Rhythmic Jaw Movements Research Articles

Masseter muscle activity in diagnosed sleep bruxists compared with non-symptomatic controls.

All-night polysomnographic recordings were made of clinically diagnosed sleep bruxists (n = 23) and non-symptomatic controls (n = 6). The total duration of masseter contraction (MC) episodes during sleep was 11.6 minutes per night in bruxists and 6.6 in controls (P < 0.01). The mean frequency of MC episodes was 11.0 per hour of sleep in bruxists and 6.4 in controls (P < 0.05). The mean relative amplitude of MC episodes reflecting clenching strength was 0.81 in bruxists and 0.56 in controls (P < 0.01). The percentage distribution of mixed and phasic MC episodes was 94% among bruxists and 88% in controls. The remaining activity was classified as tonic in both groups. The subclassification of rhythmic jaw movements (RJM), defined as three or more separate rhythmic contractions during MC episode were also evaluated. The frequency of those MC episodes with RJM was 3.6 per hour of sleep in bruxists and 1.1 in controls (P < 0.001). The difference in the relative amplitude between the two study groups suggests that the amount of clenching force is the primary factor responsible for the harmful effects of sleep bruxism on the masticatory apparatus. The most significant difference was seen in those phasic and mixed MC episodes which also included the subclassification of rhythmic jaw movement. The result suggests that relative amplitude and rhythmicity of MC episodes can be used as a basis to confirm the diagnosis and to evaluate the treatment effects of suspected sleep bruxists.

Inhibitory effect of sympathetic stimulation on activities of masseter muscle spindles and the jaw jerk reflex in rats.

1. To evaluate sympathetic effects on jaw muscles, the discharges of masseter muscle spindle afferents, jaw muscle electromyographic (EMG) activities and blood flow changes were compared in anaesthetized decerebrate rats before and during electrical stimulation of the cervical sympathetic trunk. 2. To eliminate the possibility of efferent control from the trigeminal motoneurones, muscle spindle activity was recorded from the cut peripheral end of the masseter nerve. The absence of a sympathetic component in the masseter nerve was confirmed by the horseradish peroxidase method. 3. Electrical stimulation of the sympathetic nerve at frequencies within the physiological range reduced muscle spindle afferent discharges evoked by passive jaw opening. 4. Sympathetic stimulation also reduced the EMG activity evoked by the jaw jerk reflex, which may reflect a sympathetic effect on spindle afferents. After cessation of stimulation, a transient increase in EMG activity was observed, which may be due to efferent supply from the trigeminal motoneurones. During rhythmical jaw movements, no sympathetic effect on EMG activity was detected. 5. The above sympathetic effect on muscle spindle afferents and EMG activity was independent of blood flow changes.

Modulation of spinal monosynaptic reflexes during rhythmical jaw movements and its central neural mechanisms

The present study was carried out to investigate whether there was any modulation of the spinal monosynaptic reflexes during mastication in the healthy humans and urethane-anesthetized rabbits and to elucidate the central neural mechanisms responsible for the modulation in the urethane-anesthetized rabbits. 1. Human soleus H-reflex was significantly facilitated during the rhythmical jaw movements and rhythmical gum chewing. 2. There was no significant difference in the magnitude of the facilitation between the jaw-opening and -closing phases. 3. In the rabbits, monosynaptic reflex (MSR) volleys recorded from the tibial nerve (TIB) innervating the crural extensors and the common peroneal nerve (CP) innervating the crural flexors tonically increased in amplitude during the masticatory movements induced by either intraoral stimulation or repetitive stimulation of the cortical masticatory area (CMA). 4. Antidromic field potentials in the anterior horn of the lumbar cord evoked by the stimulation of TIB and CP increased during the CMA-induced actual and fictive mastication. 5. There was no significant change in the amplitude of the directly evoked antidromic potential in the primary afferents by intraspinal stimulation during the CMA-induced mastication compared with the resting state. It was concluded that (1) the MSR of the crural muscles undergoes a tonic non-reciprocal facilitation during mastication, (2) the stomatognathic afferents induced during mastication are not essential for the facilitation, and (3) an increase in the excitability of motoneurons is mainly involved in the facilitation of the MSR during mastication.

