Surface intraoral genioglossus EMG recording technique for kinesiologic studies

Electromyographic (EMG) recordings from intraoral genioglossus surface electrodes were compared to fine-wire recordings of the left genioglossus muscle during selected activities that involved (1) rest, (2) tongue protrusion without resistance, (3) isometric tongue protrusion, (4) jaw opening without resistance, (5) isometric jaw opening, and (6) swallowing. Right and left lateral protrusions of the tongue were evaluated also. Recordings from both surface and fine-wire configurations showed similar onset times, relative amplitude changes, and cessation times of EMG activity during unresisted tongue protrusion and isometric tongue protrusion. Swallowing EMG activity occurred somewhat earlier and was longer in duration in the surface electrode recordings than the fine-wire recordings; however, maximum amplitudes occurred at similar times. Neither type of electrode recorded significant EMG activity during jaw opening or isometric jaw opening. These findings support the validity of recording EMG activity of the genioglossus muscle by surface recording electrodes supported by an acrylic appliance. The development of such an appliance may be an important biofeedback tool to control genioglossus activity during such activities as tongue thrusting.     

Mandibular joint control of genioglossus muscle activity in the cat (Felis domesticus) and monkey (Macaca irus).

Jaw position had a profound influence on genioglossus (GG) muscle activity, the amount of activity varying with the amount of jaw opening. Mandibular rotations of 21 from tooth contact position increased the GG activity which was maintained as long as the jaw was opened. The response could be abolished by bilateral local anaesthesia of the mandibular joint (MJ). Bipolar stimulation of the small branches of the auriculotemporal nerve which supply the joint elicited a reproducible reflex response in the ipsilateral GG muscle with a latency of approximately 10 ms. The reflex could be abolished by administering a muscle relaxant or by severing or anaesthetizing the branches of the auriculotemporal nerve central to the stimulating electrode. Anaesthetizing or severing the nerves supplying the masseter or temporalis muscles did not abolish the GG reflex response to the nerve stimulus or the response to jaw opening. A prior threshold glossopharyngeal (IX) or superior laryngeal (SL) nerve stimulus produced an early facilitation of the GG reflex elicited by threshold MJ stimulation. In contrast, a supra-threshold lingual, IX or SL nerve stimulus resulted in a long-lasting inhibition of the GG reflex. A number of functional relationships may explain the significance of these excitatory and inhibitory responses in the GG muscle.     

Properties of tongue and jaw movements elicited by stimulation of the orbital gyrus in the cat

The cortical area for rhythmic tongue and jaw movements is located at the anterior part of the orbital gyrus. The frequency of cortically induced rhythmic movements of tongue and jaw was always identical, and the tongue protruded either when the jaw opened or closed, but the tongue retracted only when the jaw opened. Patterns of these cortically induced movements were slightly different from each stimulus spot, but they were strongly influenced by alteration of stimulus parameters. Stimulation of subcortical structure also elicited rhythmic tongue and jaw movements, but their patterns and threshold were a little different from those of cortical stimulation. Cortically and subcortically induced tongue movements were facilitated by simultaneous stimulation of the lingual nerve.These findings suggest that the orbital gyrus is not essential for rhythmic tongue and jaw movements, but it can control functions of a “rhythm-generating centre of chewing movements”. Impulses via the lingual nerves can also affect activities of this centre.     

Generation of rhythmical ingestive activities of the trigeminal, facial, and hypoglossal motoneurons in in vitro CNS preparations isolated from rats and mice

