what is the relationship of the esophagus to the respiratory system

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Central Points

• Animate and swallowing processes are closely interrelated in their fundamental control and are highly coordinated.

• Many muscles and structures have dual roles in respiration and swallowing.

• Neural control centers responsible for coordination of breathing and swallowing are contained in the dorsomedial and ventrolateral medullary regions of the brainstem.

• Cortical structures also play an important role in facilitating and modulating the coordination of breathing and swallowing.

• Relationship of the stage of respiration (i.e., Inspiratory, Expiratory, Transition) and duration of the apneic phase associated with swallowing have been extensively investigated.

• Studies of swallowing dynamics and pulmonary function are needed that will investigate the clinical relevance of integrated animate and swallowing function on the health and nutritional outcomes of dysphagic patients and patients with pulmonary disorders.

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Introduction

Respiration and swallowing are physiologic processes that demonstrate specialization in their neural networks and peripheral functions and yet showroom a finely tuned partnership in the execution of their function in bones survival. Information technology is well established that breathing and swallowing do not occur simultaneously in infant or adult animals and humans. These observations have led clinicians to assume that the functions are mutually exclusive. Contempo studies, all the same, are clarifying this simplistic functional separation and highlight the complementary and overlapping nature of ane function with some other. The development of creature models and observations from clinical studies cited in this review has laid a foundation for this new thinking regarding the coordination between respiratory and swallowing function.

The term respiration, operationally divers as the procedure of moving and exchanging oxygen from inhaled air and releasing carbon dioxide via exhalation, and the central regulation of that process, is used synonymously with breathing in this review of the literature. This is washed to avoid confusion with the multiple implications of respiration, such as cellular or metabolic respiration. The discussion is restricted to components of respiration that are virtually pertinent to breathing and swallowing coordination, that is, those necessary to avert pulmonary contagion via aspiration and to ensure adequate ingestion and swallowing of secretions, liquids, and foods.

Clinical and experimental evidence support the existence of neurophysiologic, structural, and functional interdependence between respiration and swallowing. Health care professionals who care for patients with swallowing disorders are able to modify abnormal swallowing physiology using compensatory techniques that involve both peripheral alterations in breathing and swallowing. Compensatory postures and maneuvers used in the treatment of patients with dysphagia often require voluntary modification of both breathing and swallowing.^{1, ii} Farther, modifications in the volume and texture of swallowed materials are also routinely made by speech-language pathologists because these variables take shown an firsthand influence on swallowing beliefs observed during videofluoroscopic or endoscopic imaging.^{three, 4, 5, 6, 7, eight} What is not understood is the impact of behavioral modifications or swallowing treatments, such as exercise, on the overall transfer effect of these strategies direct impacting on the central controllers of animate and swallowing coordination. Changes in respiration, ventilation, and swallowing occur with normal development and crumbling, and with multiple disease processes. The arbitrary segmentation of these functions in the majority of the basic and clinical studies, however, has non significantly moved the field along toward improved agreement of the effects of these normal phase-of-life conditions on respiratory and swallowing coordination. This is related in part to the relative difficulty of conducting concurrent studies on the primal control of respiration and swallowing in various animal models, and to the limitations of directly applying these models to man species. A few studies have been able to examine perturbations in breathing and swallowing in humans that likely imply modification in the central controllers. Clinical studies of the impact of neurologic,^{9, 10, eleven} pulmonary,^{12, 13, 14, fifteen} and oncologic diseases^{sixteen, 17} on the coordination of breathing and swallowing are besides outset to emerge that are leading basic scientists back to the laboratory to determine what aberrations in central control may be contributing to the functional observations made in the clinic.

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Cardinal Control of Respiration and Swallowing

Brainstem Regulation

The electric current knowledge regarding the central control of breathing and swallowing coordination has been derived from lesion studies and electrophysiologic, neuroanatomic, and pharmacologic data obtained from experimental studies with decerebrate animals. Early investigations demonstrated that swallowing motor activity could be initiated via electric stimulation of the superior laryngeal nerve (SLN) branch of the vagus nerve [cranial nerve (CN) X] in anesthetized animals. Jean⁵⁰ pointed out that the stimulated swallows elicited from these animal models relate to "stereotyped basic swallowing move [every bit in a reflexive response, rather] than to physiological motor activity" (p. 935). The fauna model studies showed that SLN-stimulated swallows are anticipated and relatively consistent. In the human consume, even so, there is considerable variability in the early on components of swallowing that is not reflected in an SLN-stimulated swallow. This is an important stardom that must exist appreciated when attempting to interpret the findings from creature studies to human swallowing behavior. However, the model has allowed for considerable understanding regarding the potential neural control because the majority of the muscle contractions of human swallows are included in the stimulated swallows in animals.^fifty

Information technology was postulated a century ago by Meltzer^{51, 52} that swallowing was under the control of a central pattern generator. Doty⁵³ later proposed that the motor patterns involved in swallowing were controlled by a center with 3 arms: (1) an input arm—peripheral afferents; (2) an organizing arm—commanding interneurons); and (3) an output arm—motoneurons. Doty explained that a control centre must have a selective mechanism at its afferent portal that allows the centre to exist activated past appropriate stimuli. He explained that the consume middle must demonstrate a filtering arrangement to give preferred matching only to those stimuli matching the spatiotemporal code to elicit swallowing. Other simple reflexive synergies, many of which protect the airway, such equally coughing and gagging, recruit the same muscles used by swallowing but are filtered out by the eye defined by Doty. The synergy of pharyngeal swallowing would be released simply by a stimulus pattern specific to it.

