Functional properties of single motor units in inferior head of human lateral pterygoid muscle: task relations and thresholds

Pterygoid muscle activity in speech: A preliminary investigation

BACKGROUND

Speaking depends on refined control of jaw opening and closing movements. The medial pterygoid muscle (MPT), involved in jaw closing, and the lateral pterygoid muscle (LPT), involved in jaw opening, are two key mandibular muscles in mastication and are likely to be recruited for controlled movements in speech.

OBJECTIVES

Three hypotheses were investigated, that during speech the MPT and LPT: (1) were both active, (2) but exhibited different patterns of activity, (3) which fluctuated with the vowels and consonants in speech.

METHODS

Intramuscular EMG recordings were made from the right inferior head of the LPT and/or the right MPT in five participants during production of 40 target nonsense words (NWs) consisting of three syllables in the form /V₁ C₁ V₂ C₂ ə/ (V = vowel; C = consonant; ə = unstressed, reduced vowel), spoken by each participant 10 times per NW; analysis focussed on the target syllable, C₁ V₂ .

RESULTS

Both MPT and LPT exhibited robust increases in EMG activity during utterance of most NWs, relative to rest. Peak LPT activation was time-locked to the final part of the target consonant (C₁ ) interval when the jaw begins opening for the target vowel (V₂ ), whereas peak MPT activation occurred around the temporal midpoint of V₂ , when the jaw begins closing for C₂ . EMG amplitude peaks differed in magnitude between "high" vowels, i.e., for which the tongue/jaw are high (e.g., in SEEK), and "low" vowels, i.e., for which the tongue/jaw are low (e.g., in SOCK).

CONCLUSIONS

These novel findings suggest a key role for the LPT and MPT in the fine control of speech production. They imply that speech may impose major synergistic demands on the activities of the MPT and the LPT, and thereby provide insights into the possible interactions between speech activities and orofacial activities (e.g. mastication) and conditions (e.g. Temporomandibular Disorders) that involve the masticatory muscles.

Masticatory Myofascial Pain Syndrome: Implications for Endodontists

INTRODUCTION

Masticatory myofascial pain syndrome can present similarly to other dental conditions in odontogenetic structures. Endodontists should be familiar with the symptomology and pathophysiology of masticatory myofascial pain syndrome to avoid misdiagnosis, incorrect treatment, and medicolegal repercussions. The aim of this review was to provide a foundational summary for endodontists to identify and correctly manage masticatory myofascial pain syndrome.

METHODS

A narrative review of the literature was performed through a MEDLINE search and a hand search of the major myofascial pain textbooks.

RESULTS

Masticatory myofascial pain syndrome is a musculoligamentous syndrome that can present similarly to odontogenic pain or refer pain to the eyebrows, ears, temporomandibular joints, maxillary sinus, tongue, and hard palate. Currently, the most comprehensive pathophysiology theory describing masticatory myofascial pain syndrome is the expanded integrated hypothesis. The most widely accepted diagnostic guidelines for masticatory myofascial pain syndrome are the Diagnostic Criteria for Temporomandibular Disorders; however, their diagnostic capability is limited. There is no hierarchy of treatment methods because each patient requires a tailored and multidisciplinary management aimed at regaining the muscle's range of motion, deactivating the myofascial trigger points, and maintaining pain relief.

CONCLUSIONS

The pain patterns for masticatory myofascial pain syndrome are well-known; however, there is a lack of consensus on the most proper method of trigger point diagnosis or pain quantification. The diagnostic strategies for masticatory myofascial pain syndrome vary, and the diagnostic aids are not well developed.

Threshold variations of medial pterygoid single motor units during vertical or horizontal force tasks

OBJECTIVES

To test the hypotheses that (a) the force thresholds at onset of medial pterygoid muscle single motor unit (SMU) activity do not decrease with an increase in the rate of force generation in standardised vertical or horizontal jaw-force tasks, and (b) there is evidence for functional heterogeneity within the medial pterygoid muscle.

METHODS

In 14 healthy participants, electromyographic recordings of the right medial pterygoid muscle were performed with intramuscular fine-wire electrodes during four isometric force tasks: vertical, horizontal contralateral, horizontal protrusion and horizontal ipsilateral, performed at two rates of force development (slow ramp, fast ramp). Computer tomography scans confirmed electrode location within the muscle, which was divided into medial and lateral parts. Force thresholds of onset of discriminated SMUs were compared between rates in each task; significance accepted at p < 0.05.

RESULTS

Of 45 SMU force thresholds studied in one or more tasks, there was no significant difference between slow and fast ramp within each force task, except slow ramp thresholds from the lateral part during the vertical force task were significantly higher than fast ramp thresholds. Reversals of recruitment order between tasks provided evidence for functional heterogeneity within the muscle. Force thresholds of the vertical tasks (range: 1-292.6 N) were mostly higher than for the horizontal tasks (range: 0.1-12.5 N).

