Reflex organization of cat masticatory muscles

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.

Feeding disorders research and teams: a new challenge for dentistry

Feeding disorders, which include masticatory and cerebral disabilities, present a major problem for disabled persons, but only in the last decade have health professionals shown an increased interest in the diagnosis and management of patients with such problems. Dentists are experts in the diagnosis and treatment of disorders of the oral-facial region, yet they have either worked in intradisciplinary isolation or have not demonstrated their interest in patients with feeding disorders. Consequently, those involved in the assessment and treatment of persons with feeding disorders are not aware of the contributions dentists have made to the basic and clinical data base of feeding assessment and therapy. Dentistry is simply not mentioned in rehabilitation. The opportunity now exists for dentistry to inform others of our expertise in the field from both a research and a clinical perspective. The basic and clinical base of dental therapeutics and its role in the diagnosis and treatment of disorders of the masticatory apparatus and of swallowing are reviewed here. It is hoped that this review will stimulate interest among dentists to take part in the rehabilitation of patients with feeding disorders through the use of the extensive data base within the profession.

Amygdaloid projections to commissural interneurons for masticatory motoneurons

Injections of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the trigeminal motor nucleus resulted in retrograde labeling of neurons, called commissural interneurons for masticatory motoneurons, in the contralateral supratrigeminal region. Further HRP studies, in which WGA-HRP injections were made into the amygdala and supratrigeminal region, indicated that the supratrigeminal region receives fibers from the central nucleus of the amygdala ipsilaterally. These findings raised the possibility of direct connections between the amygdala and commissural interneurons. In order to confirm the connections, electrolytic lesions in the central nucleus of the amygdala and WGA-HRP injections into the contralateral trigeminal motor nucleus were made on the same animal and electron microscopic observation was carried out on the supratrigeminal region. Of particular interest was that degenerating amygdalotegmental fibers synapsed upon HRP-labeled neurons.

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)

Phase-linked modulation of excitability of presynaptic terminals of low-threshold afferent fibers in the inferior alveolar nerve during cortically induced fictive mastication in the guinea pig

Excitability of presynaptic terminals of low-threshold primary afferent fibers in the inferior alveolar nerve was tested in the trigeminal spinal nucleus of the ketamine-anesthetized, paralyzed guinea pig, by Wall's method. Fictive mastication was induced by repetitive stimulation of the cortical masticatory area, and was monitored by rhythmical burst activity in the jaw-opening anterior digastric motoneuron pool. The excitability was rhythmically modulated in a phase-linked manner during the masticatory cycle: it was decreased coincidentally with the digastric burst activity (jaw-opening phase) and increased during the middle and late periods of the interburst phase (jaw-closing phase) of the masticatory cycle. The results imply that presynaptic modulation of synaptic transmission of peripheral inputs from primary afferents to interneurons in the jaw-opening reflex pathway may contribute to the rhythmical modulation of the jaw-opening reflex evoked by innocuous stimulation of the intraoral structures during mastication; presynaptic inhibition contributing to the depression of the jaw-opening reflex during the jaw-closing phase and presynaptic facilitation to its enhancement during the jaw-opening phase.

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.

Lesions rostral and caudal to the trigeminal motor nucleus and their effects on the jaw-opening reflex of the cat

Brain stem lesions placed immediately rostral to the trigeminal motor nucleus of the cat abolish the visible jaw-opening reflex produced by electrical stimulation of the periodontal ligaments. These lesions also greatly diminish the amplitude of the electromyographic response recorded from the anterior belly of the digastric muscle. This attenuation could not be modified by injections of systemic strychnine (a glycine antagonist). Lesions placed immediately caudal to the trigeminal motor nucleus also abolished the visible reflex and greatly reduced the amplitude of the digastric response, but in this case, the attenuated response was slightly augmented by strychnine. These data suggest that glycinergic inhibitory neurons form part of the rostral pathway that regulate jaw-opening motor neurons.

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.

