Erich S. Luschei

Evidence that the human jaw stretch reflex increases the resistance of the mandible to small displacements.

1. Small ‘step’ or sinusoidal displacements were imposed on the mandible while human subjects maintained an average biting force of 10 N. Phase‐related changes in the force resisting sinusoidal displacement were used to determine the mechanical stiffness of the human mandibular system as a function of the frequency of stretching. 2. Jaw‐muscle electromyographic (e.m.g.) responses to ‘step’ stretches were of 8 msec latency and generated a very substantial force response. Jaw‐muscle e.m.g. responses having longer latency were not observed. 3. The mechanical stiffness of the human mandible was relatively constant as a function of the frequency of stretching, having a typical magnitude of about 15 N/mm (+/‐ 200 micrometers stretch) or 10 N/mm (+/‐ 1500 micrometers stretch) at mean biting forces of 10 N. The force resisting displacement was phase‐advanced at all frequencies. 4. Modulation of jaw muscle electrical activity evoked by sinusoidal stretches increased in amplitude as a function of increasing stretch frequency. E.m.g. modulation was 60‐‐100 degrees advanced at frequencies of 1‐‐10 Hz, but the phase decreased at higher frequencies, becoming negative (lagging stretch) at frequencies of 30 Hz and above. These characteristics are consistent with the idea that the jaw stretch reflex is dependent on jaw muscle spindle afferent fibres exciting jaw‐closing motoneurones by relatively direct (but not necessarily monosynaptic) connexions. 5. The relationship between jaw‐muscle activity and voluntary fluctuations of isometric biting force suggests that human jaw muscles can be modelled as a second‐order linear ‘filter’. The corner frequency for human jaw muscle is about 3 Hz; thus it would appear to be considerably slower than jaw muscle of monkeys. 6. The reflex stiffness of the human mandible, estimated quantitatively on the assumption that human jaw muscle stiffness is similar to the intrinsic stiffness of the gastrocnemius of the cat, ranges between 5 and 9 N/mm at frequencies between 1 and 8 Hz. Since this reflex stiffness is about the same as muscle stiffness in this frequency range, we conclude that the stretch reflex of the human mandible contributes functionally to its postural stability. 7. Reflex stiffness appears to be greater in the monkey mandible relative to muscle stiffness than in the human mandible. The difference is argued to be a manifestation of the difference in jaw muscle contraction speed between the two species. 8. The fact that the mandibular stretch reflex appears to be stronger than the stretch reflex of the limbs of intact animals and humans is discussed in terms of the special anatomical and functional features of the mandible.

Alterations in the pattern of mastication after ablations of the lateral precentral cortex in rhesus macaques

Abstract Patterns of mandibular movement and jaw muscle activity in two monkeys were recorded during mastication of natural foods before and after ablations of the lateral precentral cortex. Bilateral, but not unilateral, lesions that included the “face” area of the precentral cortex produced permanent changes in patterns of mandibular movement during mastication, including decreased jaw opening and lateral deviation. The basic chewing rate, however, was not greatly altered. Paresis of the tongue and facial muscles, possibly combined with loss of the ability to produce complex jaw movements, hindered manipulation and positioning of food required for mastication. Bilateral lesions of the far-lateral precentral cortex, a region that evokes rhythmic jaw movement when electrically stimulated, initially produced symptoms like those described above, but after a period of recovery, patterns of mastication became normal. The results indicate that the precentral cortex, particularly the face area, is involved in coordination of the tongue, jaw, and facial muscles necessary for normal, efficient mastication. At the same time, the results confirm the existence, in the nonhuman primate, of a subcortical pattern generator capable of producing a pattern of rhythmic jaw movement similar in some respects to masticatory jaw movements of normal monkeys.

Temporomandibular joint meniscectomy - Effects on joint structure and masticatory function in Macaca fascicularis

The postoperative effects of unilateral temporomandibular joint meniscectomy on joint structure and masticatory function were evaluated in four mature Macaca fascicularis and compared with one control. Mandibular movement during mastication was monitored objectively with an optoelectronic tracking system within four months and again within twelve months postmeniscectomy in each animal. Temporomandibular joint structure was documented radiographically and histologically. Results showed that degenerative joint disease was produced in the postsurgical joint, and that the morphological changes were location-dependent. Fibrous ankylosis was observed histologically in three animals. The contralateral joints were not affected morphologically, except for bony resorption of the articular eminence noted microscopically in one animal. Radiographically, however, the joint appeared normal. Variable alterations in masticatory patterns were observed following unilateral meniscectomy. While there appeared to be an association between temporomandibular joint structure and masticatory function, radiographic and microscopic observation of morphological alterations in the joint did not result in predictable functional limitations in chewing pattern.

Human and Macaque Mastication: A Quantitative Study

Significant differences exist between human and Macaca fascicularis patterns of mandibular movement during mastication. Macaque patterns display less asymmetry, more uniformity, and limited lateral excursions when compared to humans for mastication of the same food. Different anatomical structures between the two species offer explanations of the different patterns that were observed. Researchers should use caution when using macaques as models for human mastication.

