Bilateral effects of the afferent impulses from the masseteric muscle on the trigeminal motoneuron of the cat

Neuronal activities of the vestibular nuclear complex during mechanically induced rhythmic jaw movements in rats

We studied the neuronal activities of the vestibular nuclear complex (VN) neurons during rhythmic jaw movements in rats anesthetized with urethane. Rhythmic jaw movements were induced by mechanical stimulation of the palate mucosa. The firing rate of approximately 25% of VN neurons increased significantly, and that of 10% of VN neurons decreased significantly, during these rhythmic jaw movements. There was no correlation between the change in the firing rate and the phase of the rhythmic jaw movements (jaw-opening and jaw-closing phases). The neurons that were affected were intermingled in the VN. These results suggest that the VN neurons are involved in controlling jaw movements.

Modulation of cortically induced rhythmic jaw movements in rats by stimulation of the vestibular nuclear complex

We study whether stimulation of the vestibular nuclear (VN) complex can modulate rhythmic jaw movements in rats anesthetized by urethane. Rhythmic jaw movements were induced by repetitive electrical stimulation of the orofacial motor cortex. Stimulation of the medial vestibular nucleus (MVN) during the jaw-closing phase increased the amplitude of the jaw-closing movement. (This is not a movement that continues to closure.) Stimulation of the MVN during the jaw-opening phase disturbed the rhythm of jaw movements and induced a small jaw-closing movement. Stimulation of the superior VN (SVN) and the lateral VN (LVN) during the jaw-closing phase did not affect the amplitude of the jaw-closing movement. Stimulation of the SVN and the LVN during the jaw-opening phase increased the amplitude of the jaw-opening movement, however. Stimulation of the inferior VN during the jaw-closing and the jaw-opening phase, respectively decreased the amplitude of the jaw-closing and the jaw-opening movements. Stimulation applied outside the VN did not modulate the amplitude of the jaw movements. These results imply that the VN is involved in the modulation of rhythmic jaw movements induced by stimulation of the orofacial motor cortex.

Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.

Forced mouth opening reaction: a primitive reflex released from cortical inhibition

We report the case of a 6-year-old girl with congenital adrenal hyperplasia, who showed 'forced mouth opening reaction' during the course of acute encephalopathy due to adrenal crisis. When an object was moved towards her mouth, or when the corner of her mouth was stroked with a tongue depressor, she would immediately open her mouth fully and hold it open. This reaction appeared transiently during the course of her illness in association with other frontal release signs including the rooting, groping and palmomental reflexes. Magnetic resonance imaging showed bilateral widespread lesions involving the gray and white matters in the frontal lobes, and less severe lesions in the temporal and parietal areas. We propose that this unique reaction is a sign of a release phenomenon, and represents the emergence of primitive reflexes in the absence of cortical inhibition in some types of encephalopathies.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

Facilitation of the jaw reflexes by stimulation of the red nucleus in the rat

The effects of the red nucleus (RN) stimulation on the jaw-opening reflex (JOR) and the masseteric monosynaptic reflex (MMR) were studied in anesthetized rats. The JOR was evoked by electrical stimulation of the inferior alveolar nerve. The MMR was evoked by electrical stimulation of the mesencephalic trigeminal nucleus. The JOR and the MMR were recorded as electromyographic responses of the anterior belly of the digastric and the masseter muscles, respectively. The conditioning electrical stimulation of the RN facilitated both the JOR and the MMR bilaterally. The facilitatory effect on the JOR was much larger than that on the MMR. Additionally, microinjection of monosodium glutamate into the RN also elicited facilitation of the JOR and the MMR. The results suggest the RN plays an important role in reflex control of jaw movements.

