J Appl Physiol 104: 682–693, 2008.
First published December 13, 2007; doi:10.1152/japplphysiol.01043.2007.
Influence of tongue muscle contraction and dynamic airway pressure
on velopharyngeal volume in the rat
Ralph F. Fregosi
Departments of Physiology and Neurobiology, College of Medicine, The University of Arizona, Tucson, Arizona
Submitted 18 November 2007; accepted in final form 5 December 2007
THE PHARYNX IS A MUSCULAR tube with little skeletal support and
is thus subject to collapse as its transmural pressure becomes
negative during inspiratory contractions of the diaphragm and
accessory inspiratory muscles. Various skeletal muscles residing within the pharyngeal walls (e.g., pharyngeal constrictor
muscles), or lying within and adjacent to the pharynx (e.g.,
tongue and palatal muscles), are capable of dilating and/or
stiffening it. Indeed, the posterior segment of the tongue
comprises a large portion of the anterior pharyngeal wall, so
movement and/or stiffening of the tongue can have a marked
impact on the dimensions and compliance of the oropharynx
and velopharynx (VP). For both anatomic and physiological
reasons, neuromuscular activation is inadequate in at least
some patients with primary snoring, upper airway resistance
syndrome, or obstructive sleep apnea (41, 54). These patients
cannot maintain a positive pharyngeal transmural pressure
during sleep, leading to periodic pharyngeal collapse with
associated hypoventilation and hypoxia throughout the night.
At present, the favored treatment option for these patients is
application of positive pressure to the pharyngeal lumen (continuous positive airway pressure). Although continuous positive airway pressure therapy is effective when consistently
used, patient compliance is poor because it is uncomfortable,
loud, and associated with disagreeable drying of the oral and
nasal mucosa (53). As a consequence, there has been considerable interest in developing novel methods to alleviate obstructive apnea, including electrical stimulation of pharyngeal
muscles (12, 15, 16, 27, 35–37, 42, 45, 46). The success of
electrical stimulation therapy for obstructive sleep apnea requires a detailed understanding of the airway regions that are
most vulnerable to collapse in a given subject and the particular
muscles that must be activated to obtain optimal changes in
airway dimensions and compliance.
The VP and oropharynx are believed to be the regions most
vulnerable to collapse in patients with obstructive sleep apnea
(24 –26, 37, 41, 54). However, we know little about the
influence of tongue muscle contraction on VP volume changes
in either human subjects or animal models, and recent studies
suggest that the VP is a complex pharyngeal region that does
not behave uniformly along its length in cats or rats (7, 52). For
example, the VP in the adult rat is only ⬃10 mm long, but it
shows differences in compliance between its most rostral and
caudal regions (52). This observation suggests that volume
changes evoked by tongue muscle contraction along the VP
will not be uniform, and that there is likely a small, highly
compliant region that is vulnerable to collapsing pressure, but
also amenable to expansion and/or stiffening with contraction
of the tongue muscles. Several investigators have hypothesized
the existence of such a region (14, 20, 24, 38, 39, 43, 47– 49,
51), and, although its boundaries are purported to be small, it
should be amenable to precise localization with MRI.
Accordingly, the goal of the present study was to evaluate
the influence of extrinsic tongue muscle contraction and dynamic changes in pharyngeal pressure on regional VP volume.
VP volume changes, measured at 1-mm intervals throughout
the length of the rat VP, were estimated with MRI as dynamic
pharyngeal pressure was reduced (negative, collapsing transmural pressures) or increased (positive, distending transmural
pressures). Tongue protrudor and retractor muscles were stimulated either independently or simultaneously at five discrete
levels of pharyngeal transmural pressure. I found that the
largest effects of both pressure and tongue muscle contraction
act posterior to the caudal tip of the soft palate, suggesting that
Address for reprint requests and other correspondence: R. F. Fregosi, Dept.
of Physiology, Gittings Bldg., The Univ. of Arizona, Tucson, AZ 85721
(e-mail: [email protected]).