Pharmacologically induced changes in the latency of digastric reflexes in 1-7 day old rabbits

1. The naturally occurring change in latency of the digastric reflex from long (40-70 msec) to short (15-35 msec) in the first week of life was studied in rabbits using drugs known to affect GABAergic and glycinergic transmission. 2. Usually the long or the short latency reflex response appeared alone but after strychnine both appeared together. 3. The long latency responses were favoured by the GABA blockers bicuculline and picrotoxin; periodically rhythmic activity was also elicited. 4. The short latency responses were favoured by the GABA agonists pentobarbitone and diazepam.

Central projections of the mesencephalic nucleus of the fifth nerve: an autoradiographic study

Projections of cells of the mesencephalic nucleus of the fifth nerve (Mes V) to brainstem structures in the cat were studied by labelling Mes V cells with tritiated leucine. Before the leucine injections were made, however, large kainic acid lesions were produced in the vicinity of Mes V cells because these neurons are resistant to being killed or injured by this neurotoxin. Thus Mes V cells were selectively labelled by the leucine even though they are scattered among many other neurons. Leucine injections near Mes V cells located in the mesencephalon, which are primarily the somata of jaw muscle spindle afferent fibers, produced essentially the same pattern of terminal labelling as injections near a caudally located group of Mes V cells that includes the somata of many tooth mechanoreceptive afferents. Labelling was dense above the trigeminal motor nucleus in the nucleus supratrigeminalis and in the most medial portion of the principal trigeminal sensory nucleus. A scattering of labelled axons and diffuse label was seen along the length of the tract of Probst, which follows the medial border of the descending trigeminal sensory nucleus as far caudally as the dorsal motor nucleus of the vagus. Labelling within most of the trigeminal motor nucleus, which is known to receive direct synaptic input from Mes V cells, was very light. The only reasonably dense region of label was confined to a small dorsolateral portion of the motor nucleus. Although Mes V has generally been supposed to be involved with jaw control in a direct, reflexive manner, the extensive projections to nucleus supratrigeminalis and parts of the trigeminal sensory system draw attention to the potential proprioceptive sensory contribution of Mes V.

Morphology of masticatory motoneurons stained intracellularly with horseradish peroxidase

Masticatory motoneurons were identified electrophysiologically and stained with horseradish peroxidase (HRP). The masseter motoneurons could be divided into 3 groups on the basis of their dendritic morphology. In contrast, the digastric or mylohyoid motoneurons showed a similar dendritic configuration. These neurons had much developed dendritic trees in the dorsomedial than ventrolateral direction. The first group of the masseter motoneurons had their dendritic trees which extended radially in all directions with a slight preference to project rostrally. These somata were located in the center of the subdivision containing the masseter motoneurons. In the second group, their dendritic arbores had a polarity extending hemispherically. These neuronal somata were located in the medial, ventral, and lateral regions of the subdivision. For the masseter motoneurons in the two groups and jaw-opening motoneurons, the dendritic swellings were frequently observed in the distal branches. The third group had their dendritic trees which were much simpler in configurations with less tapering or branching than those of other neurons examined. Furthermore, a wide variety of dendritic spines and appendages, and no dendritic swellings, observed in the third group were distinct from other neurons stained. The dendritic trees of the jaw-closing and -opening motoneurons were confined to the individual subdivisions. There were no instances in which axon collaterals were observed for well-stained 16 axons.

Direct and relayed projection of periodontal receptor afferents to the cerebellum in the ferret

Field potentials in the cerebellar cortex of the ferret have been studied in response to stimulation of alveolar, muscular and cutaneous branches of the trigeminal nerve. Responses from the alveolar nerves are unusual in their very short latency. Evidence based on latency analysis, frequency following and comparison with other well-known inputs supports the view that the earliest field potentials are due to direct, unrelayed afferents, which terminate as mossy fibres. There is, in addition, a monosynaptically relayed afferent path via mossy fibres. The alveolar nerve afferents concerned with the direct projection are shown to come from periodontal mechanoreceptors and not from cutaneous receptors. No such connections are found from jaw-muscle spindle afferents. The direct and relayed periodontal pathways are both ipsilateral and crossed. They terminate in the cerebellar cortex in the parvermal region of lobules IV, V and VI. The functional significance of the direct periodontal afferent projection is considered particularly in the light of parallels with the vestibular system, which also has direct and relayed cerebellar projections.

