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Departments of 1 Physiology and 2 Pediatrics, Medical College of Wisconsin, Zablocki Veterans Affairs Medical Center, Milwaukee 53226; and 3 Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin 53201
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The purpose of this study was to determine whether neurons in the facial (FN), gigantocellularis reticularis (RGN), and vestibular (VN) nuclei contribute to the regulation of breathing, swallowing, and the coordination of these two functions. Microtubules were chronically implanted bilaterally in goats. Two weeks later during wakefulness, 100-nl unilateral injections were made of mock cerebral spinal fluid or an excitatory amino acid receptor agonist or antagonists. When the agonist, N-methyl-D-aspartic acid, was injected into any nuclei, breathing and swallowing increased transiently (15-30%; P < 0.05), whereas only injections of the antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo-(f)quinoxaline into VN increased swallowing (20%; P < 0.05). The phase of breathing in which the swallows occurred was not altered by any injections. However, more importantly, injections of the agonist and the antagonists significantly altered (P < 0.05) by 5-50% the respiratory phase-dependent timing and tidal volume effect of swallows on breathing relative to mock cerebral spinal fluid injections. In addition, these effects were not uniform for all three nuclei. We conclude that the FN, RGN, and VN are part of a neural circuit in the rostral medulla that regulates and/or modulates breathing, swallowing, and their coordination in the awake state.
respiration; deglutition; pharyngeal muscles; rostral medulla; excitatory amino acid receptors
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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SWALLOWING AND BREATHING ARE two important yet neuromechanically opposing functions that utilize the pharyngolaryngeal airway. As a result, the control and activation of the upper airway muscles must be precisely coordinated to prevent aspiration. The sites for the "triggering" and "patterning" of swallowing and breathing are thought to be in or near the nucleus ambiguus (NA) and nucleus tractus solitarius (NTS) in the medulla oblongata [reviewed by Jean and co-workers (12, 15) and Miller (22)]. The site for coordination of these functions has been postulated to be in the area around the NA (2) as well as the area lateral to the NTS (9-11, 15, 20, 25, 26), but no firm evidence is available in support of this postulate.
Recent studies using decerebrate and anesthetized preparations suggest that the facial (FN), gigantocellularis reticularis (RGN), and vestibular (VN) nuclei can all influence breathing (4, 16, 23, 39, 40). In addition, our laboratory has recently shown in awake goats that injections of excitatory amino acid (EAA) receptor antagonists at some sites within these rostral nuclei alters CO2 sensitivity and the exercise hyperpnea (38). In the course of these studies, changes in swallowing after some of these injections were also observed. Previous studies have shown that EAA receptor agonist [N-methyl-D-aspartic acid (NMDA)] injections into the lateral ventral NTS (area of the putative swallowing generator) and the para-NA areas increased swallowing (12, 18, 19). Furthermore, in this latter region (thought to contain "switching" neurons for the swallowing pattern generator), injections of EAA receptor antagonists depressed or eliminated swallowing (12). However, the changes in swallowing that were observed after EAA receptor agonist and antagonist injections suggested that rostral medullary nuclei are also involved in the control of swallowing via EAA receptors.
We therefore began a systematic, retrospective investigation of the effects on swallowing and breathing of EAA receptor agonist and antagonist injections into these rostral medullary nuclei. On the basis of our preliminary findings on the effects of injections on swallowing, we hypothesized that altered neural function in these rostral medullary nuclei would affect the normal coordination or interaction of breathing and swallowing.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals. The data from 13 adult goats of various breeds were examined in this study. All received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" formulated by the National Research Council, 1996. The study protocol was approved by the Institutional Animal Care Committee of the Medical College of Wisconsin.
Surgical preparation. All surgeries were performed under sterile conditions. An initial surgery was performed to implant chronic electromyographic (EMG) electrodes in the diaphragm (EMGDia), thyropharyngeus (EMGTP), and stylopharyngeus (EMGSP) muscles and to elevate the carotid arteries. In addition, during the initial surgery in four goats, the upper airway was isolated by creating a tracheostoma at the level of the eighth tracheal ring. A second surgery was performed ~4 wk later to implant microtubules bilaterally into the FN, RGN, or VN. Implants were bilateral to increase the amount of data obtained from each goat. Anesthesia was induced with an intravenous injection of ketamine (Ketaset) and xylazine (12:1 vol/vol ratio, 15 mg/kg). After intubation for mechanical ventilation, anesthesia was maintained with 1.5% halothane in oxygen (sufficient to eliminate the withdrawal reflex and any signs of pain.) Both EMG electrode placement (7) and microtubules implantation have been described previously (38).