The central pattern generator (CPG) for masticatory movements has been located in the medial bulbar reticular formation, by using cortically induced rhythmical jaw and tongue movements as a model. To analyze how stationary input from the cortex is transformed into rhythmical output in the neuronal population comprising the CPG, rhythmical neural activities representing rhythmical food ingestive movements were experimentally induced in vitro. Bath-application of N-methyl-D-aspartate (NMDA) induced rhythmical activities in the trigeminal (V), facial (VII) and hypoglossal (XII) nerves of in vitro brainstem-spinal cord preparations isolated from newborn rats and mice. This paper will review evidence for the notion that (1) the rhythmical XII nerve activity represents rhythmical sucking movements; (2) the population of neurons critically involved in the rhythm generation of the XII nerve is localized in the ventromedial medulla oblongata on both sides, and can induce rhythmical XII nerve activity on the same side independently of each other; (3) the rhythmical activities in the V, VII, and XII motoneurons are induced by separate CPGs, which are located segmentally at the respective level of the V motor, VII, and XII nuclei. In addition, rhythmical masticatory-like EMG activity of jaw muscles can be induced by repetitive stimulation of the pyramidal tract in the in vitro brainstem isolated from adult mice together with the oral-facial structures. We propose that the in vitro brainstem preparation is a useful tool for longitudinal analysis of postnatal development of the central pattern generation of food ingestive movements, including conversion from sucking to mastication.     

Human jaw–tongue reflex as revealed by intraoral surface recording

summary The purpose of this study was to examine if there is a human jaw–tongue reflex. This study was carried out in seven healthy adult males and recorded the genioglossus muscle activity during various functions by using a miniature intraoral surface electrode, which is comparable with intramuscular fine-wire electrodes, but without pain or disturbance of the tongue movement. The ipsilateral masseteric and digastric muscle activities were simultaneously recorded with the surface electrodes. Tonic genioglossus muscle activity was recorded during clenching. A passive jaw opening elicited the stretch reflex in the masseteric muscle and increased genioglossus muscle activity. Electrical stimulation of the lower lip inhibited the tonic activity in the masseteric and genioglossus muscles during both clenching and tongue protrusion. Moreover, the latency of the inhibition in the genioglossus muscle activity was shorter during clenching than during tongue protrusion. Based on these findings, the authors conclude that the human jaw-tongue reflex exists and that the jaw-closing muscle is involved in evoking the reflex.     

A method of studying adaptive changes of the oropharynx to variation in mandibular position in patients with obstructive sleep apnoea

The aim of this study was to develop a method of studying the effects of mandibular advancement on oropharyngeal airway dimensions in the sagittal plane in conscious, supine patients. Six white, dentate, male patients with proven obstructive sleep apnoea had sagittal fluoroscopic recordings taken in the resting supine position. Images were recorded at four frames per second as the mandible was advanced with the teeth in contact to maximum protrusion and then opened. Software in the fluoroscopic imaging system permitted measurement of the change in mandibular position together with oropharyngeal airway dimensions expressed as the narrowest dimension observable in the post-palatal and post-lingual sites. Plotting of airway dimensions during mandibular advancement enabled estimation of the degree of protrusion associated with maximal airway benefits. Progressive mandibular advancement produced variable adaptive changes in the post-palatal and post-lingual regions of the oropharynx. The amount of airway opening appeared to be related to the horizontal and vertical relationships of the face and to the dimensions of the soft palate. The changes in post-palatal and post-lingual airway dimensions were not always identical, despite the observation that both tongue and soft palate were seen to move in unison, with close contact being maintained between the two structures. Jaw opening resulted in synchronous posterior movement of both tongue and soft palate, with consequent narrowing of oropharyngeal airspace. Fluoroscopy is a simple method of assessing upper airway changes with mandibular advancement in the conscious patient. The technique should facilitate the selection of subjects for whom mandibular advancement would seem advantageous. The nature of the adaptive response is dependent on individual structural variation. It is suggested that, where artificial mandibular advancement with dental devices is considered beneficial, jaw opening should be kept to a minimum.     

Brainstem segmental arrangement of sucking rhythm generators for trigeminal, facial and hypoglossal motoneurons]

