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Biomedical subjects

C G Widmer

Publications and source records attributed to C G Widmer.

At least 19 recordsLinked to original sources
PubMed2006

Developmental and functional considerations of masseter muscle partitioning.

The masseter muscle participates in a wide variety of activities including mastication, swallowing and speech. The functional demands for accurate mandibular positioning and generation of forces during incising or a power stroke require a diverse set of forces that are determined by the innate muscle form. The complex internal tendon architecture subdivides the masseter into multiple partitions that can be further subdivided into neuromuscular compartments representing small motor unit territories. Individual masseter compartments have unique biomechanical properties that, when activated individually or in groups, can generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint. The myosin heavy chain (MyHC) fibre-type distribution in the adult masseter is sexually dimorphic and is influenced by hormones such as testosterone. These testosterone-dependent changes cause a phenotype switch from slower to faster fibre-types in the male. The development of the complex organization of the masseter muscle, the MyHC fibre-type message and protein expression, and the formation of endplates appear to be pre-programmed and not under control of the muscle nerve. However, secondary myotube generation and endplate maturation are nerve dependent. The delayed development of the masseter muscle compared with the facial, tongue and jaw-opening muscles may be related to the delayed functional requirements for chewing. In summary, masseter muscle form is pre-programmed prior to birth while muscle fibre contractile characteristics are refined postnatally in response to functional requirements. The motor control mechanisms that are required to coordinate the activation of discrete functional elements of this muscle remain to be determined.

Animals↗
PubMed2003

Differential activation of neuromuscular compartments in the rabbit masseter muscle during different oral behaviors.

The rabbit masseter muscle is composed of multiple anatomical partitions that produce different mechanical actions. The purpose of this study was to test the hypothesis that these compartments are differentially activated during the performance of different oral behaviors. Rhythmic activation of the masticatory muscles was elicited by stimulating the cortical masticatory area (CMA) while recording forces generated at the incisors in three dimensions with the mandible immobilized. Torques about the right temporomandibular joint (TMJ) were calculated using these forces recorded during isometric function. A set of 1-15 unique rhythmic behaviors was identified for each rabbit using torque phase plot patterns. Electromyographic recordings were made at nine different compartments in the right masseter, two compartments in the left masseter, two regions in the right digastric, and single locations in the left digastric and right and left medial pterygoid muscles. In activation cycles producing similar mechanical actions, activity patterns at the 16 recording sites were clustered into three to six groups using principal component analysis (PCA). To test for similarities in the activation of masseter compartments, pair-wise comparisons of the PCA assignment for the nine masseter compartments were conducted and frequencies of common assignment were compiled for each unique rhythmic behavior for each rabbit. Masseter muscle compartments were found to vary significantly in their PCA from the expected distribution of 100% common principal component (PC) assignment (i.e., similar activation pattern). This finding is consistent with the independent activation of masseter compartments.

Animals↗
PubMed2002

Modeling rabbit temporomandibular joint torques during a power stroke.

Little information exists regarding the effects of changes in mandibular form as a result of orthognathic surgery on torques produced about the temporomandibular joint (TMJ). In this study, we have modeled torques produced about the working side TMJ by selected compartments of the rabbit masseter muscle based on published electromyographic activity. The masseter muscle is composed of multiple subregions or compartments that have unique mechanical actions. In a previous study, forces were elicited by electrical stimulation of each compartment and were recorded by a multiaxis force transducer attached to the anterior mandible. Torques were calculated using mandibular lever arms measured from the center of the TMJ. We have extended this modeling to include variations in mandibular width, length, or height to determine the torques that would be generated with variations in mandibular form. Three superficial masseter compartments on the working side and one posterior deep compartment from the balancing side masseter were examined using data collected from a companion study. It was found that the working and balancing side compartments were synergists for pitch torque components but were antagonists for roll and yaw. In modeling an increase of each mandibular dimension by 20%, nonuniform changes in compartment-generated torques were found. The largest increase was found for the posterior superficial masseter yaw torque component. The effects of changing mandibular form on torques produced about the TMJ may be greater than predicted by previous models that assumed a single line of force produced by each jaw muscle.

Analysis of Variance↗
PubMed2002

Evidence for functional partitioning of the rabbit digastric muscle.

