateral nucleus of the rostral medulla, infraolivary superior nucleus, ventrolateral parabrachial nucleus, and dorsomedial nucleus of the intercollicular complex. In all these nuclei, except XIIts, retrogradely labeled neurons were also found, indicating reciprocity of the connections. These results suggest the possibility of monosynaptic connections between inspiratory premotor neurons and inspiratory motoneurons, which, together with connections of IR neurons with other brainstem respiratory-vocal nuclei, seem likely to mediate the close coordination that exists in birds between the vocal and respiratory systems. The distribution of IR neurons in birds is similar to that of the rostral ventral respiratory group (rVRG) in mammals. J. Comp. Neurol. 379:347–362, 1997. © 1997 Wiley-Liss, Inc.     

On the relation of PAG neurons to laryngeal and respiratory muscles during vocalization in the monkey

Despite evidence from previous unit recording, microstimulation, lesioning and anatomical studies, the functions of the midbrain periaqueductal gray (PAG) remain unclear. We attempted to clarify the function of the PAG by recording activity of PAG units along with laryngeal and respiratory electromyograms (EMG) during vocalization in awake monkeys. PAG units were classified with respect to vocalization on the basis of their discharge patterns as 'early burst', 'late burst', 'tonic-increase' and 'tonic-off', with the vast majority being of the early- and late-burst type. Early-burst cells were correlated most frequently with inspiratory muscles of the respiratory system and laryngeal abductor muscles. Late-burst cells were most clearly correlated with laryngeal adductor and expiratory respiratory muscles. Data from spike-triggered averaging and parametric correlations indicate that most cells are related to single muscles, but a significant number were related to functionally related groups of two or more muscles. The results suggest that the PAG determines qualitative aspects of vocalization by the multisynaptic action its cells have on laryngeal and respiratory motoneurons.     

Influence of trigeminal nasal afferents on bulbar respiratory neuronal activity

This study examined the influence of nasal trigeminal afferents, the anterior ethmoidal nerve (AEN) and posterior nasal nerves (PNN) on the spike discharges of respiratory-related neurons recorded in the ventral respiratory group (VRG) (2.6-3.5 mm lateral to the midline, from 1 mm rostral to 3 mm caudal to the obex and at depth of 2-4 mm below the dorsal surface). Electrical stimulations to the AEN and PNN were administered to 10 pentobarbital anaesthetized cats and to 8 ketamine anaesthetized, vagotomized, curarized and ventilated cats. Single shock stimulations of either nerve evoked transient and total inhibition of inspiratory activities. Expiratory-related neurons of the VRG presented three patterns of activity in response to stimulation:excitation, inhibition or inhibition followed by excitation. More generally, expiratory units are activated with a short latency. In the course of repetitive stimulation of the AEN and PNN we observed a prolongation of the spontaneous inspiratory discharge which presented transient, short inhibition in response to each shock. Most expiratory units presented a short activation which was synchronous with the transient inhibition of inspiratory activities. When repetitive stimulation provoked a sneeze-like response, we observed a progressive increase in the duration of transient inspiratory inhibition first, associated with a progressive reinforcement of transient expiratory activation. Secondarily, just before the expiratory thrust, we noted a stronger inhibition of the inspiratory activity which preceded a high-frequency (400 Hz) expiratory discharge. Nasal afferents exert a forceful excitatory effect on bulbospinal (BS) and non-bulbospinal-non-vagal (NBS-NV) expiratory cells of the VRG. The effects due to vagotomy and curarization are discussed.     

Effects of electrical and chemical stimulation of the B?tzinger complex on respiratory activity in the cat

