Ml Broekhuijsen

FACTORS INFLUENCING JAW POSITION SENSE IN MAN

The relative contribution of muscle sensors, joint sensors and trigeminal exterosensors to the position sense of the mandible was investigated. Subjects compared the sizes of gauges placed between the teeth with the size of a previously explored standard gauge of 4.75 mm. The experimental conditions were either anaesthesia of both mandibular joints and the oral exterosensors, or load on the jaw-closing muscles, or adaptation to an extreme jaw position. The results were compared with control experiments under normal conditions. (1) All subjects matched to standard fairly well in control conditions, the imprecision of the match being approximately 0.5 mm. Under anaesthesia, the matches were 1-1.5 mm greater than the controls, the imprecision being almost unaffected. Anaesthetizing the joints and oral exterosensors before or after exploring the standard gauge did not affect the match. Imposing various loads (less than or equal to 3.4 kg) did not change the match to the standard. Adaptation to a wide open position introduced a considerable mismatch of the standard, leaving the imprecision of the match unaffected. (2) The experiments provide evidence that muscle afferents are important sources in sensing jaw position and fulfil the criteria for an independent position servo in the trigeminal system. It is suggested that the engram of the standard is built up mainly from muscle sensor information.

Peripheral influences on the central pattern-rhythm generator for tongue movements in the rat.

Rhythmic activity in the tongue and other oral muscles was evoked by mechanical stimulation of the hard palate in ketamine-anaesthetized rats. The relation between neural discharges of single hypoglossal motoneurones and activity in the masseter and anterior digastric muscle, and stimulus parameters was analysed. By varying the stimulus parameters, hypoglossal-motoneurone activity was modulated from phasic reflex activity into rhythmic activity. The tongue and other recorded muscles are likely to be controlled by the same pattern-rhythm generator, when rhythmic activity is induced by palatal stimulation. Exteroceptive oral stimuli can activate the tongue pattern-rhythm generator independently of proprioceptive feedback. Proprioceptive input from oral structures influenced the burst duration of rhythmic hypoglossal motoneurone activity. The cycle duration and the number of discharges/burst, however, was not affected by proprioceptive input. The hypoglossal-motoneurone pool may be influenced by more than one pattern-rhythm generator because the burst characteristics of rhythmically-firing hypoglossal motoneurones depend upon stimulus conditions. These hypoglossal pattern-rhythm generators have possible hierarchic relations because a shift from one burst pattern into another can be evoked by changing stimulus conditions. The pattern-rhythm generators for oral movements may be composed of a neural-pattern generator and a neural-rhythm generator, which can be modulated separately by peripheral inputs.

A HIERARCHY OF NEURAL CONTROL OF MASTICATION IN THE RAT

In rats anaesthetized with ketamine, rhythmic jaw-opening and jaw-closing movements were induced by palatal stimulation. The two masseter muscles (jaw-closing) and the four digastric muscles (jaw-opening) were fitted with electrodes, which could be used either for electrical stimulation or for recording electromyographic responses. Electrical stimulation of the masseters in the phase when the digastrics were the contracting muscles, caused responses in the digastrics. The amplitude of these responses was dependent on whether the stimulated masseters were active or not. The responses in digastric persisted when contraction of the masseters during stimulation was prevented by dantrolene sodium but they disappeared when the masseteric nerves were blocked with xylocaine. The responses in digastric are thus reflexes from stimulating afferent fibres in the masseteric nerves. Likewise, electrical stimulation of the four digastrics in the phase when the masseters were contracting, caused responses in the masseters. The amplitude of these responses, however, was independent of the state of activity of the stimulated digastrics. Furthermore, the responses in masseter disappeared when contraction of the digastrics was prevented by dantrolene sodium; but they persisted when the digastric nerves were blocked with xylocaine, provided the digastrics continued to twitch to the electric stimuli. The responses in masseter are thus reflexes in masseter caused by mechanical stretch transmitted from the digastric twitches. In the rhythmic preparation, prevention of contraction of the masseters of digastrics by dantrolene sodium or xylocaine leaves the overall frequency and amplitude of the evoked rhythmic activity unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of the digastric muscles to the control of bite force in man.

The contribution of the (co-contracting) digastric muscles to the rapid decline in bite-force magnitude after unloading of a static bite was investigated by asking participants to perform two different biting tasks with sudden unloading, and correlating the degree of co-contraction of the digastrics (as derived from their electromyograms) with the impact force, the impact velocity (as measured after a travel distance of 5 mm), and the residual force when the jaw system was in static conditions again after the impact. Co-contraction of the digastrics was varied by asking participants to perform the biting task while controlling bite force (force-controlled experiments) or jaw position (position-controlled experiments). In half of the experiments, participants co-contracted their digastrics more strongly in the position-controlled than the force-controlled experiments. However, there was no clear relation between the level of co-contraction and the magnitude of the impact force, the impact velocity and the residual force. The results imply that co-contraction of the digastric muscles is not sufficient to explain the reduction in bite force and the low impact velocity after an unexpected jaw-closing movement. Two other possible mechanisms that reduce forces in an unloaded jaw system are: (1) force velocity properties of the activated jaw muscles in conjunction with creep of the aponeurotic sheets of the jaw muscles, resulting in a slow partial recovery of the biting force after impact: (2) force length properties of jaw-opening muscles, an activity not recorded here.