Ultrastructural anatomy of physiologically identified jaw-muscle spindle afferent terminations onto retrogradely labeled jaw-elevator motoneurons in the rat

Neuronal microcircuits involving jaw-muscle spindle afferents and jaw-elevator motoneurons were studied via retrograde and intracellular labeling in rats. Initially, trigeminal motoneurons were retrogradely labeled from horseradish peroxidase (HRP) injections into the temporalis and masseter muscles. The intracellular response of jaw-muscle spindle afferent neurons was then characterized during palpation, ramp and hold, and sinusoidal stretching of the jaw-closing muscles. Biotinamide was injected into these neurons, and the tissue was processed for the visualization of HRP and biotinamide. The ultrastructure of 243 intracellularly stained jaw-muscle spindle afferent boutons located within the trigeminal motor nucleus (Vmo) was examined. Eighty-five of these boutons synapsed with motoneurons retrogradely labeled with HRP, and 158 boutons synapsed with unlabeled structures within the Vmo. All spindle afferent boutons contained clear, spherical synaptic vesicles. Although the majority of boutons were S type, a few labeled jaw-muscle spindle afferent boutons possessed a long, narrow cleft, with a subsynaptic cistern comparable to previous descriptions of C-type boutons. Sixty-eight percent of spindle afferent boutons synapsed with large or medium-sized, retrogradely labeled motoneuron dendrites, and 32% synapsed with retrogradely labeled somata. In numerous instances, spindle afferent boutons synapsed with trigeminal motoneuron dendritic or somatic spines. Most of the synapses between spindle afferent boutons and trigeminal motoneuron dendrites were asymmetric, and the greatest percentage of axosomatic synapses between spindle afferents and trigeminal motoneurons were symmetric. Approximately 24% of spindle afferent boutons constituted the intermediate element of a axoaxodendritic or axoaxosomatic assemblage, implying that some jaw-muscle spindle afferent synapses with trigeminal motoneurons are presynaptically modulated.

Morphological analysis of cat masseteric motoneurons after intracellular staining with horseradish peroxidase

Intracellular injection of horseradish peroxidase (HRP) into 58 masseteric motoneurons identified by antidromic activation was performed in cats under pentobarbital anesthesia. Monosynaptic EPSPs were evoked by masseteric nerve stimuli in 52 cells, and were absent in the remaining six cells. The antidromic nature of the evoked spikes was confirmed by IS-SD separation observed at high frequency (50 Hz) stimulation. Motoneurons with monosynaptic excitation from masseter afferents showed IPSPs following stimulation of lingual and inferior alveolar nerves. Motoneurons which did not show monosynaptic excitation from masseter afferents showed no IPSPs from the above nerves. There were no differences in cell size or the number of stem dendrites between motoneurons with and without monosynaptic EPSPs. No recurrent collaterals were observed in any motor axons. Motoneurons with monosynaptic EPSPs were located at all rostrocaudal levels throughout the trigeminal motor nucleus, whereas motoneurons without such EPSPs were encountered only at the middle level. Dendrites of motoneurons with monosynaptic EPSPs did not extend into the medial portion of the nucleus where motoneurons innervating the anterior belly of the digastric muscle were located. In contrast, motoneurons without monosynaptic EPSPs had dendrite branches extending well into the medial part. The results show that there are two subpopulations of masseteric motoneurons that differ in peripheral inputs as well as dendritic morphology.

Physiological tremor in human jaw-muscle system

Small, quasi-rhythmical tremor of the jaw occurs at rest and during voluntary movements. In peripheral limbs, tremor consists of a component due to mechanical resonance properties of the system, and a neurogenic component mediated by a central pacemaker or neural loops. The present study attempted to determine if these components were present in jaw tremor measured with a position transducer held lightly between the incisors. When weights were suspended from the mandible, the tremor frequency was unaltered; sharp taps delivered to the jaw did not elicit any kind of damped oscillations of the system. These findings indicated the absence of mechanical resonance in the system. No correlation of the tremor signal with the electrocardiogram was found. However, a strong correlation was found between the rectified electromyographic signals recorded over the masseter muscles and the tremor signal, where the electromyographic signal preceded jaw movement by 20-30 msec. Frequency-domain analysis also showed positive peaks in a majority of coherence functions between electromyographic and tremor signals. These results suggested a strong neurogenic component of the tremor.

GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat

The distribution of GABAergic and glycinergic premotor neurons projecting to the trigeminal motor nucleus (Vm) was examined in the lower brainstem of the rat by a double labeling method combining retrograde axonal tracing with immunofluorescence histochemistry. After injection of the fluorescent retrograde tracer, tetramethylrhodamine dextran amine (TRDA), into the Vm unilaterally, neurons labeled with TRDA were seen ipsilaterally in the mesencephalic trigeminal nucleus, and bilaterally in the parabrachial region, the supratrigeminal and intertrigeminal regions, the reticular formation just medial to the Vm, the principal sensory and spinal trigeminal nuclei, the pontine and medullary reticular formation, especially the parvicellular part of the medullary reticular formation, the alpha part of the gigantocellular reticular nucleus, and the medullary raphe nuclei. Some of these neurons labeled with TRDA were found to display glutamic acid decarboxylase (the enzyme involved in GABA synthesis)-like or glycine-like immunoreactivity. Such double-labeled neurons were seen mainly in the supratrigeminal region, the reticular region adjacent to the medial border of the Vm, and the dorsal part of the lateral reticular formation of the medulla oblongata; a number of them were further scattered in the intertrigeminal region, the alpha part of the gigantocellular reticular nucleus, the nucleus raphe magnus, the principal sensory trigeminal nucleus, and the interpolar subnucleus of the spinal trigeminal nucleus. These neurons were considered to be inhibitory (GABAergic or glycinergic) neurons sending their axons to motoneurons in the Vm, or to local interneurons within and around the Vm.

Modulation of human rhythmical jaw motor function by perioral argon laser stimuli

The influence of peripheral afferent activity on human jaw motor function was examined in this study. Painful argon laser stimuli were applied to the upper lip in 10 subjects during rhythmical jaw movements and the effects on the jaw-closer electromyogram (EMG) and jaw movements were studied. The duration of the open-close cycle (1 sec) and the voluntary level of the jaw-closer EMG burst were standardized by auditory and visual feedback, respectively. Laser stimuli (0.2 sec) were delivered at predetermined percentages of the open-close cycle. Laser stimulation consistently had an excitatory effect on the jaw-closer EMG burst. The root-mean-square (RMS) value of the stimulus-related EMG burst was significantly larger than the preceding and the following EMG bursts. The excitatory effects on the jaw-closer EMG burst were modulated with respect to where in the open-close cycle the laser stimulus was applied, the voluntary force of the jaw-closer burst, and the laser stimulus intensity. Thus, the largest excitatory effects were seen during the start of jaw closure, with the most forceful voluntary contractions, and with the most painful laser stimuli. The stimulus-related changes in the jaw-closer EMG bursts were always associated with a significant shortening (30-90 msec) of the duration of the open-close cycle. The present results suggest that argon laser-induced activity in sensory afferents may generally have an excitatory effect on rhythmical jaw-closer motor performance.

The influence of low back pain on muscle activity and coordination during gait: a clinical and experimental study

Chronic low back pain (CLBP) is a major clinical problem with a substantial socio-economical impact. Today, diagnosis and therapy are insufficient, and knowledge concerning interaction between musculoskeletal pain and motor performance is lacking. Most studies in this field have been performed under static conditions which may not represent CLBP patients' daily-life routines. A standardized way to study the sensory-motor interaction under controlled motor performances is to induce experimental muscle pain by i.m. injection of hypertonic saline. The aim of the present controlled study was to analyze and compare electromyographic (EMG) activity of and coordination between lumbar muscles (8 paraspinal recordings) during gait in 10 patients with CLBP and in 10 volunteers exposed to experimental back muscle pain induced by bolus injection of 5% hypertonic saline. When the results are compared to sex- and age-matched controls, the CLBP patients showed significantly increased EMG activity in the swing phase; a phase where the lumbar muscles are normally silent. These changes correlated significantly to the intensity of the back pain. Similar EMG patterns were found in the experimental study together with a reduced peak EMG activity in the period during double stance where the back muscles are normally active. Generally, these changes were localized ipsilaterally to the site of pain induction. The clinical and experimental findings indicate that musculoskeletal pain modulates motor performance during gait probably via reflex pathways. Initially, these EMG changes may be interpreted as a functional adaptation to muscle pain, but the consequences of chronic altered muscle performance are not known. New possibilities to monitor and investigate altered motor performance may help to develop more rational therapies for CLBP patients.