Immunohistochemical localization of glutamate and glutaminase in guinea pig trigeminal premotoneurons

Previous electrophysiological experiments in guinea pigs from our laboratory [11,36,37] have suggested that synaptic transmission between last-order interneurons (premotoneurons) and trigeminal motoneurons during reflex activation or cortically induced rhythmical jaw movements is mediated by excitatory amino acids (EAAs). In the present study, we performed a series of double-labeling experiments in guinea pigs to determine the location of neurons which contain glutamate or glutaminase and project to the trigeminal motor nucleus (Mo5). This was accomplished by combining immunohistochemical staining and standard retrograde tract-tracing techniques. Injections of a retrograde tracer, colloidal-gold bound to inactivated WGA-HRP (gWGA-HRP), into the trigeminal motor nucleus labeled a column of neurons originating adjacent to Mo5, including the supratrigeminal nucleus, intertrigeminal nucleus and the mesencephalic nucleus of V. The column extended caudally into the parvocellular reticular formation and adjacent trigeminal sensory nucleus oralis and oralis γ subdivision. In all of these regions, immunoreactivity to glutamate or glutaminase was observed co-localized with gWGA-HRP.

Resetting of cortically induced rhythmical jaw movements by stimulation of the cerebellar interpositus nucleus in the guinea pig

Effects of stimulation of the cerebellar interpositus nucleus on fictive rhythmical jaw movements induced by stimulation of the cortical masticatory area were studied in ketamine-anesthetized, paralyzed guinea pigs. A short pulse-train applied to the interpositus nucleys caused a phase shift in cortically induced rhythmical jaw movements. A phase transition curve indicated that interpositus stimulation can reset the cortically induced rhythmical jaw movements.

Diverse firing properties of single motor units in the inner and outer portions of the guinea pig anterior digastric muscle

Microwire recordings from the histochemically heterogeneous inner compartment of the guinea pig anterior digastric muscle (ADG) revealed tonic firing of single motor units, which were spontaneously active and could also be recruited following orofacial afferent stimulation and during rhythmic jaw movements (RJM). As units with tonic firing were not observed in the homogeneously fast-twitch outer ADG, the tonic units were classified as slow-twitch motor units. Irregular patterns of motor-unit firing at variable frequencies were observed after orofacial stimulation and during RJM in the outer and inner compartments. The irregular firing pattern of units in the fast-twitch outer compartment was characterized by shorter and less variable bursts than that of units in the heterogeneous inner compartment. A phasic, centrally driven firing pattern was observed during RJM in outer and inner ADG units. The firing frequency of some of these units was modulated during the rhythmical bursts. It is suggested that, as in limb muscles, functionally specialized ADG motor units are recruited in an orderly sequence, starting with spontaneously active, slow-twitch units in the inner compartment, continuing with fast-twitch units recruited upon enhancement of the synaptic drive (as in the case of orofacial stimulation), and ending with massive, rhythmical recruitment of slow- and fast-twitch units during RJM.

Coordination of cortically induced rhythmic jaw and tongue movements in the rabbit