CONCLUSION

The data are consistent with the proposal that the medial pterygoid muscle stabilises the jaw in the vertical plane during isometric force generation in the jaw closing, as well as horizontal directions.

Evaluating the morphological features of the lateral pterygoid insertion into the medial surface of the condylar process

OBJECTIVE

The lateral pterygoid is vital in coordinating multidimensional jaw movements. Since a vector in three-dimensional (3D) space is defined by two 3D points, the role of the lateral pterygoid in 3D jaw movements is defined by its origin and insertion. Reportedly, the lateral pterygoid is inserted into not only the pterygoid fovea but also into the medial surface of the condylar process. The objective was to investigate the morphological features of the region of the lateral pterygoid that inserted into the medial surface of the condylar process.

MATERIAL AND METHODS

Ten head halves of seven cadaveric donors were analyzed. In all specimens, the insertion area on the medial surface of the condylar process was measured based on microcomputed tomography images. Muscle fibers were separated into ≥50 small bundles, and positional relationships between the origins and insertions were investigated.

RESULTS

Overall, 28.8 ± 5.0% of the insertion area of the lateral pterygoid was situated on the medial surface of the condylar process. Moreover, muscle fiber arrangement revealed that origins of the muscle bundles inserting into the medial surface in seven specimens posteriorly occupied the lateral plate of the pterygoid process longitudinally, whereas those in three specimens mainly occupied the posteroinferior portion.

CONCLUSIONS

Muscle bundle inserting on the medial surface of the condylar process had a broad insertion area and a distinct origin on the posterior region of the lateral plate of the pterygoid process. This muscle bundle could act as one of the significant subunits within the lateral pterygoid. Therefore, anatomical coordination mechanisms underlying jaw movement cannot be elucidated without considering this subunit.

Functional properties of single motor units in the human medial pterygoid muscle: Thresholds

BACKGROUND

Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements.

OBJECTIVES

The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle.

METHODS

Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P < .05.

RESULTS

A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening-closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties.

CONCLUSIONS

The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.

Single motor units from the medial pterygoid muscle can be active during isometric horizontal and vertical forces

OBJECTIVES

To determine (a) whether the medial pterygoid muscle is active in an isometric vertical force task and in isometric horizontal force tasks in the contralateral, protrusion and ipsilateral directions; (b) whether the same single motor units (SMUs) could be active across different directions of isometric force generation; and (c) whether different regions of the medial pterygoid muscle exhibit different patterns of SMU activation during the generation of any one direction of isometric force.

METHODS

Intramuscular electromyographic (EMG) recordings were made from the right medial pterygoid muscle in 15 healthy participants during isometric force tasks: vertical and horizontal contralateral, protrusion and ipsilateral. A computed tomography scan divided the EMG recording site into a medial or lateral part in each participant. Single motor units were discriminated in each task.

RESULTS

Medial pterygoid SMU activity was recorded in 100% of participants for the vertical biting tasks, 86% of participants for the horizontal contralateral and horizontal protrusion tasks and 57% of the horizontal ipsilateral tasks. Of the 72 SMUs that were discriminated, 36% were active in all tasks; 18% were active only in the vertical tasks and 17% were active in the vertical, horizontal contralateral and horizontal protrusion tasks. The proportion of SMUs that was active in at least 1 horizontal task in the lateral part (33/39) was significantly higher than the proportion (21/33) in the medial part (Chi-Square, P < 0.05).

CONCLUSION

The data are consistent with a stabilisation role for the medial pterygoid muscle in isometric jaw forces in the vertical and horizontal planes.

Reorganization of motor unit activity at different sites within the human masseter muscle during experimental masseter pain

The aims were to test the hypotheses that experimental masseter muscle pain leads to recruitment and/or derecruitment of motor units at different sites within the masseter and that the patterns of change in motor unit activity differ between sites. Single motor unit (SMU) activity was recorded at two sites within the right masseter [superior/anterior, inferior/posterior (IP)] during isometric biting tasks (ramp, step level) on an intraoral force transducer in 17 participants during three experimental blocks comprising no infusion (baseline), 5% hypertonic saline infusion (pain), or isotonic saline infusion (control). A visual analog scale (VAS) was used to score pain intensity. The VAS scores were statistically significantly greater during infusion of hypertonic saline than during infusion of isotonic saline. No significant differences in force levels and rates of force change were found between experimental blocks. In comparison with isotonic saline infusion, SMUs could be recruited and derecruited at both sites during hypertonic saline infusion. The frequency of recruitment or derecruitment, in comparison with no change, was statistically significantly greater at the IP site than at the superior/anterior site. Experimental noxious masseter stimulation results in a reorganization of motor unit activity throughout the muscle, and the pattern of reorganization may be different in different regions of the muscle.