Both respiratory and swallowing responses have been elicited in experimental animals via electrical stimulation to the internal co-operative of the SLN of (CN) 10.^{54, 55, 56, 57, 58, 59, 60, 61} Stimulation of the SLN produced not simply the stereotyped swallow pattern only as well secretion of mucus in the throat, larynx, esophagus, and trachea.^{62, 63}. The blazon of the elicited response was dependent on the nature of the stimulus (i.e., frequency, intensity, and blueprint) that was applied to the SLN.^{54, 55} These early on studies demonstrated that the inhibition of respiration, in the expiratory phase, was easier to elicit with electrical stimulation of the SLN than were swallow responses in various species of experimental animals.⁵⁴ It was explained that this phenomenon was due to the beingness of optimal and limiting frequencies of the electrical stimulation for elicitation of swallowing that varied beyond species. The optimal frequency was that which resulted in the greatest number of repetitive swallows with the shortest latency at the everyman stimulus intensity. Frequencies above or below the optimal frequency were not equally effective in eliciting continuous swallowing. Doty⁵⁴ demonstrated that respiratory effects were more than easily elicited than swallowing at these limiting frequencies. Electrical stimulation to the glossopharyngeal nerve (CN 9) in dogs, rabbits, and cats also resulted in the elicitation of swallowing just only when anesthesia was sufficiently lite.⁵⁴ Stimulation of Ix at various frequencies, durations, and intensities could not induce swallowing in anesthetized sheep, just did evoke curt latency potentials in the nucleus of the solitary tract (NTS).⁶⁴ In the same experiment, a conditioning stimulus practical to IX facilitated swallowing elicited by SLN stimulation. The investigators ended that the afferent fibers running in Nine do non have sufficient input on the medullary swallowing neurons to induce the swallowing motor sequence, merely likely play a facilitative part. Sinclair^{65, 66} demonstrated through selective sectioning of IX and the pharyngeal co-operative of X that Nine was the primary afferent pathway of the swallow response initiated from the throat in the dog. Additionally, he demonstrated from dissection experiments that sections of IX and SLN anastomose inside the pharyngeal plexus. He suggested that the modest sensory fibers of IX and the larger fibers of SLN interact centrally to explain the observation that electric stimulation of SLN elicits swallowing more readily than stimulation to the Nine cranial nerve. Conspicuously, this after model appears to relate best to human part whereby the eat typically triggers when the sensory afferents in the oropharynx are stimulated. Different branches of these same afferent portals (i.e., IX and 10) also acquit sensory information from critical sensory stop organs for the control of respiratory rhythm, including the pulmonary stretch receptors, carotid and aortic body chemoreceptors, and vascular baroreceptors.^{67, 68, 69, lxx} Anatomic and electrophysiologic studies have demonstrated that afferent inputs from the peripheral swallowing and respiratory regions arise to the NTS in the medullary region of the brainstem. The NTS is a main sensory nucleus, lying intimately with the descending fibers of the tractus solitarius, and is the first to receive special and general viscerosensory information from regions of the upper and lower digestive, respiratory, and cardiac systems.⁷¹

Ventral and dorsal regions of the medulla have been implicated as including the centers, or fundamental decision-making neurons, responsible for programming the consume motor output and respiratory rhythmogenesis. The exact location of these central command centers varies based on the beast model that has been studied. Early on lesion studies in the dog, true cat, and monkey reported that the interneurons relevant to pharyngeal and esophageal phases of swallowing resided in different regions of the medulla, and that the central blueprint generators or cadre interneurons for swallowing were located on each side of the brainstem in the medial reticular germination between the posterior pole of the facial nucleus and the rostral pole of the inferior olive.⁵⁵