Rhythmical sucking like activity (RSA) is known to be induced in the hypoglossal nerve (XII n) in an isolated brainstem-spinal cord preparation from newborn rats with bath application of N-methyl-D-aspartate (NMDA). Since natural sucking consists of well-coordinated rhythmical activities of the tongue, face, and jaw muscles, it is assumed that the trigeminal (V) and facial (VII) motoneurons would show rhythmical activities during the RSA. The purpose of this study was (1) to induce rhythmical activities in the V and VII ns in addition to XII n during bath application of NMDA, and (2) if they are induced, to locate the structures essential for generation of NMDA-induced rhythmical V, VII, and XII n activities using sectioning techniques. Experiments were performed on brainstem-spinal cord preparations isolated from newborn mice with or without the orofacial structure. Neural activities were recorded from the V, VII, and XII ns with suction electrodes. Movements of the tongue and jaw were recorded with a CCD camera system. It was found that (1) bath application of NMDA (5-20 microM) induced RSA in the V, VII, and XII ns, (2) it also induced rhythmical sucking-like movements of the jaw and tongue, and (3) after complete transections of the brainstem between the V and VII ns as well as at the pontomedullary junction, NMDA still induced the RSA in the V, VII, and XII ns. These results demonstrate that separate rhythm generators for rhythmical NMDA-induced activities in the V, VII, and XII motoneurons are located at the respective levels in the brainstem.     

Facilitatory effect of jaw opening on somatosensory (SI) cortical neurones sensitive to tooth pressure in the cat

In anaesthetized cats, an upper canine tooth was stimulated mechanically at two different levels of jaw opening, the resting position and an open position with 20 ± 2 mm between the upper and lower canines. The evoked field potentials and neuronal discharges were recorded from the caudal part of the contralateral coronal gyrus (SI cortex). The waveforms of the evoked potentials appeared in a positive-negative sequence. There were no significant changes in them when the jaws were open. Discharge patterns elicited in the cortical neurones by mechanical stimulation of the teeth consisted of initial ‘burst’ discharges, inhibitory pauses and/or large after-discharges. Jaw opening did not influence any phases of these responses to suprathreshold stimulation, spontaneous activities, or the sizes of the receptive fields. However, jaw opening did affect the initial ‘burst’ phase of the response to threshold stimulation, i.e. that which caused the neurones to fire with a probability of 30–50% with the jaw closed. Jaw opening enhanced this response probability in half (6/12) of the units that had very small receptive fields restricted to the canine tooth, but did not influence it in the majority (21/24) of the units that had larger receptive fields including the oral mucosa and the facial skin. There was no difference in distribution in the coronal gyrus between the two groups of neurones categorized by whether or not they were influenced by jaw position.     

Functional heterogeneity in the superior head of the human lateral pterygoid

The activity of the superior head of the human lateral pterygoid muscle (SHLP) is controversial. Given the non-parallel alignment of some SHLP fibers, the SHLP may be capable of differential activation. The aims were to clarify SHLP activity patterns in relation to location within SHLP. In 18 subjects, SHLP single motor units were intramuscularly recorded at computer-tomography-verified sites during horizontal (e.g., protrusion) and vertical (e.g., opening) jaw tasks (recorded by a jaw-tracking device) and at resting postural jaw position. None of 92 units was active at the resting postural position. Medially located units (21) showed activity during contralateral movement, protrusion, and opening; 5 were also active on jaw closing. There was a significant association between unit location and the number of units active during vertical tasks (i.e., jaw closing and clenching). Analysis of the data suggests differential activation within SHLP and raises the possibility of functional heterogeneity within SHLP.     

The effect of prior jaw motion on the plot of electromyographic amplitude versus jaw position

Fabrication of interocclusal splint at a thickness determined by the vertical dimension at which the jaw muscle EMG amplitude is minimum has been recommended. However, the effect of prior jaw motion and the effect of the recording site on the EMG amplitudes and on the vertical dimension of minimum EMG activity have not been documented. IEMG amplitudes at various static jaw positions achieved during opening and during closing were analyzed in nine subjects. Surface IEMGs were recorded over the left anterior temporal muscle, left masseter and left suprahyoids muscles, and by nonspecific EMG recording as described by Rugh and Drago. The jaw position was recorded in 5 mm increments by a kinesiograph. After 30 seconds of relaxation, 10 successive IEMG reading at 4-second integration times were obtained at each recording site. These 10 recordings at each requested jaw position were averaged and analyzed. The IEMG activity changed with different jaw position. As the jaw opened from centric occlusion, the IEMG from jaw closing muscles decreased to a minimum and then increased with further opening. Moreover, the IEMG for a particular jaw position differed depending on the history of the jaw movement, that is, whether the position was achieved after an opening step or after a closing step. Two factors, the amount of jaw opening and the history of jaw movement to reach that position, seemed to influence the IEMG differently in each of the recorded muscles.     