Cortically induced masticatory rhythm in masseter motoneurons after blocking inhibition by strychnine and tetanus toxin

In the first series of experiments, we studied whether or not strychnine (STR)-resistant inhibition of masseter motoneurons (MASS . MNS) was involved in their rhythmical inhibition that occurs during masticatory activity, induced by repetitive stimulation of the cortical masticatory area (CMA) in the cat. After systemic STR injection, repetitive CMA stimulation induced rhythmically alternating activity in the masseteric and anterior digastric nerves with a shorter cycle time than before STR-administration. The short-latency IPSPS in the MASS . MNS evoked by single shocks applied to the CMA were abolished. In contrast, repetitive CMA stimulation still induced a rhythmical alternation of EPSPS and IPSPS in the MASS . MNS, although the IPSPS were significantly reduced in amplitude. In the second series, we attempted to abolish the STR-resistant component of the rhythmical IPSP with tetanus toxin (TT). This was injected into one superficial masseter muscle of the guinea pig. In the majority of animals, repetitive CMA stimulation induced a tonic EMG superimposed by rhythmical bursts in the TT-intoxicated masseter muscle. Repetitive CMA stimulation induced a rhythmical sequence of EPSPS and superimposed spikes in the MASS . MNS innervating the TT-intoxicated masseter muscle in paralyzed guinea pigs. It was concluded that: (1) the cortically-evoked short-latency inhibition of MASS . MNS is STR-sensitive, as is part of the rhythmical inhibition during CMA-induced mastication; and (2) rhythmical inhibition is not essential for the central generation of the rhythmical activity in the MASS . MNS.

Responses of neurons in nucleus supratrigeminalis to sinusoidal jaw movements in the cat

Single neurons in the mesencephalic nucleus of the fifth nerve and in the rostral brain stem of barbiturate-anesthetized cats were studied in terms of their response to sinusoidal jaw movements at different frequencies and amplitudes. Units were quantitatively characterized by magnitude and phase of their firing rate modulation as a function of frequency, the linearity of their response at 5 Hz, and the variability of the interpsike intervals. Units were qualitatively characterized in terms of their response to gentle palpation of jaw muscles. Neurons were found in the nucleus supratrigeminalis that were selectively driven by jaw movement and that responded to gentle palpation to one or more ipsilateral jaw muscles. The magnitude of the sensitivity of these neurons as a function of frequency was not significantly different from cells of the mesencephalic nucleus of the fifth nerve. The phase of the sensitivity was, however, significantly more advanced than were those cells at all but the highest frequencies. The typical interspike interval variability of cells in nucleus supratrigeminalis was about 40% of the mean interval, whereas that variability of neurons of the mesencephalic nucleus of the fifth nerve was only about 10% of the mean interval. Supratrigeminal cells could relay jaw proprioceptive information to the cerebellum or thalamus.

Effect of inhibitory amino acid antagonists on masseteric reflex suppression during active sleep

We examined the pharmacological basis for the suppression of the masseteric (jaw-closer) reflex which occurs during the behavioral state of active sleep. Accordingly, the masseteric reflex was recorded in intact, unanesthetized, normally respiring cats during naturally occurring states of wakefulness and active sleep. The amplitude of the reflex during these states was determined before and after strychnine, picrotoxin, and bicuculline methiodide were applied, by microinjection, to the trigeminal motor nucleus. The effectiveness of each drug in blocking the active sleep-related suppression of the masseteric reflex was examined and compared with the degree of suppression evoked, during wakefulness, by stimulation of the inferior alveolar nerve. Microinjection of strychnine (50 microM to 20 mM) reduced the degree of suppression of the masseteric reflex during active sleep, but was markedly more effective in blocking reflex suppression that was induced by stimulating the inferior alveolar nerve. Picrotoxin and bicuculline methiodide (10 microM to 5 mM) produced a nonspecific increase in the amplitude of the masseteric reflex during both states. Thus, these substances did not appear to reduce the degree of reflex suppression induced by inferior alveolar nerve stimulation or that occurring spontaneously during active sleep. We concluded that strychnine-sensitive postsynaptic inhibition does participate in the suppression of masseter motor activity during active sleep, but that it is not the exclusive factor responsible for atonia of the masseter musculature during this state.

