Effect of the stimulation of sensory inputs on the firing of neurons of the trigeminal main sensory nucleus in the rat.

Mastication can be triggered by repetitive stimulation of the cortex or of sensory inputs, but is patterned by a brain stem central pattern generator (CPG). This CPG may include the dorsal part of the principal trigeminal sensory nucleus (NVsnpr), where neurons burst repetitively when the extracellular concentration of Ca(2+) ([Ca(2+)](e)) drops. We examined the effects of repetitive stimulation of sensory afferents of the trigeminal tract on activity of NVsnpr neurons recorded extracellularly in vitro under physiologic [Ca(2+)](e) (1.6 mM). Spontaneously active cells had either a tonic (n = 145) or a bursting (n = 46) firing pattern. Afferent stimulation altered burst duration and/or burst frequency in bursting cells and firing frequency in most tonic cells. In 28% of the latter, the firing pattern switched to rhythmic bursting. This effect could be mimicked by local application of N-methyl-d-aspartate and blocked by APV but not DNQX. Detailed analysis showed that rhythm indices (RIs) of 35 tonic neurons that were negative (nonrhythmic) before stimulation became significantly rhythmic (RI > or = 0.01) after stimulation. Mean and median bursting frequency of these units were 8.32 +/- 0.72 (SE) Hz and 6.25 Hz (range, 2.5-17.5 Hz). In seven instances, two units were recorded simultaneously, and cross-correlation analysis showed that firing of six pairs was rhythmic and synchronized after stimulation. Optimal stimulation parameters for eliciting rhythmic bursting consisted in 500-ms trains of pulses delivered at 40-60 Hz. Together, our results show that repetitive stimulation of sensory afferents in vitro can elicit masticatory-like rhythmic bursting in NVsnpr neurons at physiological [Ca(2+)](e).

Respiratory rhythms generated in the lamprey rhombencephalon.

Brainstem networks generating the respiratory rhythm in lampreys are still not fully characterized. In this study, we described the patterns of respiratory activities and we identified the general location of underlying neural networks. In a semi-intact preparation including the brain and gills, rhythmic discharges were recorded bilaterally with surface electrodes placed over the vagal motoneurons. The main respiratory output driving rhythmic gill movements consisted of short bursts (40.9+/-15.6 ms) of discharge occurring at a frequency of 1.0+/-0.3 Hz. This fast pattern was interrupted by long bursts (506.3+/-174.6 ms) recurring with an average period of 37.4+/-24.9 s. After isolating the brainstem by cutting all cranial nerves, the frequency of the short respiratory bursts did not change significantly, but the slow pattern was less frequent. Local injections of a glutamate agonist (AMPA) and antagonists (6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or D,L-amino-5-phosphonopentanoic acid (AP5)) were made over different brainstem regions to influence respiratory output. The results were similar in the semi-intact and isolated-brainstem preparations. Unilateral injection of AP5 or CNQX over a rostral rhombencephalic region, lateral to the rostral pole of the trigeminal motor nucleus, decreased the frequency of the fast respiratory rhythm bilaterally or stopped it altogether. Injection of AMPA at the same site increased the rate of the fast respiratory rhythm and decreased the frequency of the slow pattern. The activity recorded in this area was synchronous with that recorded over the vagal motoneurons. After a complete transverse lesion of the brainstem caudal to the trigeminal motor nucleus, the fast rhythm was confined to the rostral area, while only the slow activity persisted in the vagal motoneurons. Our results support the hypothesis that normal breathing depends on the activity of neurons located in the rostral rhombencephalon in lampreys, whereas the caudal rhombencephalon generates the slow pattern.

Modulation of respiratory activity by locomotion in lampreys.