Jean⁷² likewise concluded that the controlling regions for unlike areas of the swallowing tract were located in different regions of the medulla. The location of the areas housing the central decision-making neurons in sheep, however, differed considerably from those described in Doty's piece of work. Jean demonstrated that lesions to the dorsomedial NTS in sheep abolished the esophageal components of swallow, whereas lesions placed more rostrally in the NTS eliminated swallows elicited by SLN stimulation. These findings, together with fundamental microelectrode recordings following SLN stimulation in the same species, resulted in the location of 2 key swallowing regions: (ane) a dorsal region that included NTS and adjacent reticular formation, and (2) a ventral region equanimous of the lateral reticular formation in and around the nucleus ambiguus (NA)^{56, 57} (Figure 1). The portion of the NTS receiving afferent input from CNs IX and X corresponded to Doty's⁵³ afferent portal, whereas the interneurons in NTS corresponded to the system arm of the network.⁷³ The ventral region around and including the NA contained interneurons that were described as commanding the motoneurons involved in swallowing. Other early electrophysiologic studies support the anatomic findings of neuronal projections betwixt NTS and NA in various species.^{67, 70, 74, 75}

Similar to the swallowing research, several ablation, stimulation, and recording techniques have been used to locate the respiratory control areas in the brain. In that location is general understanding that the product of basic respiratory rhythmicity occurs in the medulla.⁷⁶ Simply as the ii regions of swallowing neurons were identified co-ordinate to their synchronous activity with swallowing muscle contraction, there have been ii large concentrations of respiratory neurons identified and characterized by the temporal relationships of their firing activity with phrenic nerve belch: a dorsal respiratory group (DRG) and a ventral respiratory group (VRG). A region ventrolateral to the lonely tract, DRG is composed of a heavy density of primarily inspiratory neurons.⁷⁶ Merrill⁷⁷ divided these cells into iii categories: (1) respiratory neurons unaffected by lung aggrandizement; (2) respiratory neurons activated by lung inflation; and (3) pump cells that fired with the pump strokes of a respirator in paralyzed, artificially ventilated animal preparations. The dorsal respiratory region of the NTS receives afferent projections from the larynx, extrathoracic trachea, intrathoracic trachea, primary bronchus, and lung.^seventy Expiratory phase switching neurons accept also been found in this region leading to the implication that the NTS lone is capable of generating the respiratory rhythmogenesis.⁷⁸

The VRG intermingles with NA motoneurons and has been named the nucleus retroambigualis (NRA). The NRA has not been implicated in the neural control of swallowing, just receives projections from the NTS.^{67, 70, 75, 77} Kickoff in the spinal cord at the level of the get-go cervical vertebra and extending to the near rostral border of the medulla, Merrill⁷⁷ distinguished three major subdivisions of the NRA: (1) an area of almost exclusively expiratory neurons, (two) a mainly inspiratory part, and (3) a region near the pontomedullary border composed primarily of expiratory neurons. Previously labeled the retrofacial nucleus, the latter farthermost rostral expanse containing the expiratory neurons is labeled the Botzinger circuitous.^{77, 79, 80} More than recent studies have supported the subdivision of the VRG into functionally distinct compartments and accept identified an surface area labeled the pre-Botzinger complex, which may comprise the primal rhythm generating network for breathing.^{81, 82} Inputs from the pre-Botzinger circuitous have also been shown to play a critical office in initiating the normal ventilatory response to hypercapnia and hypoxia⁸³ (Figure 2).

Effigy 2: Dorsal view of brainstem and cervical spinal cord indicating regions involved in control of breathing and progression of labeling with a viral tracer injected into the phrenic nerve.

KF, Kölliker-Fuse nucleus; PB, parabrachial nuclei; NA, noradrenergic A5 area; RTN, retrotrapezoid nucleus; PGi, paragigantocellular reticular nucleus; BötC, Bötzinger Circuitous; preBotC, preBötzinger Complex; rVRG, rostral ventral respiratory group; cVRG, caudal ventral respiratory group. (Source: Annual Review of Physiology, Volume 60, with permission. ©1998 by Annual Reviews. www.annualreviews.org.)

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Doty'due south⁵³ final benchmark for a control eye of swallow muscular activity is that of an efferent portal. He reported that the center must take an uncommonly constructive access to motoneurons, and must exist able to inhibit competing centers. Evidence for the beingness of this third criterion has been illustrated from neurophysiologic studies of central respiratory and swallowing action.

The swallow-related motor output to musculature of the mouth, pharynx, and larynx is transmitted by axons whose cell bodies reside in the brainstem. These include the trigeminal motor nucleus located toward the cranial function of the brainstem near the level of the mid-pons, the facial motor nucleus located at the level of the caudal pons, the NA running rostrocaudally in the medulla, and the hypoglossal motor nucleus. The NA consists of not only interneurons (i.e., premotor commanding neurons) just as well large motor neurons that somewhen distribute to the striated musculature innervated past CNs Nine and X. Fibers that emerge from its upper cease join IX, and those that emerge at the lower level join fibers of X and the cranial role of the accessory CN (Eleven). The NA gives ascension to fibers of Ten that are eventually distributed to the constrictor muscles of the pharynx and the intrinsic muscles of the larynx. A grouping of fibers in NA also give rise to IX fibers that innervate the stylopharyngeus muscle. From this anatomic description, it appears that the organizational arm of the swallow, as defined by Doty, does have direct admission to the motor nuclei that precisely initiate and fourth dimension the movements of the muscles that they supply.