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.
apnea; functional electrical stimulation; hypoglossal nerve; magnetic
resonance imaging; pharynx; upper airway
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8750-7587/08 $8.00 Copyright © 2008 the American Physiological Society
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Fregosi RF. Influence of tongue muscle contraction and dynamic airway pressure on velopharyngeal volume in the rat. J Appl
Physiol 104: 682–693, 2008. First published December 13, 2007;
doi:10.1152/japplphysiol.01043.2007.—The mammalian pharynx is a
collapsible tube that narrows during inspiration as transmural pressure
becomes negative. The velopharynx (VP), which lies posterior to the
soft palate, is considered to be one of the most collapsible pharyngeal
regions. I tested the hypothesis that negative transmural pressure
would narrow the VP, and that electrical stimulation of extrinsic
tongue muscles would reverse this effect. Pressure (⫺6, ⫺3, 3, and 6
cmH2O) was applied to the isolated pharyngeal airway of anesthetized
rats that were positioned in a 4.7-T MRI scanner. The volume of eight
axial slices encompassing the length of the VP was computed at each
level of pressure, with and without bilateral hypoglossal nerve stimulation (0.1-ms pulse, one-third maximum force, 80 Hz). Negative
pressure narrowed the VP, and either whole hypoglossal nerve stimulation (coactivation of protrudor and retractor muscles) or medial
nerve branch stimulation (independent activation of tongue protrudor
muscles) reversed this effect, with the greatest impact in the caudal
one-third of the VP. The dilating effects of medial branch stimulation
were slightly larger than whole nerve stimulation. Positive pressure
dilated the VP, but tongue muscle contraction did not cause further
dilation under these conditions. I conclude that the narrowest and most
collapsible segment of the rat pharynx is in the caudal VP, posterior
to the tip of the soft palate. Either coactivation of protrudor and
retractor muscles or independent contraction of protrudor muscles
caused dilation of this region, but the latter was slightly more
effective.
NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
this is the narrowest and most collapsible segment of the rat
pharyngeal airway.
METHODS
where they were connected to a stimulator (Grass S88) via separate
constant-current stimulus isolation units (Grass PSIU6). Nerves were
stimulated with 0.1-ms pulses at 90 Hz. The threshold was defined as
the lowest current that caused visible tongue movement (9, 52); the
current above which tongue protrusion or retraction appeared to be
maximal defined the maximal current level. A stimulus level of
one-third to one-half maximal was used for subsequent stimulation
trials (range: 50 ␮A to 5 mA).
MRI protocol. Rats were placed in a MRI scanner (Bruker Instruments, Billerica, MA) (Fig. 1) and positioned so that the pharyngeal
airway was at the iso-center of the gradient coil (4, 9, 52). The MRI
protocol involved two sequences: gradient echo fast imaging (GEFI)
and rapid acquisition with relaxation enhancement (RARE), as described in detail previously (4, 9, 52). GEFI images were used to
visualize tongue movements, to optimize stimulus current levels, and
to check the viability of the nerve preparation before and after each set
of RARE images was obtained. (Please paste the following link into
your web browser for an example of a GEFI sequence: http://
www.physiology.arizona.edu/labs/rnlab/GEFI.mov. Be sure to press
the “play” button to see how the tongue moves with stimulation.) The
RARE technique was used in concert with a stimulus-gated acquisition protocol to obtain axial images of the VP during pressure
application, with and without stimulation, achieved by triggering the
acquisition software with the stimulator output pulse (4, 9, 52).
Data analysis. The nasal VP was defined as the region on the dorsal
side of the soft palate, extending caudally from the junction of the
hard and soft palate to the tip of the soft palate, as shown in Fig. 2. The
oral VP was defined as the portion of the airway on the ventral side of
the soft palate encompassing the oral airway lumen between the
tongue and the soft palate (Fig. 2).
The cross-sectional area of eight consecutive images spanning the
VP from the caudal edge of the soft palate to its rostral extent was
measured (refer to Fig. 2 to visualize these anatomic landmarks). Each
axial slice was 1 mm thick, so measurements of cross-sectional area
and the volume of each slice are equivalent. I evaluated the influence
of whole nerve (n ⫽ 6), medial branch (n ⫽ 6), and lateral branch
(n ⫽ 6) stimulation at atmospheric pressure, and at positive (3 and 6
cmH2O) and negative (⫺3 and ⫺6 cmH2O) pharyngeal pressures. All
pressure and stimulation/no-stimulation trials were randomized.
I also estimated the regional compliance of the nasal airway at each
1-mm segment of the VP, in each of the three groups of rats (i.e.,
whole nerve, medial branch, and lateral branch stimulation groups).