Supra- and juxtatrigeminal inhibitory premotor neurons with bifurcating axons projecting to masseter motoneurons on both sides

Inhibitory neurons participating in the bilateral disynaptic inhibition of jaw-closing motoneurons by stimulation of unilateral trigeminal sensory branches were searched for in the reticular formation around the trigeminal motor nucleus in cats anaesthetized with pentobarbital. Extracellular recordings were made from neurons which responded orthodromically after a monosynaptic latency to single shock stimulation of the ipsilateral infraorbital and/or inferior alveolar nerves. Direct inhibitory connection with contralateral masseter motoneurons was demonstrated in reticular neurons by the spike-triggered averaging technique, i.e., by averaging the intracellular potentials of a contralateral masseter motoneuron with respect to spontaneously occurring spikes of a reticular interneuron. By intraaxonal injection of neurobiotin, electrophysiologically identified inhibitory premotor reticular neurons were found to project to and to terminate in the trigeminal motor nuclei on both sides. Termination in the contralateral motor nucleus was demonstrated for four neurons that showed the peripheral input pattern stated above. The results provide hard evidence for contralaterally projecting interneurons in the reticular formation, participating in peripherally evoked disynaptic inhibition of masseter motoneurons on the contralateral side. Given the previously reported findings that the supratrigeminal region contains neurons which project to the ipsilateral motor nucleus and mediate disynaptic inhibition of masseter motoneurons, it is suggested that the supratrigeminal region contains bilaterally projecting interneurons, mediating peripherally evoked disynaptic inhibition of masseter motoneurons on both sides.

An electrophysiological study of trigeminal commissural interneurons in the anaesthetized rabbit

The physiological characteristics of intertrigeminal area, nucleus oralis tau and supratrigeminal interneurons terminating in the contralateral-trigeminal motor nucleus were studied. Their antidromic conduction velocities were found to be between 3.7 and 16.3 m/s and most units had ipsilateral oral and peri-oral low threshold mechanoreceptive fields. Many received convergent inputs from both mandibular and maxillary divisions of the trigeminal nerve as well as the sensorimotor cortex.

Monosynaptic connexions of single V interneurones to the contralateral V motor nucleus in anaesthetised rats

We have used the extracellular spike triggered averaging method to obtain evidence for a monosynaptic connexion of single V (trigeminal) interneurones, located in the region immediately caudal to the V motor nucleus, onto neurones within the contralateral V motor nucleus. The extracellular fields recorded in the contralateral nucleus are of smaller amplitude than those detected within the ipsilateral nucleus and the implications of this are discussed.

Candidate interneurons mediating peripherally evoked disynaptic inhibition of masseter motoneurons of both sides

Location and axonal projection of interneurons presumed to mediate disynaptic inhibition evoked from the trigeminal sensory nerve in the ipsi- and contralateral masseter motoneurons were studied in pentobarbital anesthetized cats. Neurons monosynaptically excited from the periphery and antidromically activated from the contralateral trigeminal motor nucleus at low current intensity, hence probably terminating there, were found in the supratrigeminal region. Intracellular staining of such cells revealed collaterals terminating in the ipsilateral masseter motor nucleus. It is suggested that both the crossed and uncrossed disynaptic inhibition of masseter motoneurons are at least in part relayed by the same neurons in the supratrigeminal region.

