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PubMed1983

Cephalometric analysis of adaptations after lengthening of the masseter muscle in adult rhesus monkeys, Macaca mulatta.

This relationship between the function of the muscles of mastication and craniofacial form was investigated in young adult monkeys by increasing the functional length of the elevator muscles of the mandible non-invasively by a bite-opening splint cemented to the maxillary dentition. The major adaptations to increased vertical dimensional were (1) marked superior and some anterior displacement of the maxillary complex, (2) dental intrusion and (3) rotation of the mandible. These alterations produced a shortening of the lengthened masseter muscle, i.e. a reduction in the amount of muscle stretch brought about by the appliance. Migration of the masseteric insertion along the ramus did not contribute significantly to the pattern of adaptation. The role of the masseter muscle in craniofacial adaptations to altered vertical dimension was determined by detaching and re-attaching the insertion of the masseter muscle in one group of experimental animals. The myotomized monkeys experienced significantly less anterior displacement of the maxilla than the non-myotomized monkeys, indicating that the surgery may have lessened some of the anteriorly-directed distracting forces of the lengthened masseter. Masseter myotomy alone was not sufficient to eliminate the vertically-directed distracting forces of the remainder of the mandibular elevator muscles brought about by increasing the vertical dimension of the lower face.

Adaptation, Physiological↗
PubMed1988

A comparison of reflex depressions of activity in jaw-closing muscles evoked by intra- and peri-oral stimuli in man.

The effects on activity in the masseter muscle of applying electrical stimuli to discrete areas within the mouth or on peri-oral skin were studied electromyographically in 8 subjects. In all subjects, the intra-oral stimuli produced two phases of depressed masseteric activity with mean latencies of 14 ms and 47 ms. By contrast, this shorter latency response was generally not obtained with peri-oral stimuli: responses evoked from vermilion-border skin had an intermediate latency (mean 25 ms) and consisted of either one or two periods of depressed activity, while the responses to stimulation of the hairy skin of the lip generally consisted of a single phase of depressed activity, the time course of which was similar to the later phase produced by intra-oral stimuli (mean latency, 48 ms). These findings were consistently obtained with both single and short trains of electrical stimuli and contrast with reports that intra- and peri-oral stimuli produce similar reflexes. The finding that the short-latency reflex depression of masseteric activity could be evoked only by stimulating nerves supplying intra-oral tissues is consistent with the notion that this reflex may play a role in functions such as mastication, and indeed may indicate that the reflex exists specifically for such a purpose.

Adult↗
PubMed1988

Mechanical capabilities of the human jaw muscles studied with a mathematical model.

The human muscles of mastication have complex shapes with large attachment areas. This suggests a variety of bite force directions and magnitudes. The possible range of these and the concomitant joint force of each individual muscle were determined by a mathematical model describing static equilibrium conditions in the sagittal plane. The range of force directions for each muscle was defined by the action lines of the most anterior and most posterior (for the lateral pterygoid, most superior and most inferior) muscle fibre bundles. Calculations from the various directions of the reaction force in the temporomandibular joint demonstrated that each muscle can produce a unique variety of bite force directions. Except for the lateral pterygoid and posterior temporalis, the range and orientation of possible bite forces was closely related to the orientation of the joint force. In general, at the canine tooth the bite forces were directed more posteriorly than at the second molar. Within a muscle, distinct portions may produce considerably different bite force magnitudes; the largest bite forces are produced at horizontal and vertical joint force directions. The posterior portions of the deep masseter and temporalis muscles and the lateral pterygoid muscle have the largest mechanical advantage. In the majority of muscles the magnitude of the joint reaction force is smallest at an oblique joint force direction.

Aged↗
PubMed1989

Comparison of jaw-opening reflex response elicited by anterior and posterior tooth tapping in the cat.