Studies take supported the early observations that the NA contains the premotor neurons that command swallowing. In sheep it has been demonstrated that just the swallowing neurons in the ventral group (region surrounding NA) could be antidromically activated by stimulation of the trigeminal motor nucleus, whereas none of the neurons in the dorsal region exhibited this antidromic activation.⁸⁴ Anatomic studies also showed that the trigeminal motor nucleus received axon terminals from medullary neurons in the reticular formation surrounding NA.⁸⁵ The ventral medullary region around NA was also found to receive projections from the homologous contralateral medullary region, and from the bilateral facial (CN Seven), X, and XII motor nuclei that are all involved in swallow related motor activeness. These information showed that the ventral region contains the premotor neurons (i.e., motor command area), which accept precise access to the muscles involved in deglutition. Neurons in the ventral medullary region have also been institute to play a critical role in the control of respiratory muscle activeness.

Intracellular recordings from intercostal motor neurons in the ventral horn of the spinal cord reveal the presence of descending excitatory and inhibitory activity. Projections from brainstem respiratory regions, VRG and DRG, to the spinal cord have been identified.^{76, 77, 86} Respiratory impulses from the brainstem descend to various segments of the spinal cord where they are integrated with intrasegmental and intersegmental data. This descending neural activity alternatively depolarizes and hyperpolarizes the motor cells, leading to rhythmic increases and decreases in their excitability.²² The proposed fundamental controllers of respiration, such equally those for swallowing, have directly access to the spinal motor neurons supplying the muscles of respiration. The major muscle of inspiration, the diaphragm, is supplied by the phrenic nerve arising from cervical levels 4 through half dozen. The external and internal intercostals muscles that help in inspiration and expiration, respectively, are controlled by descending axons that synapse at the levels of the cervical and thoracic spinal motoneurons in the ventral horn of the spinal cord.

It is more often than not accepted that the neural control centers responsible for breathing and swallowing coordination are contained in the dorsomedial and ventrolateral medullary regions of the brainstem. Given the complexity of the intramedullary control of swallow initiation, execution, respiratory rhythm, and ventilation, it is not surprising that few neurophysiologic studies be that simultaneously examine the central control centers. An early investigation of single neurons in the medulla of cats demonstrated that pouring water into the pharynx usually, merely not consistently, caused swallowing movement, and ever inhibited the discharge of inspiratory neurons.⁸⁷ The swallow evoked a brief outburst of impulses in inspiratory neurons followed by a period of inhibition when the swallowing act was elicited by water pouring. When the inhibitory effect of the swallowing neurons on the inspiratory cells was prolonged, the succeeding discharge volley of the inspiratory neuron was longer in elapsing and higher in frequency. The expiratory neurons that intermingled with the inspiratory neurons began to discharge instantaneously every bit the inspiratory neurons stopped their discharge, and were inhibited only transitorily when the swallowing act occurred.⁸⁷

Sumi⁸⁸ showed that motor fibers from NA and XII demonstrated a momentary dominance by the swallowing pathway during eat; however, this dominance was lost during apoplexy of the intratracheal tube. During asphyxia, as the oxygen fractional pressure level decreased and the partial pressure of carbon dioxide increased, the motor fibers that are usually silent during a swallow began to discharge rhythmically during either inspiration or expiration. With continued asphyxia, the consume fibers no longer demonstrated motor discharge to stimulation of the pharyngeal or laryngeal mucosa or to electrical stimulation of the SLN. The two physiologic events reversed in importance, with a patent airway becoming the priority of the motor nuclei and their muscles.^{71, 89, ninety} Neurons firing during inspiration and expiration were identified and characterized into different types depending on their discharge blueprint when water was poured on the pharyngeal mucosa. Sumi'due south findings indicated that two types of interneurons were agile in respiration: those inhibited by swallowing input and those that continue their activeness and may serve functions of both swallowing and respiration.⁸⁸

Recent studies have built upon Sumi'south⁸⁸ early notion that central neurons may play dual roles. Experiments have demonstrated not merely that the central pattern generation of breathing and swallowing may anatomically overlap, but that sure groupings of medullary interneurons may have more than 1 role.^{50, 58, 91, 92, 93} Interneurons in the dorsal and ventral regions of the swallowing central design generator (CPG) have been found to discharge during different behaviors such as swallowing, respiration, mastication. and vocalisation.^{91, 94, 95, 96, 97, 98, 99, 100, 101} Jean^l reports:

Common motoneurons might therefore be triggered by mutual pools of interneurons. These results indicate that in mammals, the neurons liable to be involved in pattern generation tin can belong to different CPGs. Multifunctional neurons of this kind would brand for groovy functional flexibility. (p. 959)

If the concept of multifunctional neurons holds truthful in humans, there are implications regarding plasticity of brainstem structures that may contribute to recovery of functional swallowing and breathing coordination in patients recovering from brainstem stroke.