Suppression of the jaw-opening reflex by periaqueductal gray stimulation is decreased by paramedian brainstem lesions

Electrical stimulation of the midbrain periaqueductal gray region (PAG) suppresses the tooth pulp-evoked jaw-opening reflex (TP-JOR). In the present study the pathways that mediate this suppression were investigated by placing brainstem lesions in lightly anesthetized cats. Parasagittal lesions that interrupted the afferent and efferent connections of the medullary and pontine raphe nuclei attenuated (but did not abolish) suppression of the TP-JOR evoked by PAG stimulation. This result provides further evidence that medial brainstem structures partially mediate the effects of PAG stimulation in the trigeminal system.

Location of, and peripheral convergence on, the interneuron in the disynaptic path from the coronal gyrus of the cerebral cortex to the trigeminal motoneurons in the cat

Primary afferent and descending cortico-bulbar convergence on 186 interneurons located in the intertrigeminal area was investigated. The experiments were performed on cats anaesthetized with chloralose. Nerves from the three trigeminal dermatomes were stimulated electrically at intensities below and above twice the threshold level. Nerves from oral, perioral and periorbital structures, and afferents from the masseteric and digastric muscles were included. The surface of the cerebral cortex was stimulated electrically in systematically selected, maximally receptive points within the trigeminal primary projection fields. The intertrigeminal neurons generally responded to stimulation of low-threshold afferents from periodontal, lingual or perioral cutaneous receptors with a polysynaptic latency. Inputs from 3-5 nerves were common but one afferent input was usually most effective. The neurons were generally discharged from two or more cortical points, as a rule those of the oral and perioral projection fields in areas 3a and 3b of the coronal gyrus. The fastest path from the cerebral cortex to the intertrigeminal area was monosynaptic. However, the median latency was 4-5 ms which indicates an oligosynaptic path. The path went through the pyramid at the pontine level. The discharge pattern of the intertrigeminal neurons was 1-4 spikes in 54% of the neurons and a high frequency train of spikes in 46%. Cortical excitation followed by inhibition of the neurons was observed. The neurons were not discharged by electrical stimulation in the defence-attack area of the hypothalamus. Transsynaptic responses evoked from the mesencephalon were seen in 1/3 of the tested neurons.

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)

Properties of postsynaptic potentials evoked in hypoglossal motoneurons by inferior alveolar nerve stimulation

The properties of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) produced in cat hypoglossal motoneurons by inferior alveolar nerve stimulation were studied. The percentage magnitude of a short- and a long-lasting component in inferior alveolar-induced IPSPs was measured by application of double shocks separated by 80- to 90-ms intervals to the inferior alveolar nerve. As a result, the protruder motoneurons received synaptic input primarily from the afferent fibers in both the ipsilateral and the contralateral inferior alveolar nerve generating the long-lasting IPSP. In addition, the retractor motoneurons received afferent fibers from the bilateral inferior alveolar nerves to generate the short- and the long-lasting IPSP. With respect to the properties of EPSPs, we have demonstrated that the amplitude of inferior alveolar-induced EPSPs decreased greatly with depolarization.

Inhibition of the nociceptive jaw opening reflex by the lateral reticular nucleus (LRN) in the rabbit

The effect of stimulation of the lateral reticular nucleus (LRN) on the jaw opening reflex (JOR), evoked by tooth pulp or by palatal tissue stimulation, was studied in anesthetized rabbits. The results show an inhibitory effect on the JOR evoked by tooth pulp stimulation, which is considered a nociceptive reflex, while no effect is shown on the JOR evoked by palatal tissue stimulation. The maximum effect was observed at intervals of 30-50 ms between the two stimuli and the more effective site of inhibition in the nucleus was found to be the ventrolateral portion. The result of a selective effect of LRN stimulation on nociceptive reflexes supports the view of a role of this nucleus in the central mechanisms of pain control.