In vertebrates, locomotion is associated with changes in respiratory activity, but the neural mechanisms by which this occurs remain unknown. We began examining this in lampreys using a semi-intact preparation of young adult Petromyzon marinus, in which respiratory and locomotor behaviors can be recorded simultaneously with the activity of the underlying neural control systems. Spontaneous fictive respiration was recorded with suction electrodes positioned over the glossopharyngeal or the rostral vagal motor nucleus. In this preparation, locomotor activity, characterized by symmetrical tail movements (electromyogram recordings), was evoked by mechanical stimulation of the skin. During locomotion, the mean respiratory frequency and the mean area of the motor bursts were significantly increased (81.6+/-28.6% and 62.8+/-25.4%, respectively; P<0.05). The frequency returned to normal 92+/-51 s after the end of locomotion. There were fluctuations in the instantaneous respiratory and locomotor frequencies that were rhythmical but antiphasic for the two rhythmic activities. The changes in respiratory activity were also examined during bouts of locomotion occurring spontaneously, and it was found that a modification in respiratory activity preceded the onset of spontaneous locomotion by 3.5+/-2.6 s. This suggests that the early respiratory changes are anticipatory and are not caused by feedback generated by locomotion. The increase in respiratory frequency during locomotion induced by sensory stimulation persisted after removal of the mesencephalon. When both the mesencephalon and spinal cord were removed, resulting in the isolation of the rhombencephalon, changes in the respiratory activity were also present following skin stimulations that would have normally induced locomotion. Altogether, the results suggest that respiratory changes are programmed to adjust ventilation prior to motor activity, and that a central rhombencephalic mechanism is involved.

Measuring chewing ability in randomized controlled trials with edentulous populations wearing implant prostheses.

In this article, the authors review methods commonly used to measure the masticatory ability of patients wearing conventional and implant prostheses. Measurement approaches are viewed in two broad categories: laboratory based and patient based. The laboratory based measures discussed are mastication time, particle size, force measurements, patterns of jaw movement and electromyographic (EMG) bursts and total EMG activity. Patient based measures include satisfaction and chewing difficulty. Oral health-related quality of life measures and diet/nutrition variables are briefly mentioned. Patient-based outcomes are recommended as the most appropriate variables of masticatory efficiency.

Morphological and immunohistochemical characterization of interneurons within the rat trigeminal motor nucleus.

Three series of experiments were carried out to characterize interneurons located within the trigeminal motor nucleus of young rats aged 5-24 days. Cholera toxin injections were made bilaterally into the masseter and, sometimes, digastric muscles to label motoneurons. In the first set of experiments, thick slices were taken from the pontine brainstem and cholera toxin-positive and cholera toxin-negative neurons located inside the trigeminal motor nucleus were filled with biocytin through whole-cell recording patch electrodes. Positively identified motoneurons (cholera toxin+) of various shapes and sizes always had a thick, unbranched axon that entered the motor root following a tight zigzag course. Many cholera toxin-negative neurons were also classified as motoneurons after biocytin filling based on this particularity of their axon. These are probably either fusimotor motoneurons or motoneurons supplying other jaw muscles. The cholera toxin-negative neurons classified as interneurons differed markedly from motoneurons in that they had thin, usually branched axons that supplied the ipsilateral reticular region surrounding the trigeminal motor nucleus (peritrigeminal area), the main trigeminal sensory nucleus, the trigeminal mesencephalic nucleus, the medial reticular formation of both sides, and the contralateral medial peritrigeminal area. Most often, their dendrites were arranged in bipolar arbors that extended beyond the borders of the trigeminal motor nucleus into the peritrigeminal area. Immunohistochemistry against glutamate, GABA and glycine was used to further document the nature and distribution of putative interneurons. Immunoreactive neurons were uniformly distributed throughout the rostro-caudal extent of the trigeminal motor nucleus. Their concentration seemed greater toward the edges of the nucleus and they were scarce in the digastric motoneuron pool. Glutamate- outnumbered GABA- and glycine-immunoreactive neurons. There was no clear segregation between the three populations. In the final experiment, 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyanine perchlorate crystals were inserted into one trigeminal motor nucleus in thick slices and allowed to diffuse for several weeks. This procedure marked commissural fibers and interneurons in the contralateral trigeminal motor nucleus. Together these results conclusively support the existence of interneurons in the trigeminal motor nucleus.

Inputs to nucleus pontis caudalis from adjacent trigeminal areas.

Recent studies suggest that the nucleus pontis caudalis (nPontc) plays a role in patterning mastication through interactions with the adjacent lateral tegmentum. In this study, we used in vitro intracellular recording and staining to describe the basic membrane properties and morphology of nPontc neurones and to further explore interactions with adjacent structures, using coronal sections of the brainstem of 78 rats, aged 9-28 days. Neurones were large, with dendrites that spread in all directions, and about 64% fired tonically even in the absence of synaptic inputs. Tonic neurones were predominant in the centre of the nucleus. Electrical stimulation of all regions of the nPontc produced mixed excitatory and inhibitory effects on interneurones of lateral tegmental nuclei. Focal inactivation of the dorsal nPontc with injections of tetrodotoxin also had mixed effects on the spontaneous firing of both interneurones and motoneurones but similar injections in the ventral nPontc produced mostly increases of firing. Sixty-five percent of nPontc neurones received synaptic inputs from the lateral tegmental areas and most of these (68%) were excitatory and mediated by glutamatergic receptors. Inhibitory postsynaptic potentials were mediated by GABA(A) or glycinergic receptors. Although most responses occurred at relatively long latencies (> 2 ms), they could follow relatively high-frequency stimulation (> 50 Hz). Excitatory and inhibitory connections between ipsi- and contralateral nPontc neurones were also documented, which could contribute to bilateral coordination of jaw movements. This study provides evidence that the nPontc exerts both tonic and phasic influences on the premotor components of the masticatory central pattern generator.