The effects of electrical and chemical stimulation of the expiratory neuronal population in the region of the retrofacial nucleus, the so called 'B?tzinger complex' (B?t. c.), on respiratory activity were investigated in vagotomized cats under pentobarbitone anaesthesia. Some of the experiments were performed on paralyzed or bilaterally thoracotomized, artificially ventilated animals. Sustained tetanic electrical stimulation (20 to 100-Hz, 0.5-ms current pulses at intensities of 5-60 microA) induced strong depressant effects on the inspiratory motor output which could lead to complete apnoea. The apnoeic response was accompanied by tonic activation of expiratory muscles; the appearance and the strength of tonic expiratory activity were dependent upon the frequency of stimulation. Brief tetani (40 to 100 ms trains of 0.5-ms rectangular pulses at 100-300 Hz) timed either during the inspiratory or the expiratory phase caused depression of inspiratory activity and prolongation of expiratory time, respectively. These effects increased gradually as the onset of stimulation was progressively delayed during each respiratory phase. The effects of sustained tetanic stimulation were mimicked by microinjections (25-100 nl) of 0.5 M L-glutamate or 0.16 M DL-homocysteic acid in the same region, thus indicating that they were the result of the stimulation of cell bodies and not of axons of passage. The present results support the hypothesis that B?t. c. neurons play an important role in the control of the breathing pattern by exerting inhibitory influences on inspiratory activity and, possibly, by contributing to the off-switch mechanisms. Furthermore, they suggest that these neurons are involved in the central control of expiratory activity.     

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.     

On the role of the reticular formation in vocal pattern generation

This review is an attempt to localize the brain region responsible for pattern generation of species-specific vocalizations. A catalogue is set up, listing the criteria considered to be essential for a vocal pattern generator. According to this catalogue, a vocal pattern generator should show vocalization-correlated activity, starting before vocal onset and reflecting specific acoustic features of the vocalization. Artificial activation by electrical or glutamatergic stimulation should produce artificially sounding vocalization. Lesioning is expected to have an inhibitory or deteriorating effect on vocalization. Anatomically, a vocal pattern generator can be assumed to have direct or, at least, oligosynaptic connections with all the motoneuron pools involved in phonation. A survey of the literature reveals that the only area meeting all these criteria is a region, reaching from the parvocellular pontine reticular formation just above the superior olive through the lateral reticular formation around the facial nucleus and nucleus ambiguus down to the caudalmost medulla, including the dorsal and ventral reticular nuclei and nucleus retroambiguus. It is proposed that vocal pattern generation takes place within this whole region.     

Discharge of respiratory neurons in sneezes resulting from ethmoidal nerve stimulation

Sneezes were induced in pentobarbital-anesthetized cats by stimulating the ethmoidal nerve (10 to 20 Hz). Activity of medullary respiratory neurons was recorded extracellularly, before and during development of sneezes. Onset of ethmoidal nerve stimulation was attended by premature termination of ongoing inspiration and inhibition of a majority of inspiratory neurons. Such stimulation was also accompanied by immediate short-latency driving of expiratory neurons and recruitment of numerous, previously silent, expiratory neurons. Despite the activation of numerous expiratory neurons, active expiration was not, and could not be, immediately provoked. Active expiration ocurred only after the first deep inspiration (“preparatory inspiration”). Such active expiration was preceded by high-frequency bursting discharge of expiratory neurons. Two groups of experiments suggested that lung inflation occurring in the preparatory inspiration played a part in triggering active expiration. First, stimulation of the ethmoidal nerve in the artificially respirated, succinylcholine-paralyzed cat provoked expiratory neuronal bursting only when the lungs were strongly inflated. Second, unilateral and bilateral vagotomy resulted in defective triggering (delayed) of active expiration so that sneezes were much slower after vagotomy.     

Respiratory modulation of sympathetic activity

Sympathetic activity recorded from cardiac and renal nerves was correlated with phrenic and internal intercostal nerve activity under normocapnea and hypercapnea. Cats were anesthetized with halothane for surgery switching to chloralose for recording. Both vagal and carotid sinus nerves were cut, animals were paralyzed and artificially ventilated. We found that sympathetic activity followed the rhythmic pattern of phrenic nerve discharge fairly closely except in two important respects: first, sympathetic activity was significantly depressed during early inspiration and second, it reached a minimum during post inspiration while phrenic activity was decaying but still active. These effects were accentuated when PACO2 was raised. In one cat early inspiratory depression was the only manifestation of respiratory modulation of sympathetic activity superimposed on an otherwise tonic pattern. In 4 cats sympathetic activity increased in an augmenting fashion in parallel with the augmenting discharge of expiratory alpha motoneurones. We suggest that respiratory-related, excitatory and inhibitory inputs modulate sympathetic activity at the brainstem level. Inspiratory and possibly expiratory interneurones may be the source of activation, and inhibitory inputs may derive from early inspiratory and postinspiratory interneurones. The inhibitory effects may be the only manifestation of respiratory modulation during strong tonic drive of the sympathetic activity.     