Anatomical and electromyographic studies of the lateral pterygoid muscle

The relationships of the lateral pterygoid muscle within the infratemporal fossa were observed by conventional dissections and by examination of specimens sectioned in the horizontal and frontal planes. The following less well-known features were noted. At the origins of the superior and inferior heads there are regions in which the fibres are interlaced or closely overlapped by fibres of either the temporalis muscle or the medial pterygoid muscle. Fibres of the superior head insert not only into the meniscus of the temporomandibular joint, but also into the pterygoid fovea at the neck of the mandibular condyle. Specimens sectioned through the origin of the inferior head of the muscle show internal tendon lamellae consistent with a pennate structure. Electromyographic (EMG) activity was recorded in five healthy subjects using concentric needle and fine-wire electrodes. Strong to very strong activity was consistently observed in the superior head during clenching and tooth gnashing. The inferior heads were silent or had negligible to slight activity most of the time during ipsilateral movements or clenching, but were co-activated bilaterally, with strong to very strong activity during jaw opening, protrusion, swallowing, tooth gnashing and during passive retrusion. They showed marked activity unilaterally during contralateral movements.     

Tongue and jaw muscle activity in response to mandibular rotations in a sample of normal and anterior open-bite subjects.

In order to investigate the effect of mandibular rotations in human subjects, the electromyographic activity from the left GG, left masseter, and left orbicularis oris muscles was recorded during voluntary opening movements of the mandible that were monitored by an electronic transducer. For each muscle, a computer-based system was used to calculate a threshold incisor-separation position corresponding to a 20 percent increase in base line muscle activity. The mean threshold level for the GG muscle in a sample of twenty subjects with normal occlusions was 53.0 percent (S.D. +/- 22.8)of maximum jaw opening. By contrast, a sample of nine subjects with anterior open-bite malocclusions had a statistically significant (p less than 0.001) lower mean threshold of 5.9 percent (S.D. +/- 4.5) of maximum jaw opening. No significant differences in masseter or orbicularis oris muscle activity were found between the two samples. Since postural tongue activity could play an important role in the development of the anterior dentition, the lower threshold for the GG muscle activity in response to jaw opening in anterior open-bite subjects may be of significant clinical importance.     

Cortically induced masticatory rhythm and its modification by tonic peripheral inputs in immolbilized cats.

Repetitive cortical stimulation induced rhythmical masticatory jaw movement and masseteric nerve activity in cats with the spinal cord sectioned at the C2 level. Jaw depression alone did not induce any rhythmical jaw movement, but it turned the irregular jaw movement evoked by subthreshold cortical stimulation into the regular jaw movement as evoked by suprathreshold cortical stimulation. The cortically induced rhythmical masseseteric nerve activity could be induced even in immobilized condition and after cerebellectomy, although the rhythmical nerve activity was depressed in amplitude and the rhythm became slower than before immobilization. Tonic jaw depression after immobilization facilitated the tonic activity in the masseteric nerve and also induced a remarkable increase in the amplitude of the cortically evoked rhythmical nerve activity and a slight acceleration of the rhythm, leading to the same masticatory rhythm as before immobilization. It was concluded (1) that the cortically induced masticatory rhythm in cats is basically generated centrally and (2) that the tonic inputs from the muscle spindle in the jaw-closing muscles participate in the rhythm formation by supplying tonic excitatory inputs mainly to the jaw-closer motoneuron and also to the central rhythm generator.     

A cephalometric study of orofacial structures during secondary palate closure in the rat