For at least 24 h after surgery for microtubule implantation, laboratory personnel frequently inspected the animals. Some goats were unable to maintain normal sternal recumbent posture for 3-6 h after surgery, and some were unable to stand for up to 4 days after the implant. In these latter goats, food and water intake was decreased over this time period; thus fluids and nutrition were administered intravenously. To minimize brain edema after implantation surgery, the goats were medicated three times per day with dexamethasone (0.4 mg · kg
1 · day1 for 2 days,
then decreasing by 0.05 mg · kg1 · day1). To
minimize infection, chloramphenicol (20 mg/kg) was administered three times per day for 3 days. Thereafter, the goats received daily
intramuscular injections of antibiotics (ceftiofur sodium, 2 mg/kg;
gentamicin, 3 mg/kg). Buprenorphine was administered 3 and 12 h
after implantation to minimize pain. The animals rectal temperature,
eating habits, and behavior were monitored daily. These measures
indicated that the goats were in normal good health by at least the
fourth day after surgery.
Methods of measurement. In the goats without a tracheostoma, a tight-fitting custom mask was taped to the snout, and a two-way breathing valve was attached to the mask. In the goats with a tracheostoma, an 8-Fr cuffed tracheostomy tube was inserted into the trachea, the cuff was then inflated, and the tube was connected to a two-way breathing valve. Inspiratory airflow was measured by a pneumotachograph attached to the inspiratory side of the breathing valve. The pneumotachograph was connected via tubing to a differential pressure transducer. The proximal ends of the EMG wires were connected via microclips to a Grass recorder for signal processing and recording on paper. The EMG signals were filtered at a band pass of 3-500 Hz. The airflow and raw EMG signals were sent for display, digital recording, and analysis to a CODAS computer data acquisition system at a sampling rate of 250 Hz.
Experimental design. Before any data collection, the goats were thoroughly familiarized with all aspects of the experimental condition. At least 7 days after the initial surgery (before microtubule implantation surgery), quiet breathing and spontaneous swallows during wakefulness were monitored for a minimum of 60 min. A similar period of data collection was performed on a day with no injections at least 14 days after microtubule implantation. Even though the goats were in apparent good health, previous studies had shown that ventilation, arterial blood gases, and rectal temperature were not stable or at control levels until 10-14 days after brain surgery (38). All studies were performed during the awake state.
On or after the 15th day after surgery for microtubule implantation, mock cerebrospinal fluid (mCSF) and subsequently 100 mM NMDA (mixed in mCSF) were unilaterally microinjected (100 nl) through the microtubules and into the tissue. The mCSF injection was intended as a control fluid injection while the agonist NMDA was injected to indicate whether neurons at the injection site were part of the respiratory or swallowing neural circuitry. It has been previously shown that this concentration of NMDA is effective in awake goats (8, 38). During these studies, respiratory airflow, EMG activity, heart rate, and arterial blood pressure were continuously monitored over an initial 15-min control period and for the subsequent 2-3 h. Injections were made at 30-min intervals except when additional time was needed for measured values to fully recover from the previous injection. Over several subsequent days, mCSF, 250 mM kynurenic acid (KynA), 5 mM 2-amino-5-phosphonovalerate (AP5), or 0.3 mM 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo-(f)quinoxaline (NBQX) was microinjected (100 nl) unilaterally. These antagonists (each mixed in mCSF) have been shown to be effective in awake goats for several minutes after injection, thus creating a prolonged period of altered neuronal function (8, 38). The rationale for injections of different EAA receptor antagonists was as an attempt to distinguish between NMDA and non-NMDA receptor contribution to the control of breathing, swallowing, and their interaction. At least 4 h were allowed for recovery before a microinjection was made in the contralateral microtubule. The order of unilateral antagonist injections was randomized. In the subset of goats with tracheostomies (n = 4), we also examined the effect of stimulated swallowing during eupnea and hypercapnia (7% inspired CO2 fraction) before and after injections of mCSF and EAA receptor antagonists. For these studies, a second unilateral injection was made in the same microtubule 60 min after the initial injection. Fifteen minutes after this later injection, hypercapnia was created by increasing inspired CO2 by 2.5% every 5 min to a maximum of 7.5%. Stimulated swallows were evoked by injecting tap water (1 ml/s) for 30 s via a 5-Fr feeding catheter that was passed nasally and placed just below the velopharyngeal airway.Data analysis. Respiratory airflow and EMG signals were processed and analyzed (WinDaq, DATAQ Instruments). Raw EMG data were full-wave rectified and passed through a moving-time averager (time constant of 0.1 s) to obtain an integrated EMG signal. The integrated EMGTP signal was analyzed to obtain the occurrence of swallowing. The time of the occurrence of the swallow was set at 0.15 s before the peak of a 10-fold increase in the moving time average of the EMGTP activity without signs of movement artifact.
