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PubMed1986

Inhibition of jaw-closing muscles by electrical stimulation of the ophthalmic division in man.

High-intensity stimulation of the supraorbital region elicits, together with a blink reflex, a reflex inhibition of the jaw-closing muscles in normal man. The response differs from the well known inhibition obtained by intra- and perioral stimulation in two main features. Firstly, it consists of a single late silent period (SP), only occasionally preceded by a short and partial decrease of the background EMG activity; secondly, the inhibitory response appears at a rather high threshold, requiring a stimulus intensity which is 4-fold the sensory perception threshold and 3-fold that required to evoke the blink reflex. Electrical and mechanical stimulation of the cornea failed to evoke a significant inhibitory reflex. The silent period and the blink reflex were similarly affected by local anaesthetic infiltration of the supraorbital skin, suggesting that the afferents subserving the two reflexes belong to the same fibre group; the higher threshold of the supraorbital inhibitory response may be explained by the need for a larger spatial summation. The considerable latency gain and relatively rapid habituation shown by the supraorbital inhibitory response imply a multisynaptic circuit, similar to that responsible for the second silent period which occurs following 'oral' stimulation. A common interneuronal net for these two reflexes is suggested by the results of interaction experiments employing combined supra- and infraorbital stimulation.

Adult↗
PubMed1986

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei.

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.

Animals↗
PubMed1987

The histaminergic innervation of the mesencephalic nucleus of the trigeminal nerve in rat brain: a light and electron microscopical study.

Histaminergic fibers in the mesencephalic nucleus of the trigeminal nerve of Long-Evans rats were examined by light and electron microscopy after peroxidase-antiperoxidase immunocytochemical staining for histidine decarboxylase (HDC) as a marker. By light microscopy, neurons in the mesencephalic nucleus of the trigeminal nerve were seen to be surrounded by a number of HDC-like immunoreactive (HDCI) fibers, suggesting the presence of axo-somatic contact. This finding was supported by immunoelectron microscopic demonstration of synaptic contact of some HDCI fibers with the soma of the neurons in this nucleus. These findings indicate that histamine is involved in the sensory regulation of movement of the masticatory muscles at the level of the trigeminal mesencephalic nucleus.

Animals↗
PubMed1988

Interactive periodontal and acoustic influences on the masseteric post-stimulus electromyographic complex in man.

Post-stimulus electromyogram (EMG) complexes (PSECs) were studied in the full-wave rectified and averaged EMGs of the masseter muscles in 15 subjects, who clenched at a controlled level. The PSECs, a series of downward- and upward-going waves reflecting inhibitory and excitatory influences upon the masseteric motoneurones, were elicited by mechanical stimulation of a tooth. The stimuli selectively activated mechanoreceptors in the periodontium and, by bone-conduction, acoustic receptors. Application of acoustic masking during the periods of stimulation revealed a series of inhibitory and excitatory acoustic influences in the PSEC, which were absent after local electrical stimulation of receptors in the periodontium or their afferents. By applying local anaesthesia to the periodontium of a mechanically stimulated tooth, the durations of the acoustic influences were on the average reduced by 76%. In subjects whose PSECs consistently included a second inhibitory period, the duration of the acoustic influences with respect to that of the PSEC (30%) was larger than otherwise (13%), suggesting a central gating of periodontal pathways which can block both periodontal and acoustic influences. The acoustic influences, of which the appearance in the PSEC largely depends upon activated periodontal pathways, represent a new finding of audio-motor reflexes.

Acoustic Stimulation↗
PubMed1988

Morphology of single mesencephalic trigeminal neurons innervating masseter muscle of the cat.

The morphology of functionally identified single axons of mesencephalic trigeminal neurons was studied in the cat by the method of intra-axonal injection of horseradish peroxidase (HRP). Each axon can be divided into united (U), peripheral (P) and central branches (C). The united axon (U) descends from its soma within the tract of the trigeminal mesencephalic nucleus to the dorsal aspect of the trigeminal motor nucleus (Vmo), where it splits into peripheral and descending central branches with a Y-shaped bifurcation. The peripheral axon (P) joins the motor root of the trigeminal nerve to exit the brainstem. The central axon (C) travels caudally within the juxtatrigeminal regions (or lateral reticular formation). All 3 branches issue axon collaterals that distribute terminal boutons within the dorsolateral subdivision of Vmo, supra- and intertrigeminal regions. Collaterals emanating from the central axon (C) except for its proximal segment travel ventrolaterally within the juxtatrigeminal regions, and send their terminal branches into the lateral boundaries adjacent to the spinal trigeminal nucleus. The trajectory of terminal branches distinguishes group Ia afferents from the possible group II afferents. The majority of terminal boutons are found to distribute in the supra- and intertrigeminal regions for group II afferent fibers and in the dorsolateral subdivision of Vmo for group Ia afferents.

Afferent Pathways↗
PubMed1989

Masseter inhibitory periods and sensations upon electrical tooth-pulp stimulation in children and adults.