The relationships between orofacial structures prior to and subsequent to secondary palate closure were evaluated in midsagittal sections of 30 foetal rats. Efforts were made to differentiate between the effects of shelf elevation and the effects of growth of individual orofacial components.In the day before palatal closure, the growth rates (percentage increase per hour) of the tongue and Meckel's cartilage were greater than those of the mandible and the maxilla. The tongue, Meckel's cartilage and the mandible grew in a downward and forward direction similar to the angulation of the incline formed by the nasal septum and the medial palatal process of the primary palate. Space for growth of the lower face was provided by maxillary growth, opening of the cranial base, and by extension of the head during the day before palatal closure.Palatal closure took place when the length of Meckel's cartilage exceeded the length of the oronasal cavity measured to the medial palatal process. During palatal closure, the medial palatal process increased in infero-posterior prominence, the cross-section of Meckel's cartilage and the anterior portion of the mandible rotated forward, counter-clockwise, head facing right, and the vertical distance of Meckel's cartilage from the primary palate increased. Anterior tongue protrusion was present during the day of palatal closure but the degree of protrusion relative to the mandible did not change when the shelves elevated. In the day after palatal closure, mandibular growth became more horizontally directed and the rotation of Meckel's cartilage and the anterior portion of the mandible continued. The rate of growth of the mandible was similar to its rate before palatal closure.Growth of the tongue, Meckel's cartilage, the mandible and the medial palatal process appeared to be important factors in creating the environment conducive to secondary palatal closure. Rotation of the anterior portion of the mandible and Meckel's cartilage beginning at the time of palatal closure may produce a change in position of the mid-portion of the mandible, allowing for tongue and palatine shelf exchange.     

Disc-condyle Relationships during Class II Treatment with the Functional Mandibular Advancer (FMA)

OBJECTIVE: The objective of this study was to verify the effects of treatment to correct Class II malocclusion with the Functional Mandibular Advancer (FMA) on the relative positions of the articular disc and mandibular condyle. In particular, we aimed to find out whether the disc-condyle relationship changed between baseline and post-treatment, in temporomandibular joints initially exhibiting a physiological relationship and alternatively, in temporomandibular joints initially presenting anterior displacement of the articular disc. PATIENTS AND METHODS: Treatment progress in 15 patients was monitored at defined points in time by manual structural analysis (MSA) and magnetic resonance (MR) imaging. The disc-condyle relationship was assessed by examining the parasagittal MR images made up of three slices each (lateral, central, medial) taken in habitual intercuspation and maximum-open mouth position. The MR images were metrically analyzed to determine the sagittal positional relationship of the articular disc and mandibular condyle using two methods on the central slices of the images taken in closed-mouth position. MSA was used in particular to determine the disc-condyle relationships and to metrically record the extent of active movement of the mandible during mouth opening, protrusion, laterotrusion, and retrusion. RESULTS: Comparison of baseline and post-treatment findings revealed that none of the joints exhibited a treatment-induced deterioration in the disc-condyle relationship, while the relationship improved in five joints. After categorization of the joints according to groups according to disc position, metric analysis of the MR images showed significantly-improved post-treatment disc positions in the joints that had initially exhibited anterior disc displacement. Post-treatment findings for maximum-open mouth position, protrusion, and laterotrusion corresponded to the baseline values recorded before bite-jumping, and the extent of maximum active retrusion increased significantly. CONCLUSIONS: Functional jaw orthopedics for correction of skeletal Class II with the rigid fixed FMA leads to side-effects reflected in the disc-condyle relationship in the temporomandibular joints: no adverse effects were observed in joints presenting an initial physiological disc-condyle relationship, whereas the disc position may improve in joints with initial partial or total anterior disc displacement. In comparison with the baseline findings, we observed no post-treatment restriction in the extent of maximum mouth opening, protrusion, and left and right laterotrusion. Maximum active retrusion increased due to the treatment. MR imaging and MSA only partly cover the same aspects of temporomandibular joint diagnostics.     

The effect of intra-oral stimulation on the human masticatory cycle

Experiments were carried out on 8 adult subjects to measure the influence of transient, intra-oral electrical stimulation upon jaw displacement and chewing rhythm during natural, unilateral chewing in man. Jaw movements throughout continuous gum-chewing sequences were monitored with a non-invasive transducer designed to record incisor point displacement in 3 dimensions. The displacement signals were sampled by a computer which delivered non-noxious, and in some subjects noxious, stimuli in a pseudo-random sequence to the upper gingiva of the ipsilateral side at fixed points in the chewing cycle. Statistical comparison of paired test and control cycles revealed that the disruptive effects of the stimuli became more common and greater as the stimulus strength increased. Marked changes were observed in the duration of the masticatory cycle, especially with painful stimulation. Whereas stimuli delivered during jaw opening usually evoked either lengthening or shortening of subsequent phases in the cycle that were consistent (although different) for individuals, those delivered during closure consistently produced a lengthening of that part of the cycle at or near the intercuspal position of the teeth. Stimulation during the jaw-closing phase did not prevent the teeth from achieving the control intercuspal position, whereas strong stimuli during jaw opening often altered the coordinates of the maximum jaw-opening position. Thus the response to such stimulation during chewing depends upon the phase of the cycle at which the stimulus is delivered, the most predictable result being a lengthening of the cycle by stimulation just before the onset of intercuspation.     