A two-state computer algorithm was used for the breath-by-breath calculations of tidal volume (VT), duration of inspiration (TI), and duration of expiration (TE) from the airflow signal. Minute ventilation (
I) was
calculated for each breath using the equation I = VT · [(TI + TE)/60]. The
absolute time for the start of inspiration was also recorded, along
with the absolute time for each swallow. As a result, swallows were
categorized as expiratory (SWE), the transition from
expiration to inspiration (SWEI), inspiratory
(SWI), or the transition from inspiration to expiration
(SWIE; see Fig. 1). The
percentage of the total number of swallows during SWE,
SWEI, SWI, and SWIE was computed. The respiratory parameters (TI, TE,
VT, and I) during SWE,
SWEI, SWI, and SWIE were also
calculated and expressed as a percentage of the immediate control
values using breaths without swallows.
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I were expressed
as the changes from the immediately preceding 60-s control period.
Assessment of microtubule location. After the death of the goat (by Beuthanasia), the brain was perfused with phosphate-buffered saline (pH 7.3) and 4% paraformaldehyde fixative and then removed. Frozen, transverse sections (20 µm) were cut, stained (hematoxylin and eosin), and examined microscopically. The distal end of the microtubule lesion identified the site of the injection into the targeted nuclei (6). The location of each unilateral microtubule was assessed and grouped into the FN, RGN, or VN.
Statistical analysis.
A Student's t-test was performed on the pre-MT and post-MT
swallowing interval to determine whether the rate of swallowing statistically changed after implantation surgery (P < 0.05). To determine whether the percentage of SWE,
SWEI, SWI, and SWIE after implantation surgery changed, a Student's t-test was
similarly performed on the pre-MT and post-MT data. The effect of
SWE, SWEI, SWI, and
SWIE on respiration, pre-MT and post-MT, was evaluated by
using a Student's t-test to test whether the means of the
TI, TE, VT, and
I values were significantly different
(P < 0.02) from control and from pre-MT and post-MT.
I for breaths without swallows
(expressed as percentage of the previous control values) for each
10-min interval were significantly different (P < 0.05) from zero. To assess the effect of injections on the interactions
between swallowing and breathing, the means of TI, TE, VT, I during
SWE, SWEI, SWI, and
SWIE [expressed as the difference from the findings of
mCSF injections, e.g.,
TI(mCSF)/TI(control) 
TI(NMDA)/TI(control)] was calculated. A
Student's t-test was then performed to test whether these
values were significantly different (P < 0.05) from
zero for each 10-min interval after injections.
To test the effect of stimulated swallowing with and without
hypercapnia, a one-way ANOVA with repeated measures was performed on
the number of coughs and swallows, and the onset of swallowing for each
mCSF and EAA receptor antagonist injections was grouped for the
placement of microtubules into either the FN, RGN, or VN. The effect of
stimulated swallowing on TI, TE,
VT, and I after injections was
statistically examined by using a Student's t-test to test
whether these values were significantly (P < 0.05) different from zero.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Location of microtubule implantation.
In the 13 goats studied, 11 microtubules were implanted in or near the
FN, 9 in or near the RGN, and 4 in or near the VN (Fig. 2). A total of 24 rather than 26 sites
was studied because in one goat one of the microtubules was
inadvertently occluded and in another goat the microtubule was placed
at a site other than the FN, RGN, or VN. In the four goats with
tracheostomies, five of the microtubules were in the FN and two
microtubules were placed in the RGN.
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Effect of microtubule implantation on swallowing and ventilation.
Microtubule implantation did not significantly alter (P > 0.05) the rate of swallowing (Fig.
3A). The mean pre-MT
swallowing interval was 21.2 ± 4.3 s, whereas the post-MT
mean swallowing interval was 27.5 ± 10.9 s. Similarly, the
percentage of swallows during the different phases of respiration in
the post-MT studies was not significantly different (P > 0.05) from that in pre-MT studies (Fig. 3B). Because the
occurrences of SWIE were so infrequent, this category of
swallowing during respiration was excluded from further analysis.
Although swallows during different phases of respiration
affected TI, TE, VT, or
I, no significant (P > 0.05) differences were found between pre-MT and post-MT values (Fig. 3C).
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Effects of injections on swallowing.
Swallowing frequency was increased transiently after EAA receptor
agonist (NMDA) injections into the FN, RGN, and VN (P < 0.015; Fig. 4). In contrast,
swallowing frequency was unaltered (P > 0.015) after
mCSF and most EAA receptor antagonist (AP5, KynA, NBQX) injections into
the nuclei. The exception was NBQX injections into the VN. The phase of
respiration in which swallows occurred was not altered by these
injections in any of the three nuclei.
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Effects of injection on respiration.
In breaths with no swallows, injections of mCSF into FN or RGN
significantly decreased TI, TE, and
VT (8-10%) and increased I
(14-17%) relative to control breaths (P < 0.015). These changes persisted for 10-30 min, and the results
were reproducible. In contrast, the only effect of mCSF injection into
the VN was to slightly increase VT (~10% in the first
10-min interval) and TE (8-10% in the second and
third 10-min interval; P < 0.015).