Suprabulbar Influence

Even though electrically stimulated swallows and respiratory rhymicity persist in experimental animals later removal of suprabulbar structures, there is increasing prove that cortical structures play a office in facilitating and modulating the coordination of animate and swallowing in adult humans. Certainly the employment of various compensatory maneuvers directed toward improving airway protection in patients with dysphagia must involve cortical centers. Patients are often trained to "think swallow or swallow on cue," "agree your breath, and then swallow," etc. These instructions must involve recruitment of premotor and motor cortex for effective initiation.

Cortical stimulation to the frontal orbital gyrus and lateral precentral gyrus has elicited swallowing or facilitated swallowing elicted by other means.¹⁰² Information technology was further demonstrated that stimulation to the motor cortex did non evoke swallowing in monkeys, nonetheless swallowing was readily elicited via electrical stimulation to the precentral area.¹⁰³ Lesion studies in animals and observations of humans post-obit cortical harm lend support for the cortical influence on swallowing. Experimental lesions made in the lateral precentral region of monkeys resulted in pregnant impairment in the initiation and progression of the oral aspects of swallowing.¹⁰⁴ It has besides been shown that destruction to Brodmann'due south area vi in monkeys resulted in persistent difficulty in making the transition betwixt chewing and swallowing.¹⁰⁵ Neurophysiologic studies accept identified projections from cortical regions to the medullary swallowing CPG in sheep. Electric stimulation to the frontal orbital cortex resulted in an antidromic response in the NTS region at the entrance of the peripheral swallowing afferents (i.e., the SLN)¹⁰⁶. It was also shown that swallowing could be elicited upon stimulation to the frontal orbital region, but this cortically induced swallowing was absent after destruction of the NTS.¹⁰⁷ It was further demonstrated that initial excitatory activity could exist elicited in the NTS dorsal swallowing heart with a unmarried pulse to the frontal cortex, merely could only be elicited in the ventral NA region subsequently tetanization of the cortex. These findings seem to suggest that the cortical swallowing regions have an influence on the initiation of the consume plan or on the early stages of eat, merely exercise not contain the cardinal controlling neurons responsible for the swallow programme.¹⁰⁸

Anatomic studies take supported these neurophysiologic findings. Cortical projections have been found from the lateral precentral cortical regions to the NTS in humans, monkeys, and chimps,^{109, 110} and to the NA in humans.¹⁰⁹ Other anatomic studies in laboratory animals have traced connections between the NTS in several forebrain and infralimbic structures, such as the hypothalamus, amygdala, and insular regions.^{111, 112} These findings were varied, and may have represented the central mediation of feeding and casualty behaviors. Subcortical sites have besides been associated with swallowing activities in neurophysiologic studies.^{113, 114, 115, 116, 117} More recent studies support this early on work regarding a role of the cortex in swallowing activity. Two studies accept shown that neurons in the primary motor cortex are active during swallowing.^{118, 119} Innovative technologies such every bit cortical evoked potentials,¹²⁰ transcranial magnetic stimulation,¹²¹ functional magnetic resonance imaging (fMRI).¹²² and positron emission tomography (PET)¹²³ are pointing to multiple cognitive regions (sensorimotor cortex, insula, temporal cortex, and cerebellum) involved in the cardinal modulation of swallowing. Though the exact influence of these cortical and subcortical structures to the swallowing CPG remain unclear,⁵⁰ the multiple cortical and subcortical regions that have been implicated in their swallowing roles may be reflected in the complex swallowing problems seen in patients with focal lesions, every bit in the instance of stroke, or in dysphagic patients with progressive neurologic diseases.

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Translation to Homo Function

A torso of literature reporting results from studies of breathing and swallowing coordination in humans has grown over the past twenty years. These studies showroom improved composure of simultaneous recording methods and assay. However, the instrumentation used in many of these studies was assembled in, and was unique to, a detail laboratory, and generally pocket-size sample sizes were employed. Investigators have attempted to compensate for the modest number of subjects past including the recording and assay of several swallows per subject field. The problem with this method, however, is that it does not account for the known variability in swallowing behavior betwixt individuals. Further, the majority of the studies implemented indirect methods, such every bit submental EMG (sEMG) or records of consume sounds from a stethoscope placed on the lateral neck^{9, x, 11, 124, 125, 126, 127, 128, 129, 130, 131} to mark swallowing activity without visual confirmation. The signals were synchronized using a timer counter, and judgments were made regarding the phase of swallowing activity that was associated with preswallow or postswallow activity. These problems have express the ability to generalize the findings to the population of adult humans. A number of studies have confirmed the swallowing onset and physiologic events related to swallowing and respiration via simultaneous recordings of visual images with either videoendoscopy or videofluoroscopy^{16, 17, 132, 133, 134, 135, 136, 137, 138, 139} and a respiratory signal, adding greater external validity and reliability to the collected data. The studies of breathing and swallowing coordination located by this author in Medline have been summarized chronologically in Table ane, which demonstrates the unlike methodologies (i.e., recording, bolus texture and volume, swallowing tasks, and study end points).¹⁴⁰

