Two neuronal groups for NaCl with differential taste response properties and topographical distributions in the rat parabrachial nucleus.

It is crucial for animals to discriminate between palatable (safe) and aversive (toxic) tastants. The mechanisms underlying neuronal discrimination of taste stimuli remain unclear. We examined relations between taste response properties (spike counts, response duration, and coefficient of variation [CV]) and location of taste-sensitive neurons in the pontine parabrachial nucleus (PBN). Extracellular single units' activity in the PBN of Wistar rats was recorded using multibarrel glass micropipettes under urethane anesthesia. Forty taste-sensitive neurons were classified as NaCl (N)-best (n = 15), NaCl/HCl (NH)-best (n = 14), HCl (H)-best (n = 8), and sucrose (S)-best (n = 3) neurons. The net response to NaCl (15.2 ± 2.3 spikes/s) among the N-best neurons was significantly larger than that among the NH-best (4.5 ± 0.8 spikes/s) neurons. The response duration (4.5 ± 0.2 s) of the N-best neurons to NaCl was significantly longer than that of the NH-best (2.2 ± 0.3 s) neurons. These differences in the spike counts and the response durations between the two neuronal types in the PBN were similar to that previously reported in the rostral nucleus of the solitary tract (rNST). The CVs in the N-best and the NH-best neurons were significantly smaller in the PBN than those in the rNST. Histologically, most N-best neurons (12/13, 92%) were localized to the medial region, while NH-best neurons (11/13, 85%) were primarily found within the brachium conjunctivum. These results suggest that NaCl-specific taste information is transmitted by two distinct neuronal groups (N-best and NH-best), with different taste properties and locations within rNST to PBN tractography. Future studies on the higher order nuclei for taste could reveal more palatable and aversive taste pathways.

Changes in masseter muscle fibers by liquid diet rearing in rabbits and recovery by chewing of solid diet.

OBJECTIVE: To investigate the effects of liquid diet on the development of masseter muscle fibers and whether the changes in the masseter muscle can be recovered by chewing of solid diet. DESIGN: Masseter muscles from 40 rabbits (solid- and liquid-diet groups, n = 30; unweaned group, n = 5; recovery group, n = 5) were histochemically examined at 4, 12, 18, and 33 weeks after birth. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were distinguished via mATPase staining. Muscle fiber diameter and fiber type composition were measured and compared between groups. RESULTS: In the liquid diet group, the diameter of types IIAB (solid group: 81.7 µm, liquid group: 60.9 µm) and IIB (solid group: 89.3 µm, liquid group: 68.8 µm) and the fiber type composition of type I (solid group: 18.4%, liquid group: 9.6%) decreased significantly at 33 weeks of age. In the recovery group, the fiber type composition of type I fibers recovered to 16.5%, while no recovery of type IIAB (56.6 µm) and IIB (64.6 µm) fiber diameter was observed. CONCLUSIONS: Liquid diet caused atrophy of muscle fibers and an increase in the proportion of fast-twitch fibers. Although the diameter and ratio of slow-twitch fibers were recovered by chewing of solid diet, recovery was not observed for fast-twitch fibers. Our findings are relevant for dental medicine as it explored the possibility of masticatory muscle function recovery by hard food.

Development of medial pterygoid muscle fibers in rabbits fed with a liquid diet.

OBJECTIVE: This study aimed to investigate the influence of decreased functional load on the medial pterygoid muscle during mastication in rabbits fed with a liquid-diet. MATERIALS AND METHODS: Medial pterygoid muscles from 54 rabbits (solid- and liquid-diet groups, n=48; unweaned group, n=6) were histochemically examined at 4, 9, 12, 18, and 33 weeks after birth. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were distinguished via mATPase staining. RESULTS: Significant increases in the diameters of all fiber types were seen up to 33 weeks of age in the solid-diet group; however, no significant increase was noted in fiber types I and IC, from 4 to 33 weeks of age, in the liquid-diet group. The proportion of slow fibers increased up to 12 weeks followed by an increase in the number of fast fibers in the solid-diet group, whereas in the liquid-diet group, the number of slow fiber declined after weaning. CONCLUSIONS: Liquid-diet consumption caused muscle fiber atrophy and an increase in the number of fast fibers during early developmental stages after weaning. Furthermore, the growth pattern of the medial pterygoid muscle in the liquid-diet group was different from that in the solid-diet group.

Histochemical study of rabbit medial pterygoid muscle during postnatal development.