The response elicited by various levels of controlled mechanical stimuli to maxillary teeth was studied in 8 adult cats anaesthetized with alpha-chloralose. When an anterior tooth was tapped, short (4-6 ms) and long (16-20 ms) latency responses were recorded from the mylohyoid nerve. On the other hand, when a posterior tooth was tap-stimulated, only the long-latency response was induced. The effects of ether narcosis on the responses were studied. As the short-latency response was induced only by anterior tooth tapping and its amplitude increased according to increases in stimulus intensity, the response may be related to the protection of the mouth from entrance of harmful materials. The long-latency response was induced by light tapping on both anterior and posterior teeth and its amplitude increased according to increases in stimulus intensity to the posterior teeth rather than the anterior teeth. As it was readily inhibited by ether, the long-latency response may have a reflex centre in an area like the brain stem reticular formation which is particularly sensitive to ether and may be related to the regulation of jaw movement in mastication.

Action Potentials↗
PubMed1991

Changes in mandibular condyle morphology related to tooth wear in a prehistoric human population.

The exact relationship between tooth wear and changes in the shape of the articulating surface of the mandibular condyles is unknown. It is assumed that to some degree these changes are in response to the loads on the joint generated during chewing. Excavated skeletal remains from sites of primitive human habitation provide an opportunity to study the masticatory system of human beings in their natural context, with the features of form and function expressed to their full potential. Accurate measurements of the condylar head can also be made on such remains. A collection of skulls from an excavation site known as Oakhurst on the South African coast provided material for this study. Occlusal wear was measured using the scale devised by Molnar; approximal wear was assessed by measuring the size of the wear facet between the first and second molars. The width of the condyles were measured directly, and a method was devised to measure changes in the proportion of medial and lateral articulating facets. An increase in the size of the lateral facet of the condyle was found to be associated with increased tooth wear. It is suggested that the direction of condylar remodelling may give an indication of the reaction at the joint to optimal mastication.

Adult↗
PubMed1991

Ultrastructural changes in the masseter muscle of Macaca fascicularis resulting from intramuscular injections of botulinum toxin type A.

Intramuscular injections of botulinum toxin type A (Oculinum) is used to treat strabismus and focal dystonias affecting orofacial muscles. However, the toxin-induced morphological changes that underlie the therapeutic alterations of tone in the muscles of mastication have not been described. In this study, paired intramuscular injections of botulinum toxin (10 units) were made in three adult monkeys (Macaca fascicularis) allowed to survive 14, 28 and 63 days. Another monkey received multiple injection-pairs over 84 days. Animals were killed by deep pentobarbital anaesthesia before transcardiac perfusion-fixation. Tissue sampled from comparable regions of the injected masseter, the uninjected masseter and an uninjected animal was processed for ultrastructural analysis. Few changes were found 14 days post-injection. However, muscle fibres showed myofibrillar dissolution, aberrations in the Z-line, and enlarged mitochondria in the region of the I-band by 28 days. In the 63-day and 84-day animals, the injected muscle was considerably smaller than the uninjected, contralateral muscle. Regions of the injected muscle contained fibres with markedly reduced cross-sectional area. Internalization of myonuclei, loss of myofibrillar organization, and helical complexes were common. Toxin-induced changes, though similar to those that follow denervation by axotomy, were not accompanied by degeneration of neuromuscular junctions. Instead, morphological evidence for axonal sprouting in the region of the neuromuscular junction, possibly contributing to functional recovery, was seen as early as 14 days in toxin-treated muscles.

Actin Cytoskeleton↗
PubMed1992

Masticatory muscle fatigue: endurance times and spectral changes in the electromyogram during the production of sustained bite forces.

The purpose of this experiment was to determine if the characteristics of fatigue observed in the limb muscle system are also evident in the muscles of mastication, specifically, the masseter and temporalis. Surface electrodes were placed bilaterally over the masseter and temporalis muscles of 10 adults. Each subject was instructed to maintain maximal and six levels of submaximal incisal bite forces for as long as possible. The power density spectrum of the electromyographic signals for each muscle was calculated at the onset and failure of the task. The decrease in endurance time with an increase in bite force followed a pattern similar to that in limb muscles, and each muscle was characterized by a consistent reduction in the mean power frequency of the power density spectrum and a variable change in r.m.s. power. The variability of changes in r.m.s. power (which is inconsistent with changes found in the limb muscles) was explained in terms of either changes in motor-unit firing rate or the muscle's relative contribution to the generation of bite force. The analyses also demonstrated a curvilinear relationship between bite force and r.m.s. power, but no relationship between bite force and the mean power frequency of the power density spectrum.

