Functional role of ventral respiratory group expiratory neurons during vocalization

Firing properties of medullary expiratory neurons during fictive straining in cats

Expiratory (E) neurons in the caudal nucleus retroambigualis extend descending spinal axons to the lumbar and sacral spinal cord. Discharge rates of single E neurons were recorded to examine differences in activity of E neurons projecting to the lumbar or sacral spinal cord during fictive straining induced by distention of the colon with a balloon. Firing frequencies of E neurons with descending axons in the thoracic and lumbar spinal cord increased during the repetitive rise of rectum pressure, whereas those of E neurons with descending axons in the sacral spinal cord decreased. E neurons with descending axons in the thoracic/lumbar and sacral spinal cord exhibit different firing characteristics during the repetitive rise of rectum pressure when straining during defecation. The activity of abdominal nerves during fictive straining is in phase with changes in rectum pressure, but out of phase with the activity of E neurons.

Coordination of breathing with nonrespiratory activities

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

Characterization of respiratory neurons in the rostral ventrolateral medulla, an area critical for vocal production in songbirds

Much is known about the neuronal cell types and circuitry of the mammalian respiratory brainstem and its role in normal, quiet breathing. Our understanding of the role of respiration in the context of vocal production, however, is very limited. Songbirds contain a well-defined neural circuit, known as the song system, which is necessary for song production and is strongly coupled to the respiratory system. A major target of this system is nucleus parambigualis (PAm) in the ventrolateral medulla, a structure that controls inspiration by way of its bulbospinal projections but is also an integral part of the song-pattern generation circuit by way of its "thalamocortical" projections to song-control nuclei in the telencephalon. We have mapped out PAm to characterize the cell types and its functional organization. Extracellular single units were obtained in anesthetized adult male zebra finches while measuring air sac pressure to monitor respiration. Single units were characterized by their discharge patterns and the phase of the activity in the respiratory cycle. Several classes of neurons were identified and were analogous to those reported for mammalian medullary respiratory neurons. The majority of the neurons in PAm was classified as inspiratory augmenting or preinspiratory, although other basic discharge patterns were observed as well. The well-characterized connectivity of PAm within the vocal motor circuit and the similarity of its neural firing patterns to the rostral ventral respiratory group and pre-Bötzinger complex of mammals make it an ideal system for investigating the integration of breathing and vocalization.

Avian nucleus retroambigualis: cell types and projections to other respiratory-vocal nuclei in the brain of the zebra finch (Taeniopygia guttata)

In songbirds song production requires the intricate coordination of vocal and respiratory muscles under the executive influence of the telencephalon, as for speech in humans. In songbirds the site of this coordination is suspected to be the nucleus retroambigualis (RAm), because it contains premotor neurons projecting upon both vocal motoneurons and spinal motoneurons innervating expiratory muscles, and because it receives descending inputs from the telencephalic vocal control nucleus robustus archopallialis (RA). Here we used tract-tracing techniques to provide a more comprehensive account of the projections of RAm and to identify the different populations of RAm neurons. We found that RAm comprises diverse projection neuron types, including: 1) bulbospinal neurons that project, primarily contralaterally, upon expiratory motoneurons; 2) a separate group of neurons that project, primarily ipsilaterally, upon vocal motoneurons in the tracheosyringeal part of the hypoglossal nucleus (XIIts); 3) neurons that project throughout the ipsilateral and contralateral RAm; 4) another group that sends reciprocal, ascending projections to all the brainstem sources of afferents to RAm, namely, nucleus parambigualis, the ventrolateral nucleus of the rostral medulla, nucleus infra-olivarus superior, ventrolateral parabrachial nucleus, and dorsomedial nucleus of the intercollicular complex; and 5) a group of relatively large neurons that project their axons into the vagus nerve. Three morphological classes of RAm cells were identified by intracellular labeling, the dendritic arbors of which were confined to RAm, as defined by the terminal field of RA axons. Together the ascending and descending projections of RAm confirm its pivotal role in the mediation of respiratory-vocal control.