The medial pterygoid muscle is a layered structure like the masseter muscle. This study aimed at investigating the regional differences in fiber type composition and fiber diameter of the medial pterygoid muscle in the rabbit from birth until 33 weeks of age. Histochemical analysis of the medial pterygoid muscle was performed during five developmental stages (4, 9, 12, 18, and 33 weeks after birth) in 30 male Japanese white rabbits. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were identified by mATPase staining. An increase in diameter was observed in fiber types I and IC until 9 weeks of age, and in fiber types IIC, IIA, IIAB, and IIB until 33 weeks of age. No significant differences in fiber diameter were noted in the different regions of the pterygoid muscle. Moderate fast to slow fiber type shifts occurred from weeks 4-12; thereafter, a rapid slow to fast fiber type shift was observed. Significant differences in fiber type composition based on regional differences were noted at 4 weeks of age. However, there was no difference in fiber type composition between regions at 33 weeks. In conclusion, it was clear that the diameter and proportion of fast fibers had increased even after reaching sexual maturity in rabbits. In addition, the medial pterygoid muscle tissues appeared to be homogenous at 33 weeks of age with very few differences between regions.

Postero-inferior condylar movement induced by artificial occlusal interference on the balancing side during fictive mastication in rabbits.

OBJECTIVE: Tooth contact does not occur on the balancing side during mastication. Hence, it is possible that the presence of occlusal interference on the balancing side causes mandibular rotation followed by atypical condylar movement because the jaw-closing muscle activity on the working side is greater than on the balancing side. The aim of the present study was to investigate the relationship between occlusal contact on the balancing side and condylar movement during mastication. METHODS: EMG activity of the masseter (MS), lateral pterygoid (LP) and digastric (DG) muscles and jaw movements were recorded. Condylar movements in the sagittal plane were recorded using a high speed charge-coupled device (CCD) camera. Incisal point movements were recorded using a magnet on the mentum and a magnetometric sensor on the nasal bone. A removable biting plate was used to introduce an artificial occlusal interference on the balancing side. RESULTS: Nine of the 10 rabbits showed large postero-inferior condylar movement (Pi-Cm) when a biting plate was applied on the balancing side. Five rabbits showed very small Pi-Cm when a biting plate was applied on the balancing side. In the small Pi-Cm group, MS activity decreased markedly and LP and DG transient activity appeared in the early occlusal phase in the presence of the biting plate. CONCLUSION: Interference on the balancing side always caused Pi-Cm on the ipsilateral side during mastication. However, the degree of Pi-Cm was often decreased by a jaw opening reflex response.

Unusual postero-inferior condylar movements that depend on the position of occlusal contact during fictive mastication in rabbits.

OBJECTIVE: The mandible can be modelled as a triangular plate supported at two joints and the point of occlusion. The mandible is stable if the vector of the jaw-closing muscle forces lies within the triangle of support. If this vector lies outside of the triangle of support, one of the three contact points will tend to separate as the mandible rotates around a line connecting the other two points. Here, we examined whether postero-inferior condylar movements (Pi-Cm) due to mandibular rotation may occur during fictive mastication in anaesthetized rabbits. METHODS: EMG activities of the masseter (MS) and lateral pterygoid (LP) muscles and movements of the condyle and incisal points were recorded. Condylar movements in the sagittal plane were recorded using a high speed CCD camera. RESULTS: Pi-Cm were observed on the working side during occlusal phase in half of the rabbits (altered-movement group), if the biting point was restricted at the posterior most tooth (M3) on the working side using a metal biting plate. Pi-Cm appeared in the period between the estimated maximum force of the MS and the LP during late occlusal phase. The MS EMG ratio between the working and balancing sides in the altered-movement group was significantly less than that in the unaltered-movement group. CONCLUSION: Since the space lying between the condyle and the articular eminence expanded during the Pi-Cm, it is likely that the posterior band of the articular disc tended to slip anteriorly. The clinical significance of the Pi-Cm is discussed concerning the anterior dislocation of the disc in patients with TMJ disorder.

Effects of medullary lesions on conditional pacemaker activity of neonatal rat hypoglossal motoneurons in vitro.

