Charles G. Widmer

Use of a novel thermal operant behavioral assay for characterization of orofacial pain sensitivity

&NA; Orofacial pain has been well‐characterized clinically, but evaluation of orofacial pain in animals has not kept pace. The objective of this study was to describe behavioral responses to facial thermal stimulation and inflammation with/without an analgesic using a novel operant paradigm. Animals were trained to voluntarily place their face against a stimulus thermode (37.7–57.2 °C) providing access to positive reinforcement. These contingencies present a conflict between positive reward and tolerance for nociceptive stimulation. Inflammation was induced and morphine was provided as an analgesic in a subset of animals. Six outcome measures were determined: reward intake, reward licking contacts, stimulus facial contacts, facial contact duration, ratio of reward/stimulus contacts, and ratio of facial contact duration/event. Animals displayed aversive behaviors to the higher temperatures, denoted by a significant decrease in reward intake, total facial contact duration, and reward licking events. The number of facial contacts increased with increasing temperature, replacing long drinking bouts with more frequent short drinks, as reflected by a low ratio of facial contact duration/event. The number of reward licking/facial contact events was significantly decreased as the thermal stimulus intensity increased, providing another pain index derived from this operant method. These outcomes were significantly affected in the direction of increased nociception following inflammation, and these indices of hyperalgesia were reversed with morphine administration. These data reflect an orofacial pain behavior profile that was based on an animals responses in an operant escape paradigm. This technique allows evaluation of nociceptive processing and modulation throughout the neuraxis.

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.

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.

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.

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.

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.

Sex Differences in Rabbit Masseter Muscle Function

The proportions of fibers of different phenotypes in the rabbit masseter muscle differ strikingly in adult males and females. Muscles from females contain similar proportions of small fibers that express both the slow/β and cardiac α myosin heavy chain (MyHC) isoforms and larger fibers containing the IIa MyHC isoform. In muscles from males, nearly 80% of fibers are of the IIa phenotype. To evaluate the functional significance of these sex differences, we used finely graded intramuscular microstimulation to study the contractile properties of masseter motor units in >6-month-old male and female rabbits. Twitch forces and torques in males were significantly greater in magnitude than those of females. Greater proportions of units that produced larger forces/torques were encountered in the males. The same motor units produced force or torque more rapidly in males than in females, principally because units in which twitch rise times were >22 ms were found only in females. The forces applied to the mandible and the torques generated about the right mandibular condyle were studied during cortically evoked rhythmic activation of the masticatory muscles. The overall range of torque rise times and the magnitudes of the peak torques did not differ between sexes. The mean rise time was significantly shorter and the mean peak torque was significantly greater in males. We conclude that sex differences in fiber phenotype proportions are reflected in sex differences in motor unit properties and in the function of these motor units during rhythmic activation.

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.

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.

Limb, Respiratory and Masticatory Muscle Compartmentalization: Developmental and Hormonal Considerations

Neuromuscular compartments are subvolumes of muscle that have unique biomechanical actions and can be activated singly or in groups to perform the necessary task. Besides unique biomechanical actions, other evidence that supports the neuromuscular compartmentalization of muscles includes segmental reflexes that preferentially excite motoneurons from the same compartment, proportions of motor unit types that differ among compartments, and a central partitioning of motoneurons that innervate each compartment. The current knowledge regarding neuromuscular compartments in representative muscles involved in locomotion, respiration, and mastication is presented to compare and contrast these different motor systems. Developmental features of neuromuscular compartment formation in these three motor systems are reviewed to identify when these compartments are formed, their innervation patterns, and the process of refinement to achieve the adult phenotype. Finally, the role of androgen modulation of neuromuscular compartment maturation in representative muscles of these motor systems is reviewed and the impact of testosterone on specific myosin heavy chain fiber types is discussed based on recent data. In summary, neuromuscular compartments are pre-patterned output elements in muscle that undergo refinement of compartment boundaries and muscle fiber phenotype during maturation. Further studies are needed to understand how these output elements are selectively controlled during locomotion, respiration, and mastication.