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Functional Heterogeneity in the Superior Head of the Human Lateral Pterygoid
Jaw Function and Orofacial Pain Research Unit, Faculty of Dentistry, University of Sydney, Level 3, Professorial Unit, Westmead Centre for Oral Health, Westmead Hospital, Westmead, NSW 2145, Australia; Correspondence: 3 corresponding author, gregm{at}mail.usyd.edu.au
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.
Key Words: computer tomography • functional heterogeneity • jaw movement • single motor unit
We have a very limited understanding of the normal function of the superior head of the human lateral pterygoid muscle (SHLP), and there is controversy among studies. For example, some studies report that the SHLP is active during ipsilateral movement, jaw closing, and retrusion (Grant, 1973; McNamara, 1973; Mahan et al., 1983; Gibbs et al., 1984; Wood et al., 1986; Hirabaet al., 2000; for reviews, see Klineberg, 1991; Miller, 1991), while other studies report that the SHLP may be active during jaw opening, protrusion, or contralateral jaw movements (Sessle and Gurza, 1982; Miller, 1991; Murray et al., 1999). Another conflicting issue is whether the SHLP is maintained in a mild state of contraction that results in a slight anterior and medial force on the disc when the jaw is at the resting postural jaw position (Okeson, 1998; Møller, 2001). Some investigators (Mahan et al., 1983; Murray et al., 2001) suggest that the SHLP is silent at resting posture in normal asymptomatic subjects. Possible explanations for the inconsistencies between SHLP studies include the absence in most previous studies of reliable verification methods. It is possible that some of the recordings in these earlier studies may have been obtained from other adjacent muscles, such as the deep temporalis, or may have been incorrectly attributed to a particular head of the lateral pterygoid (Widmalm et al., 1987; Hiraba et al., 2000). Second, most previous studies involved multi-unit electromyographic recordings, where it can be difficult to draw conclusions as to the relative levels of muscle activity. A third explanation could relate to the possibility of functional heterogeneity within the SHLP. Functional heterogeneity, which refers to selective activation of different subcompartments of a muscle, has been well-described in the masseter and temporalis muscles (for review, see Hannam and McMillan, 1994), and recent evidence suggests that the inferior head of the lateral pterygoid is functionally heterogeneous (Phanachet et al., 2001a). The presence of functional heterogeneity within the SHLP would allow the brain to take advantage of the non-parallel fiber alignment that has been described (Troiano, 1967) and provide the possibility of a range of force vectors (i.e., magnitude and direction) on the condyle from the SHLP. If the SHLP is functionally heterogeneous, then recordings at different sites within the muscle would yield different functional properties. This may well be an explanation for the inconsistencies among studies as to the task relations of the SHLP. The aims of the study, therefore, were to use single motor unit (SMU) recordings from computer-tomography-verified sites in the SHLP (a) to clarify the normal function of the SHLP by identifying the tasks to which SMUs in SHLP were related, and (b) to identify whether there were different patterns of activity in different parts of the SHLP.
Volunteers (age, 18-63 yrs; 12 males, 6 females) without temporomandibular disorders or history of chronic pain or neuromuscular condition participated. All gave informed consent; experimental procedures were approved by the Western Sydney Area Health Service Ethics Committee and the Human Ethics Committee, University of Sydney. The following reviews previously described methods (Orfanos et al., 1996; Phanachet et al., 2001a,b).
Electrode Placement within SHLP Based on histological studies (Meyenberg et al., 1986; Widmalm et al., 1987), SHLP was defined to be 5 mm thick superior-inferiorly. For the purposes of assessing electrode recording site, we arbitrarily divided the SHLP into medial, middle, and lateral parts. A second computer tomography session, after recordings, provided electrode location in relation to SHLP boundaries. The amount of bend-back of the wires (2-3 mm) was taken into account.