The medial pterygoid muscle: a stabiliser of horizontal jaw movement

There is limited information of the normal function of the human medial pterygoid muscle (MPt). The aims were to determine whether (i) the MPt is active throughout horizontal jaw movements with the teeth apart and (ii) whether single motor units (SMUs) are active during horizontal and opening-closing jaw movements. Intramuscular electrodes were placed in the right MPt of 18 participants who performed five teeth-apart tasks: (i) postural position, (ii) ipsilateral (i.e. right) jaw movement, (iii) contralateral movement, (iv) protrusive movement and (v) opening-closing movement. Movement tasks were guided by a target and were divided into BEFORE, OUT, HOLDING, RETURN and AFTER phases according to the movement trajectories recorded by a jaw tracking system. Increased EMG activity was consistently found in the OUT, HOLDING and RETURN phases of the contralateral and protrusive movement tasks. An increased RETURN phase activity in the ipsilateral task indicates an important role for the MPt in the contralateral force vector. Of the 14 SMUs active in the opening-closing task, 64% were also active in at least one horizontal task. There were tonically active SMUs at the postural jaw position in 44% of participants. These new data point to an important role for the MPt in the fine control of low forces as required for stabilisation of vertical mandibular position not only to maintain postural position, but also throughout horizontal jaw movements with the teeth apart. These findings provide baseline information for future investigations of the possible role of this muscle in oro-facial pain conditions.

Finite element analysis of the human mastication cycle

The aim of this paper is to propose a biomechanical model that could serve as a tool to overcome some difficulties encountered in experimental studies of the mandible. One of these difficulties is the inaccessibility of the temporomandibular joint (TMJ) and the lateral pterygoid muscle. The focus of this model is to study the stresses in the joint and the influence of the lateral pterygoid muscle on the mandible movement. A finite element model of the mandible, including the TMJ, was built to simulate the process of unilateral mastication. Different activation patterns of the left and right pterygoid muscles were tried. The maximum stresses in the articular disc and in the whole mandible during a complete mastication cycle were reached during the instant of centric occlusion. The simulations show a great influence of the coordination of the right and left lateral pterygoid muscles on the movement of the jaw during mastication. An asynchronous activation of the lateral pterygoid muscles is needed to achieve a normal movement of the jaw during mastication.

3-Dimensional Physiologic Postural Range of the Mandible: A Computerized-Assisted Technique—A Case Study

Previous studies demonstrated that while the mandible assumes its resting position in space, antagonistic muscles should assume minimal muscle activity within a spatial range. This zone of mandibular rest has been mapped using physiologic parameters of muscle activity and incisal spatial kinematics. This case study expands on previous research by monitoring incisal and posterior jaw position and includes lateral pterygoid muscle activity, thus allowing for determining the spatial range including additional relevant coordinates and muscle activity. Four positions were evaluated: a maximum physiologic open position, a maximum physiologic closed position, physiologic rest position, and maximum physiologic protrusion position. Within the physiologic zone of rest formed by these 4 positions, the vertical and anterior borders of the envelope of function may be documented for the incisal and posterior mandible in true 3-dimensional fashion to assist the clinician in determining a physiologic interocclusal freeway space and vertical dimension of occlusion. Advantages and limitations are discussed.

Masseter Motor Unit Recruitment is Altered in Experimental Jaw Muscle Pain

Some management strategies for chronic orofacial pain are influenced by models ( e.g., Vicious Cycle Theory, Pain Adaptation Model) proposing either excitation or inhibition within a painful muscle. The aim of this study was to determine if experimental painful stimulation of the masseter muscle resulted in only increases or only decreases in masseter activity. Recordings of single-motor-unit (SMU, basic functional unit of muscle) activity were made from the right masseters of 10 asymptomatic participants during biting trials at the same force level and direction under infusion into the masseter of isotonic saline (no-pain condition), and in another block of biting trials on the same day, with 5% hypertonic saline (pain condition). Of the 36 SMUs studied, 2 SMUs exhibited a significant ( p < 0.05) increase, 5 a significant decrease, and 14 no significant change in firing rate during pain. Five units were present only during the no-pain block and 10 units during the pain block only. The findings suggest that, rather than only excitation or only inhibition within a painful muscle, a re-organization of activity occurs, with increases and decreases occurring within the painful muscle. This suggests the need to re-assess management strategies based on models that propose uniform effects of pain on motor activity.