Respiratory rhythm generator neurons in medulla of brainstem-spinal cold preparation from newborn rat

Rhythmic neuronal activity preceding C4 inspiratory activity (Pre-I neuron activity) was recorded in rostral ventrolateral (near ventral surface) medulla of brainstem-spinal cord preparation isolated from newborn rat. Vagal stimulation inhibited C4 activity but not Pre-I neuron activity. Rhythmic Pre-I neuron-like activity was still recorded in the block of rostral medulla after transection. Results suggest that Pre-I neurons generate the basic respiratory rhythm and trigger inspiratory activity.     

Activity of neurons in ventrolateral respiratory groups during swallowing in decerebrate rats

To elucidate the neuronal basis of the coordination between swallowing and respiration, we examined the swallowing-related activity of respiratory neurons in the ventrolateral respiratory groups of the medulla oblongata of decerebrate, paralyzed and artificially ventilated rats (n = 14). Extracellular recording was made during fictive swallowing evoked by the electrical stimulation of the superior laryngeal nerve from a total of 141 neurons with respiratory rhythm (99 expiratory and 42 inspiratory neurons). The burst of discharge by the hypoglossal nerve was used to monitor the pharyngeal phase of swallowing. The decrementing-expiratory (E-DEC) neurons (n = 62) were activated during (n = 46) or after (n = 10) the hypoglossal bursts, or showed no swallowing-related activity (n = 6). All of the augmenting-expiratory (E-AUG) neurons (n = 37) were silent during the hypoglossal bursts but were activated after each swallow. Inspiratory neurons showed either no swallowing-related bursts (n = 27), or were activated after the hypoglossal bursts (n = 15). Activation of the majority of E-DEC neurons may be related to the arrest of respiration during swallowing, and the post-swallow activation of E-AUG neurons may correspond to the expiratory phase that follows swallowing. We suggest that these behaviors of expiratory neurons are essential in the phase resetting of the respiratory cycle in association with the swallowing.     

Influences of neck afferents on sympathetic and respiratory nerve activity

It is well established that the vestibular system influences the sympathetic nervous system and the respiratory system; presumably, vestibulosympathetic and vestibulorespiratory responses participate in maintaining stable blood pressure and blood oxygenation during movement and changes in posture. Many brainstem neurons that generate vestibulospinal reflexes integrate signals from the labyrinth and neck muscles to distinguish between head movements on a stable body and whole body movements. In the present study, responses were recorded from the splanchnic (sympathetic), hypoglossal (inspiratory) and abdominal (expiratory) nerves during stimulation of the C2 dorsal root ganglion or C2 or C3 nerve branches innervating dorsal neck muscles. Stimulation of neck afferents using low current intensities, in many cases less than twice the threshold for producing an afferent volley recordable from the cord dorsum, elicited changes in sympathetic and respiratory nerve activity. These data suggest that head rotation on a stable body would elicit both cervical and vestibular inputs to respiratory motoneurons and sympathetic preganglionic neurons. The effects of cervical afferent stimulation on abdominal, splanchnic and hypoglossal nerve activity were not abolished by transection of the brainstem caudal to the vestibular nuclei; thus, pathways in addition to those involving the vestibular nuclei are involved in relaying cervical inputs to sympathetic preganglionic neurons and respiratory motoneurons. Transection of the C1-3 dorsal roots enhanced responses of the splanchnic and abdominal nerves to pitch head rotations on a fixed body but diminished responses of the hypoglossal nerve. Thus, neck and vestibular afferent influences on activity of respiratory pump muscles and sympathetic outflow appear to be antagonistic, so that responses will occur during whole body movements but not head movements on a stationary trunk. In contrast, neck and vestibular influences on tongue musculature are complementary, presumably to produce tongue protrusion either during movements of the head alone or of the whole body.