On the self-perception of jaw positions in man

A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined, verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.

Impact Velocities of the Teeth after a Sudden Unloading at Various Initial Bite Forces, Degrees of Mouth Opening, and Distances of Travel

A potentially dangerous situation arises when an individual bites on hard and brittle food which suddenly breaks, since the impact velocity of the lower teeth onto the upper teeth after the food is broken can be high and may cause dental damage. The present experiments were designed to study the magnitude of the impact velocity after a sudden unloading at various initial bite forces, degrees of mouth opening, and distances of travel. Subjects were asked to perform a static biting task during which the resistance to the bite was suddenly removed. The upward mandible movement was arrested after a certain distance. The velocity of the lower teeth at impact was calculated just before the mandible came to a standstill in combinations of 4 different bite forces (100, 80, 60, and 40 N), 4 different initial degrees of mouth opening (33.5, 30.5, 27.5, and 24.5 mm), and 3 different distances of travel of the mandible (4.5, 3.0, and 1.5 mm). We found that the bite force rapidly declined after the unloading, resulting in a small impact velocity of the lower front teeth. This impact velocity largely depended on the magnitude of the initial bite force and the distance traveled; it was barely sensitive to variations in degree of initial mouth opening. The maximal velocity of the lower teeth was 0.43 m/s (at an initial bite force of 100 N). This maximum was reached after a distance of travel of about 4 mm in 12 ms. The data suggest that the rapid decline in bite force coupled with a limitation of impact velocity is due to the force-velocity properties of the active jaw muscles and is not caused by neural control.

Psychophysical investigations of the preferred vertical dimension of occlusion in edentulous patients

Two psychophysical methods were tested to obtain information on the long-term accuracy and stability of the vertical relation of occlusion as determined by the technique described by Lytle and Timmer (PVDO). One method (Békésy) was unsatisfactory. The other method (forced-choice) produced stable results. The data collected showed that, over a brief period of time, all test subjects could define their PVDO within a margin of less than 1 mm. Some subjects were unable to determine a precisely defined PVDO. These subjects also showed considerable variations in the PVDO over longer periods. Other subjects were able to establish a constant and remarkably stable value for the PVDO over many weeks. For these types of patients, precise determination of the vertical relation of occlusion is of the utmost importance.

Masseter, Digastric and Omohyoidal Responses from Weak Mechanical Oral Stimuli in the Chewing Rat

In rats anesthetized with ketamine, effects of jaw jerks (rise time 5 ms, strength 70 mN) applied in the open direction were studied both in the quiescent preparation and during rhythmic jaw-opening and jaw-closing movements (induced by mechanical stimulation of the palate). The left masseter, posterior and anterior digastric and omohyoid muscles were fitted with electrodes for recording electromyographic responses. In the quiescent state a jaw jerk causes responses in all recorded muscles. In the rhythmic preparation masseter responses persisted but digastric and omohyoidal responses were suppressed. This suppression was most prominent when jerks were applied during closing. However, responses in digastrics and omohyoid persisted when masseter nerves and periodontal afferents of the lower incisors were blocked with xylocaine. The suppression of digastric and omohyoidal responses therefore originates--at least partly - from masseter and periodontal afferents. When the periodontal afferents alone were blocked with xylocaine only the response in the opening phase persisted. The suppression of digastric and omohyoidal responses during opening are therefore reflexes from periodontal receptors. These observations support the concept that oral activity is controlled by two relatively independent systems. The first program-oriented system generates the basic movements whereas the second, environment-oriented, system has access to the program for correcting perturbations. There is evidence that muscles poorly supplied with muscle spindles (digastrics and omohyoid) act as pure effectors. Modulation of their activity is prompted by exteroceptors and muscle receptors in the masticatory muscles.

THE PRECISION OF MEASUREMENT AND THE STABILITY OF THE PREFERRED VERTICAL DIMENSION OF OCCLUSION IN MAN

A longitudinal study on the preferred vertical dimension of occlusion (PVDO) established with edentulous subjects shows that these subjects can be divided into two groups, one in which there is significant instability of PVDO, and a second where the instability of PVDO is not greater than can be expected from the imprecision of the PVDO matches. It is concluded that the subjects in the latter group must have referred to a reasonably fixed standard during their PVDO determinations.

Sensitivity of anterior digastric and intrinsic tongue muscle reflexes to electrical stimulation of various areas in the rat palate.

Abstract The relationship between the sensitivity of various palatal sites to electrical stimulation and threshold reflex activity of the anterior digastric and the intrinsic tongue muscles was studied in decerebrated rats. For the anterior digastric muscle, threshold levels increased in antero-posterior direction; the papilla incisiva and the medial sites on the first antemolar ruga showed the lowest thresholds; medial test spots had lower thresholds than lateral ones. For the intrinsic tongue muscles the papilla incisiva had the lowest thresholds; otherwise thresholds decreased towards postmolar sites; but, in the posterior part of the postmolar region thresholds were markedly raised. Inter-muscle comparison of thresholds showed that in the antemolar region and the most posterior area the digastric had the lowest thresholds. In the intermolar field, no differences between digastric and tongue thresholds were found.