G. M. Murray

Properties and plasticity of the primate somatosensory and motor cortex related to orofacial sensorimotor function.

1. The lateral pericentral region of the cerebral cortex has been well documented in primates to be important in sensorimotor integration and control and in the learning of new motor skills.

Electromyographic recordings of human jaw-jerk reflex characteristics evoked under standardized conditions.

A device for imparting reproducible chin taps was employed to evoke monosynaptic jaw-closing reflexes in subjects without (group A) and with (group B) overt muscle-joint pain dysfunction. Latency, duration and amplitude values obtained were consistent within an individual at constant tap force at a single-recording session but varied among subjects. Latency differences between sides were greater in subjects exhibiting mild to moderate dysfunction (group B) than in group A, but there were not corresponding differences in duration and amplitude. Stimuli were delivered in 5 subjects whilst the jaw was firmly held at postural jaw position with the aid of acrylic resin indices secured with adhesive to the maxillary and mandibular teeth. Recordable reflex responses were evoked in the masseter muscles of one subject only, indicating that vibration alone was not an adequate stimulus consistently to evoke a jaw-closing reflex at postural jaw position. Spindle stretch is needed, unless postural motoneurone excitability is at a sufficiently high level. Reproducible jaw-closing reflexes were evoked following standardized stimuli; subtle variations in motoneurone excitability, such as reflected by differences in jaw-jerk latency between sides, may then become apparent.

Placement and verification of recording electrodes in the superior head of the human lateral pterygoid muscle

Previous studies in man have relied only on electromyographic (EMG) activity patterns and anatomical landmarks to confirm electrode placement in the superior head of the lateral pterygoid. Computed tomography (CT) was used here to determine whether indwelling fine wires could be visualized in that muscle head in man and thus provide evidence for correct placement before EMG. Two imaging sessions were conducted in six individuals. First, a series of contiguous axial CT slices was obtained to calculate the trajectory of insertion of fine wire electrodes into the superior head. For each individual, the trajectories for the location of the fine wires in the approximate mediolateral and anteroposterior centre of the superior head were different. Further axial CT slices were taken after electrode insertion; in each case, these slices clearly showed the wire ends located within the muscle. It was concluded that fine wires in the lateral pterygoid can be imaged by CT. Furthermore, imaging is necessary to accommodate interindividual anatomical variations and to confirm the position of the electrodes within the muscle.

An electromyographic analysis of orofacial motor activities during trained tongue-protrusion and biting tasks in monkeys

This study sought to characterise the electromyographic (EMG) activity patterns of orofacial-muscles during trained tongue-protrusion and biting tasks in two awake monkeys (Macaca fascicularis). Chronic or acute EMG electrodes were placed in the anterior digastric (DIG), genioglossus (GG), masseter (MASS), platysma (PLAT), zygomaticus major (ZYGO), orbicularis oris superior (OOS), and orbicularis oris inferior (OOI) muscles and their EMG activity as well as the force signals of the tongue-protrusion and biting tasks were recorded. A total of 327 tongue-protrusion task trials and a total of 210 biting-task trials were successfully completed in several recording sessions and the EMG patterns were generally consistent between the different sessions. For the tongue task, the mean onset time of increase in GG activity significantly (p < 0.0001) led the mean onset time of increase in the force. The DIG, GG, and OOI (and also the OOS in one of the monkeys) showed a significant (p < 0.0001) increase in mean EMG amplitude during the holding phase, but the GG in both monkeys had the highest mean EMG amplitude ratio (MAR), i.e. the mean EMG amplitude during the holding or dynamic phase divided by the mean EMG amplitude during the pre-trial period. A similar EMG pattern was documented for different directions of the tongue-protrusion task (right, symmetrical, and left) and changes in the levels of EMG activities occurred in GG and OOI as the direction of the tongue-protrusion task changed from left to right. The task at different forces was associated with no apparent change in MAR for the holding phase for each muscle recorded. However, during the dynamic phase, only the GG showed a significant increase in EMG activity as the forces were increased. For the biting task, the mean onset times of the MASS activity and force were not significantly different. The MASS and ZYGO muscles (and the PLAT in one of the monkeys) showed a significant increase in mean EMG amplitude during the holding phase compared with the pre-trial period, and the MASS showed the highest MAR. It was also the only muscle showing a significant increase in the EMG activity when the bite-force level was increased. These findings reveal that certain orofacial muscles are selectively recruited during the two different orofacial tasks.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor Control of Masticatory Muscles