Respiratory drive in hindlimb motoneurones of the anaesthetized female cat

Anatomical studies have shown a monosynaptic projection from nucleus retroambiguus (NRA) to semimembranosus (Sm) motor nucleus in female cats, which is stronger in oestrus. Expiratory bulbospinal neurones are the best documented functional cell type in the NRA. If these cells participate in this projection, an expiratory drive would be expected in Sm motoneurones and this drive would be expected to be stronger in oestrus. In anaesthetized, paralyzed, ovariohysterectomized female cats, artificially ventilated to produce a strong respiratory drive (as monitored by phrenic nerve discharges), intracellular recordings were made from Sm motoneurones and from motoneurones in the surrounding hindlimb motor nuclei that are outside the focus of the NRA projection. The animals comprised two groups: either treated for 7 days with oestradiol benzoate (oestrous) or untreated (non-oestrous). Central respiratory drive potentials (CRDPs) were observed in most motoneurones of both groups, with amplitudes larger for the oestrous than for the non-oestrous group (1.58+/-1.34 mV versus 0.89+/-0.79 mV, mean+/-S.D.). However, the CRDPs most often consisted of a maximum depolarization in early expiration, which declined in late expiration and into inspiration. This pattern is different from the incrementing firing pattern of most expiratory bulbospinal neurones. The CRDPs in Sm and semitendinosus motoneurones (located in the same motor column) were of similar size and frequency to CRDPs in motoneurones outside that column. The hypothesis that expiratory bulbospinal neurones are significantly involved in the projection was rejected. Alternative sources and possible functional roles for the CRDPs are discussed.

Functional Heterogeneity Among Neurons in the Nucleus Retroambiguus With Lumbosacral Projections in Female Cats

Nucleus retroambiguus (NRA), in the caudal medulla, projects to all spinal levels. One physiological role is abdominal pressure control, evidenced by projections to intercostal and abdominal motoneurons from expiratory bulbospinal neurons (EBSNs) within NRA. The roles of NRA projections to the lumbosacral cord are less certain, although those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we physiologically characterized NRA projections to the lumbosacral cord. Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO2gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L6, L7, or sacral segments and to microstimulation in the region of semimembranosus motor nucleus or ON. All units were found at sites showing expiratory discharges. Units that showed collisions between antidromic and spontaneous spikes (all in late expiration) were identified as EBSNs. These were common from the ventrolateral funiculus (VLF) of L6(42.5%) or L7(32.9%), but rare from the sacral VLF or the motor nuclei. Antidromic latencies revealed a subthreshold respiratory drive in some non-EBSNs. This group had lower conduction velocities than the EBSNs. The remainder, with a negligible respiratory drive, had even lower conduction velocities. A new population of NRA neurons has thus been defined. They are not active even with a strong respiratory drive, but may provide most of the synaptic input from NRA to lower lumbar and sacral segments and could subserve functions related to mating behavior.

Physiology of neuronal subtypes in the respiratory-vocal integration nucleus retroamigualis of the male zebra finch

Learned vocalizations, such as bird song, require intricate coordination of vocal and respiratory muscles. Although the neural basis for this coordination remains poorly understood, it likely includes direct synaptic interactions between respiratory premotor neurons and vocal motor neurons. In birds, as in mammals, the medullary nucleus retroambigualis (RAm) receives synaptic input from higher level respiratory and vocal control centers and projects to a variety of targets. In birds, these include vocal motor neurons in the tracheosyringeal part of the hypoglossal motor nucleus (XIIts), other respiratory premotor neurons, and expiratory motor neurons in the spinal cord. Although various cell types in RAm are distinct in their anatomical projections, their electrophysiological properties remain unknown. Furthermore, although prior studies have shown that RAm provides both excitatory and inhibitory input onto XIIts motor neurons, the identity of the cells in RAm providing either of these inputs remains to be established. To characterize the different RAm neuron types electrophysiologically, we used intracellular recordings in a zebra finch brain stem slice preparation. Based on numerous differences in intrinsic electrophysiological properties and a principal components analysis, we identified two distinct RAm neuron types (types I and II). Antidromic stimulation methods and intracellular staining revealed that type II neurons, but not type I neurons, provide bilateral synaptic input to XIIts. Paired intracellular recordings in RAm and XIIts further indicated that type II neurons with a hyperpolarization-dependent bursting phenotype are a potential source of inhibitory input to XIIts motor neurons. These results indicate that electrically distinct cell types exist in RAm, affording physiological heterogeneity that may play an important role in respiratory-vocal signaling.