I (P < 0.05) in the first 20 min
after injections. NMDA injections into the RGN had no effect on
I even though VT was slightly elevated (8-10%; P < 0.015) in the later portion of the
30-min recording period. In the VN, injections of NMDA produced a
transient decrease (~10%; P < 0.05) and then
increase (~8%; P < 0.05) in TE and a transient increase in VT and I (~17%
and ~20%, respectively; P < 0.05). EAA receptor
antagonist injections into the FN, RGN, or VN produced only a small
number of changes in respiration. In the FN, NBQX increased
I ~15% (P < 0.05) through either
a decrease in TI or an increase in VT. In the
RGN, AP5 increased I (12%: P < 0.05) primarily because of a decrease in TI and TE during the 30-min period.
Effect of swallows on breathing after injections.
After injections of mCSF into the three nuclei, the effect of swallows
on breathing were generally similar to the effect without injections
(Fig. 3C); namely the primary effect of swallows during expiration was to increase TE, whereas swallows during
inspiration increased TI and decreased I
(P < 0.05). On the other hand, swallows during the
transition from expiration to inspiration transiently decreased
TI, TE, and VT and increased
I (P < 0.05) after mCSF injections,
which differed from the increased TE during swallows
without any injections. Accordingly, because of these effects by mCSF
injections, the effects of swallowing on breathing after EAA receptor
agonist and antagonist injections are referenced to the results of mCSF injections.
I and
decreased TE by 10-35% more than after mCSF injections (P < 0.05). On the other hand for
SWE, TI, TE, and VT
were reduced and I increased 10-35% more
(P < 0.05) after NBQX injections than after mCSF
injections. In contrast, after NMDA injections (Fig. 5A),
the major effect was during SWEI in which TI
and TE were reduced (P < 0.05) relative to
the effect of mCSF injections.
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I
(P < 0.05). In contrast, AP5 injection into the RGN
(Fig. 6B) was characterized by a significant (P < 0.05) biphasic decrease and then an increase in
I (relative to mCSF) primarily because of changes in
TI and TE. The primary effect of NBQX was on
SWE 20 min after the injections, which was to increase
TI and TE (10-15%; P < 0.05), thereby decreasing I (15%; P < 0.05).
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I (Fig.
7B) relative to mCSF
injections. AP5 injections primarily resulted in a decrease
(P < 0.05) in TI and VT (20 and 40%, respectively) during SWI in the first 20 min
after injections. In contrast, NMDA injections (Fig. 7A)
produced opposite biphasic responses with SWI and
SWEI relative to mCSF injections.
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Effects of EAA injections on stimulated swallowing and breathing
and their interactions.
Although we observed an increase in coughs with stimulated swallows in
the four goats examined, no other significant changes in the number of
swallows or coughs or in the onset of swallowing between eupneic and
hypercapnic conditions were observed in the absence of microinjections
(Table 1). Similarly, in both the FN
(Table 2) and RGN (not shown), we did not
observe significant changes in these swallowing parameters after any
injections, with or without stimulated breathing.
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I during stimulated
swallowing were consistently decreased (P < 0.05) 15 min after NBQX injections into the FN compared with the highly variable
response after injections of mCSF, AP5, and KynA (Fig.
8). During hypercapnia, the response to
stimulated swallowing was increased (P < 0.05) after
mCSF, whereas there was very little change in breathing after NBQX
injection (Fig. 8). Because of the limited number of microtubules in
this nucleus, no significant changes were observed in the RGN.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Our data suggest that the FN, RGN, and VN are involved in the interactions between swallowing and breathing. These findings are particularly notable considering that some EAA receptor antagonists did not alter breathing or swallowing but did alter the interaction between the two functions. In addition, the timing and magnitude of the ventilatory response with swallows after injections were different between the three nuclei, which suggests that each of these rostral medullary nuclei contributes differently to the coordination of swallowing and breathing.
Effects of mCSF and EAA receptor agonist and antagonist injections. Previously, our laboratory has reported that the injections into the medulla affected neurons in a spherical pattern with a radius that approximated 1.5 mm (38). This area is large compared with the spread of the injectate reported in studies on rats and cats but is small considering that the medulla of a 35-kg goat is 160% the size of the medulla in a 2-kg cat (6). We are thus confident that the major or all the effect of an injection was in the nucleus of the implanted microtubule.