This chapter focuses on the brainstem and higher brain center mechanisms involved in the execution, initiation, reflex regulation, and sensorimotor coordination of the masticatory musculature. A brief overview is given of masticatory musculoskeletal biomechanics, but other chapters may be consulted for general aspects of biomechanics related to motor control and for the structural and functional features of this musculature and its motor units and muscle fibers and sensory innervation. Mastication is a complex motor function that involves the simultaneous bilateral coordinated activation and/or inactivation of the jaw, tongue and face muscles. Jaw opening occurs by downward traction of the mandible by the anterior belly of the digastric muscle and the mylohyoid muscle and anterior traction of the condyles by the lateral pterygoid muscle. Jaw closing occurs by activation of the masseter, temporalis, and medial pterygoid muscles. Jaw protrusion requires activation of the lateral pterygoid, the anterior fibers of the temporalis and the superficial masseter muscles, and jaw retrusion is brought about by activation of the posterior fibers of the temporalis muscles. During mastication, the tongue muscles (e.g., genioglossus—tongue protrusion; hyoglossus—tongue depression; styloglossus—tongue retrusion; palatoglossus—tongue elevation) assist in maneuvering the food bolus from side to side, and the lip muscles (e.g., orbicularis oris—perioral sphincter, zygomaticus major—elevation and retraction of the modiolus) and cheek muscles (buccinators—retraction of the modiolus), along with the tongue muscles, assist in maintaining the food bolus within the mouth on the occlusal table (Dubner et al. 1978; Lang 1995; Miles et al. 2004).

Jaw‐movement smoothness during empty chewing and gum chewing

A major goal of motor coordination is the production of a smooth movement. Jerk-cost, which is an inverse measure of movement smoothness, has been evaluated during gum chewing in previous studies. However, the effect of the gum bolus is still unclear. The aims of this study were to compare the jerk-cost values of normal gum chewing with those of empty chewing. Thirteen subjects undertook, empty chewing, then chewing of gum, and then a second empty chewing. Jerk-cost was calculated from an accelerometer attached to the skin of the mentum. There was a significantly higher smoothness (i.e. lower jerk-cost, P < 0.05) during the opening and second-half closing phases in empty chewing compared with gum chewing. There were no significant differences in jerk-costs (i.e. opening or closing) between the first and the second empty-chewing sequences. These results suggest that the influence of the mechanical effects of tooth contact on jerk-cost is not restricted just to the occlusal phase of chewing, but rather the effect influences the entire opening and closing phases of chewing.

Isotonic resistance jaw exercise alters jaw muscle coordination during jaw movements

The aim was to investigate the effects of isotonic resistance exercise on the electro-myographic (EMG) activity of the jaw muscles during standardised jaw movements. In 12 asymptomatic adults surface EMG activity was recorded from the anterior temporalis and masseter muscles bilaterally and the right anterior digastric muscle during right lateral jaw movements that tracked a target. Participants were randomly assigned to a Control group or an Exercise group. Jaw movement and EMG activity were collected (i) at baseline, before the exercise task (pre-exercise); (ii) immediately after the exercise task (isotonic resistance at 60% MVC against right lateral jaw movements); (iii) after 4 weeks of a home-based exercise programme; and, (iv) at 8-weeks follow-up. There were no significant within-subject or between-group differences in the velocity and amplitude of the right lateral jaw movements either within or between data collection sessions (P > 0.05). However, over the 8 weeks of the study, three of the tested EMG variables (EMG Duration, Time to Peak EMG from EMG Onset, and Time to Peak EMG activity relative to Movement Onset) showed significant (P < 0.05) differences in the five tested muscles. Many of the significant changes occurred in the Control group, while the Exercise group tended to maintain the majority of the tested variables at pre-exercise baseline values. The data suggest a level of variability between recording sessions in the recruitment patterns of some of the muscles of mastication for the production of the same right lateral jaw movement and that isotonic resistance exercise may reduce this variability.

Cerebral Cortical Control of Primate Orofacial Movements: Role of Face Motor Cortex in Trained and Semi-Automatic Motor Behaviours

Despite the extensive literature on the role of the sensorimotor cortex in limb motor control, only limited details were available until recently regarding the cortical mechanisms in orofacial motor behaviour in the primate. Over the past several years, we have used a combination of techniques to elucidate the organisation and neuronal properties of the primate lateral pericentral cortex in relation to orofacial sensorimotor function. In particular, we have shown that reversible cold block of the awake monkey’s face motor cortex (MI), including tongue-MI, significantly reduces the successful performance of a trained tongue protrusion task, but causes little disruption of a biting task (Murray, Lin, Moustafa & Sessle, 1991). These findings are consistent with intracortical microstimulation (ICMS) data indicating a large tongue motor representation in MI but only a minor jaw-closing representation (see Huang, Sirisko, Hiraba, Murray & Sessle, 1988). They also agree with findings that single neurones located at ICMS-defined tongue-MI sites exhibit activity patterns selectively related to the tongue task, including directional specificity, whereas MI neurones with activity related to the biting task are comparatively scarce (Murray & Sessle, 1992 b, c). We have also shown that many tongue-MI neurones are characterised by a close spatial matching of somatosensory input and motor output (Huang, Hiraba & Sessle, 1989a; Murray & Sessle, 1992a). While these findings point to a role for many face-MI neurones in the generation and fine control of voluntary orofacial movements, our ICMS data also revealed that semi-automatic movements (such as those in mastication and swallowing) can be evoked from face MI (see Huang, Hiraba, Murray & Sessle, 1989b). Since this finding suggests that MI might participate in the initiation or control of semi-automatic as well as trained motor behaviours, further studies have been carried out to test this possibility.