The musculus pterygoïdeus proprius: an in-vivo approach with magnetic resonance imaging

There is a limited understanding of the normal function of the pterygoïdeus proprius muscle and the role that this muscle may have in temporomandibular disorders. Despite a well-described anatomical in-vitro approach to this muscle, there are still difficulties in investigating the fossa pterygopalatina. This study reveals an alternative in-vivo approach by magnetic resonance imaging to visualise the muscle in the fossa pterygopalatina on 78 head halves, describe the connections with the musculus temporalis and pterygoïdeus lateralis as well as report the incidence without dealing with the known inconveniences of the dissection approach. The results show an incidence of 12.82% for the musculus pterygoïdeus proprius equally divided between both genders. Two different types of bridging between the musculus temporalis and musculus pterygoïdeus lateralis were also found: (i) 'O' shape (6.41%) and (ii) 'Y' shape (6.41%). This study suggests the use of magnetic resonance imaging to investigate the different connections between vascular and muscular structures in the fossa pterygopalatina. Further research with this approach to link the appearance of the muscle with neurovascular entrapment syndromes is warranted.

Masticatory motion is controlled in humans by a limited set of muscle synergies

The masticatory motion, whereby food introduced into the mouth is processed into a bolus suitable for swallowing, can be divided into successive masticatory cycles, each comprising downward and subsequent upward movements of the mandible. The present study deals with the problem of the existence of muscle synergies in mastication, that is whether some of the muscles involved in mastication receive common motor drives, rather than controlled individually. Evidence for muscle synergy during mastication is scarce, partly due to the difficulties in simultaneous recording of the electromyographic (EMG) activities from all the muscles involved. Thus, we analyzed the variability of the mandibular motion during mastication rather than to examine the EMG patterns, based on the hypothesis that a motion elicited by a limited set of muscle synergies can be approximated as a superposition of the same number of independent motions. Mandibular motion paths were recorded from 8 healthy males (25-31 years), who chewed gum or gummy candy. A morphometric technique, which describes the shape of a closed curve by using normalized elliptic Fourier descriptors and reduces the variance of the shape by using principal component analysis, was applied to analyze the variability of the mandibular motion paths. We found three independent variations of the motion paths, whose linear combinations accounted for an average of 93% (range, 88-96%) of the total variance. The extracted variations were similar among the subjects. These findings provide indirect evidence for the existence of a limited set of muscle synergies for mastication in humans.

Superior head of human lateral pterygoid muscle: Single motor unit firing rates during isometric force

The superior head of the human lateral pterygoid muscle (SHLP) has been classically considered to have functions that are independent of the inferior head of the lateral pterygoid (IHLP). Recent evidence however suggests that some of the functional properties of the SHLP are similar to those of the IHLP. The aim was to determine whether the functional properties in terms of single motor unit (SMU) firing rates within the SHLP vary with horizontal isometric force (400-800gwt) and direction (i.e., contralateral (CL), protrusive (P), ipsilateral (IL) and intermediate directions, CL-P, IL-P) in a manner similar to those identified for the IHLP, and as would be expected if both SHLP and IHLP should be regarded as one muscle. In eight subjects, the firing rates of 40 SMUs were recorded from computer tomography (CT)-verified SHLP sites while each subject exerted horizontal isometric forces with their lower jaw onto a force transducer in the five directions. Firing rates increased significantly with horizontal isometric force from 400 to 800gwt. Firing rates also changed significantly (p<0.01) with direction with CL, CL-P and P having comparable firing rates (13.3, 12.6 and 12.6impulses/s, respectively) which were significantly higher than IL-P. The similarity of these data to previous IHLP data, provide additional support for the hypothesis that the SHLP and the IHLP should be regarded as two parts of one muscle.

A new method for lateral pterygoid electromyographic electrode placement

STATEMENT OF PROBLEM

Making electromyographic recordings of the lateral pterygoid muscle (LP) is difficult because of potential electrode damage to, for example, the maxillary artery and long buccal nerve, and because of pain and reduced jaw mobility characteristic of many orofacial pain patients.

PURPOSE

The purpose of this study was to develop a reliable intraoral placement technique for the inferior head of the lateral pterygoid (IHLP) that minimizes jaw displacement.

MATERIAL AND METHODS

In 2 dried skulls and 7 human cadavers, it was estimated that, with the mandible in an ipsilateral closed position, a straight needle could be used to position fine-wire electrodes into the midportion of IHLP by inserting the needle through the mucosa adjacent to the distal root of the maxillary second molar, towards the external auditory meatus and parallel to the buccal alveolar bone of the maxilla. The needle avoided the maxillary artery and long buccal nerve. Using this approach in 5 adults, 2 fine-wire electrodes were placed into the IHLP. Placement was verified by computer tomography (CT) and electromyography.