1. Rhythmic movements of the jaw, tongue, and hyoid that were induced by stimulation of the cortical masticatory area (CMA) were recorded cineradiographically in the anesthetized rabbit. Jaw movements were also recorded by a laser position detector. 2. The evoked jaw movements were classified into four types: small circular (type A), large circular (type B), large vertical (type C), and crescent-shaped (type D). Among these, types B and D resembled the jaw movements of the food transport cycle and those of the chewing cycle in a masticatory sequence. 3. Each type of jaw movement was associated with a particular pattern of tongue and hyoid movements. In general, the tongue protruded during jaw opening and retracted during jaw closure. The hyoid generally moved upward and forward during jaw opening but downward and backward during jaw closure. 4. Electromyograms (EMGs) were recorded from jaw muscles [masseter (Ma) and digastric (Di) muscles], extrinsic tongue muscles [styloglossus (Sg) and genioglossus (Gg) muscles], and hyoid muscles [sternohyoid (Sh) and geniohyoid (Gh) muscles] during cortically induced rhythmic jaw and tongue movements (CRJTMs). These muscles were classified into two groups: group 1 was activated mainly in the jaw opening phase, and group 2 was activated mainly in the jaw closing and power phases. The Di, Gg, and Gh were included in the former, and the Ma, Sg, and Sh were included in the latter. 5. The timings of EMG activation to a jaw movement cycle were relatively constant for the muscles of group 1, irrespective of the types of CRJTMs, whereas those for the muscles of group 2 altered considerably with the different types of CRJTMs. 6. Relationships of the integrated muscle activity between the Di and Gg and between the Di and Gh were significant, whereas those between the Ma and Sg and between the Ma and Sh were not. 7. When a small strip of polyurethane form of various degrees of hardness was inserted between the opposing molars during CRJTMs, EMG activity of the muscles of group 2 increased with the hardness of the strip. On the other hand, EMG activities of the muscles of group 1 were less affected by the same intraoral stimuli. 8. Two conclusions were reached: first, physiological properties of the CRJTMs and cortically induced rhythmic movements of the hyoid were essentially similar to those observed in natural mastication. This fictive mastication might thus be regarded as a suitable model for simulating natural mastication.(ABSTRACT TRUNCATED AT 400 WORDS)

Trigeminal premotor neurons in the bulbar parvocellular reticular formation participating in induction of rhythmical activity of trigeminal motoneurons by repetitive stimulation of the cerebral cortex in the guinea pig.

1. Single-unit activity was recorded from neurons in the bulbar parvocellular reticular formation (PCRF) dorsal and dorsolateral to the gigantocellular reticular nucleus near its caudal boundary, and the roles of these reticular neurons in induction of rhythmical activity of trigeminal motoneurons by repetitive stimulation of the cerebral cortex (the cortical masticatory area, CMA) were studied in the paralyzed guinea pig anesthetized with urethan or with ketamine and chlorpromazine. 2. One hundred nine PCRF neurons were activated antidromically by microstimulation in either the masseter (MA) or anterior digastric (AD) motoneuron pool in the ipsilateral trigeminal motor nucleus, and orthodromically by stimulation in the contralateral CMA. Repetitive CMA stimulation induced rhythmical burst activity in these PCRF neurons in association with the rhythmical field potential in the contralateral AD motoneuron pool induced by the same CMA stimulation. The burst was synchronous with the rhythmical AD field potential in 81 neurons, 44 and 37 of which responded antidromically to stimulation in the MA and AD motoneuron pools, respectively. The remaining 28 neurons antidromically responded to stimulation in the MA motoneuron pool, and their burst corresponded in time with the period between successive AD field potentials. 3. Spike-triggered averaging of the intracellular potentials of MA and AD motoneurons (MNs) by simultaneously recorded spontaneous spikes of the PCRF neurons, which showed rhythmical burst responses during the jaw-opening phase to repetitive CMA stimulation, revealed a monosynaptic inhibitory postsynaptic potential in MA.MNs in 12 of 34 tested pairs and a monosynaptic excitatory postsynaptic potential (EPSP) in AD.MNs in 14 of 26 tested pairs. An EPSP was also found in MA.MNs after a monosynaptic latency from triggering spikes in 11 of 37 tested PCRF neurons that showed burst activity during the jaw-closing phase. 4. We conclude that both excitatory and inhibitory premotor neurons projecting to MA.MNs as well as excitatory premotor neurons projecting to AD.MNs are located in the PCRF, and that these premotor neurons relay the output of the central rhythm generator for rhythmical jaw movements in the medial bulbar reticular formation to trigeminal motoneurons, and thus participate in induction of rhythmical activities of trigeminal motoneurons by repetitive CMA stimulation.

Jaw movements and EMG activities of limb-licking behavior during grooming in rabbits

Jaw movements and masticatory muscle activity were recorded in freely moving rabbits during limb-licking, chewing, and drinking. The limb-licking movement was characterized by rhythmical jaw movement and digastric burst and relatively small masseter activity. The mean cycle time was 292 ms ± 11.0 (mean ± SE: n = 7) and was longer than that in chewing (267 ms ± 20.8, p < 0.01) or drinking (208 ms ± 15.2, p < 0.01). Jaw movements in the frontal plane of the ingestive behaviors coincided with those reported in previous studies; however, that of the licking movements was featured here. The basic cycle included; a) an opening stroke in which the mandible moved downward and laterally, b) a lateral excursion stroke in which the mandible moved medially and either upward or downward, and c) a closing stroke in which the mandible moved upward to the closing position. During opening, the digastric activity on the working side was determined to be larger than that on the other side ( p < 0.001).