Physiological and morphological characteristics of cat masticatory motoneurons--intracellular injection of HRP

The physiology and morphology of masticatory motoneurons of adult cats were examined by the methods of intracellular recording and intracellular injection of horseradish peroxidase. Masseter and jaw-opening motoneurons were identified by intracellular recordings of the antidromic response following stimulation of the masseter and mylohyoid nerves, respectively. An excitatory postsynaptic potential (EPSP) was recorded from masseter neurons by stimulation of the masseter nerve with stimulus intensity below threshold for antidromic response. In contrast, the EPSP was not recorded from jaw-opening motoneurons by stimulation of the mylohyoid nerve with stimulus intensity below threshold for antidromic response. Patterns of postsynaptic potentials (PSPs) in the masseter motoneurons following stimulation of the tooth pulp or periodontal afferents were classified into 4 types: hyperpolarization (n = 40), depolarization-hyperpolarization (n = 9), hyperpolarization-depolarization (n = 5), and depolarization with spike potentials (n = 10). On the other hand, patterns of the PSPs in the jaw-opening motoneurons following stimulation of the same afferents were classified into two types: depolarization with spike potentials (n = 19), and hyperpolarization (n = 5). Twenty-five masseter and 7 jaw-opening motoneurons and an intranuclear neuron were reconstructed from serial sections in the transverse plane. On the basis of dendritic morphology, the masseter motoneurons could be classified into two major groups, type I (n = 15) and type II (n = 9), whereas two neurons were found to constitute a separate category of the masseter motoneuron. The dendritic distributions of all the jaw-opening motoneurons examined were generally similar and there was no indication of the existence of subtypes, whereas there were 2 or 3 subgroups in type I and type II masseter motoneurons. Type I masseter neurons had primary dendrites which extended radially in all directions, and the whole profile of their dendritic trees presented a spherical and an egg-shaped appearance. In type II masseter neurons, the origin of primary dendrites was bipolar or tripolar, and the whole profile of their dendritic trees presented a hemispherical and mirror-imaged, funnel-shaped appearance. The other two masseter motoneurons had a particular dendritic tree which was much simpler in configuration, with less tapering or branching than those of other neurons examined. In contrast, the dendritic profiles of all the jaw-opening motoneurons were similarly organized and showed vertically oriented dendritic trees which were more developed in the dorsomedial than in the ventrolateral direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological and functional properties of trigeminal nucleus oralis neurons projecting to the trigeminal motor nucleus of the cat

Horseradish peroxidase (HRP) was injected into the somata located in the rostrodorsomedial part (Vo.r) of the trigeminal nucleus oralis; an axonal projection to the trigeminal motor nucleus (Vmo) was demonstrated in two Vo.r neurons. The two neurons differed in their morphological and functional properties. The first Vo.r neuron responded to stimulation of low-threshold mechanoreceptors and its stem axon gave off massive axon collaterals that issued terminal branches to the dorsolateral subdivision of Vmo, Vo.r, and the medial and lateral parts of the lower brainstem reticular formation. The second Vo.r neuron was activated by stimulation of the tooth pulp or lingual nerve at twice longer latency than that of the first neuron. This stem axon was divided into two main ascending and one descending branches, and one of the main ascending branches was further bifurcated into two branches. The main non-bifurcated ascending branch gave off 4 collaterals, two of which sent terminal branches into the dorsolateral subdivision of Vmo and others into the Vo.r and juxta-trigeminal regions. The somato-dendroarchitectonic differences were also described in the two Vo.r neurons stained.