Most investigations of breathing and swallowing relationships in developed humans targeted two principal stop points: (1) phase(due south) of respiration (i.e., inspiratory, expiratory, transition) that were associated with oropharyngeal swallowing activity, and (ii) elapsing of the apneic interval. The methodologies used in these clinical experiments, as viewed in Tabular array 1, varied in study sample characteristics, bolus characteristics, mode of oral intake, swallow task, and instrumentation. Respiratory bands, nasal or oral masks with airflow, or temperature-sensitive transducers were typically employed to record the respiratory activity while the subject was administered some type of liquid or food bolus. The respiratory signal was displayed on a monitor and recorded for subsequently demarcation and temporal analysis. Many of these studies involved indirect marking of the swallowing point using the noninvasive techniques described above.

Phase Relationships and Swallow Apnea

Respiratory Phase

The results from the studies listed in Tabular array 1 confirm that the expiratory phase of respiration is the favored limb of the respiratory bicycle surrounding the oropharyngeal swallow, regardless of the variation in study method. Solid food tends to differ from liquids and viscous materials in terms of the regularity of respiration, yet the cycle of breathing interrupted by the swallow (i.e., the apneic interval) continues to occur at some indicate in the expiratory phase of respiration.^{137, 141} The predominant respiratory design surrounding swallowing activeness in healthy adults reported in the majority of these studies is the EX/EX design (expiration earlier the swallow and expiration after the eat), followed next in lodge of frequency of occurrence by IN/EX (inspiration before the consume and expiration later on the swallow), and rarely by the EX/IN (expiration earlier the swallow and inhalation later the swallow) or IN/IN (inhalation earlier the swallow and inhalation after the swallow) patterns. McFarland et al.¹³⁰ showed, all the same, that the segment of the expiratory limb during which swallowing occurs tin be influenced by the changes in the whole trunk posture of the private in adult humans. This study discovered that when human adults attempted to swallow with the hands and knees planted on the ground (i.eastward., "on all fours"), the swallow occurred during the early function of the respiratory cycle and when standing, swallow occurred late in the expiratory cycle. It should be mentioned that the pattern preference of expiration preceding swallowing is reversed in nigh animal models. Swallowing has been shown typically to occur during the inspiratory limb of respiration in unanesthetized^{141, 142} and anesthetized^{47, 143} animals. In baby humans, the production of spontaneous swallows reportedly is equally distributed between the expiratory and inspiratory phases of respiration.¹⁴⁴

The differences amongst animal, human infant, and homo developed swallows may exist related to the differences in neural structures, neural development and maturation, and the overall head and neck anatomy. In animals and homo infants, the larynx is seated high in the neck and the epiglottis opposes the base of the tongue and soft palate. This anatomic position is a favorable feeding, breathing, and swallowing arrangement that allows bolus entry into the pharynx around the side channels of the larynx formed by the aryepiglottic folds. Nasal airflow during breathing can be maintained during the sucking and feeding process, yet all of the animal studies mentioned take observed a conspicuously distinguishable apneic interval that follows a sucking/feeding sequence. In dissimilarity to animals and babe humans, the larynx of developed humans descends with development and reaches its final position around the time of puberty. The lowered laryngeal position provides a unique resonating bedchamber for homo vocalisation and speech production, even so comprises the once anatomically protected airway from liquid and food entry during swallowing. This optimal anatomic configuration for a resonant vocalisation requires that the hyoid and larynx be lifted and pulled forrad to prevent aspiration of a flowing bolus through the pharynx.

A minor group of investigators have taken the study of the respiratory phases and cycles surrounding swallowing action a step further toward understanding the influence of the eat CPG on the respiratory CPG. Surely this data will assist scientists in the translation of the peripheral coordination of these basic physiologic behaviors to the operation of their primal controllers.^{125, 131, 134, 137, 145} These studies included an analysis of the timing in the respiratory phase when swallowing occurred, and calculated the duration of the preswallow, swallow, and postswallow breaths. In unanesthetized humans,¹²⁵ both spontaneous and h2o-induced swallows during expiration increased expiratory time and total time of the swallow breath. The tidal book of the postswallow breath immediately later the consume was increased. Martin¹³² also constitute increased expiratory time in pre- and postswallow breaths when compared to basal respiration. McFarland and Lund¹³¹ too reported that swallowing occurred during the expiratory phase and the duration of that phase was prolonged. Charbonneau et al.¹⁴⁵ also demonstrated that swallowing significantly increased the duration of respiratory cycles of the consume breath and in subsequent respiratory cycles. This notion was carried a step further in the classic study past Paydarfar et al.¹³⁴ that demonstrated that swallows produced a "true" resetting of the respiratory rhythm.