Functional electrical stimulation of laryngeal adductor muscle restores mobility of vocal fold and improves voice sounds in cats with unilateral laryngeal paralysis

Functional electrical stimulation (FES) has been proposed as a potential treatment for restoring motor functions of denervated motor systems. We investigated whether FES of paralyzed laryngeal adductor muscles could restore adduction to the vocal folds. In addition, we studied the effect of stimulated vocal fold adduction on the intensity and overall quality of voice production. We recorded movement of the vocal fold, electromyographic activity of muscles recruited for vocalization, and sound production in unanesthetized decerebrate cats during FES of the paralyzed thyroarytenoid (TA) muscle. FES of the paralyzed TA muscle induced adduction of the vocal fold. Appropriate stimulus parameters for induction was 1.5-3.0 mA intensity pulses delivered at a frequency of 30-50 pulses per second (pps). FES of the paralyzed TA muscle prolonged phonation time and increased intensity of voice sounds during vocalization induced by electrical stimulation (0.2 ms, 20-50 microA, 50 pps) of the periaqueductal gray (PAG). The quality of voice sounds evaluated by sound spectrography was shown to improve during vocalization with FES. We conclude that FES of the paralyzed laryngeal adductor muscle was effective in restoring adduction of the vocal fold and improving voice sounds impaired by unilateral laryngeal paralysis.

Membrane potential changes in vocal cord tensor motoneurons during breathing, vocalization, coughing and swallowing in decerebrate cats

We studied the patterns of membrane potential changes in vocal cord tensor motoneurons, i.e. cricothyroid muscle motoneurons (CTMs), during fictive breathing, vocalization, coughing, and swallowing in decerebrate paralyzed cats to determine the nature of central drives to CTMs during these behaviors. CTMs were identified by antidromic activation from the superior laryngeal nerve. During breathing, CTMs always depolarized during the inspiratory phase, and sometimes depolarized during the expiratory phase as well. During vocalization, CTMs strongly depolarized. During coughing, CTMs exhibited depolarizations during both inspiratory and expiratory phases, but it was interrupted by a transient repolarization between the last part of the inspiratory phase and the first part of the abdominal burst during which chloride-dependent inhibitory postsynaptic potentials were revealed. During swallowing, most CTMs hyperpolarized, and this hyperpolarization was sometimes followed by a weak depolarization. We conclude that the main role of the cricothyroid muscle is vocalization but the functional roles in coughing and swallowing are minor, and that the CTM activity during resting breathing and vocalization are primarily controlled by excitatory inputs, while during coughing and swallowing, inhibitory inputs play roles in shaping membrane potential trajectories.

Do respiratory neurons control female receptive behavior: a suggested role for a medullary central pattern generator?

Nucleus retroambiguus (NRA) consists of a column of neurons in the caudal medulla with crossed descending axons that terminate in almost all spinal segments. Many of these neurons transmit the drive for expiratory movements to the spinal cord. The same neurons are also known to participate, however, in other motor acts, such as vomiting and abdominal straining, for which it appears that the medullary circuits controlling the respiratory pattern are reconfigured. Plasticity in projections from the NRA to hindlimb motor nuclei provides evidence that some of these projections are involved in yet another motor act, female receptive behavior. Here, we present the hypothesis that the medullary circuits are also reconfigured to act as a central pattern generator for this behavior. In addition, we suggest that during estrus, plasticity is shown not only in spinal cord connections, but also in a selected membrane property of hindlimb motoneurons.