Cost-effectiveness of mandibular two-implant overdentures and conventional dentures in the edentulous elderly.

Implementation of new therapies is usually governed by financial considerations, so efficacy studies should also include cost comparisons. The cost and effectiveness of mandibular conventional dentures (CD, n = 30) and two-implant overdentures (IOD, n = 30) were compared in elderly subjects. Effectiveness (Oral Health Impact Profile, OHIP-20) and cost were measured up to one year post-treatment. Data for subsequent years were estimated by the Delphi method. Using an average life expectancy of 17.9 years, the equalized annual costs (in Canadian dollars) were dollar 399 for CD and dollar 625 for IOD (p < 0.001), and the equalized annual values for the outcome (OHIP-20) were 47.0 for CD and 31.3 for IOD treatment (p < 0.05). These values translate into a yearly additional cost for IOD treatment of dollar 14.41 per OHIP-20 point. These results are key to the implementation of programs to provide this form of therapy for edentulous adults.

Speech with maxillary implant prostheses: ratings of articulation.

Speech is often perturbed after placement of maxillary implant-retained prostheses. We tested the hypothesis that the rate of speech errors varies with prosthetic design. Thirty edentulous subjects with mandibular implant prostheses entered two within-subject crossover trials. Subjects wore maxillary fixed prostheses and removable long-bar overdentures (Trial 1), or overdentures with and without palates (Trial 2). Test words from a French language speech battery were recorded after each prosthesis had been worn for two months. The percentages of stops, fricatives, and vowels correctly perceived by lay judges were calculated. Subjects produced a significantly higher percentage of sounds correctly with overdentures than with fixed prostheses. Between-treatment differences were significant for stops and fricatives (p < 0.01), but not for vowels. There were no significant differences in error rates between the two overdentures. In conclusion, maxillary implant overdentures with and without palates enable patients to produce more intelligible speech than fixed prostheses.

Anatomical and physiological study of respiratory motor innervation in lampreys.

The innervation of gill muscles of lampreys was investigated in a semi-intact preparation in which the respiratory rhythm was maintained for more than 2 days. Lesion experiments showed that the muscles of gill 1 are innervated by nerves VII (facial) and IX (glossopharyngeal), and those of gill 2 by nerve IX and the first branchial branch of nerve X (vagal). The other gills are supplied by the other branchial branches of nerve X. Retrograde tracers, injected in peripheral respiratory nerves, showed that branchial muscles are innervated by VII, IX and X motoneurons. Within the X nucleus, the motoneuron pools were branchiotopically organized, but with considerable rostro-caudal overlap. Electrophysiological recordings were used to show that the onset of activation of the branchial muscles was increasingly delayed with the distance from the brainstem, but that motoneuronal activity recorded with surface electrodes began at approximately the same time in all pools. The conduction velocity of VII and caudal X motor axons was found to be the same. Differences in the length of motoneuron axons appear to account for the rostro-caudal delay in gill contraction. The data presented here provide a much needed anatomical and physiological basis for further studies on the neural network controlling respiration in lampreys.

Effect of lidocaine and NMDA injections into the medial pontobulbar reticular formation on mastication evoked by cortical stimulation in anaesthetized rabbits.