Our laboratory has also previously reported that injections of mCSF into some rostral medullary nuclei alter breathing (38). For the present manuscript, the ventilatory data were reanalyzed on a breath-by-breath basis and binned into 10-min epochs. The detailed breath-by-breath analysis yielded a greater number of significant effects of the mCSF than the previous analysis. In addition, the analysis of mCSF injections was completed on data from two separate injections (different days) into each medullary site in each goat; both data sets yielded the same results, demonstrating reproducibility. Moreover, mCSF injections were consistently associated with a time-dependent increase in ventilation, which is opposite to our observations during noninjection control periods in which we would see no changes or a decrease in ventilation. The cause of these changes is unknown, but conceivably they could be due to an alteration in the extracellular milieu of the affected neurons (ion and/or neurotransmitter concentrations). The injections of EAA receptor agonist (NMDA) and some of the antagonists also affected breathing, but the observed changes (e.g., TI, TE, and VT) differed from mCSF injections, indicating that they were specific to the injectate. In contrast, injections of mCSF and most EAA receptor antagonists did not affect the rate of swallowing when injected into any nuclei. On the other hand, NMDA injections consistently increased swallowing. Moreover, mCSF, NMDA, and some antagonists significantly altered the effect of swallows on breathing. Rather than present the effects of the agonist and antagonists relative to just each respective noninjection control period, we present these effects relative to the mCSF effects (Figs. 5-7). Both ways of analysis and/or presentation of the data lead to the same conclusion: injections of the agonist and antagonist altered the interaction between swallowing and breathing. EAA receptor agonists and antagonists should have opposite effects on physiological functions. In this and a companion publication (38), we report that in many cases the agonist and antagonists had qualitatively the same effects. We cannot explain the apparent paradox other than it could be due to metabolism of an antagonist to an agonist, for example KynA to quinolinic acid (34). Another potential explanation is that there may have been subtle differences in diffusion of the agonist and antagonist that resulted in the agonist and antagonist affecting different neurons differently over time within the same nucleus. This possibility is suggested by our laboratory's previous finding that the antagonists did not have a uniform effect on CO2 sensitivity or exercise hyperpnea when injected into any one of these rostral medullary nuclei. This finding led us to the conclusion that there is a heterogeneous population of neurons in these nuclei. EAA receptor agonist and antagonist play an important role in the generation and transmission of the swallowing signal to the pharyngeal motoneurons. Injections of NMDA into the dorsal and ventral swallowing groups (DSG and VSG, respectively) have been shown to increase swallowing (12, 18, 19). Kessler (17) has shown that swallowing was depressed or eliminated with injections of EAA receptor antagonists in or near the rostral VSG that contains switching neurons for swallowing and motoneurons that supply pharyngeal muscles (12). The sites of injections (Fig. 2) in our study that increased swallowing were located not in the vicinity of the DSG and VSG but in other nuclei within the rostral medulla. An important finding in this study is that both NMDA and non-NMDA receptors are important in the coordination of swallowing and breathing. However, the effects of the non-NMDA receptor antagonist (NBQX) were more prevalent than those of the selective NMDA receptor antagonist (AP5) or the nonselective (KynA) antagonist. These findings may suggest that non-NMDA receptors are more important than NMDA receptors.Medullary coordination of breathing and swallowing. The coordination between swallowing and breathing is important for the maintenance of pharyngolaryngeal function and the prevention of pulmonary aspiration, but the neural basis of this coordination remains unclear (12, 22, 24). Bianchi et al. (2) postulated that the intermediate VRG and the immediate surrounding regions contain propriobulbar neurons, which are important in the coordination of pharyngeal and laryngeal muscles during respiration, swallowing, coughing, and upper airway mechanoreceptor activation. Several studies have shown that ambigual, trigeminal, and hypoglossal motoneurons activated during swallowing are also activated in phase with respiration, suggesting that a common set of motoneurons are driven by both swallowing and breathing central pattern generators (9, 10, 26, 29). Moreover, some of the interneurons within the DSG and VSG have been shown to fire during respiration, further demonstrating an input source to the aforementioned cranial motoneurons (5, 20, 28, 36, 37). Similarly, respiratory interneurons in the dorsal and ventral respiratory groups have been shown to be active during swallowing (9-11, 22, 28).
