GABAergic control of action potential propagation along axonal branches of mammalian sensory neurons.
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The main axons of mammalian sensory neurons are usually viewed as passive transmitters of sensory information. However, the spindle afferents of jaw-closing muscles behave as if action potential traffic along their central axons is phasically regulated during rhythmic jaw movements. In this paper, we used brainstem slices containing the cell bodies, stem axons, and central axons of these sensory afferents to show that GABA applied to the descending central (caudal) axon often abolished antidromic action potentials that were elicited by electrical stimulation of the tract containing the caudal axons of the recorded cells. This effect of GABA was most often not associated with a change in membrane potential of the soma and was still present in a calcium-free medium. It was mimicked by local applications of muscimol on the axons and was blocked by bath applications of picrotoxin, suggesting activation of GABA(A) receptors located on the descending axon. Antidromic action potentials could also be blocked by electrical stimulation of local interneurons, and this effect was prevented by bath application of picrotoxin, suggesting that it results from the activation of GABA(A) receptors after the release of endogenous GABA. We suggest that blockage is caused mainly by shunting within the caudal axon and that motor command circuits use this mechanism to disconnect the rostral and caudal compartments of the central axon, which allows the two parts of the neuron to perform different functions during movement. (+info)
Stresses on the cervical column associated with vertical occlusal alteration.
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The biomechanical effects on cervical vertebral columns (C1-C7) during mastication were calculated using a three-dimensional (3D) finite element method. To verify the biomechanical influences of vertical occlusal alteration to the cervical column, three finite element models (FEM) showing a normal (model A), a steep (model B), and a flat occlusal plane (model C) were constructed. The occlusal stress distribution showed various patterns for the three models; the stress extended to the anterior area as the occlusal plane became steeper. The plots of the stresses on the mid sagittal section of the cervical columns showed different patterns for the three models; the stress converged at the odontoid process in models A and B, whereas the stresses at C7 in model B tended to decrease compared with model A. Concentrated stress was observed at C5 in model C, supporting the hypothesis that vertical occlusal alteration could influence stress distribution in the cervical columns. (+info)
Neurobiological mechanisms involved in sleep bruxism.
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Sleep bruxism (SB) is reported by 8% of the adult population and is mainly associated with rhythmic masticatory muscle activity (RMMA) characterized by repetitive jaw muscle contractions (3 bursts or more at a frequency of 1 Hz). The consequences of SB may include tooth destruction, jaw pain, headaches, or the limitation of mandibular movement, as well as tooth-grinding sounds that disrupt the sleep of bed partners. SB is probably an extreme manifestation of a masticatory muscle activity occurring during the sleep of most normal subjects, since RMMA is observed in 60% of normal sleepers in the absence of grinding sounds. The pathophysiology of SB is becoming clearer, and there is an abundance of evidence outlining the neurophysiology and neurochemistry of rhythmic jaw movements (RJM) in relation to chewing, swallowing, and breathing. The sleep literature provides much evidence describing the mechanisms involved in the reduction of muscle tone, from sleep onset to the atonia that characterizes rapid eye movement (REM) sleep. Several brainstem structures (e.g., reticular pontis oralis, pontis caudalis, parvocellularis) and neurochemicals (e.g., serotonin, dopamine, gamma aminobutyric acid [GABA], noradrenaline) are involved in both the genesis of RJM and the modulation of muscle tone during sleep. It remains unknown why a high percentage of normal subjects present RMMA during sleep and why this activity is three times more frequent and higher in amplitude in SB patients. It is also unclear why RMMA during sleep is characterized by co-activation of both jaw-opening and jaw-closing muscles instead of the alternating jaw-opening and jaw-closing muscle activity pattern typical of chewing. The final section of this review proposes that RMMA during sleep has a role in lubricating the upper alimentary tract and increasing airway patency. The review concludes with an outline of questions for future research. (+info)
Maximum bite force, muscle efficiency and mechanical advantage in children with vertical growth patterns.
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This study correlated maximum bite force and masticatory muscle electromyography (EMG) activity with craniofacial morphology and mechanical advantage of children with vertical growth patterns. From lateral cephalograms of 30 females and 17 males (9.3 +/- 3.6 years of age), 13 morphological and eight biomechanical measurements were recorded. Two maximum bite forces and 12 submaximal bite forces along with their associated EMG muscle activity were recorded at the right mandibular first molar. Muscle efficiency was evaluated using the relationship between bite forces and EMG activity levels. There were no significant sex differences (P > 0.05) for any of the morphological, functional or biomechanical variables. Factor analyses reduced: (1) the 13 morphological variables to four factors explaining 82.8 per cent of the morphological variance; (2) six functional variables to two factors explaining 68.8 per cent of the functional variance; and (3) 11 biomechanical variables to three factors explaining 90.9 per cent of the biomechanical variance. The vertical size factor was negatively correlated with the muscle efficiency factor (r = -0.39; P = 0.006) and positively correlated with the moment arm factor (r = 0.67; P < 0.001). The morphological divergence factor was negatively correlated with the bite force factor (r = -0.34; P = 0.019) and the mechanical advantage factor (r = -0.32; P = 0.028). The muscle efficiency factor (functional) was negatively correlated with the moment arm factor (r = -0.33; P = 0.023). It is concluded that: (1) independent of chronological age, children with larger faces have larger moment arms and require less muscle activity to attain any given force, and (2) greater hyperdivergence is related to poorer mechanical advantage and lower maximum bite force. These data support the relationships between bite force, muscle strength and morphology in children, similar to those reported for adults. (+info)
Temporomandibular disorders, occlusion and orthodontic treatment.