RESULTS

In all subjects, the ideal insertion depth to place the electrodes in the middle of IHLP was 29 mm.

CONCLUSIONS

This technique is a reliable method for IHLP electrode placement for patients with impaired jaw function, minimizing risk of damage to major structures.

Activity of superior head of human lateral pterygoid increases with increases in contralateral and protrusive jaw displacement

The hypothesis was that the superior head of human lateral pterygoid muscle (SHLP) plays a similar role in jaw movement as the inferior head of human lateral pterygoid muscle (IHLP). The aims were to determine the functional properties of SHLP single motor units (SMUs) and root mean square activity (RMS) of the SHLP during contralateral and protrusive jaw movement tasks and to compare these features with those identified previously for the IHLP. In 22 human subjects, SMUs were recorded intramuscularly from computer tomography-verified sites within the SHLP during standardized contralateral and protrusive jaw movement tasks recorded by a jaw-tracking device. Of the 50 SMUs discriminated, 39 were active during contralateral and 29 during protrusive jaw movements. The firing rates and RMS of the SHLP motor units increased with an increase in jaw displacement. The RMS activity across the entire trial during contralateral jaw movement was significantly greater than that during protrusion. Similarly to conclusions previously identified for the IHLP, the data are consistent with an important role for the SHLP in the control of contralateral and protrusive jaw movements. The similarities in SHLP and IHLP functional properties support the proposal that both heads should be regarded as a system of fibers acting as one muscle.

Threshold properties of single motor units in superior head of human lateral pterygoid muscle

The superior head of the human lateral pterygoid muscle (SHLP) may play a similar role in jaw movement as the inferior head (IHLP).

OBJECTIVE

The aim was to determine whether threshold properties of single motor units (SMUs) within SHLP during jaw tasks were comparable to those identified for IHLP.

DESIGN

In 24 human subjects, SMUs were recorded intramuscularly from computer-tomography verified sites within SHLP during standardised jaw tasks recorded by a jaw-tracking device.

RESULTS

Of the 69 SMUs discriminated, 54 were active during contralateral, 52 during protrusive and 8 during ipsilateral jaw movements. The thresholds, at which SMUs commenced firing, decreased (p<0.05) as speed of contralateral or protrusive tasks increased. The data suggest an important role for SHLP in generation and control of contralateral and protrusive jaw movements. A number of lines of evidence were consistent with functional heterogeneity within SHLP.

CONCLUSIONS

The similarities in SHLP and IHLP functional properties support the proposal that both heads should be regarded as a system of fibers acting as one muscle.

The human lateral pterygoid muscle

One of the jaw muscles particularly implicated in temporomandibular disorders (TMD), a common form of non-dental chronic orofacial pain, is the human lateral pterygoid muscle. The precise role of this muscle in TMD is unclear as is the nature of the interaction between pain and motor function particularly involving this muscle. This research group has adopted a two-stage approach to studying the effects of pain on motor function. The first is to study normal orofacial motor function through recordings of jaw movement and electromyographic (EMG) activity from a number of jaw muscles (including recordings from the lateral pterygoid muscle; verification of electrode location achieved through computer tomography imaging) during a number of standardised jaw movements. These studies have defined the detailed functional properties of, in particular, the lateral pterygoid muscle, whose physiology and function is not well understood. In summary, the data are consistent with the hypothesis previously proposed that the lateral pterygoid should be regarded as a system of fibres that acts as one muscle, with varying amounts of evenly graded activity throughout its entire range, and with the distribution of activity within the muscle being determined by the biomechanical demands of the task. Our second approach has been to study the effects of experimental masseter muscle pain on the detailed functional properties (e.g., root-mean-square EMG activity) of the jaw muscles, especially the lateral pterygoid muscle. Preliminary data from these pain studies point towards significant effects of human experimental muscle pain on jaw muscle activity and jaw movement.

A method for studying jaw muscle activity during standardized jaw movements under experimental jaw muscle pain

This paper describes a method for studying superficial and deep jaw muscle activity during standardized jaw movements under experimental jaw muscle pain. In 22 healthy adults, pain was elicited in the right masseter muscle via tonic infusion of 4.5% hypertonic saline and which resulted in scores of 30-60 mm on a 100-mm visual analogue scale. Subjects performed tasks in five sessions in a repeated measures design, i.e., control 1, test 1 (during hypertonic or isotonic saline infusion), control 2 (without infusion), test 2 (during isotonic or hypertonic saline infusion), control 3 (without infusion). During each session, subjects performed maximal clenching and standardized jaw tasks, i.e., protrusion, lateral excursion, open/close, chewing. Mandibular movement was recorded with a 6-degree-of-freedom tracking system simultaneously with electromyographic (EMG) activity from the inferior head of the lateral pterygoid muscle with fine-wire electrodes (verified by computer tomography), and from posterior temporalis, the submandibular muscle group and bilateral masseter muscles with surface electrodes. EMG root mean square values were calculated at each 0.5 mm increment of mandibular incisor movement for all tasks under each experimental session. This establishes an experimental model for testing the effects of pain on jaw muscle activity where the jaw motor system is required to perform goal-directed tasks, and therefore should extend our understanding of the effects of pain on the jaw motor system.