The effects of swallowing on respiratory timing and total output have been demonstrated in a few studies using unanesthetized, intact animals. In awake rabbits, swallows during expiration significantly increased TE and the preceding TI, whereas swallows during inspiration significantly increased TI and the subsequent TE (23). A similar respiratory phase-dependent effect was seen in unanesthetized humans, with both spontaneous and water-induced swallows. In these studies, swallows during expiration increased TE and the successive VT. Swallows during inspiration reduced TI, VT, and the subsequent TE (26). Our laboratory has previously shown in goats (7) that swallows during expiration also increased TE, but in contrast to data in humans (26) the ensuing TI and VT were significantly reduced, suggesting that an inhibitory effect of swallows persisted in the following inspiratory timing and output. In goats, swallows during inspiration increased TI (7), which is similar to that found in rabbits (21) but is in contrast to findings in humans, in whom swallows decreased TI (24). It is conceivable that the neural substrate for the specific phase-dependent interaction between swallowing and breathing exists downstream from the central pattern generators for swallowing and respiration between the premotoneurons of the dorsal and ventral respiratory and swallowing groups. The interaction at this level could potentially modulate the effect of swallows on the pattern of respiratory output to the cranial motor and phrenic nerves. However, the changes in respiratory timing within a breath observed in this and other studies (7, 21, 24), as well as the type I resetting of the respiratory rhythm network after swallows (7), suggest a shared neural network and/or an input from the triggering center for swallowing to the respiratory rhythm generator. Although there are some data suggesting shared interneurons and/or neural networks (5, 9-11, 20, 26, 33, 35), a functional neuronal connection between these centers has yet to be shown. Our data demonstrate that rostral medullary nuclei not only have a role in regulating breathing and swallowing but also modulate the effect of swallowing on respiratory timing, VT, and total ventilation. The different effects in the three nuclei may indicate that each nucleus has a unique regulatory function. Data from previous studies also suggest these nuclei contribute to the control of breathing. The VN has been shown to have projections into the RGN (23, 27, 39, 40). Neurons from the RGN have been shown to innervate inspiratory and expiratory premotor neurons and to modulate respiratory chemoreflexes (16, 30, 31); they are also involved in the ventilatory response to exercise (16, 31). Stremel et al. (35) demonstrated that both electrical and chemical excitation (glutamate and kainic acid) of the RGN caused a decrease in TI, TE, VT, andI. Both Xu et al. (39) and our
laboratory (38) found that injection of a neurotoxin into
the RGN transiently increased breathing. Our laboratory has also
reported that chronic ibotenic acid lesions in the RGN and FN altered
eupneic breathing, CO2 sensitivity, and the hyperpnea of
exercise (38). Moreover, Xu et al. found that neurotoxic
lesions in the RGN eliminated the effects on breathing of stimulating
the cerebellar fastigial nucleus. In addition, Yates et al.
(40) found that the effects on breathing of electrical
stimulation in the vestibular nucleus are dependent on functional RGN
neurons and that the RGN is part of the central command circuitry for
exercise (16). Finally, other studies using pseudorabies
virus for retrolabeling interneurons and motoneurons have shown
connections between swallowing-activated neurons in the NTS and the RGN
(5, 27, 32, 33). The question remains to what degree do
the known pathways and sites of integration contribute to the specific
response after EAA receptor agonist and antagonist injections.
An emerging concept based on data from several laboratories is that the
RGN is a site of integration (convergence, coordination) for stimuli
from different sources (Fig. 9). The
unique data from the present study further enhance and also expand this
concept by providing evidence that the interaction or coordination of two behaviors, swallowing and breathing, might occur or be affected by
the RGN. The uniqueness and importance of our data are that the
findings were obtained during physiological conditions. We speculate
that the potential role of the FN and VN in this interaction is due to
these nuclei being part of the neural circuit, via the RGN, that
regulates not only swallowing and breathing but also other behaviors
(vomiting, coughing, sneezing, etc.) that utilize the so-called
respiratory muscles.
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Flexibility of coordination. The data reported herein demonstrate considerable flexibility in the breathing by goats to accommodate swallowing. First, goats are somewhat unique among mammals in that they can swallow successfully during virtually every stage of a breathing cycle. Second, with altered neuronal function of rostral medullary nuclei, there are marked changes in the effect of swallowing on breathing. However, judging from the absence of coughs, the coordination of the behaviors is still achieved. Lastly, even when the coordination system is stressed, as with stimulated swallowing during hypercapnia, there is minimal increase in coughing. Accordingly, it appears that for at least this species there is considerable reserve in the system before aspirations would occur. If the same applies to humans, then when aspirations occur clinically, this must indicate considerable pathology in the coordination of swallowing and breathing. However, because goats are ruminants, their frequency, control, and other aspects of swallowing may differ from humans, which should be considered in any extrapolation to humans.
We conclude that although the FN, RGN, and VN are not considered primary sites for regulation of breathing, swallowing, and the coordination of theses two functions, these rostral medullary nuclei are part of a neural circuit that in the awake state can affect these physiological functions. Furthermore, these data support the strategy of using agents to induce reversible altered neuronal function and measuring multiple pattern-generator outcomes (including their interactions) with in vivo awake animal models.| |
ACKNOWLEDGEMENTS |
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The authors thank Nancy Schlick, Alex Serra, and Leenne Klum for technical assistance in preparation of the manuscript.
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FOOTNOTES |
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This study was supported by National Heart, Lung, and Blood Institute Grant HL-25739 and by the Veterans Affairs.
Address for correspondence: H. V. Forster, Medical College of Wisconsin, 8701 Watertown Plank Rd., Milwaukee, WI 53226 (E-mail: bforster{at}mcw.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
March 15, 2002;10.1152/japplphysiol.01268.2001
Received 28 December 2001; accepted in final form 12 March 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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1.
Amri, M,
Car A,
and
Jean A.
Medullary control of the pontine swallowing neurones in sheep.
Exp Brain Res
55:
105-110,
1984[ISI][Medline].
2.
Bianchi, AL,
Denavit-Saubié M,
and
Champagnat J.
Central control of breathing in mammals: neuronal circuitry, membrane properties, and neurotransmitters.
Physiol Rev
75:
1-26,
1995
3.
Billig, I,
Foris JM,
Card JP,
and
Yates BJ.