The neuronal circuit of augmenting effects on intrinsic laryngeal muscle activities induced by nasal air-jet stimulation in decerebrate cats

We previously demonstrated that during nasal air-jet stimulation, both the activities of intrinsic laryngeal adductor and abductor muscles persistently increase, whereas the respiratory cycle prolongs and the activity of diaphragm decreases [Am. J. Rhinol. 9 (1995) 203-208; Neurosci. Res. 31 (1998) 137-146]. The purpose of this study was to clarify the neuronal circuit underlying the augmentation of intrinsic laryngeal muscles evoked by nasal air-jet stimulation. The immunohistologic analysis of Fos-expression was reported to determine the distribution of activated neurons in cat brainstem evoked by sneeze-inducing air puff stimulation of the nasal mucosa [Brain Res. 687 (1995) 143-154]. In sneezing cats, immunoreactivity was evoked in projection areas of the ethmoidal afferents, e.g. the subnuclei caudalis, interpolaris and in interstitial islands of the trigeminal sensory complex. Immunoreactivity was also enhanced in the solitary complex, the nucleus retroambiguus, the pontine parabrachial area and the lateral aspect of the parvocellular reticular formation [Brain Res. 687 (1995) 143-154]. In the present study, we focussed on the parvocellular reticular nucleus (PRN) as a relay of the neural circuit contributed to the augmentation of intrinsic laryngeal muscles evoked by nasal air-jet stimulation. We recorded the neuronal behavior of PRN during the nasal air-jet stimulation in precollicular-postmammillary decerebrate cats. As the results, 24% (17/71) of recorded neurons which were activated orthodromically by the electrical stimulation to anterior ethmoidal nerve, increased their firing rates in response to the nasal air-jet stimulation. Furthermore, spike-triggered averaging method revealed that four of these 17 PRN neurons activated intrinsic laryngeal muscles, suggesting that such neurons have excitatory projections to the intrinsic laryngeal muscle motoneurons in the nucleus ambiguus. These results suggest that the some of PRN neuron play a role in augmentation of the intrinsic laryngeal muscles activities during nasal air-jet stimulation.

Respiratory sinus arrhythmia during speech production

The amplitude of the respiratory sinus arrhythmia (RSA) was investigated during a reading aloud task to determine whether alterations in respiratory control during speech production affect the amplitude of RSA. Changes in RSA amplitude associated with speech were evaluated by comparing RSA amplitudes during reading aloud with those obtained during rest breathing. A third condition, silent reading, was included to control for potentially confounding effects of cardiovascular responses to cognitive processes involved in the process of reading. Calibrated respiratory kinematics, electrocardiograms (ECGs), and speech audio signals were recorded from 18 adults (9 men, 9 women) during 5-min trials of each condition. The results indicated that the increases in respiratory duration, lung volume, and inspiratory velocity associated with reading aloud were accompanied by similar increases in the amplitude of RSA. This finding provides support for the premise that sensorimotor pathways mediating metabolic respiration are actively modulated during speech production.