This was done by constructing linear regression curves of the volumepressure relationship under stimulated and nonstimulated conditions
for each animal. The regression analysis was based on all four
dynamic pressure levels (⫺6, ⫺3, 3, and 6 cmH2O). After the curves
for each animal were constructed, average values for each of three
nerve stimulation groups were obtained (see Fig. 12).
Results for each measured variable were analyzed with two-way
repeated-measures ANOVA, with VP segment (i.e., slices 1– 8) and
stimulation/no-stimulation the main factors. If the results of ANOVA
were significant, differences between stimulation and no-stimulation
conditions for each slice were evaluated with Bonferroni post hoc tests
(Graph Pad Prism Software, San Diego, CA). A P value of ⱕ0.05 was
considered significant for all tests.
RESULTS
Fig. 1. Schematic representation of the experimental configuration used in this
and in previous studies from our laboratory (4, 7, 52). The bottom portion
shows an enlargement of the ventral neck region and depicts the location of the
tracheotomy tube, hypopharyngeal catheter, and site of application of positive
and negative pressures. Sites for sampling hypopharyngeal (Php) and pharyngeal airway pressures and the approximate location of the cuff electrodes
(represented by boxes), proximal to the bifurcation of the XII nerves, are also
shown.
J Appl Physiol • VOL
Comparison of nasal and oral airway volume along the rat
VP under static conditions at atmospheric pressure. Figure 3
shows nasal and oral VP volume at 1-mm intervals, starting at
the tip of the soft palate caudally and extending rostrally to the
junction of the hard and soft palate. The nasal airway was
widely patent throughout the VP, whereas the oral air space
was large near the tip of the soft palate, but narrowed significantly starting at a point 3 mm above the tip of the soft palate
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Animal preparation. Eighteen male Sprague-Dawley rats (324.6 ⫾
10.8 g, mean ⫾ SE) were used in accordance with guidelines established by the Institutional Animal Care and Use Committee (IACUC)
at the University of Arizona. All experimental procedures and manipulations were reviewed and approved by the IACUC at The University
of Arizona. The experimental preparation was described in detail in
recent papers (4, 52) and is shown schematically in Fig. 1. Animals
were anesthetized with urethane (1.2 g/kg iv) and breathed through a
tracheotomy tube. A second tracheotomy was made ⬃5 mm caudal to
the vocal folds, through which a nonmagnetic pressure transducer
(Millar Instruments) was advanced rostrally to a point ⬃3–5 mm
caudal to the left nares, to measure nasopharyngeal pressure. The left
nare remained open to allow airflow; the right nare was sealed with
super glue. A catheter was advanced through the same tracheotomy to
a point just rostral to the vocal folds (i.e., in the hypopharynx) and
connected in series to a pressure or vacuum source and a second
differential pressure transducer (Validyne, model DP45–28) that provided an estimate of the pressure applied at the hypopharynx. I studied
the airway at four dynamic, nasopharyngeal pressure values, ⫺6, ⫺3,
3, and 6 cmH2O, by applying either suction (via a vacuum source) or
positive pressure (compressed air). Baseline measurements under
static conditions at atmospheric pressure were also made. The duration of pressure application corresponded to the duration of imaging
time, ⬃2 min. The application of each pressure level was fully
randomized, and all pressure levels were studied in each animal, once
with and once without stimulation of the hypoglossal nerves or its
main branches.
Wire electrodes were placed around the hypoglossal nerves bilaterally (activation of all tongue muscles), or on the medial (protrudor
muscle activation) or lateral (retractor muscle activation) branches.
The electrode lead wires were shielded and passed outside the magnet
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
(Fig. 3). The nasal airway was significantly wider than the oral
airway at all points more rostral than 3 mm above the tip of the
soft palate (Fig. 3).
Whole hypoglossal nerve and nerve branch stimulation with
the airway under static conditions at atmospheric pressure.
Representative axial images showing the influence of stimulating the whole hypoglossal nerves, or the medial or lateral
nerve branches, on airway dimensions at atmospheric pressure
are shown in the middle panels of Figs. 4, 5, and 6, respectively
(for orientation, the arrows on the bottom right panel of Fig. 4
denote the nasal and oral airways). The average data is shown
in Fig. 7, wherein the data points represent slice-by-slice data
through the VP, starting 1 mm above the tip of the soft palate
and extending rostrally for 8 mm. Whole hypoglossal nerve
and medial branch stimulation had a negligible influence on the
oral airway, with the changes confined to a modest dilation 3– 4
mm above the tip of the soft palate (Fig. 7, left top and middle).