Adult↗
PubMed1983

A crossed rubrobulbar projection in the snake Python regius.

In the present study a distinct crossed rubrobulbar projection has been demonstrated in the snake Python regius, a limbless vertebrate which lacks a rubrospinal tract. This rubrobulbar projection is presumably involved in the neural control of mastication. The red nucleus may relay cerebellar influence to the trigeminal and facial nuclei.

Animals↗
PubMed1985

Physiological properties of intradental mechanoreceptors.

A major role of tooth receptors in signaling overt or impending tissue damage (nociception) has been previously established by substantial evidence from mechanical, thermal and chemical stimulation of exposed dentin. We report evidence showing that some intradental receptors in canine teeth of the cat detect mechanical transients applied to intact enamel. This new finding suggests that dental innervation may play an important non-nociceptive role in oral function such as detecting tooth contact during mastication and swallowing.

Animals↗
PubMed1988

Functional [14C]2-deoxyglucose mapping of progressive states of status epilepticus induced by amygdala stimulation in rat.

Electrical stimulation of rat amygdala induced self-sustained steady-state seizures (status epilepticus (SE] within 60 min. These SE states varied in behavioral severity from mere alteration of motility to frank clonic convulsions. Four distinct behavioral states were observed: immobility, exploration, mastication and clonus. These SE states were associated with [14C]2-deoxyglucose (2-DG) autoradiography anatomic patterns that were correspondingly more extensive and complex. Four distinct 2-DG activation patterns were observed: a restricted pattern involving several discrete limbic nuclei, including amygdala; more extensive patterns involving numerous limbic areas, first unilaterally, then bilaterally; finally the most extensive pattern involving widespread areas of forebrain. These data imply a systematic progression of seizure activity: originating in the amygdala, then spreading to some direct amygdala projection areas, and from there to a restricted network of interconnected ipsilateral limbic nuclei. This restricted network then recruits most of the remaining limbic structures, first ipsilaterally, then contralaterally. Finally, most of the basal ganglia, thalamus and neocortex are recruited.

Amygdala↗
PubMed1989

Limbic seizures without brain damage after injection of low doses of kainic acid into the amygdala of freely moving rats.

Kainic acid (KA, 8-15 ng) was injected into the amygdala of conscious freely moving rats via chronically implanted fused silica cannulas. At 15-25 min after the injection, most rats suffered a limbic seizure attack of short duration, consisting of mastication, forelimb clonus, and raising on hind limbs, behaviorally indistinguishable from kindled seizures. Typically, the attack was followed by stereotypies, intense exploration, and by 1 or 2 more attacks. About 60 min after the injection, most rats appeared normal again and histopathological changes in their brains did not exceed those seen in vehicle-injected rats. In 3 cases, however, recurrent seizures culminated in behavioral status epilepticus 60-90 min after the injection. The status epilepticus was stopped by i.p. injection of diazepam (10 mg/kg) after a duration of 10 min (1 case) and 30 min (2 cases), respectively. After 10 min status epilepticus, we observed marginal neuronal damage with slight gliosis in both hippocampi (CA3 and CA1); after 30 min, hippocampal histopathology was more pronounced, with additional necrosis of the ipsilateral piriform cortex. After 0.8 microgram KA, a hundredfold higher dose, the incidence of limbic seizures during the first 40 min was not significantly higher (9/12) than after the lower KA doses (13/19). However, a significantly higher proportion of rats exhibited long-lasting seizure activity, associated with confluent destruction of CA3 pyramidal cells and additional seizure-related brain damage. Our results show that limbic motor seizures do not inevitably lead to histopathological changes in the brain, provided they do not culminate in a state of permanent seizure activity.