Capsaicin induced rhythmical jaw movements in the rat

Capsaicin induced rhythmical jaw movements in the rat

Genioglossus EMG activity during rhythmic jaw movements in the anesthetized guinea pig

The electromyograph (EMG) activity of the left anterior digastric and the genioglossus muscles was studied in ketamine-anesthetized guinea pigs under 3 separate jaw movement paradigms. The first paradigm has been previously named spontaneous rhythmic jaw movements. These jaw movements occur 1–2 h after the onset of ketamine anesthesia. After spontaneous rhythmic jaw movements began, a single dose of apomorphine caused a new, second jaw movement paradigm to occur, apomorphine-induced rhythmic jaw movements. The final paradigm, cortically-evoked rhythmic jaw movements, was elicited by electrical stimulation of the masticatory area of the cerebral cortex. Genioglossus EMG activity was complex and highly variable in spontaneous rhythmic jaw movements; however, apomorphine-induced jaw movements were characterized by simultaneously occurring rhythmic EMG bursts of approximately 230 ms duration in both the digastric and genioglossus muscles. In 4 of 5 animals, genioglossus muscle activity onset preceded digastric muscle activity onset by approximately 20 ms. These results support the hypothesis that apomorphine-induced rhythmic jaw movements are an analog of lapping in the awake animal. In cortically-evoked rhythmic jaw movements, both digastric and genioglossus EMG activity were time-locked to the cortical electrical stimulation, with an onset latency of approximately 11 ms for the digastric EMG activity and of 16 ms for the genioglossus EMG activity. These results support the hypothesis that both trigeminal and hypoglossal motoneuron pools are closely coupled in certain coordinative movement patterns.

ガムのテクスチャーがそしゃく運動に及ぼす影響の多変量解析

This study was made to investigate the influence of gum texture on the chewing motion. The textures of chewed gum (Green gum, LOTTE Co.), the hardness, the cohesiveness, the gumminess, the stickiness and the adhesiveness, were quantified by a texturometer (Zenken Co.), and also the weight measured for 10, 20, 30, 60, 90, 120, 180, 240, 300 and 600 sec in 8 normal subjects and 2 patients with craniomandibular disorders (CMD). Using a kinesiograph (Model K 6, Myo-tronics Co.), the jaw movement of mandible was recorded three-dimensionally during 10min. habitual gum chewing. Multiple regression analysis was applied for investigating the effects of the change of gum texture and weight on chewing motion (criterion variables: chewing movement parameters, explanatory variables: texture and weight factors).The results obtained were as follows:1. In normal subjects, the closing and the occluding phases were observed high correlations with the hardness and the cohesiveness. On the other hand, the opening phase and the cycle time were not observed correlations with any texture and weight factors.2. In CMD patients, all parameters of chewing movement were observed high correlations with texture and weight factors.In conclusion, a central pattern generator (CPG), somewhere in the brain stem, is suggested to be responsible for rhythmical jaw movement in normal subjects. In CMD patients, however, jaw movement was easy to be affected by peripheral factors such as gum texture and weight.

Role of the basal ganglia in manifestation of rhythmical jaw movement in rats

Microinjection of picrotoxin (PTX), a selective GABA antagonist, into the rostral part of the head of the caudate-putamen complex (CPC) produced rhythmical potentials of 3–11 Hz there that lasted for about 5–15 s in the rat. Rhythmical jaw movements (RJM) were observed to be associated with these rhythmical potentials. Since the potentials were still observed even after the CPC had been isolated, they were thought to be generated intrinsically by the CPC itself. Increased and grouped neuronal discharges of the CPC were recorded in association with the rhythmical potentials. Periodic or prolonged inhibition of neurons of the pars reticulata of the substantia nigra were also recorded in association with the rhythmical potentials in the CPC. We therefore propose that the basal ganglia are involved directly in the manifestation of RJM in the rat.

