Effects of Nasal Obstruction on Maturation of the Jaw-Opening Reflex

Home , Jaw

a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8

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Effects of nasal obstruction on maturation of the

jaw-opening reﬂex in growing rats

Yukiha Funaki, Maya Hiranuma, Mai Shibata, Satoshi Kokai *,

Takashi Ono

Orthodontic Science, Department of Orofacial Development and Function, Division of Oral Health Science, Graduate

School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

a r t i c l e i n f o a b s t r a c t

Article history: Objective: Nasal obstruction during growth changes craniofacial morphology and function.

Accepted 24 February 2014 However, the etiological mechanisms of these changes are unknown. The aim of the present

study was to investigate the effects of nasal obstruction during growth on the maturation of

Keywords: the jaw-opening reﬂex (JOR) using an electrophysiological technique. We focused on the oral

Growth sensory receptors that regulate the activities and reﬂexes of the orofacial muscles.

Design: Sixty 6-day-old male Wistar rats were randomly divided into control and experi-

jaw-opening reﬂex

mental groups (n = 30 each). The experimental group underwent unilateral nasal obstruc-

Nasal obstruction

tion at 8 days of age. The JOR was evoked by bilateral, low-intensity electrical stimulation of

Oral sensory receptor

Rat the inferior alveolar nerve. The electromyographic responses were recorded bilaterally from

the digastric muscles at 5, 7, and 9 weeks of age.

Results: The latency of the JOR was signiﬁcantly longer and the peak-to-peak amplitude was

signiﬁcantly smaller in the experimental group than in the control group at each age, while

the duration was not signiﬁcantly different. Intragroup comparison of the latency, peak-to-

peak amplitude, and duration at 5, 7, and 9 weeks of age revealed no signiﬁcant differences

in either the control or experimental groups.

Conclusions: Unilateral nasal obstruction during growth may have signiﬁcant effects on

maturation of craniofacial function.

harmonious craniofacial structures that allow for adequate

1. Introduction 3

interaction between mastication and swallowing.

In orthodontic practice, patients often present with

Normal craniofacial development and occlusion depend on maxillary protrusion or an anterior open bite, problems that

various factors. For example, genetic factors inﬂuence the are frequently associated with nasal obstruction and mouth

1 4

constitution of the facial and occlusal pattern. Acquired breathing. Recent studies have indicated that the number of

factors such as caries, oral habits, and nasopharyngeal patients with breathing problems caused by obstructive sleep

conditions can also play important roles in craniofacial and apnea syndrome and allergic rhinitis is increasing, especially

occlusal development. Mammals, including humans, usually during the growth period of preadolescence and adoles-

5,6

breathe through the nose. Normal respiratory activity favors cence. A previous study in humans indicated that mouth

* Corresponding author: Tel.: +81 3 5803 5528; fax: +81 3 5803 5528.

E-mail address: [email protected] (S. Kokai).

http://dx.doi.org/10.1016/j.archoralbio.2014.02.013

a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8 531

breathing may be due to an anatomical predisposition, nostril, consequently occluding the oriﬁce of the nostril without

velopharyngeal insufﬁciency, habit, or interference of lip mechanical or chemical damage to the olfactory mucosa. After

closure. Many clinical studies have also elucidated relation- cauterization, the nostril was coated with 3% chlortetracycline

ships between nasal obstruction and craniofacial morphology. (Aureomycin Ointment; Pola Pharma, Tokyo, Japan) to prevent

For example, a lower position of the mandible caused by infection. The pups were kept warm (37 8C) for 30 min and then

mouth breathing increases the lower facial height and returned to their mothers. The control group underwent a sham

adenoidal facies. Changes in the craniofacial pattern might operation in which the cauterizing instrument was placed

induce simultaneous changes in occlusion, such as an anterior about 1–2 mm above the left nostril. The body weights were

8,10 10 10

open bite, maxillary protrusion, or posterior crossbite. measured throughout the experimental period. The JOR was

On the other hand, some studies have elucidated the effects of then recorded from the Dig muscle in control and experimental

nasal obstruction on masticatory function, such as inhibited groups when the rats were 5, 7, and 9 weeks old (n = 10 per group

masseter muscle activity and increased suprahyoid muscle per time point).

activity during mouth breathing. Moreover, hypotonia of the

lips, tongue, and buccinator muscle has been reported, while 2.2. Stimulation and recording

mouth breathing was also shown to decrease masticatory

14,15

activities. For electrophysiological recordings, all rats were anesthetized

Biochemical studies in rats suggest that nasal obstruction with 60 mg/kg of thiamylal sodium (Isozol ; Yoshitomi

is associated with reduced growth of the masseter muscle and Pharmaceutical, Osaka, Japan) administered intraperitoneally.