The rabbit digastric muscle has a single belly that opens and retracts the mandible. It does not contain connective tissue partitions, and all fibers arise from the same tendon and insert into a single broad site. Historically, it was assumed that the muscle functioned as a single unit. Since we had preliminary evidence that this might not be the case, we carried out five small studies in rabbits. First, we showed that electromyographic (EMG) activity varies between recording sites within the muscle during the masticatory cycle induced by repetitive stimulation of the sensorimotor cortex. We found that EMG activity in the caudal region sometimes began before the anterior EMG during mastication when the jaw swung to the side of the muscle, but the two regions became active at the same time during other patterns. We next showed that separate branches of the mylohyoid nerve enter the anterior, intermediate and caudal regions of the digastric. However, a separate study showed that the motor endplates were distributed across a continuous sheet, consistent with a single anatomical partition. We then stimulated single nerve branches to deplete glycogen. By comparing the optical density of fibers labeled by the periodic acid-Schiff method for glycogen, we were able to show that the three branches innervate separate regions of the muscle. Finally, we applied either FluoroGold or Fast Blue dyes to the central cut ends of the branches to label the cell bodies of the three pools of motoneurons. These were found within the middle and caudal thirds of the trigeminal motor nucleus, but there appeared to be no spatial separation of the three pools or double labeling of cells. We conclude that the digastric muscle contains two and possibly three functional subregions. The fact that the motoneurons are intermingled suggests that the distribution of motor commands to the three pools is not based on their location.

Animals↗
PubMed2002

Spatial distribution of myosin heavy-chain isoforms in mouse masseter.

There is a paucity of information regarding the anatomy and muscle fiber phenotype of the masseter. The objective of this study was to characterize the distribution of each myosin heavy-chain (MyHC) isoform within different anatomical regions of male and female mouse masseters. Masseters from male and female CD-1 mice (2-4 months old) were examined for description of the anatomical partitioning of muscle fibers and endplate distribution. The spatial distribution of MyHC isoforms--embryonic, neonatal, slow, alpha-cardiac, IIa, and IIb--was determined within the defined masseter partitions by means of Western blot analysis and immunofluorescent localization. Types IIa, IIx, and IIb were the predominant MyHC isoforms observed. Distinct differences in the spatial distribution of these MyHC isoforms were found between muscle regions and varied between sexes. The regionalization of muscle fiber types in the mouse masseter is consistent with the functional compartmentalization of the masseter observed in other species.

Animals↗
PubMed1998

Development of trigeminal mesencephalic and motor nuclei in relation to masseter muscle innervation in mice.

This study evaluated temporospatial changes in the central organization of trigeminal mesencephalic (mesV) and motor (motV) nuclei during their development. Very little is known regarding the timing of formation of these trigeminal nuclei and the role that target tissue interactions may have on their spatial organization. Cells located in motV innervate muscles of mastication while the mesV nucleus contains populations of primary afferent cells that innervate muscle spindles in jaw closing muscles and periodontal mechanoreceptors around the roots of teeth. To label mesV afferents and motV efferents during their development, lipophilic dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) or 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA), were inserted into various jaw muscles, the dorsomedial mesencephalic region or tooth buds of maxillary or mandibular teeth of embryonic or postnatal mouse pups. Parasagittal sections were evaluated under epifluorescence to determine the temporospatial organization of trigeminal nuclei and the timing of outgrowth of their processes to target tissues. Early in development, motV motoneurons were organized in columns or clusters closely associated with groups of motoneuron axon fascicles that innervated a specific muscle. Double labelling of masseter and temporalis muscles showed that the columns containing motoneurons associated with these muscles were interdigitated early in development but later condensed into respective motoneuron pools. In contrast, no spatial organization of mesV afferent cell bodies was observed throughout the developmental sequence examined. The results of this study also demonstrated that motV processes enter into jaw muscle at least 1 day prior to proprioceptive afferents. MesV afferent appearance at the tooth was further delayed by 10 days suggesting different signaling mechanisms for these two targets.

Animals↗
PubMed1997

Anatomical organization of efferent neurons innervating various regions of the rabbit masseter muscle.