A working-side change to lateral tooth guidance increases lateral pterygoid muscle activity

The inferior head of lateral pterygoid (IHLP) is thought to play a critical role in the generation and control of lateral jaw movements.

AIM

The aim was to test the hypothesis that a change to the lateral tooth guidance (working-side occlusal alteration, OA) results in a significant change in the electromyographic (EMG) activity of the IHLP during standardised lateral jaw movements (laterotrusion) tracked by a jaw-tracking system.

METHODS

Ten trials of right laterotrusion were repeated under: control 1 (before occlusal alteration), OA (after occlusal alteration placement), and control 2 (after occlusal alteration removal) conditions in 14 subjects while recording left IHLP, bilateral anterior and posterior temporalis, masseter and submandibular muscles.

RESULTS

IHLP activity was significantly (p<0.05) increased with the occlusal alteration during the outgoing (movement from intercuspal position to approximately 5mm right) and return phases of laterotrusion. The other muscles demonstrated no change or a significant decrease in activity.

CONCLUSIONS

These findings suggest that a change to the occlusion on the working-side in the form of a steeper guidance necessitates an increase in IHLP activity to move the mandible down the steeper guidance. It must be emphasised that these data cannot be used as justification for occlusal therapy.

Topographical Characteristics of Motor Units of the Lower Facial Musculature Revealed by Means of High-Density Surface EMG

The objective of this study was to systematically characterize motor units (MUs) of the musculature of the lower face. MU endplate positions and principal muscle fiber orientations relative to facial landmarks were identified. This was done by the analysis of motor unit action potentials (MUAPs) in the surface electromyogram. Thirteen specially trained, healthy subjects performed selective contractions of the depressor anguli oris, depressor labii inferioris, mentalis, and orbicularis oris inferior muscles. Signals were recorded using recently developed, 0.3-mm thin and flexible high-density surface electromyography (sEMG) grids (120 channels). For each subject and each muscle and for different low contraction levels, representative MUAPs ("MU fingerprints") were extracted from the raw sEMG data according to their spatiotemporal amplitude characteristics. We then topographically characterized the lower facial MUs' endplate zones and main muscle fiber orientations on the individual faces of the subjects. These topographical MU parameters were spatially warped to correct for the different sizes and shapes of the faces of individual subjects. This electrophysiological study revealed a distribution of the lower facial MU endplates in more or less restricted, distinct clusters on the muscle often with eccentric locations. The results add substantially to the basic neurophysiologic and anatomical knowledge of the complex facial muscle system. They can also be used to establish objective guidelines for placement of conventional (surface or needle) EMG electrodes as well as for clinical investigations on neuromuscular diseases affecting the facial musculature. The localized endplate positions may also indicate optimal locations for botulinum toxin injection in the face.

Fatigue-related changes in discharge patterns of motor units in the inferior head of the lateral pterygoid muscle in humans

OBJECTIVE

To investigate single motor unit (SMU) firing patterns of the inferior head of the human lateral pterygoid muscle (IHLP) during sustained isometric contractions, and to examine IHLP fatigability.

DESIGN

In 5 subjects, EMG was recorded from the right IHLP during generation of horizontal isometric mandibular force towards left side. Subjects tracked a target on an oscilloscope, slowly increasing the force to 25% of maximum voluntary contraction, and then maintained that force for 120 s (endurance task). Subjects rested for 30 s after each test, and three 10s recovery tests were performed every 30 s (recovery task). Fatigue data were obtained from 23 SMUs.

RESULTS

Twelve of 23 SMUs (52.2%) continued to fire throughout the 120 s, and each subject had at least one of these units. These units were classified as tonic pattern (T-unit). The firing rate of 6 of the 12 T-units (50.0%) decreased gradually with time. Comparison of the mean firing rate and coefficient of variance (CV) of interspike interval (ISI) of the 12 T-units between pre-fatigue, post-fatigue and the three recovery tasks showed that the mean firing rate decreased and CV of ISI increased after fatigue. Mean firing rate and CV of ISI recovered after 30 s of rest after the endurance task.

CONCLUSIONS

The firing patterns of IHLP units changed with the passing of time during sustained isometric contractions leading to fatigue.