Recording of Condylar and Mid-incisor Point Movement during Standardized Tasks All jaw movements were performed with the teeth apart. Before each trial, subjects swallowed and relaxed their jaws with lips lightly touching to achieve the resting postural jaw position. Jaw movements were standardized by having the subject move the position of the mid-incisor point dot to track a target that was an illuminated LED on a linear bank of LEDs positioned to the side of the mid-incisor point dot trajectory. LED illumination was controlled by CED scripts. Subjects tracked the target in single-step and multiple-step tasks. The tasks performed here also provided data for characterizing unit properties (e.g., firing patterns, thresholds); these data are not reported here. Subjects performed standardized tasks, including contralateral and ipsilateral jaw movement, protrusion, submaximal jaw opening, and jaw closing. The SHLP activity during resting postural jaw position was recorded for 15-second periods at the beginning, during, and end of the experiment. Trials for each task (i.e., single- and multiple-step tasks at different rates and directions) were repeated 5-8 times with at least one-minute rest between trials. Non-standardized retrusion and clenching in intercuspal position, without visual feedback, were also performed in most subjects. In three subjects, all tasks were non-standardized. Statistical analysis involved Pearson Chi-Square (significance at P < 0.05 level).
A total of 92 SMUs was recorded: 21 units at 4 sites from the medial part of the SHLP, 36 units (6 sites) from the middle part, and 35 units (8 sites) from the lateral part. None of the units was active at the resting postural jaw position (Figs. 1, 2  ). Not all units at each recording site contributed to all tasks labeled positive (Table). Of the 92 units, 79 (86%) were involved in more than one task, 3 participated in all seven tasks, while 13 units (14%) were active during one task only.
The 21 units recorded from the medial part of the SHLP showed activity during contralateral movement (86% of units), protrusion (95%), jaw opening (64%), and/or jaw closing (20%) (e.g., see Fig. 1  from subject B), and all were silent during ipsilateral movement, retrusion, and clenching in the intercuspal position.
In the middle part, the number of tasks for which a unit was active varied among units. Of the 6 sites, 3 (E,F,G) showed the same activity pattern as found in the medial part (Table). In subjects I and J, there was activity during all tasks except jaw closing and retrusion, whereas in subject H, there was activity during all tasks performed (Fig. 2
In the lateral part, two (Q and R in the Table) of eight sites showed activity during ipsilateral movement, retrusion, jaw closing, and clenching in the intercuspal position, two sites (K,L) demonstrated activity resembling those recorded from most units in the medial part, while the other four sites (M,N,O,P) showed activity in various patterns. The percentages of units in the lateral part contributing to each task ranged from 35% to 54%. The numbers of units activated during horizontal and vertical tasks were compared among the three regions of the SHLP to determine whether units active during each vertical or horizontal task were localized to a specific region. The horizontal tasks were protrusion, retrusion, contralateral, and ipsilateral movements. Jaw closing and clenching were defined as vertical tasks. The data from jaw opening were not included in the analysis, because the subjects performed submaximal jaw-opening, and it is possible that the units that were not active during submaximal jaw-opening would be active at maximal jaw opening. A total of 92 and 84 SMUs was included in this analysis for the horizontal and vertical tasks, respectively. The units that could not be distinguished during at least one of the vertical tasks were excluded from the analysis. Each unit was considered to be active in the horizontal (or vertical) tasks when the unit fired in at least one of the horizontal (or vertical) tasks. The percentages of units active during the horizontal tasks in the medial, middle, and lateral parts were 100%, 97%, and 94%, respectively, and 20%, 17%, and 45% during the vertical tasks. There was no significant association between the location of units and the number of units active during the horizontal tasks (P > 0.05). However, there was a significant association between the location of units and the number of units active during the vertical tasks (P < 0.05). There was a significant association between unit location and the number of units active during the contralateral task, the ipsilateral task, protrusion, retrusion, jaw opening, and clenching (each, P < 0.05), but not for jaw closing.