Previous studies have shown that the masseter muscle is supplied by motoneurons located in the anterodorsal region of the trigeminal motor nucleus and by an additional group of efferent neurons located in cell group k. The present experiments were performed on nine rabbits and were designed to establish the locations of neurons innervating the different regions of this muscle. Retrograde labeling with two fluorescent tracers (FluoroGold and Fast Blue) was applied to the central ends of cut branches of the masseter nerve. Serial coronal sections of the brainstem were viewed with fluorescence microscopy. The labeled cells were counted in all animals, and three-dimensional reconstructions of their distribution were made in five cases. In each successful experiment, labeled neurons were seen in the anterodorsal region of the trigeminal motor nucleus and in the two dorsal cell columns of cell group k (k1 and k3). Within-animal comparisons of the median position of populations innervating two distinct muscle regions in five rabbits showed that there were no significant differences in either the dorsoventral or rostrocaudal axes. However, in each case, there was a small but significant difference (83-173 microm) in the mediolateral axis within the motor nucleus but not within cell group k. Even in this axis, there was a 94-99% overlap of the two populations. Comparisons of the neuronal cross-sectional area showed that the deep regions were innervated by a larger proportion of small neurons from both nuclei than were the superficial and intermediate regions. Our results suggest that there is no simple topographical arrangement of motoneurons that corresponds to the peripheral pattern of nerve supply to the different regions of the masseter muscle.

Animals↗
PubMed1997

Physical therapy: a critique.

Some forms of physical therapy are used relatively frequently in the treatment of chronic musculoskeletal pain conditions, including the temporomandibular disorders. We found evidence that cold seems to be a useful treatment for postsurgical pain and swelling and that most patients being treated for most chronic musculoskeletal pain seem to do better with most forms of therapy. However, we agree with the authors of previous reviews that there is little evidence that these methods of management cause long-lasting reductions in signs and symptoms. Findings of recent clinical trials tend to support this conclusion, although evidence is beginning to accumulate that exercise programs designed to improve physical fitness have beneficial effects on chronic pain and disability of the musculoskeletal system.

Acupuncture Therapy↗
PubMed1997

Anatomical partitioning and nerve branching patterns in the adult rabbit masseter.

The masseter muscle is functionally heterogeneous with a complex architecture consisting of multiple tendons and a multipennate arrangement of muscle fibers. In this study, the anatomical partitioning of the rabbit masseter is described on the basis of the tendons of origin and insertion, general partition orientation relative to the zygomatic arch, motor endplate descriptions, and primary nerve branches that innervate these partitions. This work refines previous descriptions of the rabbit masseter and describes 13 anatomical partitions, each with a unique tendinous attachment. In addition, 14 naturally occurring primary nerve branches were identified and found to innervate different regions of the muscle. After correlating the anatomical partitions and the associated neural innervation pattern, it was determined that simple branch order will not adequately define all the neuromuscular compartments in the rabbit masseter.

Animals↗
PubMed1995

Validity of diagnostic and monitoring tests used for temporomandibular disorders.

Currently, diagnosis of temporomandibular disorders (TMD) depends on a comprehensive history and physical examination, supplemented, when indicated, by images of hard and soft tissues. However, there are electronic diagnostic devices being marketed to acquire other measures described as relevant to TMD and to use these for diagnosis of TMD and for monitoring the effects of treatment. This paper reviews the capacity of several devices to measure these variables accurately and reliably and to assess the theoretical basis of each of these tests. Diagnostic ability was established, when possible, according to the commonly accepted measures of sensitivity, specificity, and positive predictive values. It was found that many tests lack theoretical validity, that measurement validity tends to be poor, and that diagnostic ability can be even worse than chance, because of a high percentage of false-positive diagnoses. Based on these findings, the use of these instruments in clinical practice is inappropriate at this time and may lead to the treatment of large numbers of subjects who have no disorder.

Auscultation↗
PubMed1993

Genioglossi muscle activity in response to changes in anterior/neutral head posture.

Clinicians have acknowledged swallowing, tongue activity, and head posture as interdependent variables that must be concurrently examined. The purpose of this study was to evaluate genioglossus activity during swallowing, rest, and maximal tongue protrusion in two head positions (HPs) with a noninvasive recording device. Eight Angle Class I subjects were evaluated. Repeated measures were performed in a single session to record surface intraoral electromyographic (EMG) activity of the genioglossus muscles. Head position was measured in angular degrees from photographs. Three variables were measured in both the neutral-head position (NHP) and anterior-head position (AHP): (1) duration of genioglossus EMG during swallowing, (2) genioglossus EMG with the tongue at rest, and (3) genioglossus EMG during maximal isometric tongue protrusion. A Wilcoxin matched-pair signed-rank statistic was used for EMG analysis, and a paired sample t test statistic was used for head posture analysis. The angles measured for NHP and AHP within each subject were significantly different verifying two different head positions. Duration of swallowing was not significantly different between head positions. Resting genioglossus EMG and maximal isometric genioglossus EMG were statistically greater in the AHP. The data suggest that head positional changes may have an effect on genioglossus muscle activation thresholds. However, small differences in resting EMG activity between head positions suggests that the clinical significance needs further investigation.