Activity of inferior head of human lateral pterygoid muscle during standardized lateral jaw movements

OBJECTIVE

(a) To describe the changes in electromyographic (EMG) activity from selected jaw muscles during a standardized lateral jaw movement with the teeth together, and (b) to investigate the effects on jaw muscle activity of changes in both the rate of lateral jaw movement and the relative magnitude of jaw-closing force.

DESIGN

In 16 healthy volunteers, recordings were made using a jaw-tracking system, of mid-incisor point (MIPT) movements, as well as EMG activity from the contralateral inferior head of the lateral pterygoid muscle (IHLP), and bilateral anterior and posterior temporalis, masseter and submandibular muscles, during lateral jaw movement tasks at two speeds and two closing force levels with the teeth together.

RESULTS

The IHLP was the only muscle to show a consistent increase in activity in association with the outgoing phase of the task and a decrease during the return phase. Under high closing force at slow speed, the EMG activities of the IHLP and bilateral anterior temporalis and masseter muscles were significantly (p < 0.05) higher than those under a low closing force, while there was no significant change (p > 0.05) in bilateral posterior temporalis and submandibular muscles. The change from slow to fast lateral movement at low force did not significantly (p > 0.05) alter the mean activity except for the IHLP (increase in activity) and the contralateral anterior temporalis (decrease in activity).

CONCLUSIONS

The data suggest that the IHLP is one of the principal jaw muscles involved in a lateral jaw movement with the teeth together while the other jaw muscles may play a contributory or facilitatory role.

The human lateral pterygoid muscle: A review of some experimental aspects and possible clinical relevance

The clinical notion that some disturbance to the activity of the lateral pterygoid muscle plays a role in the aetiology of temporomandibular disorders (TMD) is still widely accepted and influences management strategies. However, there is no rigorous scientific evidence to support this clinical notion and the role of the lateral pterygoid muscle in normal function is still controversial. The classically defined functions of each head of the muscle are that the superior head is active on closing, retrusion, and ipsilateral jaw movements, while the inferior head is active on opening, protrusion and contralateral jaw movements. However, recent data indicate that these concepts are too simplistic. For example, recent evidence suggests that parts of the superior head may also be active on opening, protrusion and contralateral jaw movements, and that the superior head may consist of three mediolaterally arranged functional zones. Given these complexities, the proposal that clicking and/or locking conditions arise in the temporomandibular joint through some form of lack of co-ordination between the two heads of the muscle needs re-evaluation. Despite earlier reports to the contrary, both heads of the lateral pterygoid muscle appear to be electrically silent at the postural or resting jaw position, and therefore appear to play no role in the anteroposterior positioning of the jaw at the postural position. An important role has also been demonstrated electromyographically for progressive changes in activity in the inferior head as the direction of horizontal jaw force shifts from one side to the other. This suggests an important role for the lateral pterygoid muscle in the generation of side-to-side and protrusive jaw forces. The lateral pterygoid muscle is likely therefore to play an important role in parafunctional excursive jaw movements and also possibly a role in influencing jaw position in patients where the maxillomandibular relationship records change from session to session. The above data provide new insights into the normal function of the lateral pterygoid muscle. The proposal that the lateral pterygoid muscle plays some role in the aetiology of TMD needs now to be rigorously tested.

Minimal tonic firing rates of human lateral pterygoid single motor units

OBJECTIVE

The minimal tonic firing rates (the lowest firing rates at which motor units fire regularly; MTFR) of single motor units (SMUs) within the lateral pterygoid muscle have not been widely investigated. The aims of this study were (a) to identify MTFR of SMUs within the inferior head (IHLP) and superior head (SHLP) of the lateral pterygoid muscle during horizontal jaw movements, and (b) to determine whether these MTFR vary with movement direction.

METHODS

Twenty subjects moved the jaw to maintain SMU firing at the lowest continuous firing rate. SMU activity was recorded from computer-tomography-verified sites within the IHLP or SHLP.

RESULTS

In the IHLP, the mean (+/-SD) MTFR for contralateral movement (15.6+/-2.3 imp/s; n=22 SMUs) were not significantly different from those during protrusion (16.3+/-3.4 imp/s; n=19). In the SHLP, the mean (+/-SD) MTFR for contralateral, ipsilateral movement, and protrusion were 14.7+/-2.5 imp/s (n=10), 13.2+/-2.1 imp/s (n=8), and 16.2+/-3.7 imp/s (n=2), respectively.

CONCLUSIONS

Lateral pterygoid SMUs have greater MTFR than previously reported in the masseter and IHLP, namely 5-8 and 8-10 imp/s, respectively. The MTFR did not vary with the task within each muscle head.

SIGNIFICANCE

Some physiological properties of lateral pterygoid SMUs may be different from those in other jaw muscles.