Lateral branch stimulation had no effect on the oral airway at
any level of the VP (Fig. 6, middle, and Fig. 7, left bottom).
The nasal airway was open throughout the length of the VP
in all groups of animals (see images in Figs. 4 – 6, middle, and
the average data in Fig. 7, right). Whole hypoglossal nerve
stimulation increased nasal airway volume in the caudal VP
(Fig. 7, right top), but not in more rostral regions. In contrast,
medial branch stimulation increased nasal airway volume significantly at all levels of the VP (Fig. 5, and Fig. 7, right
middle). Lateral hypoglossal nerve stimulation had no discernible effect on nasal airway volume at any level of the VP (Fig.
6 and Fig. 7, right bottom).
Stimulation of the whole hypoglossal nerves at negative and
positive dynamic airway pressures. The influence of stimulating the whole hypoglossal nerves on VP volume at negative
and positive airway pressures is shown as images in Fig. 4,
with the average data in Fig. 8. Stimulation dilated the oral
airway at airway pressures of ⫺3 and 3 cmH2O (Fig. 8, left
J Appl Physiol • VOL
Fig. 3. Comparison of nasal and oral airway volume at 1-mm increments
across the length of the VP. Slice 1 represents the most caudal image, located
1 mm above the tip of the soft palate, with subsequent slices representing
progressively more rostral levels of the VP. ⫹P ⬍ 0.05, ⫹⫹P ⬍ 0.01, and
⫹⫹⫹
P ⬍ 0.001: different from most caudal level. *P ⬍ 0.05, **P ⬍ 0.01, and
***P ⬍ 0.001: difference between nasal and oral airway volume.
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Fig. 2. Sagittal image showing the boundaries of the velopharynx (VP;
horizontal white lines), with arrows denoting the nasal and oral air passages.
We analyzed eight 1-mm axial sections cut through the VP, as described in
METHODS.
middle two panels), although this effect was significant only at
a single location in the caudal VP (i.e., at a level 3 mm above
the tip of the soft palate). Stimulation dilated the nasal airway
at pressures of ⫺6 and ⫺3 cmH2O, but only in the caudal-most
regions of the VP (Fig. 4, left, and Fig. 8, right top two panels).
Whole hypoglossal nerve stimulation had no effect on the nasal
VP at positive airway pressures (Fig. 4, right, and Fig. 8, right
bottom two panels).
Stimulation of the medial hypoglossal nerve branches at
negative and positive dynamic airway pressures. Images showing the influence of stimulating the medial hypoglossal nerve
branches on average changes in VP volume at negative and
positive airway pressures are shown in Fig. 5, with the average
data in Fig. 9. Stimulation dilated the oral component of the VP
at an airway pressure of ⫺3 cmH2O, although this effect was
significant only at a single level located 5 mm above the tip of
the soft palate (Fig. 9, left, second from the top). Stimulation
also caused significant oral airway dilation at a pressure of 6
cmH2O, with significant effects localized to the middle of the
VP (Fig. 5, right, and Fig. 9, left bottom).
Stimulation dilated the nasal airway at all pressure levels,
with the largest effects observed at an airway pressure of ⫺6
cmH2O, where all but the rostral-most 1-mm segment of the
VP failed to dilate with stimulation (Fig. 5, left, and Fig. 9,
right top). The airway-dilating influence of medial nerve
branch stimulation at ⫺3, 3, and 6 cmH2O was confined to the
caudal-most regions of the VP (Fig. 9, right).
Stimulation of the lateral hypoglossal nerve branches at
negative and positive dynamic airway pressures. Lateral nerve
branch stimulation had no significant effects on the oral airway, although there was a clear trend for airway narrowing at
an airway pressure of 6 cmH2O (see images in Fig. 6, and
average data in Fig. 10, left bottom), but, due to highly variable
responses, the results of the ANOVA were not significant.
Significant airway narrowing with lateral nerve branch stimulation was observed in the nasal airway at pressures of ⫺3 and
3 cmH2O, but only in the rostral-most regions of the VP (Fig.
10, right middle two panels).