anterior digastric (Dig) muscle. In addition, fatigue-resistant Cannulae were then inserted into the tracheal and femoral

myosin heavy chain (MHC) ﬁber types are increased in veins. The depth of anesthesia was carefully monitored

muscles in association with nasal obstruction. In terms of throughout the experiment by checking the pupil size,

morphological changes, nasal obstruction results in a signiﬁ- withdrawal and corneal reﬂexes, and heart rate. To maintain

cant reduction in the vertical development of the nasomax- the same anesthetic level in all rats while performing the JOR

illary complex and in the skull base along the longitudinal recording, a supplemental dose (5 mg/kg, intravenously) was

axis. Although alterations of craniofacial morphology and routinely given 15 min before the start of the recording process

function during nasal obstruction have been thoroughly if a withdrawal reﬂex was elicited by pinching the paws. The

investigated, little is known about the effects of nasal rectal temperature was maintained at 37 8C with a heating

obstruction on the regulation of jaw movement. It is possible pad. Before an incision was made, the skin was injected with

that alterations in jaw movements may modify craniofacial 2% lidocaine to prevent noxious stimulation that could inhibit

sensory transmission to jaw motoneurons, since the activity the JOR.

of jaw motoneurons is dependent on sensory feedback from To bilaterally stimulate the inferior alveolar nerve (IAN), a

craniofacial structures. The jaw-opening reﬂex (JOR), a pair of stainless-steel wire electrodes (0.1 mm in diameter, 0.5

trigeminal brainstem reﬂex that can be evoked by both mm in tip exposure) was inserted bilaterally into the

nociceptive and non-nociceptive stimulation, plays an mandibular canal through the mental foramen at depths of

important role in the regulation of jaw movement during 1 and 3 mm. The bipolar electrode was kept in place by ﬁxing it

mastication. Thus, the JOR is a useful tool for investigating onto the adjacent mandibular bone with light-cured dental

alterations of sensorimotor processing in masticatory perfor- resin. Expression of the JOR was recorded bilaterally from the

mance. Dig muscles using paired stainless-steel wire electrodes with

In the present study, we focused on the oral sensory 1-mm exposed tips implanted along the direction of the

receptors that regulate the activity and reﬂexes of orofacial muscle ﬁbers. The interpolar distance was set at 3 mm for

muscles. The aim of this paper was to investigate the effects of these recording electrodes (Fig. 1A). To ensure consistent

unilateral nasal obstruction during growth on JOR maturation reproduction of this distance, a two-keyhole index, with the

using an electrophysiological technique. keyholes 3 mm apart, made of dental acrylic (UNIFAST II; GC

Corporation, Tokyo, Japan) was used to lead the paired

electrode-inserted needles, thus allowing for placement of

2. Materials and Methods the electrodes at the same interpolar distance for every

recording. The animals were then transferred to a stereotaxic

2.1. Animal preparation apparatus (models SN-2 and SM-15 M; Narishige Scientiﬁc

Instruments, Tokyo, Japan) with their bodies in prone position

Sixty 6-day-old male Wistar albino rats were used in this (Fig. 1B). Before the stimulation, Dig electromyographic (EMG)

experiment. The rats were weighed and then randomly divided baseline activity was captured for 1 min in each rat.

into the control and experimental groups (n = 30 each). At 8 days Next, the right and left IAN were alternately stimulated

of age, all rat pups were ﬁrst anesthetized by hypothermia (10 electrically (single pulse, 0.2 ms in duration) once every 5 s to

min at À18 8C). Left-sided nasal obstruction was performed in evoke the JOR. The stimulus intensity was applied gradually

17,20

the experimental group. The selected method involved in increasing in steps of 1 mA, and the lowest intensity that

cauterization of the left external nostril, which is the most constantly elicited a Dig EMG response was determined as the

common and simplest procedure allowing for the establish- JOR threshold (T). To attain comparable responses, the test

ment of nasal obstruction in neonatal animals. The tissue stimulation current was adjusted to 1.5 times the threshold

21,22

surrounding the left external nostril was burned by placing a (1.5 T). An intensity of 2 T or less can be considered as

surgical cauterizing instrument (1 mm in diameter) on the non-nociceptive stimulation, which we expected in this