Functional heterogeneity in the superior head of the human lateral pterygoid

The activity of the superior head of the human lateral pterygoid muscle (SHLP) is controversial. Given the non-parallel alignment of some SHLP fibers, the SHLP may be capable of differential activation. The aims were to clarify SHLP activity patterns in relation to location within SHLP. In 18 subjects, SHLP single motor units were intramuscularly recorded at computer-tomography-verified sites during horizontal (e.g., protrusion) and vertical (e.g., opening) jaw tasks (recorded by a jaw-tracking device) and at resting postural jaw position. None of 92 units was active at the resting postural position. Medially located units (21) showed activity during contralateral movement, protrusion, and opening; 5 were also active on jaw closing. There was a significant association between unit location and the number of units active during vertical tasks (i.e., jaw closing and clenching). Analysis of the data suggests differential activation within SHLP and raises the possibility of functional heterogeneity within SHLP.

Activity in the inferior head of the human lateral pterygoid muscle with different directions of isometric force

The aim was to test the hypothesis that this head of the muscle plays a part in the generation and fine control of horizontal mandibular forces in a range of directions. Electromyographic activity was recorded from the inferior head of the lateral pterygoid of eight individuals during the generation such forces to a target in contralateral, ipsilateral and protrusive directions, and in two intermediate directions (contralateral-protrusive, ipsilateral-protrusive). The mean electromyographic activity and the mean firing rates of 21 single-motor units were significantly affected by direction, with the highest activity being contralateral, and graded decreases occurring as the direction shifted to the ipsilateral. Firing rates were significantly correlated with force magnitude. The data suggest that the inferior head of the human lateral pterygoid is involved in the creation and fine control of mandibular forces in different horizontal directions.

Functional properties of single motor units in the inferior head of human lateral pterygoid muscle: task firing rates

The precise function of the inferior head of the human lateral pterygoid muscle (IHLP) is unclear. The aim of this study was to clarify the normal function of the IHLP. The hypothesis was that an important function of the IHLP is the generation and fine control of horizontal (i.e., anteroposterior and mediolateral) jaw movements. The activities of 50 single motor units (SMUs) were recorded from IHLP (14 subjects) during two- or three-step contralateral movement (n = 36) and/or protrusion (n = 33). Most recording sites were identified by computer tomography. There was a statistically significant overall increase in firing rate as the magnitude of jaw displacement increased between the holding phases (range of increments: 0.3-1.6 mm). The firing rates during the dynamic phases for each unit were significantly greater than those during the previous holding phases but less than those during the subsequent holding phases. For the contralateral step task at the intermediate rate, the cross-correlation coefficients between jaw displacement in the mediolateral axis and the mean firing rate of each unit ranged from r = 0.29 to 0.77; mean +/- SD; r = 0.49 +/- 0.13 (protrusive step task: r = 0.12-0.74, r = 0.44 +/- 0.14 for correlation with anterior-posterior axis). The correlation coefficients at the fast rate during the contralateral step task and the protrusive step task were significantly higher than those at the slow rate. The firing rate change of the SMUs per unit displacement between holding phases was significantly greater for the lower-threshold than for the higher-threshold units during contralateral movement and protrusion. After dividing IHLP into four regions, the SMUs recorded in the superior part exhibited significantly greater mean firing rate changes per unit displacement during protrusion than for the SMUs recorded in the inferior part. Significantly fewer units were related to the protrusive task in the superior-medial part. These data support previously proposed notions of functional heterogeneity within IHLP. The present findings provide further evidence for an involvement of the IHLP in the generation and fine control of horizontal jaw movements.

Antero-posterior activity changes in the superficial masseter muscle after exposure to experimental pain

The aim of this randomized, controlled, double-blind study was to examine how the activation pattern of the masseter muscle changes during natural function when experimental pain is induced in a discrete anterior area of the muscle. In 20 subjects, three bipolar surface electrodes and three intramuscular fine-wire electrodes (antero-posterior mapping) were simultaneously attached above and in the right masseter muscle to record the electromyographic (EMG) activity during unilateral chewing before and after infusion of a 0.9% isotonic and 5% hypertonic saline bolus in the anterior area of the muscle. The activity of the contralateral masseter muscle was registered by surface electrodes. In addition, the development of pain intensity was quantitatively measured with a numerical rating scale (NRS). While both saline concentrations caused pain, the hypertonic solution evoked stronger pain. The experiments also provided evidence of a significant although differential activity reduction of the ipsilateral masseter muscle in the antero-posterior direction. The activity reduction decreased with increasing distance from the location of the infusion. The results support the idea that the strategy of differential activation protects the injured muscle while simultaneously maintaining optimal function.