Tadashi Nagashima

Sensory components facilitating jaw-closing muscle activities in the rabbit

SummaryThe role of oral and facial sensory receptors in the control of masticatory muscle activities was assessed from the effect of acute deafferentiation on cortically induced rhythmic jaw movements (CRJMs) in anesthetized rabbits. When a thin polyurethane-foam strip (1.5, 2.5 or 3.5 mm thick) was placed between opposing molars during CRJMs, masseteric activities were facilitated in association with an increase in the medial excursion of the mandible during the power phase. The effects varied with the pattern of CRJMs, and the rate of facilitation was greater for small circular movements than for the crescent-shaped movements. Furthermore, the response of the masseter muscle was greater in the anterior half of the muscle, where muscle spindles are most dense, than in its posterior half. It was also demonstrated that the response increased with an increase in the thickness of the test strip. In contrast, the activities of the jaw-opening muscle were not affected significantly. The duration of masseteric bursts increased during application of the test strip and the chewing rhythm tended to slow down. However, the latter effect was not significant. After locally anesthetizing the maxillary and inferior alveolar nerves, the facultative responses of the masseter muscle to the test strip was greatly reduced but not completely abolished. Lesioning of the mesencephalic trigeminal nucleus (Mes V) where the primary ganglion cells of muscle spindle afferents from jaw-closing muscles and some periodontal afferents are located, also reduced the facilitative effects. Similar results were obtained in the animals with the kainic acid injections into the Mes V 1 week before electrical lesioning of this nucleus. In these animals the effects of electrical lesioning of the Mes V could be attributed to the loss of muscle receptor afferents since the neurons in the vicinity of the Mes V were destroyed and replaced by glial cells, whereas the Mes V neurons are resistant to kainic acid. When electrical lesioning of the Mes V and sectioning of the maxillary and inferior alveolar nerves were combined in animals with a kainic acid injection into the Mes V, the response of the masseter muscle to application of the strip was almost completely abolished. From these findings, we conclude that both periodontal receptors and muscle spindles are primarily responsible for the facilitation of jaw-closing muscle activities. Furthermore, it is suggested that the transcortical loop may not be the only path producing this facilitation since similar effects were induced in animals with ablation of the cortical masticatory area (CMA), when the test strip was placed between the molars during rhythmic jaw movements induced by pyramidal tract stimulation.

Properties of rhythmically active reticular neurons around the trigeminal motor nucleus during fictive masticaton in the rat

Response properties of the neurons in the reticular formation around the trigeminal motor nucleus (MoV) were examined during cortically-induced fictive mastication (CIFM) in anesthetized and immobilized rats. Forty-three neurons were rhythmically active (RA neurons) during CIFM, most of which were located in the supratrigeminal nucleus and the reticular formation medial to the oral spinal trigeminal nucleus. The firing frequency of 36 of the RA neurons was modulated in the same rhythm as that of masseteric or digastric nerve activities during CIFM. We divided these neurons into four groups according to the phase of activation: sixteen neurons fired mainly in the phase of masseteric activity (type 1), 11 fired in the transition phase from masseteric activity to digastric activity (type 2), 5 fired in the phase of digastric activity (type 3) and 4 fired in the transition phase from digastric activity to masseteric activity (type 4). Thirty-nine (91%) of the 43 RA neurons responded to at least one of the tested peripheral stimuli. The responses were mostly excitatory but inhibitory responses were sometimes obtained, especially for types-1 and 2 neurons. RA neurons in the reticular formation medial to the oral spinal trigeminal nucleus responded to stimulation of inferior alveolar nerve at a shorter latency than RA neurons in the supratrigeminal nucleus. Fifteen (48%) of 31 RA neurons responded to triple-pulse stimulation of the contralateral cortex. In contrast, only 5(26%) of the 19 RA neurons responded to the ipsilateral cortical stimulation. Stimulation of the ipsilateral MoV was performed on 24 RA neurons, of which 9 responded antidromically (A-RA neurons) at latencies of 0.4-1.4 ms. Eight (89%) of the 9 A-RA neurons received peripheral inputs. The spike triggered averaging method was applied to 4 of the 9 A-RA neurons, ad in all cases short latency field potentials were recorded in the MoV. We conclude that trigeminal premotor neurons receive convergence from central and peripheral inputs. This integration can adjust the appropriate level of motoneuronal excitability during mastication.

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.