Neurons of the dorsal nucleus reticularis pontis caudalis (nPontc) fire rhythmically during fictive mastication, while neurons of the ventral half tend to fire tonically (Westberg et al., 2001). This paper describes the changes in the pattern of rhythmical mastication elicited by stimulation of the sensorimotor cortex during inhibition or excitation of neurons in this nucleus and adjacent parts of nucleus reticularis gigantocellularis (Rgc) in the anaesthetized rabbit. Masticatory movements and electromyographic (EMG) activity of the masseter and digastric muscles produced by cortical stimulation were recorded before, during and after injections of a local anaesthetic (lidocaine) or excitatory amino acid N-methyl-d-aspartate (NMDA) into nPontc and Rgc through a microsyringe with attached microelectrode to record neuronal activity. Lidocaine inhibited local neurons and modified the motor program, and the effects varied with the site of injection. Most injections into the ventral half of nPontc increased cycle duration, digastric burst duration and burst area. The action of lidocaine in dorsal nPontc was more variable, although burst duration and area were often decreased. The effects on the muscle activity were always bilateral. Lidocaine block of the rostromedial part of Rgc had no effect on movements or on EMGs. Injections of NMDA excited local neurons and when injected into ventral nPontc, it completely blocked mastication. Dorsal injections either had no effect or increased cycle frequency, while decreasing burst duration and area. No increases in EMG burst duration or area were observed with NMDA. Our findings suggest that neurons of ventral nPontc tonically inhibit other parts of the central pattern generator during mastication, while dorsal neurons have mixed effects. We incorporated these findings into a new model of the masticatory central pattern generator.

Evidence that experimentally induced sleep bruxism is a consequence of transient arousal.

Spontaneous rhythmic masticatory muscle activity (RMMA) during sleep occurs more frequently following spontaneous transient micro-arousal in patients with sleep bruxism (SB) and normal controls. Here, we tested the hypothesis that an experimental arousal would be followed by an increase in RMMA. We identified RMMA on polygraphic recordings taken before and after sensory stimulation to induce experimental arousal in eight SB patients and eight matched normal subjects. The rate of experimental arousal and the level of resting electromyographic activity in masseter and suprahyoid muscles during sleep did not differ between the groups. In both, muscle tone and heart rate increased during the experimental arousal. Although post-arousal RMMA occurred in all SB patients, it was seen in only one normal subject. Moreover, tooth-grinding occurred during 71% of the evoked RMMA in SB patients. These results support the hypothesis that SB is an exaggerated form of oromotor activity associated with sleep micro-arousal.

Neurons of the trigeminal main sensory nucleus participate in the generation of rhythmic motor patterns.

The trigeminal principal sensory nucleus (NVsnpr) contains both trigemino-thalamic neurons and interneurons projecting to the reticular formation and brainstem motor nuclei. Here we describe the inputs and patterns of firing of NVsnpr neurons during fictive mastication in anaesthetized and paralysed rabbits to determine the role that NVsnpr may play in patterning mastication. Of the 272 neurons recorded in NVsnpr, 107 changed their firing patterns during repetitive stimulation of the left or right sensorimotor cortex to induce fictive mastication. Thirty increased their firing tonically. Seventy-seven became rhythmically active, but only 31 fired in phase with mastication. The others discharged in bursts at more than twice the frequency of trigeminal motoneurons. Most rhythmic masticatory neurons were concentrated in the dorsal part, and those which fired during the jaw closing phase of the cycle were confined to the anterior pole of the nucleus. Most of these cells had inputs from muscle spindle afferents, whereas most of those firing during jaw opening had inputs from periodontal receptors. Non-masticatory rhythmical neurons had receptive fields on the lips and face. The majority of rhythmical masticatory units were modulated during fictive mastication evoked by both the left and right cortices and only four changed their phase of firing when switching from one cortex to the other. When coupled with the finding that NVsnpr neurons exhibit spontaneous bursting in vitro[Sandler et al. (1998) Neuroscience, 83, 891], the results described here suggest that neurons of dorsal NVsnpr may form the core of the central pattern generator for mastication.

The effects of mandibular two-implant overdentures on nutrition in elderly edentulous individuals.

It is unclear whether mandibular implant overdentures improve the nutritional state of edentulous patients better than conventional dentures. In a randomized clinical trial, we tested for post-treatment differences in nutritional status between patients with mandibular two-implant retained overdentures and those with conventional complete dentures. Edentulous subjects (ages 65-75 yrs) received two-implant mandibular overdentures (IOD, n = 30) or conventional dentures (CD, n = 30). Measures of nutritional state were gathered before and 6 mos after treatment. Significant improvements in anthropometric parameters were detected in the IOD but not in the CD group, for percent body fat (p = 0.011) and skin-fold thickness at the biceps, subscapularis, and abdomen (p < 0.05), with significant decreases in waist circumference (p < 0.0001) and waist-hip ratio (p = 0.001). Significant increases were seen in concentrations of serum albumin (p = 0.015), hemoglobin (p = 0.01), and B12 (p = 0.01). No significant between-group differences were found. These results suggest that low-cost IOD treatment may improve the nutritional state of edentulous people.