The effects of nanoliter ejections of lidocaine into the pontomedullary reticular formation on cortically induced rhythmical jaw movements in the guinea pig

In the ketamine/urethane anesthetized guinea pig, electromyographic (EMG) responses of the anterior digastric muscle were studied when loci within the lower brainstem were microejected with lidocaine (2%) during rhythmical jaw movements (RJMs) evoked by repetitive electrical stimulation of the masticatory area of the cortex. The area investigated was between the trigeminal motor nucleus (Mot V) and the rostral pole of the inferior olive. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ventral-medial portion of Mot V where digastric motoneurons are known to be located, resulted in reduction or complete abolishment of the digastric EMG activity ipsilateral to the ejection with no effective change in mean cycle duration (CD) or mean percent normalized integrated amplitude of the contralateral digastric EMG. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ponto-medullary reticular formation in areas that included portions of the caudal nucleus pontis caudalis (PnC), nucleus gigantocellularis (GC), medial nucleus parvocellularis (PCRt), and dorsal paragigantocellularis (dPGC), in most cases produced a bilateral reduction in the mean normalized integrated amplitude and a bilateral increase in the mean cycle duration. In these sites, the bilateral increase in mean cycle duration of digastric EMG bursts was also associated with a significant increase of coefficient of variation in CD. In many cases, microejection of lidocaine completely abolished rhythmical digastric activity, bilaterally. HRP injections into Mot V were performed to determine the locations of trigeminal premotoneurons and their relationship to effective lidocaine sites for rhythmical jaw movement suppression. Retrogradely labeled cells were found mainly in the mesencephalic nucleus of V; trigeminal principal and spinal V sensory nuclei, bilaterally; and within the intermediate and lateral regions of reticular formation, bilaterally. No labeling was found in the medial reticular formation, including the nucleus gigantocellularis and dorsal paragigantocellularis. The results of this study indicate that regions in the lower brainstem, including portions of the nuclei gigantocellularis and dorsal paragigantocellularis, may participate in masticatory timing and pattern formation, lending further support to the previous hypothesis that (1) both GC and dPGC nuclei of the medial reticular formation contain components of the oscillatory network necessary for RJM production; (2) this region does not contain trigeminal premotoneurons, suggesting a segregation between central timing neurons within these regions of brainstem and trigeminal premotoneurons; and (3) ipsilateral cortex activates neurons located contralaterally within the brainstem reticular core which are critical for bilateral digastric EMG activity during cortically induced RJMs.

High-frequency oscillations as indicators of neural control mechanisms in human respiration, mastication, and speech

1. Electromyograms (EMGs) were recorded from the right and left ventrolateral surfaces of the chest wall to study the activity of the diaphragm during three tasks: moderately deep breathing, speech production, and speechlike breathing in which subjects mimicked their speech breathing pattern but did not vocalize. In a second experimental protocol, activity of right and left masseter muscles was recorded in three conditions: chewing, rhythmic clenching, and speech. 2. EMGs were analyzed in the frequency domain by computing the average spectrum of 128-ms intervals selected from the rising phase of the EMG in 30 repetitions of each task. Coherence functions between bilateral pairs of EMGs were computed and examined for significant coherence in the range of 20-230 Hz. 3. In the deep breathing condition, significant coherence was present in two ranges, 20-60 Hz and 60-110 Hz. Oscillations in the 60- to 110-Hz range have long been associated with the operation of the primary respiratory pattern generator, and the present results replicate earlier reports of correlated oscillations in this range in bilateral recordings of human respiratory muscles during voluntarily controlled breathing. 4. In speech and speechlike breathing, correlated oscillations in the 60- to 110-Hz range were significantly reduced. This result supports the hypothesis that the neural circuitry characterized by high-frequency oscillations (HFOs) in the 60- to 110-Hz range, possibly a central pattern generator (CPG), is not the primary source of drive to respiratory motoneuron pools in speech. 5. During chewing, highly correlated oscillations were observed in right and left masseter activity. Significant coherence occurred in the range of 20-140 Hz, with very large peaks in the 20- to 60-Hz band. This result is interpreted in light of studies of rhythmic jaw movements elicited by cortical stimulation in experimental animals and studies of the reflex organization of the jaw-closing muscles. It is possible that highly correlated oscillations in the 20- to 60-Hz band arise from a central command system, such as a pattern generator, from reflexes, or from an interaction of central and reflex inputs. 6. In most subjects, nonzero coherence in the 20- to 60-Hz band also was observed during clenching and speech; however, levels of coherence in the 20- to 60-Hz band were significantly reduced in these conditions compared with chewing. 7. Taken together, the present results suggest that coherent HFOs observed in consistent frequency ranges across subjects are characteristic of human motor behaviors that are likely to involve pattern generation circuitry.