The Riddle of the Large Loss in Bite Force after Fast Jaw-closing Movements:

In unloading experiments (in which the resistance to a forceful static bite is suddenly removed), it is shown that the residual bite force (when the jaw system is arrested shortly after the unloading) is remarkably small. For example, of a 100-N initial bite force, only 18 N is left after a jaw travel distance of 5.0 mm. The present experiments were designed to study whether the magnitude of the low residual bite force is dependent on the initial bite force, the initial degree of mouth opening, and the distance of jaw travel. Furthermore, we analyzed whether the low magnitude of the residual force can be attributed to reflex events of the jaw muscles or to the force-length properties of the jaw-closing muscles. It was found that the residual forces are largely dependent on the distance of jaw travel and are barely sensitive to variations in initial mouth-opening. The relative residual forces are independent of the magnitude of the initial bite force. The maximum residual forces are on the order of 25% of the initial bite force after a jaw travel of 4.5 mm. The low values of the residual forces cannot be attributed to reflex events, because it took about 80 ms for the masseter muscles to decrease their force to a 50% level after their excitation was switched off. Furthermore, it was shown that the force-length properties of the jaw-closing muscles are not responsible for the small values of the residual forces, since over the trajectories used in the present experiments, the sarcomere lengths of the jaw-closing muscles were beyond their optimum. It is suggested that the low residual forces are brought about by (1) a non-uniform sarcomere behavior of the jaw-closing muscles when contracting, or (2) a long-lasting change in the myofilament system of the closing muscles induced by the sudden shortening of muscle fibers.

Application of Piezography in Complete Denture. Evaluation of Impression Materials.

Piezography used mainly within France and Spain is one of the functional approaches to obtain the proper denture space in complete and partial dentures. The purpose of this study was to find impression materials suitable for Piezography. Five subjects and five operators (mean age: 27.0±1.5 years) were selected for the study. Eleven kinds of commercially available soft impression materials were examined by using Visual Analog Scale (VAS) on six items, irritation to oral soft tissues, operability, aptness to pronounce and to hear words, hardening time, and surface smoothness.As a result, FITT (Kerr Co.) used in the original Piezography was the best impression material. Tissue Conditioner (Shofu Inc.) and Evatouch H-type (Neo Dental Chemical Products Co.) also obtained high evaluation among impression materials tested in this study. In consideration of economic efficiency, it was suggested that Tissue Conditioner was the best impression material for Piezography in Japan.

Effect of Metal Surface Treatment with Gold Dust on Bonding Strength between Metal and Porcelain.

It has been proved in previous studies that the surface treatment of cobalt-chromium alloy with gold dust was effective not only to increase the bonding strength between metal and porcelain, but also to cover metal color and prevent the border coloring.In this study, two kinds of gold alloys for porcelain fusing indifferent compositions (Select 50 and Select 77 H, Ohura Precious Metal Co.) were selected to examine the effect of metal surface treatment with gold dust on the bonding strength. The size of gold dust (Super-Binder OG, Ohura Precious Metal Co.) used in this study was 1.0 μm in a uniform diameter.The results showed that both alloys exhibited stronger bonding strength after treated with gold dust, particularly in the case of Select 50. Most of the gold dust remained on the fractured surface of Select 50 specimens after the shear test, observed by scanning electron microscope. It was also found that gold dust existed on both metal and porcelain surfaces in the case of Select 77 H.The above results suggested that the treatment of a metal surface with gold dust could reinforce the bonding strength between gold alloy and porcelain. Furthermore, the bonding strength would increase greatly if the gold dust was bonded firmly to the metal surface.

Questionnaire for Craniomandibular Disorder Patients.

To elucidate the long-term development of symptoms as to craniomandibular disorders, a questionnaire was sent, in May 1992, to 200 patients referred to the clinic of removableprosthodontics of the Hospital attached to Osaka University Faculty of Dentistry, who had beentreated from 1984 to 1991. Ninety-seven (48.5%) patients answered the questionnaire. Theresults from their answers and clinical protocols were as follows:Sixty-one (63.0%) patients developed symptoms 3 to 99 months after the termination oftreatment. Their most common symptoms were TMJ sounds (38.6%), muscle dysfunction (31.1%), trismus (15.9%) and TMJ pain (11.4%), which were 97% of all. Twenty-five (28.3%) patients had recurrence of symptoms: the ratio of recurrence in the patients who had stoppedreceiving regular treatment (46.4%) was significantly higher than that in those who had continuedto the end (18.8%). Few patients had recurrence within a year after treatment, therefore, it issuggested that a clinical recall examination should at least be performed once a year from thetermination of treatment for many years.

Development of a computer system for determining retentive force of cast clasps

The purpose of this study is to develop a rational fabricating system for determining retentive force of cast clasps. The radius of curvature of a circumferential clasp arm is considered to be one of the important factors affecting dynamical properties of cast clasps. In this study, a device for determining the radius of curvature and a computer system for Akers clasps and I-bar clasps were developed. It was suggested to be very significant in partial denture designing, that the retentive force and the bending rigidity required according to various conditions around abutment teeth can be easily obtained by using a personal computer, and that the proper part of a commercially available clasp pattern required to fabricate cast clasps providing rational retentive forces can be determined in this system.