Morphology of single mesencephalic trigeminal neurons innervating periodontal ligament of the cat

The morphology of single neurons in the trigeminal mesencephalic nucleus (Vmes) that innervate periodontal ligament was studied in cats by the method of intraaxonal injection of horseradish peroxidase (HRP). Two kinds of Vmes neurons were distinguished on the basis of differences in axon profile and its central projection. The first type of Vmes neurons was unipolar in shape and its axon was divided into united (U), peripheral (P), and central axons (C). The U axon traveled caudally within the Vmes from the soma to the dorsolateral aspect of trigeminal motor nucleus (Vmo), where it split into the P and C axons with a T-shaped appearance. The P axon joined the spinal trigeminal tract across the trigeminal principal nucleus and ran within the tract and sensory root to exit the brainstem. The C axon traveled caudally within Probst's tract. All 3 axons issued axon collaterals. Axon collaterals from the U, P and the proximal C axons sent their terminal branches into the supra (Vsup) and intertrigeminal regions (Vint). Most axon collaterals from the C axon sent their terminal branches into the juxtatrigeminal regions (Vjuxta). The second type of Vmes neurons was bipolar and issued P and C axons. The C axon ran a short distance in the Vmes to leave the Vmes, and then it traveled caudolaterally in the rostrodorsomedial aspect of the Vmo. Finally, it entered in the Vmo and traveled caudally in the dorsolateral subdivision of the nucleus to its rostrocaudal mid-level. The C axon gave off massive axon collaterals.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of single mesencephalic trigeminal neurons innervating masseter muscle of the cat

The morphology of functionally identified single axons of mesencephalic trigeminal neurons was studied in the cat by the method of intra-axonal injection of horseradish peroxidase (HRP). Each axon can be divided into united (U), peripheral (P) and central branches (C). The united axon (U) descends from its soma within the tract of the trigeminal mesencephalic nucleus to the dorsal aspect of the trigeminal motor nucleus (Vmo), where it splits into peripheral and descending central branches with a Y-shaped bifurcation. The peripheral axon (P) joins the motor root of the trigeminal nerve to exit the brainstem. The central axon (C) travels caudally within the juxtatrigeminal regions (or lateral reticular formation). All 3 branches issue axon collaterals that distribute terminal boutons within the dorsolateral subdivision of Vmo, supra- and intertrigeminal regions. Collaterals emanating from the central axon (C) except for its proximal segment travel ventrolaterally within the juxtatrigeminal regions, and send their terminal branches into the lateral boundaries adjacent to the spinal trigeminal nucleus. The trajectory of terminal branches distinguishes group Ia afferents from the possible group II afferents. The majority of terminal boutons are found to distribute in the supra- and intertrigeminal regions for group II afferent fibers and in the dorsolateral subdivision of Vmo for group Ia afferents.

The central projection of masticatory afferent fibers to the trigeminal sensory nuclear complex and upper cervical spinal cord

Retrograde and anterograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) conjugate was used to study the organization of primary afferent neurons innervating the masticatory muscles. HRP applied to the nerves of jaw-closing muscles--the deep temporal (DT), masseter (Ma), and medial pterygoid (MP)--labeled cells in the trigeminal ganglion and the mesencephalic trigeminal nucleus (Vmes), whereas HRP applied to nerves of the jaw-opening muscles--anterior digastric (AD) and mylohyoid (My)--labeled cells only in the trigeminal ganglion. Cell bodies innervating the jaw-closing muscles were found with greater frequency in the intermediate region of the mandibular subdivision, while somata supplying the jaw-opening muscles were predominant posterolaterally. The distribution of their somatic sizes was unimodal and limited to a subpopulation of smaller cells. Projections of the muscle afferents of ganglionic origin to the trigeminal sensory nuclear complex (TSNC) were confined primarily to the caudal half of pars interpolaris (Vi), and the medullary and upper cervical dorsal horns. In the Vi, Ma, MP, AD, and My nerves terminated in the lateral-most part of the nucleus with an extensive overlap in projections, save for the DT nerve, which projected to the interstitial nucleus or paratrigeminal nucleus. In the medullary and upper cervical dorsal horns, the main terminal fields of individual branches were confined to laminae I/V, but the density of the terminals in lamina V was very sparse. The rostrocaudal extent of the terminal field in lamina I differed among the muscle afferents of origin, whereas in the mediolateral or dorsoventral axis, a remarkable overlap in projections was noted between or among muscle afferents. The terminals of DT afferents were most broadly extended from the rostral level of the pars caudalis to the C3 segment, whereas the MP nerve showed limited projection to the middle one-third of the pars caudalis. Terminal fields of the Ma, AD, and My nerves appeared in the caudal two-thirds of the pars caudalis including the first two cervical segments, the caudal half of the pars caudalis and the C1 segment, and in the caudal part of the pars caudalis including the rostral C1 segment, respectively. This rostrocaudal arrangement in the projections of muscle nerves, which corresponds to the anteroposterior length of the muscles and their positions, indicates that representation of the masticatory muscles in lamina I reflects an onion-skin organization. These results suggest that primary muscle afferent neurons of ganglionic origin primarily mediate muscle pain.(ABSTRACT TRUNCATED AT 400 WORDS)