Projections of two separate cortical areas for rhythmical jaw movements in the rat

The cortico-bulbar projection from two separate cortical areas which induce different types of rhythmical jaw movements (RJM), and the relationship between these cortical areas were studied with horseradish peroxidase tracing method. One area (A-area) corresponded to the primary jaw motor area and the other (P-area) was located in the agranular insular cortex. Separate descending pathways from two areas passed through the pyramidal tract and projected to the supratrigeminal nucleus, the intertrigeminal region, the dorsal part of the trigeminal sensory complex, and the reticular formation. In the reticular formation, the A-area projected more medially than the P-area did, and the ipsilateral projection from the P-area was more prominent than that from the A-area, although contralateral projections were dominant in the majority of regions. The two areas had only a sparse reciprocal connection. We suppose that the difference in patterns of RJM induced by the two cortical RJM areas may be due to the different projection patterns from these two areas.

ラットのリズム顎運動と開口反射との相互作用

Reciprocal interactions between the rhythmical jaw movements (RJM) and the jaw opening reflex (JOR) were investigated in ketamine anesthetized rats. The tooth pulp of the right lower incisor was electrically stimulated with a bipolar steel wire electrode. The electrical activities of the ipsilateral trigeminal spinal nucleus caudalis (SpNc), the ipsilateral mylohyoid nerve (Myl N) and the contralateral cerebral peduncle were recorded. Vertical jaw movements were recorded through a transducer connected to a dental steel bar fixed to the roots of the lower incisors. Neuronal activities of the cerebral peduncle were observed prior to and during the RJM and the maximum activity occurred in a few initial cycles of the RJM. The JOR related activities were modulated phase-dependently during the RJM. The initial component of the primary afferent activities in the SpNc was facilitated in the late occlusal phase and the early opening phase and progressively inhibited from the late opening phase through the closing phase to the early occlusal phase. The late component of the primary afferent activities was tonically inhibited during the RJM with a slight phase-dependency. This tonic inhibition of the late component was thought to be due to primary afferent depolarization induced by the cerebral peduncular input in the SpNc. The Myl N activity was facilitated from the late occlusal phase to the peak opening phase and inhibited in the later closing phase and the early occlusal phase. This facilitation was thought to be induced by excitatory inputs from the brainstem pattern generator. The reflex jaw opening depended not only directly on the Myl N activity but also on the jaw position, reduced during the jaw opening and increased during the jaw closure. The RJM was also modulated by the JOR. The small JOR initiated or facilitated the RJM although the large JOR inhibited the RJM fixing the jaw in an opening position. The intermediate JOR modulated the RJM phase-dependently. This inhibition of the RJM was thought to be due to strong postsynaptic inhibition of the jaw closing motoneuron. The tooth pulp afferents could participate in control of the masticatory jaw movements as the other trigeminal sensory afferents.

Two types of rhythmical jaw movements evoked by stimulation of the rat cortex.

Two different patterns of rhythmical jaw movements (RJM) resembling masticatory movements were evoked by stimulation of the cerebral cortex in the rat. The first type of RJM was evoked from the primary jaw motor area, and it always began with a sustained jaw-opening and consisted of high frequency (5-7 Hz) simple opening-closing movements of the jaw. The anterior digastric electromyographic (EMG) activity showed discharges apparently time-locked to each stimulus pulse. The second type of RJM was evoked from the ventral part of the insular cortex. In contrast to the first type of RJM, the second type started with a jaw closing movement in many cases. This type of RJM had large and complex lateral and sagittal movements and the frequency of RJM was low (3-4 Hz). The digastric EMG activity did not show the stimulus-locked component. The pattern of these RJM did not depend on the intensity or frequency of stimulation, but on the site stimulated. These two cortical RJM areas appeared to work independently from each other, because ablation of one area did not affect the pattern of the RJM evoked from the other cortical area.