Adult↗
PubMed1991

The pain-adaptation model: a discussion of the relationship between chronic musculoskeletal pain and motor activity.

Articles describing motor function in five chronic musculoskeletal pain conditions (temporomandibular disorders, muscle tension headache, fibromyalgia, chronic lower back pain, and postexercise muscle soreness) were reviewed. It was concluded that the data do not support the commonly held view that the pain of these conditions is maintained by some form of tonic muscular hyperactivity. Instead, it seems clear that in these conditions the activity of agonist muscles is often reduced by pain, even when this does not arise from the muscle itself. On the other hand, pain causes small increases in the level of activity of the antagonist. As a consequence of these changes, force production and the range and velocity of movement of the affected body part are often reduced. To explain how such changes in the behaviour come about, we propose a neurophysiological model based on the phasic modulation of excitatory and inhibitory interneurons supplied by high-threshold sensory afferents. We suggest that the "dysfunction" that is characteristic of several types of chronic musculoskeletal pain is a normal protective adaptation and is not a cause of pain.

Adaptation, Physiological↗
PubMed1990

Devices for the diagnosis and treatment of temporomandibular disorders. Part II: Electromyography and sonography.

This second article in the three-part series on temporomandibular disorder (TMD) devices compares the claimed diagnostic usefulness of electromyography and sonography with the present scientific evidence. This review concludes that there is no evidence to support the use of either surface electromyography or silent period duration for the evaluation or diagnosis of TMD. Furthermore, in view of the available evidence, sonography and Doppler ultrasound have no particular advantage over a conventional stethoscope or direct auscultation.

Auscultation↗
PubMed1989

Evidence that peaks in EMG averages can sometimes be caused by inhibition of motoneurons.

1. It has been reported that an excitatory response occurs before strong inhibition in masticatory muscles. We tested the hypothesis that this small monopolar wave in the EMG, called by us the early exteroceptive component (EEC), is in fact the first response to inhibition. 2. A mapping of the electrical activity of the masseter muscle was performed using a 3 x 4 matrix of surface electrodes with reference to the back of the neck. Subjects sat with the jaw closing muscles relaxed or contracting at approximately 75% of the maximum voluntary level. The chin was tapped to evoke a jaw jerk reflex and the EEC was elicited by electrical stimulation to the palate. 3. In addition, bipolar EMGs and jaw position were recorded at minimal bite forces and at contraction levels of 5, 10, 15, or 20 N. 4. Data were computer average, with and without rectification, for 32 stimuli. 5. The EEC [latency 12.1 +/- 1.0 (SD)ms] was found to have the same polarity, shape, and duration as the repolarizing wave of the stretch reflex. 6. After electrical stimulation, an increase of bite force was never observed during or immediately after the EEC. Instead, bite force began to decrease 5-7 ms after the onset of the EEC. 7. The amplitude of the EECs never exceeded the level of the peaks in the preceding background EMG, even when the left and right palatal electrodes were stimulated simultaneously at high intensity.(ABSTRACT TRUNCATED AT 250 WORDS)

Electromyography↗
PubMed1989

Temporomandibular joint sounds: a critique of techniques for recording and analysis.

Sonography, or the graphic recording of sounds, has been proposed as an objective measure of various pathological conditions in the temporomandibular joint. Various electronic devices have been developed to enhance our ability to auscultate the joint, monitor the timing of the sounds with jaw movement, and analyze the characteristics of the sound; the intent of these devices is to diagnose the intracapsular condition "objectively." This review paper critically evaluates the advantages and limitations of this technique. Based on the existing literature, these instruments can record sounds; however, the origin of these sounds is uncertain, since room noise, skin and hair sounds, respiration, arterial blood flow, and cross-over noises from the opposite TMJ have not been excluded as possible artifacts of the recording. More important, the diagnostic specificity, as an indicator of each type of TMJ disease, has not been clearly and consistently demonstrated with the sonographic technique.

Auscultation↗