Transneuronal tracing of neural pathways controlling an abdominal muscle, rectus abdominis, in the ferret.
Brain Res
820:
31-44,
1999[ISI][Medline].
4.
Billig, I,
Foris JM,
Enquist LW,
Card JP,
and
Yates BJ.
Definition of neuronal circuitry controlling the activity of phrenic and abdominal motoneurons in the ferret using recombinant strains of pseudorabies virus.
J Neurosci
20:
7446-7454,
2000
5.
Cunningham, ET, Jr,
and
Sawchenko PE.
Dorsal medullary pathways subserving oromotor reflexes in the rat: implications for the central neural control of swallowing.
J Comp Neurol
417:
448-466,
2000[ISI][Medline].
6.
Dean-Bernhoft, C,
Geiger L,
Sprtel B,
Ohtake P,
and
Forster HV.
An anatomical atlas of the medulla oblongata of the adult goat.
J Appl Physiol
87:
1220-1229,
1999
7.
Feroah, TR,
Forster HV,
Fuentes CG,
Lang IM,
Beste D,
Pan L,
and
Rice T.
Effects of spontaneous swallows on respiratory timing and drive in awake goats.
J Appl Physiol
92:
1923-1935,
2002
8.
Forster, HV,
Pan LG,
Lowry TF,
Feroah T,
Gershan WM,
Whaley AA,
Forster MM,
and
Sprtel B.
Breathing of awake goats during prolonged dysfunction of caudal M ventrolateral medullary neurons.
J Appl Physiol
84:
129-140,
1998
9.
Gestreau, C,
Grelot L,
and
Bianchi AL.
Activity of respiratory laryngeal motoneurons during fictive coughing and swallowing.
Exp Brain Res
130:
27-34,
2000[ISI][Medline].
10.
Gestreau, C,
Milano S,
Bianchi AL,
and
Grelot L.
Activity of dorsal respiratory group inspiratory neurons during laryngeal-induced fictive coughing and swallowing in decerebrate cats.
Exp Brain Res
108:
247-256,
1996[ISI][Medline].
11.
Hayashi, F,
and
McCrimmon DR.
Respiratory motor responses to cranial nerve afferent stimulation in rats.
Am J Physiol Regulatory Integrative Comp Physiol
271:
R1054-R1062,
1996
12.
Jean, A.
Brain stem control of swallowing: neuronal network and cellular mechanisms.
Physiol Rev
81:
929-969,
2001
13.
Jean, A.
Control of the central swallowing program by inputs from the peripheral receptors: a review.
J Auton Nerv Syst
10:
225-233,
1984[ISI][Medline].
14.
Jean, A,
and
Car A.
Inputs to the swallowing medullary neurons from the peripheral afferent fibers and the swallowing cortical area.
Brain Res
178:
567-573,
1979[ISI][Medline].
15.
Jean, A,
Car A,
and
Kessler JP.
Brainstem organization of swallowing and its interaction with respiration.
In: Neural Control of the Respiratory Muscles, edited by Bianchi AD.. Boca Raton, FL: CRC Press, 1997, chapt. 19, p. 223-237.
16.
Kerman, IA,
Hartge K,
Card JP,
and
Yates BJ.
Neurons of the medial medullary reticular formation are part of the central command circuitry.
Soc Neurosci Abstr
27 (836):
9,
2001.
17.
Kessler, JP.
Involvement of excitatory amino acids in the activity of swallowing-related neurons of the ventro-lateral medulla.
Brain Res
603:
353-357,
1993[ISI][Medline].
18.
Kessler, JP,
Cherkaoui N,
Catalin D,
and
Jean A.
Swallowing responses induced by microinjection of glutamate and glutamate agonists into the nucleus tractus solitarius of ketamine-anesthetized rats.
Exp Brain Res
83:
151-158,
1990[ISI][Medline].
19.
Kessler, JP,
and
Jean A.
Evidence that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors within the nucleus tractus solitarii triggers swallowing.
Eur J Pharmacol
201:
59-67,
1991[ISI][Medline].
20.
Larson, CR,
Yajima Y,
and
Ko P.
Modification in activity of medullary respiratory-related neurons for vocalization and swallowing.
J Neurophysiol
71:
2294-2304,
1994
21.
McFarland, DH,
and
Lund JP.
An investigation of the coupling between respiration, mastication, and swallowing in the awake rabbit.
J Neurophysiol
69:
95-108,
1993
22.
Miller, AJ.
Deglutition.
Physiol Rev
62:
129-184,
1982
23.
Mori, RL,
Bergsman AE,
Holmes MJ,
and
Yates BJ.
Role of the medial medullary reticular formation in relaying vestibular signals to the diaphragm and abdominal muscles.
Brain Res
902:
82-91,
2001[ISI][Medline].
24.
Nishino, T,
Yonezawa T,
and
Honda Y.
Effects of swallowing on the pattern of continuous respiration in human adults.