Neural pathways underlying vocal control

Vocalization is a complex behaviour pattern, consisting of essentially three components: laryngeal activity, respiratory movements and supralaryngeal (articulatory) activity. The motoneurones controlling this behaviour are located in various nuclei in the pons (trigeminal motor nucleus), medulla (facial nucleus, nucl. ambiguus, hypoglossal nucleus) and ventral horn of the spinal cord (cervical, thoracic and lumbar region). Coordination of the different motoneurone pools is carried out by an extensive network comprising the ventrolateral parabrachial area, lateral pontine reticular formation, anterolateral and caudal medullary reticular formation, and the nucl. retroambiguus. This network has a direct access to the phonatory motoneurone pools and receives proprioceptive input from laryngeal, pulmonary and oral mechanoreceptors via the solitary tract nucleus and principal as well as spinal trigeminal nuclei. The motor-coordinating network needs a facilitatory input from the periaqueductal grey of the midbrain and laterally bordering tegmentum in order to be able to produce vocalizations. Voluntary control of vocalization, in contrast to completely innate vocal reactions, such as pain shrieking, needs the intactness of the forebrain. Voluntary control over the initiation and suppression of vocal utterances is carried out by the mediofrontal cortex (including anterior cingulate gyrus and supplementary as well as pre-supplementary motor area). Voluntary control over the acoustic structure of vocalizations is carried out by the motor cortex via pyramidal/corticobulbar as well as extrapyramidal pathways. The most important extrapyramidal pathway seems to be the connection motor cortex-putamen-substantia nigra-parvocellular reticular formation-phonatory motoneurones. The motor cortex depends upon a number of inputs for fulfilling its task. It needs a cerebellar input via the ventrolateral thalamus for allowing a smooth transition between consecutive vocal elements. It needs a proprioceptive input from the phonatory organs via nucl. ventralis posterior medialis thalami, somatosensory cortex and inferior parietal cortex. It needs an input from the ventral premotor and prefrontal cortex, including Broca's area, for motor planning of longer purposeful utterances. And it needs an input from the supplementary and pre-supplementary motor area which give rise to the motor commands executed by the motor cortex.

Neuronal activity in the medulla oblongata during vocalization. A single-unit recording study in the squirrel monkey

In six squirrel monkeys (Saimiri sciureus), the medulla oblongata was explored with microelectrodes, looking for vocalization-correlated activity. The vocalizations were elicited by microinjections of glutamate agonists into the periaqueductal grey of the midbrain. Vocalization-related cells were found in greater numbers in the nucl. ambiguus (Ab) and retroambiguus (RAb), in the parvocellular, magnocellular and central reticular formation as well as in the solitary tract nucleus and spinal trigeminal nucleus. Small numbers were also found in the vestibular complex, cuneate nuclei, inferior olive and lateral reticular nucleus. A differentiation of the neuronal responses into 12 reaction types reveals that the frequency of each reaction type varies from brain structure to brain structure, thus allowing a specification of the different vocalization-related areas. According to this specification, it is proposed that initiation of vocalization takes place via the parvocellular reticular formation; vocal pattern control is mainly brought about by the parvocellular reticular formation, Ab, solitary tract nucleus and spinal trigeminal nucleus; expiratory control and respiratory-laryngeal coordination is carried out by the RAb, Ab and central nucleus of the reticular formation; vocalization-specific postural adjustments are carried out via the vestibular and cuneate nuclei.

Brain stem neural mechanisms for vocalization in decerebrate cats

In order to characterize the brain stem circuitry that produces vocalization, the activities of brain stem respiratory neurons were recorded extracellularly during vocalization induced by electrical stimulation of the periaqueductal gray in decerebrate cats. After the onset of stimulation, the respiratory rhythm ceases, and a preparatory inspiration is induced. Following this initial inspiration, vocalization characterized by increased activities of the intrinsic laryngeal adductor and the major expiratory muscles is induced. During vocalization, most of the dorsal respiratory group inspiratory neurons increase their firing rates in phase with an increase of diaphragm activity. Inspiratory neurons with a continuous discharge pattern in the rostral ventral respiratory group increase their firing rates to augment intrinsic laryngeal abductor motoneurons and bulbospinal inspiratory neurons in the dorsal respiratory group. On the other hand, most of the bulbospinal augmenting expiratory neurons in the Bötzinger complex cease firing just after the onset of periaqueductal gray stimulation for the remainder of the stimulation period. These results indicate that at least some part of the coordinated activations of intrinsic laryngeal and respiratory muscles during vocalization are mediated via the central respiratory neurons that produce breathing.