Oral surgical management of patients with mechanical circulatory support.

After the introduction of mechanical circulatory support (MCS) for treatment of patients with severe cardiogenic shock, the survival rate of these patients increased significantly. Temporary MCS may be applied over a period of several days up to months prior to heart transplantation (HTx). Oral surgical management of patients with MCS is seriously complicated by a combination of anticoagulant and antiaggregant medication. Moreover, contact of blood with artificial surfaces, i.e. polyurethane and silicone, may increase the risk of thromboembolic events and infections. A total of 32 patients with MCS (29 men and three women with a median age 48 years [range 27-60 years]) were examined for odontogenic foci, treatment needs and local and cardiovascular complications after oral surgical intervention. In 27 patients (84%) at least one potential focus was diagnosed. Following tooth extraction 4 out of 6 patients presented with local haemorrhage (n=3) and thromboembolic events (n=2). Generally MCS has to be considered a relative contraindication for elective oral surgical interventions.

Discharge patterns of neurons in the medial pontobulbar reticular formation during fictive mastication in the rabbit.

In this study, we describe functional characteristics of neurons forming networks generating oral ingestive motor behaviours. Neurons in medial reticular nuclei on the right side of the brainstem between the trigeminal and hypoglossal motor nuclei were recorded in anaesthetized and paralysed rabbits during two types of masticatory-like motor patterns induced by electrical stimulation of the left (contralateral) or right (ipsilateral) cortical masticatory areas. Sixty-seven neurons in nucleus reticularis pontis caudalis (nPontc), nucleus reticularis parvocellularis (nParv), and nucleus reticularis gigantocellularis (Rgc) were studied. These were classified as phasic or tonic depending on their firing pattern during the fictive jaw movement cycle. Phasic neurons located in the dorsal part of nPontc were active during the jaw opening phase, whilst those in dorsal nParv tended to fire during the closing phase. In most neurons, burst duration and firing frequency changed between the two motor patterns, but there was little change in phase of firing. Tonic units were mainly recorded in the ventral half of nPontc, and at the junction between Rgc and caudal nParv. Cortical inputs with short latency from the contralateral masticatory area were more frequent in phasic (82%) than tonic (44%) neurons, whilst inputs from the ipsilateral cortex were equal in the two subgroups (57% and 56%). Phasic neurons had significantly shorter mean contralateral than ipsilateral cortical latencies, whilst there was no difference among tonic neurons. Intra- and perioral primary afferent inputs activated both types of neurons at oligo-synaptic latencies. Our results show that subpopulations of neurons in medial reticular nuclei extending from the caudal part of the trigeminal motor nucleus to the rostral third of the hypoglossal motor nucleus are active during the fictive masticatory motor behaviour. Unlike masticatory neurons in the lateral tegmentum, the medial subpopulations are spatially organized according to discharge pattern.

Identification of brainstem interneurons projecting to the trigeminal motor nucleus and adjacent structures in the rabbit.

Neurons of several nuclei within the medial pontomedullar reticular formation are active during mastication, but their relationship with other elements of the pattern generating circuits have never been clearly defined. In this paper, we have studied the connection of this area with the trigeminal motor nucleus and with pools of last-order interneurons of the lateral brainstem. Retrograde tracing techniques were used in combination with immunohistochemistry to define populations of glutamatergic and GABAergic neurons. Injections of tracer into the Vth motor nucleus marked neurons in several trigeminal nuclei including the ipsilateral mesencephalic nucleus, the contralateral Vth motor nucleus, the dorsal cap of the main sensory nucleus and the rostral divisions of the spinal nucleus bilaterally. Many last-order interneurons formed a bilateral lateral band running caudally from Regio h (the zone surrounding the Vth motor nucleus), through the parvocellular reticular formation and Vth spinal caudal nucleus. Injections of tracer into Regio h, an area rich in last-order interneurons, marked, in addition to the areas listed above, a large number of neurons in the medial reticular formation bilaterally. The major difference between injection sites was that most neurons projecting to the Vth motor nucleus were located laterally, whereas most of those projecting to Regio h were found medially. Both populations contained glutamatergic and GABAergic neurons intermingled. Our results indicate that neurons of the medial reticular formation that are active during mastication influence Vth motoneurons output via relays in Regio h and other adjacent nuclei.