Functional Heterogeneity of the SHLP This study has provided the first detailed description of the activities of SMUs recorded from computer-tomography-verified sites within the human SHLP. Units located in different parts of the SHLP could be active for different combinations of tasks, and there was evidence that units active in the vertical tasks were especially localized to the lateral part. The capability of the SHLP to participate in any of the tasks studied is not in agreement with some previous studies (e.g., Kamiyama, 1961) that reported SHLP activity only during ipsilateral movement, retrusion, and jaw closing. One possible explanation for the conflicting results is that some of the previous studies may have recorded from the temporalis muscle rather than the intended lateral pterygoid muscle. Another explanation is functional heterogeneity within SHLP, as previously proposed for the lateral pterygoid (Hannam and McMillan, 1994; Foucart et al., 1998). Further, our data are consistent with the view that the lateral pterygoid constitutes a "...system of fibers (that) acts as one muscle, with varying amounts of evenly graded activity throughout its entire range, with the distribution shaded according to the biomechanical demands of the task" (Hannam and McMillan, 1994). Within the lateral pterygoid, there is a marked convergence of muscle fibers onto a small insertion site on the condylar fovea, capsule, and disc from a broad origin at the roof of the infratemporal fossa and lateral pterygoid plate (Mahan et al., 1983; Widmalm et al., 1987; Bittar et al., 1994; Schmolke, 1994; Heylings et al., 1995; Naidoo, 1996). This marked change in fiber alignment from the uppermost to the lowermost fibers and from the medial to the lateral side of each head of the muscle, together with the non-parallel fiber alignment that has been described within the SHLP (Troiano, 1967), provide, with selective activation, the possibility of a range of force vectors on the condyle from both heads of the muscle (Murray et al., 2001). For the SHLP, it is proposed that a range of force vectors is available on the condyle during jaw movements, and that these vectors are achieved through selective activation of specific regions within the SHLP with specific fiber orientations. The origin of this selective recruitment of motor units within the SHLP motor pool is unclear at present, although cortical command signals as well as differential somatosensory inputs from the TMJ capsule, ligaments, or retrodiscal area, or even muscle spindle inputs, could play a role, representing an avenue for further investigation. Our previous SMU study in the inferior head of the lateral pterygoid at verified sites confirmed that the inferior head is involved in contralateral movement, protrusion, and jaw opening (Phanachet et al., 2001a). The present findings suggest that the definition of reciprocal or simultaneous activity between the superior and inferior heads depends on the SHLP recording site and the task performed. It is acknowledged that the small sample size in the present study, together with the uncertainty as to the precise location of recording sites, precludes definitive statements as to concepts of functional heterogeneity within the SHLP. Further studies are needed to clarify the issue.
Absence of Activity at Resting Postural Jaw Position
Comments on Computer-tomography-defined Locations Anatomical studies have demonstrated that the deep temporalis frequently adjoins the lateral part of the SHLP (Widmalm et al., 1987; Akita et al., 2000). The interlacing between the SHLP and the adjacent muscle fibers could confound selective recording from the SHLP in the lateral region. However, spontaneous activity in the deep temporalis was observed in some subjects with their jaws at the resting postural position (Phanachet and Murray, unpublished observations) but was never found in the SHLP in the present study.
This investigation was supported by the National Health and Medical Research Council of Australia (Grant #990460), the Australian Research Council Small Grants Scheme, the Australian Dental Research Foundation, Inc., the Dental Board of NSW, the Dental Alumni Society of the University of Sydney, the University of Sydney Research Grants Scheme, and Canadian Institutes of Health grant 4918. We also acknowledge the photographic and art services of Westmead Hospital, and the Department of Radiology, Westmead Hospital, for the computer tomography scans. Dr I. Phanachet was a Royal Thai Government Sponsored Scholar. Dr. B.J. Sessle is the holder of a Canada Research Chair. The work is based on a thesis submitted to the Faculty of Dentistry, University of Sydney, for the PhD degree. Received for publication May 6, 2002. Revision received October 20, 2002. Accepted for publication October 31, 2002.
Journal of Dental Research, Vol. 82, No. 2,
106-111 (2003) This article has been cited by other articles:
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