The augmentation of intrinsic laryngeal muscle activity by air-jet stimulation of the nasal cavity in decerebrate cats

The purpose of this study was to examine the functional roles of nasal afferents in modulating the activity of the intrinsic laryngeal muscles. The electromyographic activities of the intrinsic laryngeal muscles and major respiratory muscles were recorded in cats during nasal air-jet stimulation. The activities of brainstem respiratory neurons were also recorded to determine which neurons transmit nasal afferent signals to the intrinsic laryngeal motoneurons. These axonal projections were identified by antidromic activation evoked by stimulation to the spinal cord at C4 level and the laryngeal nerve. The length of the respiratory cycle was prolonged and the diaphragmatic activity was decreased during air-jet stimulation of the nasal cavity. In contrast, the activities of both the intrinsic laryngeal adductor and abductor muscles were increased. Examination of the laryngeal reflexes revealed increase in the activities of intrinsic laryngeal motoneurons during both respiratory phases. Most of the respiratory neurons recorded decreased their peak firing rate during air-jet stimulation, reflecting decreased diaphragmatic activity; however, the peak firing rate of the bulbospinal expiratory neurons in the portion of the ventral respiratory group caudal to the obex did not decrease during stimulation. These findings demonstrate the nasal air-jet stimulation decreases the activities of major inspiratory muscles in order to avoid inspiration of foreign bodies into the nasal cavity and augments the activities of intrinsic laryngeal muscles, enabling prompt elicitation of the laryngeal airway reflex. Our findings also suggest that the nasal afferents suppress the major inspiratory activities by way of brainstem inspiratory neurons, but that the activities of intrinsic laryngeal muscles are controlled through undetermined pathway(s) other than the pathway through respiratory neurons.

Lombard reflex during PAG-induced vocalization in decerebrate cats

The Lombard reflex occurs when a speaker increases his vocal effort while speaking in the presence of ambient noise. The purpose of this study was to clarify whether the Lombard reflex can be evoked during controlled vocalization in an animal model. In decerebrate cats, repetitive electrical stimulation was applied to the midbrain periaqueductal gray (PAG) to evoke vocalization. Pure tone auditory stimulation was delivered through a loudspeaker. The activities of the laryngeal adductor muscle, diaphragm and external oblique abdominal muscle and the voice intensity were measured during PAG stimulation, in the presence and absence of the auditory stimulation. To clarify the effects of the auditory laryngeal reflex on the activity of laryngeal adductor motoneurons, the amplitude of the laryngeal reflex evoked by single shock stimulation of the superior laryngeal nerve was also measured during respiration, in the presence and absence of auditory stimulation. The sound made by the cats due to PAG-induced vocalization was augmented by exposure to auditory stimulation, and the activities of the laryngeal adductor muscle and external oblique abdominal muscle were also augmented. During respiration, auditory stimulation also increased the amplitude of the laryngeal reflex evoked in the laryngeal adductor muscle. These results demonstrate that the essential neuronal mechanisms for evoking the Lombard reflex exist within the brainstem.

Role of nucleus retroambigualis in respiratory reflexes evoked by superior laryngeal and vestibular nerve afferents and in emesis

An ascending projection from the medullary nucleus retroambigualis (NRA) has recently been described as important for the control of the upper airway during vocalization. We evaluated the importance of this projection in other behaviors by making localized injections of the neurotoxin kainic acid in the NRA in decerebrate cats, most of which were paralyzed and artificially ventilated. In contrast to its importance for vocalization, the NRA is not essential for activation of upper airway musculature during respiration, swallowing, vomiting, or reflexes elicited by superior laryngeal or vestibular nerve afferents. However, kainic acid injections in the NRA and adjacent reticular formation prolonged the inhibitory phrenic motoneuronal response to superior laryngeal nerve stimulation and abolished or reduced abdominal motoneuronal responses during respiration, vomiting, and superior laryngeal nerve stimulation. Thus, of the behaviors we investigated, the importance of the ascending projection from the NRA appears to be limited to vocalization, while descending projections from the NRA region are important in a number of behaviors.