Recruitment order, contractile characteristics, and firing patterns of motor units in the temporalis muscle of monkeys

Abstract Action potentials of single motor units were recorded in the temporalis muscle of rhesus monkeys trained to hold static forces on a bite bar. The average relationship between mandibular force and motor unit firing rate was determined for many motor units using a computer program, and from this relationship the threshold force at which a motor unit began to fire was determined. Several aspects of motor unit firing rate were examined in relation to recruitment threshold. Computer averaging was used to determine twitch tension and twitch contraction times for many motor units. Motor units recruited at low forces had longer contraction times and produced smaller twitch tensions than higher-threshold motor units. Motor units with small action potentials were nearly always recruited at lower force levels than those with larger action potentials. These results indicate that the motor units of the monkey temporalis muscle were recruited in an orderly fashion, which is in accord with the predictions of the “size principle”.

Alterations in the facial skeleton of the guinea pig following a lesion of the trigeminal motor nucleus

In thirty-three of sixty-six guinea pigs of the Topeka strain small, unilateral, electrolytic lesions were produced in the motor nucleus of the trigeminal nerve. Unoperated animals and the side contralateral to the muscle paralysis served as controls. Lesions were created when animals were 15 days or 75 days of age, and the animals were killed 15, 30, 45, or 60 days postoperatively. Each animal was subjected to dry skull preparations or radiographic and histologic techniques. Alterations in craniofacial form were noted in both the growing animals and the animals with little growth remaining. Alterations in form and function included paralysis and atrophy of the muscles of mastication on the lesion side, hypereruption of teeth, and reduction in growth of facial bones on the lesion side. Remodeling changes were evident in the glenoid fossa, the condylar process, and the coronoid process on the operated side but were not apparent in the contralateral control or the unoperated control animal.

Muscle potentials in reaction time

Abstract Seven human and four monkey subjects performed a reaction time task at short latency following sound or light stimuli. Electromyographic (EMG) potentials were recorded in the responding limb during performance. The EMG latency was analyzed by study of individually recorded traces and by computer averaging. Two classes of EMG activity were observed. One class of potentials was strongly correlated with the response. In human subjects, response-correlated activation of biceps occurred 80 msec following an auditory stimulus and 125 msec following a light stimulus. Slightly longer latencies were recorded for response-correlated activity in monkeys. The second class of muscle potentials was unexpected. Early EMG activity was seen in responding muscles much before the response-correlated potentials and appeared to be more closely linked to the sensory stimuli than to the response. Such early potentials were seen at latencies as brief as 25 to 50 msec following the stimulus in the arms of monkey subjects and were observed in extensor digitorum communis of one human subject when intramuscular recording electrodes were used.

Morphologic alterations in Macaca mulatta following destruction of the motor nucleus of the trigeminal nerve

In two adolescent male Macaca mulatta monkeys, small unilateral electrolytic lesions were produced in the motor nucleus of the trigeminal nerve. The side contralateral to the muscle paralysis served as the control side. The animals were killed 130 and 300 days postoperatively. One animal (animal A) was prepared as a dry skull preparation. Alterations in craniofacial form were noted clinically in both animals and included paralysis and atrophy of the muscles of mastication on the lesion side and mandibular asymmetry. In animal A (300 days postoperative survival), alterations in form included dental and mandibular asymmetry, the appearance of an anterior open-bite on the affected side, and a decrease in the size of the intratemporal fossa on the affected side. Remodeling changes were evident in the condylar process, the zygomatic arch, the orbit, and the zygomaticotemporal and zygomaticofrontal sutures on the lesion side but were not apparent on the contralateral side.

Response of Neurones in the Motor Cortex during Performance of a Simple Repetitive Arm Movement

ANALYSIS of the firing pattern of single cells in the brains of unanaesthetized animals promises to be a useful approach towards understanding brain mechanisms involved in the control of movement. Evarts1, studying reaction time in monkeys, observed that the firing rate of some pyramidal-tract neurones changes before the electromyographic potential associated with the behavioural response. Evartss study provided valuable data on the response latency of pyramidal tract cell discharge before a conditioned arm movement. The reaction time task with non-human subjects has the disadvantage that protracted training is necessary to establish fast and stable responses2, and relatively few behavioural responses are available each day for analysis in relation to unit activity.

Histochemical characteristics of mandibular muscles of monkeys

Abstract Histochemical profiles were obtained for several jaw closing muscles and for one jaw opening muscle. The jaw closing muscles were found to vary widely in their percentages of fast glycolytic (FG) fibers, fast oxidative glycolytic (FOG) fibers, and slow oxidative (SO) fibers. The masseter and the posterior portion of the temporalis muscles had high percentages of fast-contracting fibers (FG and FOG), whereas the medial pterygoid, zygomaticomandibularis (ZMM), and anterior portion of the temporalis muscles were composed largely or entirely of SO fibers. We conclude that the mandibular muscular system, like the limb muscular systems, contains both muscles which are well suited for strong phasic contractions and muscles which are well suited for the production of tonic, low force, vernier contractions. The high degree of compartmentalization of the temporalis muscle may provide a model for studying the distribution of proprioceptors within such a muscle and for studying the role which these receptors play in the control of vernier contractions.