Intracranial stimulation of the trigeminal nerve in man. II. Reflex responses

The reflex responses evoked by direct electrical stimulation of the intracranial portion of the trigeminal nerve have been studied in 16 subjects undergoing percutaneous retrogasserian thermocoagulation for the treatment of trigeminal neuralgia affecting the second or third division. In the obicularis oculi muscle, early and late responses similar to the R1 and R2 components of the blink reflex were recorded. The former could be evoked only by stimulation of the second division and its latency was consistent with intermediately fast afferents. A late reflex (50-70 ms) was occasionally recorded from the anterior belly of the digastric muscle. The response was sometimes followed by a later activity and showed the features of a polysynaptic reflex. No response was obtained in the jaw elevators when fully relaxed. With the subject voluntarily clenching his teeth, both an early "H-like" response and two silent periods in the background EMG were obtained. The second silent period was similar in the muscles ipsi- and contralateral to intracranial stimulation, while the first silent period was longer in the ipsilateral muscles. Possible mechanisms contributing to the inhibition following stimulation of the mixed portion of the nerve are discussed.

Supratrigeminal neurons mediate the shortest, disynaptic pathway from the central amygdaloid nucleus to the contralateral trigeminal motoneurons in the rat

Stimulation of the supratrigeminal area (STA) of the rat induced a monosynaptic EPSP in most mylohyoid-digastric motoneurons and a monosynaptic IPSP or EPSP in the majority of masseteric ones, contralaterally. Stimulation of the central amygdaloid nucleus induced the ipsilateral STA activity immediately followed by the contralateral mylohyoid nerve activities. The same amygdaloid stimulated excited 19 of 46 STA neurons, which were antidromically identified to project to the contralateral trigeminal motor nucleus. Nine of these were monosynaptically excited. The mean of the antidromic and monosynaptic latencies of these neurons explains the mean onset latencies of the amygdaloid influences on the contralateral trigeminal motoneurons. Therefore, the shortest crossing amygdalo-motoneuronal pathway is probably disynaptic and mediated by commissural STA neurons.

Is the jaw-opening reflex a valid model of pain?

Tooth pulp shock does not produce only pain; low intensity stimulation results in a non-painful sensation that is termed pre-pain. In animals low intensity tooth pulp shock does not evoke escape behavior; the similarity of the animal escape/detection threshold ratio with the human pain/pre-pain threshold ratio is evidence that pre-pain and pain may be present in animals as in humans. Both pre-pain and pain may arise from the activation of a common afferent modality. The TP-JOR does not correlate with the degree of pain experienced under all conditions. The TP-JOR threshold is at or near the sensory detection threshold, at stimulation intensities which evoke pre-pain. Under normal conditions both the magnitude of the TP-JOR response and the degree of pain experienced increase with increasing stimulation intensity. The TP-JOR and the tooth pulp-evoked pain are affected in parallel by sensory habituation and both appear to relay in the rostral trigeminal complex. There are no cases where the TP-JOR is suppressed and pain is still experienced from tooth pulp shock; the suppression of the TP-JOR may therefore be an accurate index of analgesia. However, in humans treatments that produce analgesia have not been shown to produce suppression of the TP-JOR. Thus, the TP-JOR that persists following analgesic treatments is not a reliable index of either analgesia or pain.

Trigeminal mesencephalic neurons innervating functionally identified muscle spindles and involved in the monosynaptic stretch reflex of the lateral pterygoid muscle of the guinea pig