Sensations and masseter inhibitory periods (MIP) evoked by electrical tooth-pulp stimulation were recorded in 5 adults and 10 children. In the adult group there was always a sensation and it was usually associated with a two-phase MIP, with an early and a late component. Sensory detection in teeth with partially formed roots in the younger children (7-9 years old) was markedly reduced, and the MIP was mostly monophasic with a predominantly early component. In the older children with fully formed roots (10-12 years old) sensations did not differ from those in the adults, but the late MIP component occurred in significantly fewer trials (P less than 0.01). It is concluded that response to electrical tooth-pulp stimulation develops in the following stages: (1) early, oligosynaptic MIP, (2) sensation, and (3) the late, polysynaptic component of the MIP.

Adult↗
PubMed1989

An HRP study of the central projections from primary sensory neurons innervating the rat masseter muscle.

Retrograde and transganglionic transport of horseradish peroxidase has been used to study the cell bodies of origin and the central projections of neurons innervating the rat masseter muscle. Labeled cell bodies were observed both in the trigeminal ganglion and in the mesencephalic trigeminal nucleus. Major central projections from mesencephalic trigeminal neurons were traced to the supratrigeminal nucleus and to the brainstem reticular formation. Smaller projections from these neurons could be followed to the borders of the solitary tract and hypoglossal nuclei as well as to lamina V of nucleus caudalis and corresponding areas in the dorsal horn at C1-C2 spinal cord segments. Labeling from trigeminal ganglion neurons was observed close to the trigeminal tract in all subdivisions of the trigeminal sensory nuclear complex and in the dorsal horn lamina I at C1 and C2 levels.

Animals↗
PubMed1989

Masseter inhibitory periods and sensations evoked by electrical tooth-pulp stimulation in subjects under hypnotic anesthesia.

Sensation and masseter inhibitory periods (MIP) to electrical tooth-pulp stimulation were recorded under hypnotic anesthesia and placebo to local anesthesia. In the first experiment, 8 subjects were tested for the effect of hypnotic anesthesia on sensory detection and MIP at non-painful stimulus levels (mean = 42.1 microA) and painful levels (mean = 86.5 microA). The percentage of detection for non-painful stimuli changed from 94.3% to 14.1% and for painful stimuli from 100% to 28%; both changes were significant (P less than 0.001). Hypnotic anesthesia blocked sensation without interrupting the initiation of the early component of the MIP, but did suppress its late component. In the second experiment, 8 subjects were tested for the perceived intensity of 5 levels of electrical tooth-pulp stimulation under hypnotic anesthesia and placebo. Sensory intensity was measured by the visual analog scale (VAS). Hypnotic anesthesia was significantly more effective than placebo (P less than 0.001) in reducing sensation. The differential effect of hypnotic anesthesia on the early and late component of the MIP lends further support to the hypothesis that hypnotic anesthesia operates primarily at suprasegmental, higher levels in the brain.

Adolescent↗
PubMed1983

Masseteric silent periods electrically evoked in normal subjects and patients with temporomandibular joint dysfunction.

The mean latency and duration of electromyographic silent periods were compared in normal subjects and patients with symptoms of temporomandibular joint (TMJ) dysfunction. The silent periods were evoked in the masseter muscle by electrical stimulation of the mental nerve area and recorded at 100% and 50% of maximum masseter EMG activity. Two silent periods were typically observed. The mean latency of the early silent period as well as the mean duration of both the early and late silent periods were shorter at 100% than at 50% of the maximal voluntary activity. In comparing the two groups, the mean latencies were shorter and the mean durations were longer for both silent periods in the TMJ group compared with the normal group. The prolonged silent periods in the TMJ group do not appear to be caused by a difference in stimulus intensity or level of voluntary activity in the masseter muscle. These results permit the speculation that the mechanism of the prolonged silent period in the TMJ patients might be central rather than peripheral.

Adult↗
PubMed1985

Effects of whole-body rotation on masseteric motoneuron excitability.

Vestibular stimulation is a popular clinical treatment for enhancing the excitability of spinal motoneurons innervating trunk and limb muscles, but whether vestibular stimulation can also influence trigeminal motoneurons is not known. We determined whether or not vestibular stimulation evoked by rotation of a seated subject would modify the excitability of masseteric motoneurons. The amplitude and frequency of occurrence of masseteric compound action potentials evoked by standard chin taps provided measures for assessing masseteric motoneuron excitability. Eleven healthy adults with no orofacial or otologic disorders served as subjects. Each sat in a motorized dental chair with his head stabilized by a halo head-piece so that chair rotation caused labyrinthine excitation. The frequency (3/s) of chin taps and their impact force were maintained constant by microcomputer control. After each tap, a 16-ms sample of EMG recorded from surface electrodes over the right masseter was digitized and stored for subsequent visual inspection. Only compound action potentials meeting rigorous criteria in terms of latency, amplitude, duration, and waveform were accepted as responses. The mean frequencies of occurrence and the mean amplitudes of the responses showed wide variability. Histogram displays of every response for each subject, however, revealed enhanced output from the masseteric motoneuron pool during the decleration and postrotation phases. In subjects not immediately retested this enhancement was persistent but decayed during the next 5 min. In five subjects the experiment was repeated after 1 min. The changes in response variables during phases 3 and 4 were significantly less than on the first trial, suggesting habituation. These results provide quantitative evidence that the dynamic input from vestibular ampullary receptors in response to rotation enhances masseteric motoneuron output.