532 a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8

Figure 1 – Experimental design. (A, B) Schematic drawing of the experimental setting. The heads of anesthetized rats were

fixed to a stereotaxic frame. The jaw-opening reflex (JOR) was evoked by electrical stimulation of the bilateral inferior

alveolar nerve (IAN), and electromyographic (EMG) activity was recorded from the anterior belly of the bilateral digastric

(Dig) muscles. (C) Data analysis. The latency (a) was defined as the amount of time between the stimulus and the first point

at which Dig EMG activity exceeded 2 SD of the baseline activity. The duration (b) was defined as the amount of time

between the onset and the point at which the response dropped below 2 SD of the baseline activity. The peak-to-peak

amplitude (c) was calculated using the Spike2 analysis function between the onset and offset in each single sweep.

study. Dig responses on the right side were recorded by averaged from the reﬂex responses after 30 consecutive

stimulating the ipsilateral IAN, and the contralateral Dig stimuli in each rat. EMG activities were full-wave rectiﬁed and

responses were recorded by stimulating the contralateral smoothed with a time constant of 20 ms. The latency and

IAN. duration were indicated as time intervals in ms. The latency

EMG activity was ampliﬁed by a 100-Hz to 3-kHz bandwidth was deﬁned as the amount of time between the stimulus and

differential ampliﬁer (DAM-80; WPI, Sarasota, FL, USA). All the ﬁrst point at which Dig EMG activity exceeded 2 standard

signals were fed into a computer by means of a CED 1401 deviations (SD) of the baseline activity (i.e., onset), whereas

interface (5000/s sampling rate) and were later analyzed the duration was deﬁned as the amount of time between the

ofﬂine with Spike2 software for Windows, version 4.02 onset and the point at which the response dropped below 2 SD

(Cambridge Electronic Design, Cambridge, UK). After record- of the baseline activity (i.e., offset). The peak-to-peak

ing, the animals were euthanized with an intraperitoneal amplitude in mV was calculated using the Spike2 analysis

overdose of thiamylal sodium. The Dig muscle was then function between the onset and offset in each single sweep

dissected to conﬁrm the location of the placed electrode. (Fig. 1 C).

2.3. Data analysis 2.4. Statistical analysis

The latency, duration, and peak-to-peak amplitude were All data are expressed as mean Æ SD. Unpaired t-test was used

measured as the JOR properties for comparison between the for statistical comparison of the weight between the two

two groups. The mean values of these parameters were groups. Repeated measures multivariate analysis of variance

a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8 533

was used for intergroup and intragroup statistical comparison. animals, the latency of the left side was 4.9 Æ 0.5 ms for the

Simple main effects analysis using the Sidak adjustment was control and 5.6 Æ 0.3 ms for the experimental group, while the

used for multiple comparisons. Statistical analysis was right side was 5.0 Æ 0.5 ms for the control and 5.8 Æ 0.5 ms for

performed using SPSS for Windows software, version 13.0 J the experimental group. The latency was signiﬁcantly longer

(SPSS Inc., Chicago, IL, USA), and statistical signiﬁcance was in the experimental group than in the control group at each

established at p < 0.05. recording age (Fig. 4A). Intragroup comparison among 5, 7, and

9 weeks of age revealed no signiﬁcant difference in either the

control or experimental group. There were also no signiﬁcant

3. Results

differences between the left and right sides in each group at

each recording age.

The mean body weight of all rats regularly increased For the 5-week-old animals, the duration of the left side

throughout the experimental period. There was no signiﬁcant was 6.1 Æ 1.0 ms for the control and 6.5 Æ 0.5 ms for the

difference between the two groups (Fig. 2). Low-intensity experimental group, while the right side was 6.0 Æ 0.7 ms for

electrical stimulation of the right and left IAN always elicited the control and 6.1 Æ 0.7 ms for the experimental group. For

JOR responses of the bilateral Dig muscles in all animals. the 7-week-old animals, the duration of the left side was 6.1 Æ

Typical examples in both groups recorded at 7 weeks of age are 0.5 ms for the control and 6.3 Æ 0.3 ms for the experimental

shown in Figure 3. The threshold was 39.6 Æ 15.0 mA (mean Æ group, while the right side was 5.9 Æ 0.4 ms for the control and

SD) in the control group and 41.7 Æ 22.2 mA in the 6.2 Æ 0.7 ms for the experimental group. For the 9-week-old

experimental group. animals, the duration of the left side was 6.0 Æ 0.6 ms for the