Statistical comparison of the effectiveness of whole hypoglossal nerve and medial branch stimulation on VP airway
volume. Since either whole hypoglossal nerve or medial nerve
branch stimulation consistently evoked nasal airway dilation,
NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
685
primarily at atmospheric and negative airway pressures, we
wished to compare the effectiveness of these two interventions.
To do this, we computed the stimulation-induced change in
nasal airway volume at airway pressures of 0, ⫺3, and ⫺6
cmH2O (Fig. 11). Medial nerve branch stimulation caused
slightly more dilation of the nasal VP at zero airway pressure,
but the difference was significant only at the midpoint of the
VP (Fig. 11, top). Similarly, there was a trend for a greater
dilating effect of medial branch compared with whole hypo-
glossal nerve stimulation at ⫺3 cmH2O, but the only significant effect was in the caudal VP, 2 mm above the tip of the soft
palate. There were no systematic differences between stimulation methods at an airway pressure of ⫺6 cmH2O.
Regional compliance. The compliance of each 1-mm segment of the VP was estimated, and the results are shown in Fig.
12. In all three nerve stimulation groups, compliance was
found to be highest near the tip of the soft plate and lowest at
the rostral end of the VP. Stimulation did not alter compliance
Fig. 5. Representative axial images through
the midregion of the VP (5 mm rostral to the
tip of the soft palate), showing the influence of
stimulating bilaterally the medial branches of
the hypoglossal nerves (Stim) at three levels
of pharyngeal pressure, under No Stim and
Stim conditions. Medial nerve stimulation dilated the nasal airway at all pressure levels.
Positive pressure opened the oral airway, but
stimulation paired with positive pressure
caused no additional dilation.
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Fig. 4. Representative axial images through
the midregion of the VP (5 mm rostral to the
tip of the soft palate). Arrows in the bottom
right image depict the nasal (arrow with
dashed shaft) and oral airway (arrow with
solid shaft) openings. Images are shown at
three levels of pharyngeal pressure, under
control conditions (No Stim) and during bilateral stimulation of the whole hypoglossal
nerves (Stim). Note influence of whole hypoglossal nerve stimulation on the oral and
nasal airway at a pharyngeal pressure of ⫺6
cmH2O. Positive pressure dilated the oral
airway, with minimal effects on the oral
airway at this level of the VP. However,
positive pressure paired with stimulation did
open the oral airway in this animal.
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
Fig. 6. Representative axial images through
the midregion of the VP (5 mm rostral to the
tip of the soft palate), showing the influence
of stimulating bilaterally the lateral branches
of the hypoglossal nerves (Stim) at three
levels of pharyngeal pressure, under No Stim
and Stim conditions. In this animal, both
nasal and oral airways were patent, but stimulation had no obvious effects at any transmural pressure.
DISCUSSION
Summary. My major finding is that stimulation of the whole
hypoglossal nerves, or of the medial branches alone, dilates the
VP under both static and dynamic flow conditions. The largest
effects occurred under conditions of negative pharyngeal transmural pressures, and the most consistent dilation was observed
in the caudal one-third of the VP. Tongue muscle contraction
consistently dilated the nasal VP, whereas effects on the oral
VP were relatively small and inconsistent. Direct statistical
comparison of the dilating influence of medial nerve branch
stimulation (independent activation of tongue protrudor muscles) with the influence of whole hypoglossal nerve stimulation
(coactivation of protrudor and retractor muscles) revealed that
the dilating effects of medial branch stimulation were larger
when the airway lumen was held at atmospheric pressure or at
a moderately collapsing transmural pressure of ⫺3 cmH2O. In
contrast, the dilating influences of whole and medial branch
stimulation were the same when airway transmural pressure
was held at ⫺6 cmH2O. Compliance was higher in the caudal
compared with more rostral levels of the VP in all groups of
animals. I conclude that the narrowest and most collapsible
segment of the rat pharynx is in the caudal VP, posterior to the
tip of the soft palate. Either coactivation of protrudor and
retractor muscles or independent contraction of protrudor muscles caused dilation of this region, with the latter slightly more
effective under conditions in which transmural pressure is
modestly negative.
Critique of methods. Although the strengths and limitations
of our experimental model have been addressed in detail
previously (4, 9, 52), here I review some technical issues that
are particularly relevant to the present experimental design.