Bilateral post-stimulus electromyographic complexes in human masseter muscles after stimulation of periodontal mechanoreceptors of bi- and unilaterally-innervated teeth

These complexes (PSECs) were studied in full-wave rectified and averaged EMG in 13 subjects, who jaw-clenched at a controlled level. The PSECs were elicited by mechanical and electrical stimulation of receptors or their afferents in the unilaterally- and bilaterally-innervated periodontium of the upper first premolars and an upper central incisor. To exclude any contribution from acoustic receptors, subjects were exposed to high-intensity noise during mechanical stimulation. Comparison of peak amplitude and area from PSEC waves in normalized EMG amplitude-time plots suggests extensive crossing of the midline by periodontal afferent information. The small variation in latency of the first inhibitory wave on the two sides suggests that there are no additional synapses in the crossed pathway. Latency differences and wave incidence on the two sides of the later inhibitory and excitatory periods varied markedly between subjects suggesting that influences from higher centres affect masseteric motoneurones. In five subjects stimulation of periodontal receptors around different teeth resulted in different PSEC wave sequences.

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)

A light and electron microscopic study of premotor neurons for the trigeminal motor nucleus

Premotor neurons sending their axons to the trigeminal motor nucleus were observed in the cat by light and electron microscopy after labeling the neurons retrogradely or anterogradely with horseradish peroxidase (HRP). After HRP injection into the trigeminal motor nucleus, retrogradely labeled neurons were seen most frequently in the parvocellular reticular formation bilaterally. Many labeled neurons were also seen contralaterally in the intermediate zone at the rostralmost levels of the cervical cord and its rostral extension into the caudalmost levels of the medulla oblongata. Additionally, some neurons were labeled ipsilaterally in the mesencephalic trigeminal nucleus, contralaterally in the main sensory trigeminal nucleus and the trigeminal motor nucleus, and bilaterally in the oral and interpolar subnuclei of the spinal trigeminal nucleus. Only a few labeled neurons were seen in the confines of the gigantocellular reticular formation. All labeled neurons were small or of medium size; no large neurons were labeled. After HRP injection into the regions around the trigeminal motor nucleus or the parvocellular reticular formation, axodendritic terminals containing HRP granules were found contralaterally within the trigeminal motor nucleus. Some of these labeled terminals were filled with round synaptic vesicles and others contained pleomorphic synaptic vesicles. The varied morphology of labeled axon terminals was considered to reflect the functional heterogeneity of the premotor neurons for the trigeminal motor nucleus.

Jaw opening reflex affected by electroacupuncture in rat

In rats, the jaw opening reflex to tooth pulp stimulation was suppressed, but that to lower lip stimulation was scarcely affected by electroacupuncture stimulation of the forefoot. After transection of the spinal trigeminal nucleus at the obex level, the magnitude of the reflex to tooth pulp stimulation was greatly decreased and electroacupuncture was less effective on the pulp-evoked reflex. By contrast, the reflex activity to lower lip stimulation was not altered after the transection. The present study suggests that the site of action of electroacupuncture on the jaw opening reflex is in the caudal part of the spinal trigeminal nucleus and that pulp evoked jaw opening reflex is a presumed nocifensive response.