Jaw-tongue reflex: afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat

The tongue position is reflexively controlled by the jaw position (the jaw-tongue reflex). The purpose of this study was to clarify the mechanism of this reflex in terms of afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat. Intracellular recordings from hypoglossal motoneurons revealed that electrical stimulation of the temporalis muscle nerve evoked excitatory and inhibitory post-synaptic potentials in hypoglossal motoneurons. The threshold of temporalis muscle nerve stimulation for evoking the synaptic potentials was higher than 2.0 times the nerve threshold. The amplitude of the potentials increased with stimulus intensity up to 5.0 times the nerve threshold. Punctate light pressure applied to the temporalis muscle induced a tonic depolarizing potential in hypoglossal motoneurons on which action potentials as well as depolarizing synaptic activation noise were superimposed. On the other hand, electrical stimulation of the temporalis muscle during jaw-opening could slightly inhibit the electromyographic activities in the genioglossus and styloglossus muscles. Lesions including the Probst's tract at the level caudal to the trigeminal motor nucleus abolished both excitation and inhibition in hypoglossal motoneurons induced by tonic depression of the lower jaw, but exerted no effects on either the tonic stretch reflex or the trigemino-hypoglossal reflex. In contrast, lesions including the trigeminal spinal tract produced no changes in either excitation or inhibition of hypoglossal motoneurons induced by temporalis muscle afferents, whereas the excitation of hypoglossal motoneurons was abolished by the lesions. We conclude that the group II muscle spindle afferents from the temporalis muscle are primarily responsible for evoking the jaw-tongue reflex.     

Changes in airway and hyoid position in response to mandibular protrusion in subjects with obstructive sleep apnoea (OSA).

This prospective clinical study examined the alterations in airway and hyoid position in response to mandibular advancement in subjects with mild and moderate obstructive sleep apnoea (OSA). Pairs of supine lateral skull radiographs were obtained for 13 female and 45 male, dentate Caucasians. In the first film, the teeth were in maximal intercuspation, while in the second the mandible was postured forwards into a position of maximum comfortable protrusion. Radiographs were traced and digitized, and the alterations in the pharyngeal airway and position of the hyoid were examined. Males and females were analysed separately. In males only, correlations were sought between the changes in hyoid and airway parameters, and the initial and differential radiographic measurements. In males, mean mandibular protrusion at the tip of the lower incisor was 5.3 mm, increasing its distance from the posterior pharyngeal wall by 6.9 mm (or 9 per cent). Movement of the hyoid showed extreme inter-subject variability, both in the amount and direction. In relation to the protruded lower jaw, the hyoid became closer to the gonion by 6.9 mm and to the mandibular plane by 4.3 mm. With respect to the upper face, a 1.3-mm upward and 1.1-mm forward repositioning was seen. The percentage alterations in airway dimensions matched or bettered the mandibular advancement. The minimum distances behind the soft palate and tongue improved by 1.0 and 0.8 mm, respectively. Despite their smaller faces, females frequently showed greater responses to mandibular protrusion than males. No cephalometric features could be identified which might indicate a favourable response of the airway to mandibular protrusion. Larger increments of hyoid movement were associated with an improved airway response, but the strength of the correlations was generally low.     