Am Rev Respir Dis
132:
1219-1222,
1985[ISI][Medline].
25.
Oku, Y,
Dick TE,
and
Cherniack NS.
Phase-dependent dynamic responses of respiratory motor activities following perturbation of the cycle in the cat.
J Physiol
461:
321-337,
1993
26.
Oku, Y,
Tanaka I,
and
Ezure K.
Activity of bulbar respiratory neurons during fictive coughing and swallowing in the decerebrate cat.
J Physiol
480:
309-324,
1994[ISI][Medline].
27.
Ootani, S,
Umezaki T,
Shin T,
and
Murata Y.
Convergence of afferents from the SLN and GPN in cat medullary swallowing neurons.
Brain Res Bull
37:
397-404,
1995[ISI][Medline].
28.
Peterson, BW,
Filion M,
Felpel LP,
and
Abzug C.
Responses of medial reticular neurons to stimulation of the vestibular nerve.
Exp Brain Res
22:
335-350,
1979.
29.
Popratiloff, AS,
Streppel M,
Gruart A,
Guntinas-Lichius O,
Angelov DN,
Stennert E,
Delgado-Garcia JM,
and
Neiss WF.
Hypoglossal and reticular interneurons involved in oro-facial coordination in the rat.
J Comp Neurol
433:
364-379,
2001[Medline].
30.
Richard, CA,
and
Stremel RW.
Involvement of the raphe in the respiratory effects of gigantocellular area activation.
Brain Res Bull
25:
19-23,
1990[Medline].
31.
Richard, CA,
Waldrop TG,
Bauer RM,
Mitchell JH,
and
Stremel RW.
The nucleus reticularis gigantocellularis modulates the cardiopulmonary responses to central and peripheral drives related to exercise.
Brain Res
482:
49-56,
1989[ISI][Medline].
32.
St. John, WM.
Influence of reticular mechanisms upon hypoglossal, trigeminal and phrenic activities.
Respir Physiol
66:
27-40,
1986[Medline].
33.
Sang, Q,
and
Goyal RK.
Swallowing reflex and brain stem neurons activated by superior laryngeal nerve stimulation in the mouse.
Am J Physiol Gastrointest Liver Physiol
280:
G191-G200,
2001
34.
Stone, TW,
and
Connick JH.
Quinolinic acid and other kynurenines in the central nervous system.
Neuroscience
15:
597-617,
1985[ISI][Medline].
35.
Stremel, RW,
Waldrop TG,
Richard CA,
and
Iwamoto GA.
Cardiorespiratory responses to stimulation of the nucleus reticularis gigantocellularis.
Brain Res Bull
24:
1-6,
1990[ISI][Medline].
36.
Travers, JB,
DiNardo LA,
and
Karimnamazi H.
Medullary reticular formation activity during ingestion and rejection in the awake rat.
Exp Brain Res
130:
78-92,
2000[ISI][Medline].
37.
Wang, W,
and
Richerson GB.
Development of chemosensitivity of rat medullary raphe neurons.
Neuroscience
90:
1001-1011,
1999[ISI][Medline].
38.
Wenninger, JM,
Pan LG,
Martino P,
Geiger L,
Hodges M,
Serra A,
Feroah TF,
and
Forster HV.
Multiple rostral medullary nuclei can influence breathing in awake goats.
J Appl Physiol
91:
777-788,
2001
39.
Xu, F,
Zhou T,
Gibson T,
and
Frazier DT.
Fastigial nuclear neurons responsible for modulating respiration project to medullary nucleus gigantocellularis in the rat.
Soc Neurosci Abstr
26 (348):
8,
2000.
40.
Yates, BJ,
Jakus J,
and
Miller AD.
Vestibular effects on respiratory outflow in the decerebrate cat.
Brain Res
629:
209-217,
1993[ISI][Medline].
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) extending from +6.0 to +11.0 mm
rostral to obex, 9 microtubules in the gigantocellularis reticularis
nucleus (RGN; 
) extending from +7 to +11.0 mm rostral
to obex, and 4 microtubules in or near the vestibular nucleus (VN;
) extending from +8.00 to 11.0 mm rostral to obex. R,
raphe nucleus; HP, hypoglossal propositus; 5ST, spinal trigeminal
tract; P, pyramids; CON, cochlear nucleus; RB, restiform body; FNm,
medial nucleus of FN; FNi, intermediate nucleus of FN.
and 
) and mean swallowing
interval (dashed lines) for 8 goats are presented. Swallowing interval
was not significantly different between pre- and post-MT implantation.
B: percent of total spontaneous swallows (means ± SD)
during SWE, SWEI, SWI, and
SWIE. Because SWIE was so rarely seen it was
excluded from further analysis. C: effect of swallows (pre-
and post-MT) on breathing. Values are presented as percent means
(±SD) of control breaths with no swallow. * Significant
(P < 0.05) change from control (dashed
lines).