Location of the neurons in the trigeminal mesencephalic nucleus innervating stretch receptors of the lateral pterygoid muscle and the mode of their synaptic connection on the lateral pterygoid motoneurons of the guinea pig were studied physiologically as well as morphologically, in comparison with the trigeminal mesencephalic neurons innervating muscle spindles in the superficial masseter muscle, with the following results: stimulation of the caudal half of the trigeminal mesencephalic nucleus evoked monosynaptic excitatory postsynaptic potentials in the ipsilateral lateral pterygoid motoneurons. Stimulation of the lateral pterygoid nerve directly evoked spike potentials in the neurons located in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, which responded with increased firing to stretch, and with silent period to twitch, of the ipsilateral lateral pterygoid muscle. Averaging of intracellular potentials of the lateral pterygoid motoneurons with extracellular spike potentials of these trigeminal mesencephalic neurons revealed excitatory postsynaptic potentials after a monosynaptic latency, but no inhibitory postsynaptic potentials. Injection of horseradish peroxidase into the lateral pterygoid muscle labeled 15-20 cells in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, while 174-228 cells retrogradely labeled by horseradish peroxidase were found throughout the whole rostrocaudal extent of the ipsilateral trigeminal mesencephalic nucleus following injection of horseradish peroxidase into the masseter muscle. It was concluded that neurons in the caudal half of the trigeminal mesencephalic nucleus send their peripheral processes to stretch receptors, presumably muscle spindles, in the ipsilateral lateral pterygoid muscle and that their central processes have excitatory synapses on ipsilateral lateral pterygoid motoneurons, thus comprising the afferent limb of a monosynaptic stretch reflex arc of the lateral pterygoid muscle of the guinea pig.

Effects of a serotonin agonist and antagonist on cortically induced rhythmical jaw movements in the anesthetized guinea pig

The effects of systemic injections of a serotonin agonist (quipazine), and antagonist (methysergide), on rhythmical jaw movements (RJMs) in the ketamine anesthetized guinea pig were examined. It was observed that quipazine, (1) significantly elevated the electrical threshold for inducing RJMs by repetitive stimulation of the masticatory cortex and (2) reduced the amplitude of the digastric and masseter EMG activity during RJMs. In contrast, the excitability of the short latency pathway from masticatory cortex to digastric motoneurons, and the excitability of the digastric reflex evoked by electrical stimulation of the mucosa of the tongue were not significantly affected by quipazine administration. Methysergide administration had little effect on the threshold for evoking RJMs, and on the excitability of the digastric reflex and short latency pathway from cortex to digastric motoneurons. These data suggest that the central pattern generator (CPG) for RJMs is not critically dependent upon serotonin receptor activation since RJMs can be induced after methysergide administration. On the other hand, excitation of the CPG by repetitive cortical stimulation can be inhibited by serotonin receptor activation. These results are interpreted as supporting our previously described model of RJM production by repetitive cortical stimulation.

Convergence of vagal and gustatory afferent input within the parabrachial nucleus of the rat

Our previous anatomical and electrophysiological studies demonstrated that first-order hepatic and gustatory afferents project to separate regions of the solitary nucleus (NST) and no intra-NST interaction of these two sensory systems could be demonstrated. However, iontophoretic injections of horseradish peroxidase into physiologically identified zones of the NST revealed that both of these regions send overlapping projections to the immediately subjacent parvocellular reticular formation as well as the postero-medial parabrachial nucleus (PBN). The present electrophysiological studies demonstrate that an interstitial zone of neurons in the caudal, medial PBN, indeed, receive convergent input from second-order gustatory and vagal afferents. Co-activation of these PBN units by the simultaneous arrival of both input sources frequently resulted in an additive interaction of evoked activity. PBN units lateral and caudal to this zone responded to vagal stimulation only, while units in the anterior and extreme medial portion of the PBN only responded to gustatory stimulation. By virtue of the efferent projections of the PBN, one might speculate that the convergence of information at this locus may, eventually, play a role in directing long term feeding behavior patterns such as learned taste aversion as well as the more transient changes in taste preference with visceral loading.