In improver to the phase relationship, and influence of swallow on the elapsing of the respiratory cycle, the glottic position associated with the phases of respiration bracketing swallow has also been explored. Simultaneous recordings of videoendoscopy and respiratory air period showed that the glottis assumes a paramedian position at the onset of laryngeal elevation (i.e., afterward the onset of apnea) in the majority of the tested swallows.^{132, 133} That is, the undulating glottic movement feature of rest animate ceased, medialized, and became fixed in the intermediate or paramedian exhalatory position just before (0.03 s) the onset of laryngeal elevation. Shaker and colleagues¹⁴⁶ similarly plant early vocal fold medialization at the onset of the swallow prior to the onset of laryngeal elevation, but the association of this posturing with respiration was not studied in their piece of work. Finally, the glottic configuration was besides described on the last video frame during which last laryngeal descent occurred. The bulk of swallows were again characterized by a paramedian, expiratory glottic configuration at the time of final laryngeal descent.^{132, 133} This study demonstrated that the glottic configuration associated with expiration, and the cycle of respiration bracketing the bulk of liquid swallow activeness (3 mL, x mL, 20 mL), may business relationship for the priority given to the expiratory bike in and around the time of swallow in immature, salubrious individuals. That is, there appears to exist a congenital-in, reliable, physiologic protection mechanism of a partially occluded glottic airway during the bicycle associated with early and belatedly oropharyngeal swallowing activity.^{125, 132, 133, 134, 136, 139, 145} It has likewise been postulated that swallowing during the expiratory stage of respiration may also facilitate elevation of the larynx because the muscles and forces responsible for the elevation take less resistance to upwardly movement when the diaphragm is relaxed.^{130, 145}

Information technology has been explained that the order of preference for respiratory-swallowing patterns in nigh studies of healthy humans is EX/EX, IN/EX, EX/IN, and IN/IN, respectively. A growing body of evidence is demonstrating, however, a reversal in the order of occurrence of these respiratory patterns in patients with advanced age and diseases oftentimes associated with crumbling. It can be seen from Table 1 that in patients with neurologic illness,^{nine, 10} chronic obstructive pulmonary illness,¹⁴⁷ and those treated for head and neck cancer,^{16, 17} the inspiratory phase of the respiratory cycle appears to increase its relationship to swallowing. The switch of expiration to inspiration preference, however, did not relate to the occurrence of aspiration pneumonia in patients with stroke¹⁰ but did relate to higher (eastward.chiliad., worsened) Penetration-Aspiration Calibration^{148, 149} scores during videofluoroscopic exams.^{sixteen, 17} The clinical significance of these findings remains uncertain until clinical trials that include larger numbers of like-patient cohorts are conducted. The inhalatory gesture surrounding the onset of the consume, and characterized by an open up laryngeal airway, would seem to pose an aspiration threat specially in patients with compromised swallowing function related to focal or generalized weakness, missing structures post-obit surgical ablation, or radiation fibrosis. A few studies^{126, 127, 129, 139, 147} take shown trends that crumbling may also disrupt the normal patterning between breathing and swallowing with a greater occurrence of inspiration surrounding swallowing. Older individuals are besides those who frequently suffer from the diseases and conditions mentioned higher up. Although these pocket-size pattern changes may not pose meaning functional business to the healthy, former individual, occurrence of stroke, pulmonary condition, or head and cervix cancer may pose an increased threat of consume-related aspiration.

Consume Apnea

The 2nd primary aspect of breathing and swallowing coordination that has been studied in the clinical arena relates to the issue of the presence and duration of the apneic interval. All adult human studies that have combined and synchronized videofluorographic and respiratory recordings take shown that an apneic interval consistently occurs with swallowing and is triggered at some point either before or afterwards initial bolus transit through the oral cavity.^{132, 133, 134, 136, 138, 139} The onset of apnea has been found to exist highly variable during cup drinking tasks^{136, 139} and during solid food mastication^{131, 137, 145} followed by swallowing. The onset of apnea, however, has been found to be much more predictable and consequent in the onset of its occurrence during syringe eat tasks.^{132, 133, 134, 138} Apnea duration has been constitute to increase with liquid bolus volume in a few small studies,^{129, 150} but other studies have not supported this notion and instead find no significant differences in apnea with bolus increments betwixt v- and 25-mL liquid swallows.^{132, 133, 138, 151} These varied findings, and the known influence of bolus book on swallowing physiology, betoken that further studies are warranted to examine the potential influence of bolus volume on apnea elapsing. If increasing volumes of liquid upwards to 25 mL, a bolus size found to be the average liquid intake in healthy adults,¹⁵² does indeed result in longer apnea, this challenge may surpass the respiratory adequacy of dysphagic patients with pulmonary disease and interfere with the extent and duration of airway closure that is required for the prevention of aspiration.