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OBJECTIVES: To prospectively and longitudinally study symptoms and signs of temporomandibular disorders (TMD) and occlusal changes in girls with Class II malocclusion receiving orthodontic fixed appliance treatment in comparison with untreated Class II malocclusions and with normal occlusion subjects. DESIGN: Prospective observational cohort. SUBJECTS: Sixty-five girls with Class II malocclusion who received orthodontic treatment, 58 girls with no treatment, and 60 girls with normal occlusion. METHOD: The girls were examined for symptoms and signs of TMD and re-examined 2 years later. Additional records were taken in the orthodontic group during active treatment and 1 year after treatment RESULTS: All three groups included subjects with more or less pronounced TMD, which showed individual fluctuation during the ongoing study. In the orthodontic group, the prevalence of muscular signs of TMD was significantly less common post-treatment. Temporomandibular joint clicking increased in all three groups over the 2 years, but was less common in the normal group. The normal group also had a lower overall prevalence of TMD than the orthodontic and the Class II group at both registrations. Functional occlusal interferences decreased in the orthodontic group, but remained the same in the other groups over the 2 years. CONCLUSIONS: (i) Orthodontic treatment either with or without extractions did not increase the prevalence or worsen pre-treatment symptoms and signs of TMD. (ii) Individually, TMD fluctuated substantially over time with no predictable pattern. However, on a group basis, the type of occlusion may play a role as a contributing factor for the development of TMD. (iii) The large fluctuation of TMD over time leads us to suggest a conservative treatment approach when stomatognathic treatment in children and adolescents is considered. (+info)
Myasthenia gravis and masticatory muscle myositis in a dog.
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A 21-month-old, castrated male Vizsla was presented for pelvic limb weakness, difficulty opening his mouth, ptyalism, voice change, and urinary incontinence. Myasthenia gravis and masticatory myositis were diagnosed. The unusual clinical findings, diagnosis, treatment, and case outcome are described, followed by a brief discussion of myasthenia gravis and masticatory myositis. (+info)
Load compensation in human masseter muscles.
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The reaction of masseter and digastric muscles to changes in the load applied during a movement was studied in human volunteers. 2. Rapid stretch of the jaw-closing muscles evoked a monosynaptic jaw-jerk reflex response in the masseteric electromyogram (e.m.g.) after a delay of 6-8 msec. 3. The averaged integrated e.m.g. activity of the masseter was increased 5-10 msec after the rapid application of a load during voluntary closure of the jaw. It was not necessary to stretch the muscle to obtain this apparently monosynaptic response. 4. A compensatory increase in the velocity of movement followed 6-10 msec after the e.m.g. peak. 5. Unloading the masseter muscle during contraction was followed by a fall in e.m.g. activity, beginning after a latency of 6-5--1 msec, with later phases of depression beginning 27--36 and 60--70 msec after unloading. 6. The spindle-poor digastric muscles were not monosynaptically activated during loading, but responded with a latency of 24--34 msec. 7. It is concluded that monosynaptic e.m.g. changes are a prominent feature of the load compensation system in human jaw-closing muscles but not in jaw-opening muscles. (+info)
Analysis of activity of muscle spindles of the jaw-closing muscles during normal movements in the cat.
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Recordings have been made of afferent activity from spindles of the jaw-closing muscles, together with jaw movement and e.m.g. from temporalis and masseter in conscious, unrestrained cats. 2. In the twenty-nine units studied, the pattern of spindle behaviour observed during eating and lapping was generally what might be expected of stretch receptors. Maximal firing frequencies were found during opening of the mouth (lengthening), while during active closing the discharge was progressively reduced or abolished. Nevertheless, changes in the relation of stretch to firing frequency in different movements indicated that fusimotor drive was not constant. 3. spindle afferents could be divided into two groups on the basis of their maximal firing frequency during eating. "High-frequency" units (range 240-600 impulses/sec) showed pronounced velocity sensitivity, which supports the proposal that they correspond to spindle primaries. 'Low-frequency" units (range 80-200 impulses/sec) showed predominantly length sensitivity and probably correspond to secondary endings. 4. Length sensitivity of low-frequency units was considerably greater in lapping movements than in eating, indicating increased static fusimotor drive in the former. Sensitivity in the opening phase of eating was indistinguishable from that recorded in deeply anaesthetized animals. 5. High-frequency units were generally silenced immediately active shortening commenced. 6. No simple relationship existed between temporalis or masseter e.m.g. and spindle firing. 7. These results imply that normal masticatory movements are not initiated or driven to any appreciable extent via the fusimotor route. Close alpha-gamma co-activation is not a feature of this situation. On the other hand, in some other movements, such as licking the lips, fusimotor drive could fluctuate so as largely to cancel the unloading effects of active muscle shortening. (+info)