Input-output relationships in the jaw and orofacial motor zones of the cat cerebral cortex

Input-output relationships of the jaw and orofacial motor zones in the cerebral cortex of lightly anesthetized cats were studied. These relationships were examined by studying the motor effects produced by intracorrtical microstimulation (ICMS) and recording from single neuron. Jaw and orofacial motor effects were evoked by ICMS of the anterior part of the coronal and lateral sigmoid gyri (C-S motor zone) and the lateral wall of the presylvian sulcus (P motor zone). ICMS of the P motor zone produced more complex movements than that of the C-S motor zone. Repetitive stimulation of the P motor zone also evoked rhythmic jaw movements. Almost all cortical cells located in the C-S motor zone responded to tactile stimulation of cutaneous skin of the orofacial regions or the tooth, whereas those of the P motor zone received no cutaneous input from the orofacial regions. Cytoarchitectonically, the C-S motor zone was restricted to areas 3a, 6aβ and occasionally to area 4γ, whereas the P motor zone was represented to area 6aβ. Therefore, it is concluded that the C-S motor zone might be involved in sensorimotor integration of the jaw and orofacial motor functions, whereas the P motor zone might function only as a command area for jaw and orofacial movements.

Intracellular analysis of trigeminal motoneuron rhythmical activity during stimulation of pontomedullary reticular formation in anesthetized guinea pig.

1. The effects of repetitive stimulation of the nucleus pontis caudalis and nucleus gigantocellularis (PnC-Gi) of the reticular formation on jaw opener and closer motoneurons were examined. The PnC-Gi was stimulated at 75 Hz at current intensities less than 90 microA. 2. Rhythmically occurring, long-duration, depolarizing membrane potentials in jaw opener motoneurons [excitatory masticatory drive potential (E-MDP)] and long-duration hyperpolarizing membrane potentials [inhibitory masticatory drive potentials (I-MDP)] in jaw closer motoneurons were evoked by 40-Hz repetitive masticatory cortex stimulation. These potentials were completely suppressed by PnC-Gi stimulation. PnC-Gi stimulation also suppressed the short-duration, stimulus-locked depolarizations [excitatory postsynaptic potentials (EPSPs)] in jaw opener motoneurons and short-duration, stimulus-locked hyperpolarizations [inhibitory postsynaptic potentials (IPSPs)] in jaw closer motoneurons, evoked by the same repetitive cortical stimulation. 3. Short pulse train (3 pulses; 500 Hz) stimulation of the masticatory area of the cortex in the absence of rhythmical jaw movements activated the short-latency paucisynaptic corticotrigeminal pathways and evoked short-duration EPSPs and IPSPs in jaw opener and closer motoneurons, respectively. The same PnC-Gi stimulation that completely suppressed rhythmical MDPs, and stimulus-locked PSPs evoked by repetitive stimulation to the masticatory area of the cortex, produced an average reduction in PSP amplitude of 22 and 17% in jaw closer and opener motoneurons, respectively. 4. PnC-Gi stimulation produced minimal effects on the amplitude of the antidromic digastric field potential or on the intracellularly recorded antidromic digastric action potential. Moreover, PnC-Gi stimulation had little effect on jaw opener or jaw closer motoneuron membrane resting potentials in the absence of rhythmical jaw movements (RJMs). PnC-Gi stimulation produced variable effects on conductance pulses elicited in jaw opener and closer motoneurons in the absence of RJMs. 5. These results indicate that the powerful suppression of cortically evoked MDPs in opener and closer motoneurons during PnC-Gi stimulation is most likely not a result of postsynaptic inhibition of trigeminal motoneurons. It is proposed that this suppression is a result of suppression of activity in neurons responsible for masticatory rhythm generation.