Though the time of apnea onset varies with the swallowing tasks, information technology is obligatory at the initiation of the pharyngeal swallow (i.east., equally marked past the onset of hyolaryngeal excursion).^{133, 134, 136, 138, 139} The reported apnea interval duration ranges from 0.5 to 3.five s. Table 1 shows that the reported average apneic interval is typically between ane.0 to 1.five s in most healthy adults. Farther, information technology has been shown that apnea offset is not e'er a postswallow phenomenon (i.e., occurs later complete descent of the larynx). In many individuals, apnea ceases during descent of the larynx at the after stages of consume and is marked by the brief exhalation that occurs during this fourth dimension.^{132, 133, 136, 139} Even though the results from the majority of studies demonstrate a predominant design of animate and swallowing coordination, age- and illness-related aberrations in the duration of the apneic interval have been reported in good for you, old individuals^{129, 139} and in patient groups when compared to young, healthy individuals or age-matched cohorts.¹²⁷

The definition of "old" varies in these studies and often mean age is reported. In full general, good for you, older adults and those with neurologic, pulmonary, and treated oncologic head and cervix cancer have longer apnea durations in the few studies that accept addressed these state of life and wellness issues on apnea duration. In the instance of crumbling and disease, the literature seems to indicate less stability in the neural respiratory-swallow phasing and patterning. The clinical implications of these findings should become clearer when breathing and swallowing patterns in these groups are found to exist true aberrations from "normal" and when the relationship of these aberrations to the overall health outcome of the patients becomes known.

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Determination

Breathing and swallowing processes are highly complex and intricately related in their cardinal control and functional coordination. This review provides just a glimpse at the vast and proliferating body of investigations that seek to determine the nature of this coordination in the brainstem, cortex, and periphery. The majority of the piece of work that has examined these integrated processes at the neural level has been conducted in the animate being model. Functional imaging studies are emerging, however, that are beginning to assist basic and clinical scientists in their understanding of the applicability of these models to human being eating and drinking behavior. Though much is yet to be understood in this regard, the contempo work in animals that implicates the presence of multifunctional neurons common to both respiratory and swallowing CPGs may have relevance for improved understanding of the function of central nervous system (CNS) plasticity. On one hand, if there were injury to the swallowing CPG, the neurons common to both respiration and swallowing may reduce the overall touch of the injury on swallowing function or vice versa. On the other mitt, if i CPG contains neurons mutual to both functions, this may place the individual at a functional disadvantage—and at greater risk—for both processes to exist afflicted in the example of an isolated injury. These notions are all speculative based on the electric current prove, all the same they create a pathway for the evolution of questions to be addressed in hereafter studies that investigate the central control of breathing and swallowing.

The seemingly artificial separation of animate and swallowing as functionally discrete behaviors is non only being questioned as to the CNS, but also with regard to peripheral part. It has been shown that animate can continue fifty-fifty afterward the onset of oral bolus ship at the beginning of the consume and ofttimes resumes prior to the completion of the pharyngeal eat.¹³⁹ This finding underscores the provocative notion mentioned in the introductory section of this review that questioned whether breathing and swallowing could occur simultaneously. The two bones physiologic processes appear to be highly integrated at a functional level and non mutually sectional, that is, an all-or-none physiologic phenomenon.

New recording technologies, methods, and analyses have improved in accuracy, reliability, and commercial availability. These factors afford investigators opportunities both to build on the current body of knowledge regarding cardinal and peripheral breathing and swallowing coordination and serve to meliorate position laboratories for implementation of human behavioral and translational studies that can be compared across study sites. Though the phenomenon of breathing and swallowing coordination, or the respective disruption in this coordination, would appear to have substantial influence on an individual'southward ability to protect the airway during swallowing, this hypothesis has not been adequately tested. Studies are warranted that will make up one's mind the relationship of breathing and swallowing coordination on the overall swallowing impairment of diverse patient populations with dysphagia. In add-on to the study of animate-swallowing phase relationships, temporal characteristics of swallow apnea, and changes in respiratory bike durations surrounding the swallow, studies are on the horizon that will investigate other potentially relevant aspects of swallowing dynamics and pulmonary function on the health and nutritional outcomes of swallowing-dumb individuals. Testify-based swallowing treatments, such every bit compensatory posturing, swallowing maneuvers, and exercises on the coordination of breathing and swallowing are too under report. Information technology is believed that these investigations will help reveal whether deviations from ascendant trends in breathing and swallowing coordination volition have an influence on the success of these treatments.

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Acknowledgements

I would like to acknowledge Martin Brodsky and Suzanne Gresle for their help in the grooming of this manuscript. This study was funded by the National Institute on Deafness and Other Communication Disorders (NIDCD R03 DC04864; 5 K23 DC005764-2) and the Mark and Evelyn Trammell Trust.

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Source: https://www.nature.com/gimo/contents/pt1/full/gimo10.html

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