Crosses and uncrossed synaptic actions on motoneurones of back muscles in the cat

Intracellular recording from motoneurones of back muscles was used to analyze their synaptic input. The sample included motoneurones located in Th13--L2 spinal segments, identified by their antidromic invasion following stimulation of medial, intermediate and lateral branches of the dorsal rami. The motoneurones were monosynaptically excited from lowest threshold ipsilateral afferents and from ipsilateral descending spinal tracts. Polysynaptic EPSPs and/or IPSPs were evoked in them from higher threshold ipsilateral and contralateral afferents and from descending spinal tracts, and recurrent inhibition was evoked from ipsilateral motor axon collaterals. There was no evidence of crossed disynaptic inhibition from group I afferents, or crossed recurrent inhibition of these neurones. Supplementary records from another group of neurones in Th13--L2 segments, unidentified but likely to innervate other back or abdominal muscles, showed monosynaptic and polysynaptic PSPs of the same origin, and in addition disynaptic IPSPs and disynaptic EPSPs from contralateral ventral roots. The crossed IPSPs had features of the crossed recurrent IPSPs, while the crossed EPSPs appeared to be more likely evoked by some afferents passing via the ventral roots. Generally, the input to the investigated neurones showed greatest similarities to the input to motoneurones of neck muscles and differed from that reported for tail motoneurones.

Subnuclear organization of the ophidian trigeminal motor nucleus. I. Localization of neurons and synaptic bouton distribution

Among the Reptilia the morphology of the trigeminal (V) motor nucleus is a rather good indicator of the sophistication of jaw kinetics. As it becomes more complex, the nucleus shifts ventrolaterally and becomes divisible into subnuclear groups. The cottonmouth moccasin, a pit viper with very finely developed jaw musculature and kinetics, has a very large V motor nucleus. It is divisible into three subnuclei: the ventral and intermediate, containing predominantly large neurons (40--60 micrometers), and the dorsal subnucleus, containing only small neurons (20 micrometers). Ultrastructural study has indicated that these subnuclei can also be characterized according to the types of boutons synapsing on the cells. The soma of neurons in the ventral and intermediate subnuclei have up to 50% of their profile covered with clusters of boutons. The neurons of the dorsal subnucleus usually have only one cluster of two to three boutons per profile. Both cell types have more boutons containing spherical vesicles in axo-dendritic synapses than those containing flattened vesicles, and approximately equal numbers of these boutons in axosomatic contacts. However, the small cells have proportionately more boutons containing spherical vesicles synapsing on them. Boutons similar to those described in mammalian spinal cord were identified in the snake V motor nucleus. Small terminals containing spherical (S) or flattened (F) vesicles and terminals associated with postsynaptic cisternae (C) or with dense bodies (T) are commonly found in the ventral and intermediate subnuclei. C- and T-boutons are rare in the dorsal subnucleus. Large terminals with multiple active sites and postsynaptic dense bodies (M) and their associated, small, preterminal boutons (P) were not observed in the snake V motor nucleus. Boutons containing only large granular vesicles (G) were also not observed. We suggest that the ventral and intermediate subnuclei consists of alpha- and possibly beta-motoneurons and the dorsal subnucleus contains gamma-motoneurons. This anatomical segregation of function may be important to the physiology of ophidian mastication, which is quite different from that of mammals. However, there do exist several morphological similarities to mammals, suggesting that the snake brainstem may be a good model for comparative structure-function correlations.

Fibers of trigeminal mesencephalic neurons in the maxillary nerve of the rat

Indirect Wallerian degeneration after sectioning the infraorbital nerve, and retrograde axonal transport following injection of horseradish peroxidase (HRP) into the maxillary nerve were studied in rats. These experiments showed the existence of primary trigeminal neurons in the pars caudalis of the ipsilateral trigeminal mesencephalic nucleus and in the supratrigeminal nucleus of Lorente de Nó. Such neurons were interpreted as being responsible for the sensitive innervation of the periodontal membrane.