Amygdala↗
PubMed1990

Differences in physical dependence induced by selective mu or delta opioid agonists and by endogenous enkephalins protected by peptidase inhibitors.

The aim of the present study was to investigate if a physical dependence could be induced by chronic activation of the endogenous enkephalinergic system. We have therefore evaluated naloxone-induced withdrawal syndrome in rats after central infusion during 7 days of comparable antinociceptive doses of RB 38 A ((R,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a mixed enkephalin catabolism blocker and of the selective mu, DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) and delta, DSTBULET (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr), opioid agonists. The responses were compared to those induced by RB 38 B ((S,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a selective inhibitor of the 24.11 neutral endopeptidase (NEP) 'enkephalinase'. DAGO induced a severe withdrawal syndrome evidenced by a large weight loss, hypothermia, jumping, mastication, teeth chattering, diarrhoea, lacrimation and salivation. In contrast, DSTBULET and RB 38 A produced only a moderate physical dependence. Only two signs were statistically different in these two groups: wet dog shakes and temperature. Chronic i.c.v. administration of DAGO, DSTBULET and RB 38 A produced a time-dependent reduction in analgesia, but 120 h after continuous infusion only RB 38 A was able to still induce a significative antinociceptive effect. The present data suggest that even in the drastic conditions used here long-term complete inhibition of enkephalin catabolism induces a weak tolerance and a moderate physical dependence, similar to that produced by delta opioid agonists. This effect was not observed after chronic selective inhibition of NEP by RB 38 B.

Animals↗
PubMed1992

Modification of medullary respiratory-related discharge patterns by behaviors and states of arousal.

The modulatory influences of behaviors and states of arousal on bulbar respiratory-related unit (RRU) discharge patterns were studied in an unanesthetized, freely behaving guinea pig respiratory model system. When fully instrumented, this model system permits concurrent monitoring and recording of (i) single units from either Bötzinger complex or nucleus para-ambiguus; (ii) electrocorticogram; and, (iii) diaphragmatic EMG. In addition to being used in surveys of RRU discharge patterns in freely behaving states, the model system also offered a unique opportunity in investigating the effects of pentobarbital on RRU discharge patterns before, throughout the course of, and during recovery from anesthesia. In anesthetized preparations, a particular RRU discharge pattern (such as tonic, incrementing or decrementing) typically displayed little, if any notable variation. The most striking development following pentobarbital was a state of progressive bradypnea attributable to a significantly augmented RRU cycle duration, burst duration and an increase in the RRU spike frequencies during anesthesia. In freely behaving states, medullary RRU activities rarely adhered to a fixed, immutable discharge pattern. More specifically, the temporal organization (such as burst duration, cycle duration, and the extent of modulation of within-burst spike frequencies) of RRU discharge patterns regularly showed complex and striking variations, not only with states of arousal (sleep/wakefulness, anesthesia) but also with discrete alterations in electrocorticogram (ECoG) activities and a multitude of on-going behavioral repertoires such as volitional movement, postural modification, phonation, mastication, deglutition, sniffing/exploratory behavior, alerting/startle reflexes. Only during sleep, and on occasions when the animal assumed a motionless, resting posture, could burst patterns of relatively invariable periodicity and uniform temporal attributes be observed. RRU activities during sniffing reflex is worthy of further note in that, based on power spectrum analyses of concurrently recorded ECoG activities, this particular discharge pattern was clearly associated with the activation of a 6-10 Hz theta rhythm. These findings indicated that bulbar RRU activity patterns are subject to change by not only behaviors and sleep/wakefulness cycles, but also a variety of modulatory influences and feedback/feedforward biases from other central and peripheral physiological control mechanisms.

Animals↗
PubMed1988

Giant cell arteritis with pulmonary involvement.