The latency, duration, and peak-to-peak amplitude when control and 6.4 Æ 0.5 ms for the experimental group, while the

the stimulus intensity was set at 1.5 T are shown in Figure 4A– right side was 6.0 Æ 0.4 ms for the control and 6.3 Æ 0.8 ms for

C. For the 5-week-old animals, the latency of the left side was the experimental group. The duration was not signiﬁcantly

5.1 Æ 0.3 ms for the control and 6.0 Æ 0.4 ms for the different between the two groups (Fig. 4B). Intragroup

experimental group, while the right side was 5.3 Æ 0.3 ms for comparison among 5, 7, and 9 weeks of age revealed no

the control and 6.1 Æ 0.6 ms for the experimental group. For signiﬁcant difference in either the control or experimental

the 7-week-old animals, the latency of the left side was 4.9 Æ group. There were also no signiﬁcant differences between left

0.6 ms for the control and 5.7 Æ 0.3 ms for the experimental and right sides in each group at each recording age.

group, while the right side was 5.1 Æ 0.4 ms for the control and For the 5-week-old animals, the peak-to-peak amplitude of

5.8 Æ 0.5 ms for the experimental group. For the 9-week-old the left side was 1.0 Æ 0.3 mV for the control and 0.7 Æ 0.3 mV

for the experimental group, while the right side was 1.1 Æ 0.2

control mV for the control and 0.8 Æ 0.2 mV for the experimental

group. For the 7-week-old animals, the peak-to-peak ampli-

experimental

tude of the left side was 1.2 Æ 0.3 mV for the control and 0.8 Æ

0.1 mV for the experimental group, while the right side was 1.2

Æ 0.1 mV for the control and 0.8 Æ 0.2 mV for the experimental

Body Weight [g] Body Weight [g] group. For the 9-week-old animals, the peak-to-peak ampli-

tude of the left side was 1.3 Æ 0.2 mV for the control and 0.9 Æ

0.2 mV for the experimental group, while the right side was 1.3

Æ 0.2 mV for the control and 1.0 Æ 0.3 mV for the experimental

5 7 9 [weeks] group. The peak-to-peak amplitude was signiﬁcantly smaller

in the experimental group than in the control group at each

Figure 2 – Changes in body weight throughout the recording age (Fig. 4 C). Intragroup comparison among 5, 7, and

experimental period. There were no significant 9 weeks of age revealed no signiﬁcant difference in either the

differences between the two groups. control or experimental group. There were also no signiﬁcant

differences between the left and right sides in each group at

each recording age.

4. Discussion

The aim of the present study was to examine how nasal

obstruction affected the regulation of JOR. The present results

indicate that unilateral nasal obstruction impacts oral

function by delaying conduction velocity and thereby delaying

the JOR.

Figure 3 – Typical examples of the jaw-opening reflex (JOR)

responses in the control and experimental groups at 7 4.1. Animal models of nasal obstruction

weeks of age. The stimulus intensity was adjusted to 1.5

times the threshold (1.5 T) in each rat. Filled circles There are numerous studies examining the effects of nasal

indicate stimulation artifacts. obstruction on systemic function. Bilateral nasal obstruction

534 a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8

A * 7 * * 7 * * * control 6 6 experimental 5 5

4 4

3 3

2 2 Latency(le) [ms] Latency(right) [ms] 1 1

0 0

5 7 9 5 7 9 [weeks] [weeks] B 8 8 7 7 6 6 5 5 4 4 3 3 2 2 Duraon(le) [ms] Duraon(right) [ms] 1 1

0 0

5 7 9

5 7 9 [weeks] C [weeks] * 1.6 * * 1.6 * * * 1.4 1.4 1.2 1.2 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0

Peak-to-peak amplitude(le) [mV]

5 7 9 Peak-to-peak amplitude(right) [mV] 5 7 9

[weeks] [weeks]

Figure 4 – Changes in the latency (ms; A), duration (ms; B), and peak-to-peak amplitude (mV; C) in the two groups. Data are

expressed as mean Æ SD. *p < 0.05.

is reportedly associated with reduced muscle growth of the the anteroposterior dimension of the cranial base were proven

16 25

superﬁcial masseter and anterior Dig muscles, respiratory to be smaller than those in normal breathing groups.

muscle ﬁber adaptation, reduced growth of the olfactory On the other hand, several researchers have investigated