Parasympathetic Vasodilator Fibers in Masseter Muscle

Research into the nervous control of blood flow to the jaw muscles controlling opening and closing of the mouth, especially the masseter muscle, has indicated that sympathetic fibers derived from the superior cervical sympathetic trunk induce vasoconstriction in the masseter muscles of a number of animal species. However, none of these reports has investigated neurally-mediated vasodilatation in the masseter muscle under physiological conditions. The present study was thus designed to examine: 1) whether there are vasodilator fibers in the masseter muscle ; and 2) if there are, to elucidate the neural pathways mediating these responses in urethane-anesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited an intensity- and frequency-dependent increase in blood flow in the masseter muscle (MBF). The MBF increase evoked via LN stimulation was reduced by hexamethonium in a dosedependent manner (1–10 mg/kg). Pretreatment with phentolamine or propranolol at a dose of 100 µ/kg had no effect on the increase in MBF through LN stimulation. Pretreatment with atropine (100 µ/kg) significantly reduced the LN stimulation-induced increase in MBE MBF elevations occurred with electrical stimulation of the trigeminal ganglion, and these increases were significantly reduced via the administration of hexamethonium. Lidocaine microinjection into the trigeminal spinal nucleus or salivatory nuclei caused a significant attenuation of the LN-induced MBF increases. When wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the masseter muscle, labeled neurons were abundantly observed in the otic ganglion. The present study indicates that there are parasympathetic cholinergic and non-cholinergic vasodilator fibers originating from cell bodies in the otic ganglion in the rat masseter muscle. The MBF increase evoked by activation of the parasympathetic fibers occurred via the trigeminal-mediated reflex, suggesting that this novel parasympathetic vasodilator response may play an important role in the regulation of jaw muscle hemodynamics. This review investigates: A) the presence of vasodilator fibers in masseter muscle ; B) the possible neural pathway leading to vasodilatation mediated by activation of these fibers when the somatic pathway of the trigeminal afferent nerve is electrically stimulated ; and C) the physiological role of neurally-mediated vasodilatation in jaw muscle hemodynamics.     

Jaw reflexes elicited by electrical stimulation of the hard palate of the rat.

Reflex responses of the oral musculature to electrical stimulation of sensory elements in the hard palate were studied in decerebrate rats. Activity in the suprahyoid muscles, the intrinsic and extrinsic tongue muscles and the jaw-closing muscles was recorded electromyographically; at the same time jaw positions were recorded. Muscle group-activity patterns depended on the place of stimulation and the strength of the stimulus.Stimulation of the antemolar region of the palate evoked:—at threshold stimulation, suprahyoid muscle activity (latency 6–7 ms) which did not result in jaw movement; with stimuli about 1.2–1.5 × threshold, enhancement of suprahyoid muscle activity resulting in a transient jaw opening (latency 15 ms, duration 35 ms) sometimes accompanied by tongue reactions (latency 15 ms, duration 5–10 ms); with stimuli about 1.5–3 × threshold, additional jaw-closing muscle reactions (latencies 5–10 ms and 15–20 ms) sometimes changing the transient jaw opening to a transient jaw closing (latency 15 ms, duration 35 ms).Stimulation of the intermolar region evoked:- at threshold stimulation, two bursts of activity in the intrinsic tongue muscles (latencies 15–20 ms and 40–60 ms) and reactions in the jaw-closing muscles (latency 50–80 ms) resulting in a long-lasting jaw closure; with stimuli about 1.2–1.5 × threshold, a gradual shift from the reactions at threshold to a suprahyoid muscle and jaw-closing muscle response of short latency (6–9 ms), causing fast transient jaw opening followed by prolonged jaw closing; with stimuli about 1.5–3 × threshold, only short latency suprahyoid and jaw-closing muscle reactions. The strongest stimuli at this level abolished the jaw-closing reflex and reinforced the jaw-opening reflex. Stimulation of the intermolar region with two equal stimuli (1.2–1.5 × threshold) and several time intervals showed the following reaction pattern:- At very short intervals (1–7 ms), the jaw-closing reflex was suppressed; intervals larger than 6 ms evoked two bursts in the anterior digastric muscles. At about 12-ms intervals, the second reflex disappeared but reappeared at intervals larger than 30–40 ms, being equal to conditioning value at intervals larger than 60 ms. At stimulus intervals up to 200 ms a second stimulus did not produce a second response of the jaw-closing muscles (long-latency closing reflex).