First, my goal was to measure responses to dynamic pressure
J Appl Physiol • VOL
changes, as opposed to the nonphysiological static changes that
have been used to measure pharyngeal compliance by others
(7, 23, 31) and us (3). This is an important issue because
collapse off the pharyngeal airway in patients with sleep apnea
is a dynamic event, occurring in the face of airflow, and
collapse can occur when flow is in either the inspiratory or
expiratory direction (43). Having said that, I am aware that,
under dynamic conditions, fluid flow through the pharynx may
lead to regional differences in pressure acting on the pharyngeal wall. However, the small variations in pressure that may
exist along the pharynx are very small, compared with the large
pressure levels that we are applying, and so these effects, if
they indeed exist, would be very small. Moreover, I computed
the cross-sectional area of each millimeter of the VP separately, so, even if there were small rostral-to-caudal variations
in pressure, these would be similar under stimulated and
nonstimulated conditions within each discrete 1-mm segment.
It is also important to note that measurements were not taken
until flow and downstream pressure had fully stabilized. This
was done by adjusting the source pressure until a stable target
pressure was obtained, resulting in steady-state flow conditions.
The second methodological issue is that the rat pharynx is
more rectilinear than the curved human pharynx, but several
observations have validated the functional similarities of the
rat and human pharynx, particularly with regard to the
neural control (1, 2, 5, 6, 10, 19, 33) and mechanical effects
(15, 16, 27, 33, 35–37, 45, 46) of extrinsic tongue muscle
contraction. The third issue is that the rat epiglottis reaches
the caudal margin of the soft palate, which contrasts with the
adult human, in whom the epiglottis and soft palate are
separated. However, the adult rodent pharynx is similar to
the human infant airway, in which the soft palate and
epiglottis are in close apposition until about the second year
of life. The mechanical interactions between the tongue and
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significantly in any of the groups, although there was a trend
for reduced compliance with stimulation in all groups.
NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
687
soft palate must also be considered, especially under conditions where tongue movements are substantial, as in this
study. Forward movement and depression of the tongue
base, evoked by either coactivation or independent protrudor muscle contraction, take pressure off the soft palate,
allowing it to move forward. Although soft palate movement
was not analyzed systematically, I did not note any significant
movements of the soft palate under any conditions in this
isolated upper airway preparation. Finally, it is important to
note that the alterations in VP geometry observed in this model
are due to direct stimulation of the tongue muscles. In contrast,
in intact unanesthetized subjects, spontaneous activation of
multiple upper airway muscles may alter the mechanical actions of the tongue muscles and produce different effects on the
VP than observed herein. Thus, although the lessons learned
from studying pharyngeal physiology in animal models may
not tell us exactly how the human airway will function, there
are enough similarities in structure and function to make these
models useful for developing hypotheses that can be tested in
human subjects.
J Appl Physiol • VOL
VP volume and hypoglossal nerve stimulation at atmospheric pressure. Stimulation of the whole hypoglossal nerves
dilated the caudal 3 mm of the nasal VP, but effects on the oral
VP were small and inconsistent. Medial hypoglossal branch
stimulation also had a modest effect on the oral VP, but a
strong and consistent effect on the nasal VP across its entire
length. These data are consistent with our previous study in the
rat (9), which showed that medial branch stimulation evoked a
greater increase in oropharyngeal volume (but not nasopharyngeal volume) than whole hypoglossal nerve stimulation, under
conditions in which the pharyngeal lumen was maintained at
atmospheric pressure. In contrast, Kuna (30) showed a greater
increase in cross-sectional area with whole nerve stimulation
compared with medial branch stimulation in a decerebrate cat
model, suggesting that coactivation of tongue protrudor and
retractor muscles is more effective in dilating the VP in the cat
(30). The reason for the different findings is uncertain, but could
be due to a species-dependent difference in pharyngeal anatomy,
different mechanical actions of the tongue muscles, the effects of
anesthesia, or differences in stimulation protocols.