25 20 24

bulbs, an initial decrease in body weight and lung growth, the effects of unilateral nasal obstruction. For example, the

and adrenal hypertrophy. Bilateral nasal obstruction also ipsilateral olfactory bulb does not grow to be as large as that on

affects craniofacial growth and development; increases in the normal side, and craniofacial growth is altered; there are

both the vertical dimension of the nasomaxillary complex and reductions in the vertical development of the nasomaxillary

a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8 535

complex and in the longitudinal axis of the skull base. These joint (TMJ) mechanoreceptor maturation is attained by the age

41,42

ﬁndings suggest that while bilateral nasal obstruction induces of 5 weeks. Both TMJ mechanoreceptors and periodontal

invasive effects in the whole body, unilateral nasal obstruction mechanoreceptors are trigeminal nerve endings. Our ﬁndings

may also affect whole body systems. Bilateral nasal obstruc- indicate that the JOR also attains maturation by 5 weeks of age

tion was associated with significant reductions in body weight without a significant influence by nasal obstruction.

in our pilot study. As such, we chose an animal model of

unilateral nasal obstruction for the present study. 4.2.2. Changes in latency

With regard to morphology in association with nasal The latency of the JOR decreases in parallel with the aging of

obstruction, previous animal studies have reported a signiﬁ- rats. In rat pups the long latency of the Dig reﬂex shortens to

cant reduction in the vertical development of the nasomax- the adult value just before the animal starts to explore solid

illary complex and in the skull base along the longitudinal food (at 14 days). This change takes place before weaning,

17 27 44

axis. Gross also demonstrated a signiﬁcant decrease in the which usually occurs about a week later (i.e., 3 weeks of age).

maxillary width and the intermolar width in rats. A narrow Latency is associated with the conduction velocity of nerve

maxilla is one of the craniofacial morphological patterns ﬁbers. The conduction velocity rises with increases in

28 45

accompanied by mouth breathing. Asahina reported the myelination and axon diameter.

effects of nasal obstruction on the growth and development of Periodontal mechanoreceptors are reportedly affected and

the craniofacial complex in young golden hamsters. According the conduction velocity of nerve ﬁbers decreases after an

28 46

to Asahina, the mandibular plane (MP) angle was signiﬁ- occlusal force reduction. The delayed conduction velocity

cantly larger than controls, as nasal obstruction caused might be explained by peripheral sensory system alteration.

signiﬁcant molar extrusion. Therefore, our animals might Maturation of peripheral A-ﬁbers, which control touch and

have had larger MP angles. pressure transduction, was recently shown to coincide with

In humans, the relationship between masticatory muscle the development of myelination. The prolongation of the

function and craniofacial morphology has been extensively latency was supported by the ﬁndings in animal models of

19 37

investigated. Findings included a negative correlation be- soft-diet feeding and an anterior open bite. No report has

29 30

tween the masseter muscle activity and occlusal force and directly revealed the effect of altered masticatory function on

the MP angle. As our experimental setup limited morphologi- the development of myelination or enlargement of ﬁber

cal investigation, these relationships between morphological diameter in these nerve ﬁbers. However, it can be assumed

and functional inﬂuences of nasal obstruction should be that nasal obstruction induces a similar condition by reducing

investigated in future studies. occlusal force. In turn, nasal obstruction delays the conduc-

Tongue pressure during nasal obstruction was reported to tion velocity and increases the latency, resulting in delayed

be signiﬁcantly greater. In the rat, nasal obstruction can lead development of reﬂex regulation.

32,33

to hypoxia. Further, hypoxia in rats increased genioglos- The interneurons of the JOR are located in the spinal

sus muscle activity. From these ﬁndings, we suggest that trigeminal subnucleus oralis (SpVo) and in the subnucleus

nasal obstruction increases the tongue-protruding muscle interpolaris (SpVi). Morphological studies have revealed the

activity, which may cause a occlusal disharmony. reciprocal connections between the vestibular nuclei (VN) and

spinal trigeminal nuclei in rats. Both the SpVo and SpVi are

4.2. Changes in JOR associated with nasal obstruction known to contain premotor neurons projecting to the trigemi-

nal motor nucleus. Moreover, the trigeminal primary afferent

4.2.1. JOR induced by non-nociceptive stimulation and ﬁbers project to the VN. A physiological study found that

maturation of JOR activation of the vestibular afferents elicited excitatory

The JOR evoked by high-intensity stimulation is related to responses in the jaw-opening motoneurons. Thus, it is

35,36

pain, and its functional signiﬁcance is well established. plausible that the VN may be involved in control of the JOR.