Giant cell arteritis (GCA) may present as pyrexia of unknown origin with profuse night sweats, pain on mastication, headache, pain in the region of the temporal arteries, polymyalgia rheumatica, myocardial infarction or dissecting aortic aneurysm. Few cases with pulmonary involvement have been described. We report a patient with temporal arteritis preceded by pulmonary vascular disease.

Aged↗
PubMed1982

Spontaneous fracture of an atrophic endentulous mandible treated without fixation.

The positive clinical decision to treat a spontaneously fractured edentulous mandible passively depends on a careful evaluation of local and general factors pertaining. These include: (1) Preferably simple fractures with minimal displacement and an intact periosteal sheath. (2) Elderly and infirm patients and others in whom surgery is contra-indicated. (3) Case in which discomfort and inconvenience with fixation is likely to be worse than without, bearing in mind that fixation does not guarantee firm bony union and that many patients wear dentures and masticate successfully in the presence of complete failure of fragments to unite. It must be stressed that it is not suggested that this line of treatment be followed routinely, but that is is valid in certain selected cases.

Aged↗
PubMed1982

Extra-articular ankylosis in fibrodysplasia ossificans progressiva.

Forty-four patients with fibrodysplasia ossificans progressiva were identified by a national survey in the United Kingdom. All patients had diagnostic skeletal malformations in addition to ectopic ossification in voluntary muscles and ligaments. Twenty-three of 34 patients (68 per cent) who were traced and examined had limitation of mandibular movement due to ectopic ossification in the muscles of mastication. In three cases such involvement was precipitated by dental therapy and in a further three by other trauma. Operative treatment of the established case was ineffective. Thus management must aim to avoid unnecessary jaw trauma.

Adolescent↗
PubMed1995

Co-treatment with MK-801 potentiates naloxone-precipitated morphine withdrawal in the isolated spinal cord of the neonatal rat.

The effects of acute and chronic administration of (MK-801: [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate) were assessed on morphine dependence in the isolated spinal cord of the neonatal rat and on behavioral measures in intact adult rats. Neonatal rats were treated chronically (3 or 4 days) with injections of either morphine, morphine + MK-801, or saline. Naloxone (10 microM) which increased baseline ventral root spontaneous firing, induced more activity in spinal cords from morphine-treated neonates than in saline controls. In spinal cords from neonates receiving MK-801 with morphine, naloxone-induced spontaneous firing was significantly greater than in saline-treated and morphine alone-treated neonates. Acute MK-801 attenuated naloxone-induced firing in the morphine-treated group. Chronic co-treatment with MK-801 increased locomotor signs of withdrawal and decreased mastication in intact adult rats which had been treated chronically with morphine. MK-801-induced enhancement of morphine withdrawal is consistent with upregulation of NMDA receptors.

Animals↗
PubMed1985

Reticular formation neurons related to tongue movement in the behaving cat.

We have found a number of cells related to tongue movement in the medial brain stem reticular formation of the unanesthetized cat. These cells constituted less than 2% of the cells tested in this region and were distributed throughout several nuclei in the medulla and pons including nucleus reticularis pontis caudalis, nucleus reticularis gigantocellularis, and the border between nucleus reticularis paramedianus and nucleus interfascicularis hypoglossi. All observed tongue movement cells (N = 6) fired maximally during protrusive tongue movements. One medullary cell discharged primarily during the protrusive tongue movement to the ipsilateral side, whereas no lateral preference was detected in the other cells. Gustatory and mechanosensory stimulation of the tongue was unnecessary for inducing discharge in these cells. Tongue movement-related cells shared several characteristics that differentiated them from adjacent reticular formation cells, including absence of response to startle-inducing auditory stimuli and low levels of spontaneous waking and sleep activity. In two pontine cells located near the trigeminal motor nucleus, spike-triggered averages of tongue EMG revealed a short-latency (5 ms) inhibitory effect on the ipsilateral genioglossus muscle by the units' discharge. We suggest that neurons of this type might be involved in tongue-jaw coordination during mastication, licking, and grooming.

Action Potentials↗