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Fig. 7. Average data showing the influence of
stimulating the whole hypoglossal nerves (top), or
the medial (middle) or lateral (bottom) branches
under static conditions, with the airway at atmospheric pressure. Each graph shows the volume of
each axial slice (y-axis), starting 1 mm above the
tip of the soft palate and extending upwards for 8
mm (x-axis), approximating the junction of the
hard and soft palate. Left: oral airway volume;
right: nasal airway volume. Stim (stimulation)
different than No Stim: *P ⬍ 0.05, **P ⬍ 0.01,
***P ⬍ 0.001.
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
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Fig. 8. Average data showing the influence of stimulating the whole hypoglossal nerves (coactivation of
tongue protrudor and retractor muscles) at negative
and positive airway pressures, as indicated on each
panel. Data for Stim (■) and No Stim control conditions (Œ) are represented by different symbols, as
indicated. Each graph shows the volume of each axial
slice (y-axis), starting 1 mm above the tip of the soft
palate and extending upwards for 8 mm (x-axis),
approximating the junction of the hard and soft palate. Left: oral airway volume; right: nasal airway
volume. Stim different than No Stim: *P ⬍ 0.05,
**P ⬍ 0.01, ***P ⬍ 0.001.
Tongue muscle stimulation and VP volume at negative and
positive pharyngeal airway pressures: protrudor stimulation
vs. coactivation of protrudor and retractor muscles. Negative
pressure application narrowed the nasal VP, and under these
conditions stimulation of the whole hypoglossal nerves or
the medial branches increased nasal VP volume, particularly in
the caudal regions of the VP. Although either coactivation of
protrudor and retractor muscles or independent activation of
protrudor muscles dilated the pharynx, there were some quantitative differences. Stimulation of the medial nerve branches
dilated the nasal VP along its entire length at a pressure of ⫺6
cmH2O, whereas at ⫺3 cmH2O only the caudal segment of the
nasal VP dilated. Contraction of the genioglossus muscle
J Appl Physiol • VOL
causes both ventral advancement (protrusion) and caudal depression of the tongue base, whereas coactivation of the genioglossus with the main retractor muscles (hyoglossus and
styloglossus) results in net tongue retraction and enhanced
depression (13, 18, 21, 22, 32, 34). Stimulation of the genioglossus muscle alone dilates the airway, but does not cause
significant changes in pharyngeal stiffness, as shown in both
animal (20, 52) and human subjects (35, 37). In contrast,
protrudor-retractor muscle coactivation evokes both pharyngeal dilation and increased pharyngeal stiffness, leading to a
more negative critical collapsing pressure in animal models
(20, 39, 50) and in human subjects (15, 27, 35–37, 45). This
flexibility allows the nervous system to activate tongue mus-
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
689
cles in a manner that can emphasize either stiffness or dilation,
in accordance with the adjustment that results in the biggest
drop in airflow resistance under a given set of conditions.
Stimulation was much less effective under conditions in
which the pharynx was exposed to positive transmural pressures. This was anticipated, given that positive pressure alone
causes substantial dilation of the airway. Nevertheless, the rat
pharynx is relatively rigid compared with the human pharynx.
Although one might predict that stimulation of the tongue
muscles would be much more effective in human subjects with
positive critical collapsing pressures, there does not appear to
be clear consensus. For example, Isono et al. (27) showed no
effect of bilateral stimulation of the tongue surface on VP
cross-sectional area at positive transmural pressures, despite a
dilating effect at negative transmural pressures. In contrast, a
recent study showed a significant increase in pharyngeal crosssectional area with direct stimulation of the genioglossus musJ Appl Physiol • VOL
cle (35). The conflicting results are likely due to the different
stimulation protocols, although in both studies it is unclear how
much of the tongue musculature was stimulated: in the former,
surface stimulation was applied to the tongue body, and it is
likely that protrudor and retractor muscles (both intrinsic and
extrinsic) were activated. Also, although stimulating electrodes
were inserted into the genioglossus muscle in the latter study,
current spread to adjacent muscles cannot be ruled out. This is
because the fibers of all seven extrinsic and intrinsic tongue
muscles are very tightly intermingled within the tongue body,
increasing the probability of current spread via volume conductance.
It is also clear that the position of the mandible has a major
influence on pharyngeal airway size and on tongue movements.
Indeed, stimulation of the tongue muscles, especially at the
relatively high intensities used herein, is associated with ventral displacement of the mandible, as shown in the accompa-
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Fig. 9. Average data showing the influence of stimulating the
medial hypoglossal nerve branches (independent activation of
tongue protrudor muscles) at negative and positive airway
pressures, as indicated on each panel. Figure conventions are
as in Fig. 5. Stim different than No Stim: *P ⬍ 0.05, **P ⬍
0.01, ***P ⬍ 0.001.