The JOR evoked by low-intensity stimulation is also thought to Another study revealed that nasal obstruction induced hypoxia

19,37 32,33 53

be important in masticatory movement. Several previous in the rat. On the other hand, Yoshida et al. reported that

studies examined the JOR evoked by low-intensity stimulation hypoxia could cause inhibition of postsynaptic components in

of the periodontal mechanoreceptors, and reported that it is the VN. Therefore, nasal obstruction may affect a component of

23,38

essential in modulation of mastication. In the IAN, there the reﬂex pathway of the JOR via hypoxia.

are ﬁbers for sensation of pain, touch, and pressure that

originate from the lips, teeth, and intraoral mucosa. In our 4.2.3. Changes in duration and peak-to-peak amplitude

study, the intensity of IAN stimulation was less than 120 mA. We found no signiﬁcant difference in the reﬂex duration

This is lower than the intensity used in awake animals between the control and experimental groups, which is

(maximum 130 mA) and could be considered to be non- supported by previous studies of soft-diet feeding and an

23,38 47

nociceptive stimulation. anterior open bite. The reﬂex duration is related to the

We found that the latency, duration, and peak-to-peak number of muscle ﬁbers contributing to the motor unit. In

amplitude in both the control and experimental groups general, the number of muscle ﬁbers decreases when muscle

remained stable throughout the experimental period. These atrophy occurs. As the reﬂex duration did not decrease in our

19,20

ﬁndings are in line with those of soft-diet feeding. A study, this suggests that muscle atrophy did not occur.

previous study suggested that the response properties of rat Therefore, it is likely that the number of muscle ﬁbers that

periodontal mechanoreceptors attain maturation by 5 weeks contribute to the motor unit activity was not signiﬁcantly

of age. Another study indicated that temporomandibular affected by nasal obstruction.

536 a r c h i v e s o f o r a l b i o l o g y 5 9 ( 2 0 1 4 ) 5 3 0 – 5 3 8

The effects of nasal obstruction on the peak-to-peak Japanese Ministry of Education, Culture, Sports, Science and

amplitude were also similar to those in studies of soft-diet Technology.

21 47

feeding and an anterior open bite; the amplitude of the JOR

did not signiﬁcantly increase with developmental age.

16 Conﬂict of interest

Gelhaye and colleagues found that in 21-day-old rats the

nasal obstruction was associated with a greater decrease of

MHC 2x compared with 2a in the anterior Dig muscle. This None declared.

indicates that the fatigue-resistant MHC ﬁber types increased

in muscles in association with nasal obstruction. It was

Ethical approval

reported that the anterior Dig muscle attained its speciﬁc

adult ﬁber-type proﬁle by approximately 6 weeks. There-

fore, we suggest that the amplitude of the JOR is not affected The experimental procedures described herein were approved

by aging from 5 to 9 weeks. by the Institutional Animal Care and Use Committee

The peak-to-peak amplitude is calculated as the voltage (#0130064A) and performed in accordance with the Animal

difference of the motor unit potential and is determined by the Care Standards of Tokyo Medical and Dental University.

diameter and number of muscle ﬁbers closest to the

electrode. The amplitude decreases when the periodontal

ligament becomes narrower, and periodontal mechanorecep- Acknowledgments

tors become degenerated. Thus, nasal obstruction may

induce the weak occlusal force, triggering a reduction in the We thank Part-time Lecturer Tadachika Yabushita, Assistant

stimulation to the periodontal mechanoreceptors, thus af- Professor Koichi Fujita, Assistant Professor Takayoshi Ishida

fecting the periodontium and decreasing the amplitude. This and Clinical Fellow Chiho Kato for useful advice. We thank

assumption should be investigated in a further study. Associate Professor Yoshiyuki Sasaki for his statistical exper-

tise. We thank Professor Kumiko Sugimoto, Associate Profes-

4.2.4. Laterality sor Koji Kino, and Lecturer Masayuki Hideshima for their

The results of the present study showed no signiﬁcant helpful comments.

differences in the latency, duration, or peak-to-peak ampli-

tude between the left and right sides. This may be explained by

the fact that the left and right nasal cavities of the rat are

anatomically connected, and the open side may compensate Appendix A. Supplementary data

for the occluded side. Therefore, we speculate that a unilateral

nasal obstruction model can simulate incomplete nasal Supplementary data associated with this article can be

breathing, which may affect the systemic craniofacial func- found, in the online version, at http://dx.doi.org/10.1016/

tion without laterality. j.archoralbio.2014.02.013.

4.3. Oral environmental changes and masticatory

performance

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