Adult↗
PubMed1993

Rabbit masseter expresses the cardiac alpha myosin heavy chain gene. Evidence from mRNA sequence analysis.

The presence of myosin alpha heavy chain in the rabbit masseter has been previously suggested at the protein level [(1991) Basic App. Myol. 1, 23-34; (1991) Histochem. J. 23, 160-170]. To confirm this finding, we cloned most of the mRNA corresponding to the myosin heavy chain S2 subfragment. PCR analysis and subsequent nucleotide sequence determination of the amplified cDNA demonstrates the presence of a myosin alpha heavy chain mRNA in rabbit masticatory muscles.

Amino Acid Sequence↗
PubMed1980

Effect of occlusal retainers on temporomandibular joint and facial pain.

The etiology of TMJ syndrome is usually multifactorial. The anatomic structures which may be involved are the dentition, the masticatory muscles, and the TMJ itself. The etiologic region is usually the dentition and its occlusal relationships. The symptoms are usually minimized during treatment, and a high percentage are eliminated. Among the diagnostic symptoms, the deviation of the mandible to the opposite side from the occlusal interference is the most frequent objective symptoms. The lateral pterygoid muscle is usually involved if the patient has (1) occlusal interference and (2) lateral deviation of the mandible. The clinical response in this study to the occlusal splint and occlusal adjustment indicate that 88% of muscle involvement improved and that 84% of the TMJ syndrome responded favorably to the occlusal splint.

Bruxism↗
PubMed1980

Quantitative electromyographic diagnosis of myofascial pain-dysfunction syndrome.

The slopes of the V/T curves of masticatory muscles were determined in patients with MPD syndrome, and the changes in the slopes were observed prior to and during electrical muscle stimulation and splint therapy. The slopes of the V/T curves in patients with MPD syndrome were significantly steeper than those of the healthy subjects. In patients with MPD the slopes were decreased by electrical muscle stimulation of moderate duration and by insertion of the splint. These findings suggest that the most simple procedure for the diagnosis of MPD syndrome is to compare the slopes in MPD patients with those of healthy subjects. The observation of the changes in slopes during electrical muscle stimulation and splint therapy permits a more quantitative diagnosis of MPD syndrome. It also permits a quantitative measurement of improvement of symptoms in patients with MPD syndrome.

Adult↗
PubMed1980

Effect of occlusal splints on the electromyographic activities of masseter muscles during maximum clenching in patients with myofascial pain-dysfunction syndrome.

Integrated EMG activities of masseter muscles during maximum voluntary isometric contraction with and without full-arch maxillary stabilization splints were observed in patients with MPD syndrome having occlusal interferences and in healthy subjects having no occlusal interferences. The masseter muscle activity was more significantly reduced in patients with MPD syndrome during maximum clenching with splints than in those patients without splints. In healthy subjects, such a significant difference could not be observed with and without splints. This finding suggests that the elimination of the occlusal interferences by means of occlusal splints could reduce the degree of sensory information from the periodontal receptors during nocturnal clenching or grinding. This could result in a decrease in masseter muscle activity giving rise to muscular relaxation.

Action Potentials↗
PubMed1981

The changes in electrical activity of the postural muscles of the mandible upon varying the vertical dimension.

The relation between BT-EMG activity and variations of VD (1 to 41 mm of interocclusal distance) was studied in the masseter, anterior temporal, and posterior temporal muscles of eight normal experimental subjects. EMG activity was recorded with surface electrodes, and the VD was measured with a specially designed device. Recordings were performed with dynamic variations (series A) and with static variations (series B) of VD. Series A shows a gradual decrease of EMG activity starting from the occlusal position, passing through a range of maximum reduction at a certain interocclusal distance, and gradually increasing to the highest values close to maximum jaw opening. Series B shows the same progression, although it points out the exact VD at which minimum basal EMG activity is observed in each muscle studied (10 mm for the masseter muscle, 13 mm for the anterior temporal muscle, and 16 mm for the posterior temporal muscle).

Adult↗
PubMed1981

The effect of voluntary activity on the masseteric silent period duration.

This study examined both TMJ and normal groups for any changes in the masseteric silent period with variation of the subjects' occlusal force. An analog meter was used to give each subject feedback on how hard the teeth were clenched together. Five silent periods were elicited at each of the four levels of occlusal force from 40% to 100% of maximum force. Results showed a small decrease in silent period duration for both groups when occlusal force was increased.

Dental Occlusion, Centric↗