N-methyl-D-aspartate (NMDA) has been demonstrated to induce rhythmic activity in various neurons, including hypoglossal motoneurons (XIIms) and converts them to conditional pacemakers. Using whole-cell patch clamp recording in a slice preparation from neonatal rats, we confirmed that some XIIms act as conditional pacemakers, with TTX-insensitivity and a burst period that is voltage-dependent during NMDA application. Other XIIms in this study only fired tonically with NMDA application. Effects of medullary structures on conditional pacemaker XIIms were assessed using lesioned preparations. As a result, NMDA-induced rhythm (NIR) in the XIIm was observed with ventral lesions (excluding inspiratory neurons) and with dorsal lesions (excluding the swallowing center located in the nucleus of the solitary tract). The NIR was also observed with lateral lesions, but with a significantly decreased burst period. These data suggest that NMDA receptor activation selects a subset of XIIms and changes them to pacemakers whose properties can be altered by their excitability. The data also demonstrate that structures fundamental to the NIR are located within the area near the XII nucleus, indicating that the NIR is distinct from inspiratory and swallowing activities. The lateral medulla is considered to be a source of modulation of the excitability of XIIms.

Movement of the mandibular condyle and activity of the masseter and lateral pterygoid muscles during masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of rabbits.

We examined the functional role of the lateral pterygoid muscle (LP) and the masseter muscle (MS) in the movement of the mandibular condyle in masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of urethane-anaesthetised rabbits. EMGs of the LP and MS were recorded along with video images of the mandibular condyle movement filmed with a high-speed CCD camera at a time resolution of 8 ms. The time required for the contractile force of the MS and LP to emerge as bite force or jaw movement was determined by direct electrical stimulation to respective muscles: 32.8+/-1.5 ms for the MS and 34.3+/-2.9 ms for the LP. The LP on the working side showed biphasic activity not only in the jaw-opening phase but also in the middle occlusal phase. It is assumed that the MS on the working side begins to exert actually the maximum mechanical influence on the bite force 32.8 ms after its EMG peak in the early occlusal phase. Such development of the mechanical effect of the MS during middle occlusal phase is appeared to be involved in stabilization of the condyle in the middle occlusal phase. Approximately 25 ms (25.1+/-2.5 ms) after the peak of mechanical influence of the MS, the mechanical influence of the working-side LP activity reached maximum in the late stage of the occlusal phase. This LP mechanical influence seems to be functionally associated with an antero-inferior movement of the working side condyle in the late stage of the occlusal phase that is coincident with a movement of the incisal point towards the balancing side across the midline.

Velocity-dependent EMG activity of masseter and sternocleidomastoideus muscles during a ballistic arm thrusting movement.

Two experiments were conducted to investigate the functional relationship between the general somatic motor function and the oral motor function. In Experiment 1, we analyzed the relationship between the amount of masseter muscle (MSS) activity and the velocity of a ballistic, 'karate-do' arm thrusting movement (ThrMov). ThrMov velocity was measured from video images taken with a high-speed CCD camera at a frequency of 500Hz. EMGs of MSS and sternocleidomastoideus (SCM) muscles as well as other related muscles were recorded simultaneously with video images in 6 varsity 'karate-do' athletes. Pearson's correlation coefficients were calculated between EMG amplitude and movement velocity. EMG activity of MSS as well as the other muscles increased as a function of ThrMov velocity in all participants, as evidenced by highly significant (p<.01) correlation coefficients, ranging from .64 to .87 (mean: .75). MSS EMG activity attained during ThrMovs performed at maximum velocity ranged between 14.6% and 113.8% of this muscle's MVC (45.7+/-39.3% MVC, mean+/-SD). SCM was also strongly active and closely associated with MSS. Besides changes in amount of EMG activity, it was further found that R-MSS EMG onset progressively shifted to the earlier phase of the ThrMov as ThrMov velocity increased. EMG onset time of R-MSS as well as R- and L-SCMs was negatively correlated with ThrMov velocity; when performed at maximum velocity MSS activation preceded the start of ThrMov by more than 100ms, whereas MSS was recruited last at approximately 150ms after the start of ThrMov when performed at moderate speed ( approximately 50% of maximum). In Experiment 2, the effects of head movement relative to the trunk on R-MSS and SCMs EMG activity were tested in both gazing and sidelong glancing conditions. A much smaller head rotation relative to the trunk was necessary during the ThrMov in the sidelong glancing condition compared to the gazing condition. R-MSS EMG activity was affected significantly by the difference between these conditions and decreased by 5.2% MVC in the sidelong glancing condition compared to the gazing condition. In association with the change in requirement for head movement between those conditions, EMG balance between the bilateral SCMs changed substantially. Finally, marked muscle activity during ThrMov was found in the MSS that was not directly involved in performing this movement, indicating a form of 'remote facilitation'.