690
NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
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Fig. 10. Average data showing the influence of
stimulating the lateral hypoglossal nerve branches
(independent activation of tongue retractor muscles)
at negative and positive airway pressures, as indicated on each panel. Figure conventions are as in
Fig. 5. *Stim different than No Stim, P ⬍ 0.05.
nying video. Although I could not quantify the magnitude of
mandibular movement, it was always observed. This marked
ventral displacement of the mandible underscores the strong
tongue depression that is evoked by coactivation of the tongue
protrudor and retractor muscles and clearly demonstrates the
complex interactions between the mandible and tongue. This is
consistent with the work of Isono et al. (28), who showed that
manual advancement of the mandible by the investigators
dilated the otherwise passive VP in human subjects with
obstructive sleep apnea. It is likely that this mechanism contributed to the VP dilation observed with whole and medial
nerve branch stimulation in the present experiments. It is also
likely that the dilation observed with stimulation of the tongue
muscles is due to ventral displacement of tissues in the lateral
J Appl Physiol • VOL
and ventral pharyngeal walls, as observed by Brennick et al.
(8), who used a sophisticated imaging paradigm in an attempt
to quantify soft tissue movements in the rat pharynx. However,
the study by Brennick et al. examined medial branch simulation only, so it is not yet clear how stimulation of the whole
hypoglossal nerves or the lateral branches will influence soft
tissue movements in the pharynx.
Locating the narrowest segment of the pharyngeal airway.
The present analysis indicates that the caudal region of the VP,
⬃2–3 mm above the tip of the soft palate, is the narrowest
segment of the rat pharyngeal airway. This region also narrowed the most with negative pressure application and dilated
the most with stimulation of the tongue muscles. The regional
compliance data show that the caudal VP is also the most
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
691
airway (11, 17, 24, 25, 29, 40, 44). Thus the rat is a useful
model for biophysical studies of the human pharyngeal airway.
In conclusion, the results of this study demonstrate that the
VP is the most vulnerable region of the mammalian pharynx,
and that electrical stimulation of tongue muscle nerves can
minimize or prevent VP narrowing. However, our analysis also
demonstrates that the efficacy of tongue muscle electrical
stimulation to alleviate human sleep apnea cannot be judged
until there are more studies that systematically vary stimulation
of different hypoglossal nerve branches, mandibular position,
and pharyngeal transmural pressure.
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Fig. 11. Direct, statistical comparison of whole hypoglossal nerve and medial
nerve branch stimulation on the change (⌬) in nasal VP volume. Each graph
shows the change in nasal VP volume as a function of VP location, starting 1
mm above the tip of the soft palate and extending upward for 8 mm. *Stim
different than No Stim, P ⬍ 0.05.
compliant region, so taken together these results indicate that
this region is the narrowest and most collapsible segment of the
rat pharynx. This conclusion, based on direct visualization and
measurement with MRI, is consistent with our previous estimates of the location of maximal pharyngeal narrowing during
negative pressure application in the rat, based on dynamic
measurement of maximal pharyngeal airflow and critical collapsing pressures (20). This is also consistent with data in
human subjects, which suggests that the caudal VP is the
narrowest and most collapsible segment of the pharyngeal
J Appl Physiol • VOL
Fig. 12. Regional compliance (C) of the rat VP for whole nerve (top), medial
branch (middle), and lateral branch (bottom) preparations. In each panel, the
solid lines indicated the No Stim conditions, and the dashed lines the Stim
condition. In all cases, compliance is highest near the tip of the soft plate, and
lowest at the rostral end of the VP. There were no differences in compliance
with stimulation of either whole, medial, or lateral hypoglossal branches,
although there is trend for compliance to be less with stimulation in all groups.
⫹
P ⬍ 0.05, ⫹⫹P ⬍ 0.01, and ⫹⫹⫹P ⬍ 0.001: different than most caudal level
of the VP (slice 1). See text for more details.
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NEUROMUSCULAR CONTROL OF THE PHARYNGEAL AIRWAY
ACKNOWLEDGMENTS
The author thanks Dr. E. Fiona Bailey, Rosaria Cabrera, Seres CostyBennett, Dr. Patrick Janssen, and Cornelius Van Zutphen for excellent technical support.
24.
GRANTS
25.
This research was supported by National Institutes of Health Grants
HL-68162 and DC-07692.
26.
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