Short and long latency jaw-opening reflex responses elicited by mechanical stimulation in man

Jaw-opening reflex responses elicited by tapping the chin during maximum clenching in incisal edge-to-edge contact position were studied in 10 healthy subjects. Stimuli were also delivered during weak clenching on a rubber stamp, separating the incisors by approx. 10 mm and protruding the mandible to the edge-to-edge incisor relationship. All four central incisors were stimulated simultaneously. With weak stimuli, there was a short-latency (9.5 ms) digastric response which may have had a disynaptic pathway. Taps of moderate strength produced long-latency (20 ms) responses, and sometimes a short-latency (9.5 ms) component as well. Strong (non-painful) taps produced an even longer-latency digastric response, 30 ms or more following the stimulus with less synchronization than earlier responses. Jaw-jerk reflexes occurred 8.5 ms following the tap, independently of the magnitude of the stimulus. Local anaesthesia of the upper and lower incisors abolished the digastric muscle response. Thus large periodontal afferents may be responsible for the early digastric reflex activity and smaller fibres for later effects. Temporal summation of the reflex response probably occurred when all incisors were stimulated simultaneously.

The effect of stimulus intensity and gape on electrically-evoked jaw reflexes in man

The changes in masseter EMG and biting force evoked by electrical stimulation of the lip were measured in normal people. Stimulation at noxious intensities elicited a characteristic, biphasic inhibitory response in the masseter EMG and a transient reduction in the biting force. The pattern of masseter inhibition and force change varied both with stimulus intensity and with inter-incisal separation. In general, the decrease in biting force was greater at intermediate jaw positions than in the open and closed positions. The duration of the EMG inhibitory period increased as the stimulus intensity increased but was independent of the jaw separation. Reflex thresholds could not be correlated with pain thresholds because subjects found it difficult to decide at what level an electrical stimulus became painful. Nevertheless this stimulation of high-threshold afferents may be useful in estimating the rate of relaxation of tension in the jaw muscles.

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching: Part I

Electromyographic recordings from the anterior temporal muscle fibers bilaterally, the posterior temporal muscle fibers bilaterally, the superficial masseter muscle bilaterally, and the left medial pterygoid muscle were used to study the effects of changing the location, size, and direction of effort on specific contact points during maximal clenching tasks in human subjects. Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle activities for all the muscles recorded. When the contact point moved posteriorly along the arch from incisors to molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks. Eccentric efforts on specific contact points generally resulted in lower activity than the corresponding vertical effort. This was usually seen in all muscles, but not all values were significant. The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles. The temporal muscles showed the most activity in retrusive clenching, with activity in the other muscles nearly nonexistent. The medial pterygoid and masseter muscles were found to be the most active muscles during protrusive and incisal clenching, while the temporal muscle activity was low. When the size and number of contacts were increased anteriorly, a generalized increase in muscle activity was seen. The same trend occurred posteriorly but was not as consistent or significant. Cross-arch contacts were associated with a slight but significant bilateral increase in masseter muscle activity and an increase in temporal muscle activity ipsilateral to the cross-arch contact when maximum vertical clenches were performed. However, no significant increases were observed when the effort was directed laterally. The findings of this electromyographic study on change of the contact point, size of contact point, and the direction of effort applied on a contact point confirm their specific associations with the activity of muscle groups. Significant data have also been made available for a biomechanic approach of the investigation of degenerative joint changes.

The distribution of afferent neurones in the mesencephalic nucleus of the fifth nerve in the cat

The objective of this study was to examine anatomically the distribution of afferent neurones in the mesencephalic nucleus of the fifth nerve (Mes V). HRP was applied, in separate experiments, to the inferior alveolar, infraorbital, and masseter nerves, and injected into the masseter muscle and periodontal ligament. Following application of HRP to the masseter muscle and masseter nerve, labelled cells were found in the ipsilateral motor nucleus of the fifth nerve and in the ipsilateral Mes V. Labelled cells in Mes V, identified as belonging to proprioceptor afferents from jaw-closing muscles, were distributed throughout the full extent of the nucleus. Following application of HRP to the inferior alveolar nerve, infraorbital nerve, and periodontal ligament, labelled cells were found in the ipsilateral trigeminal ganglion and Mes V, and the latter identified as belonging to periodontal receptor afferents. In contrast to the distribution of spindle afferent somata, they were restricted to the caudal region of Mes V. The differential distribution of afferent neurones within Mes V demonstrated in this study confirms previous electrophysiological findings, and its significance is considered.

Ethanol-induced modulation of the membrane potential and synaptic activity of trigeminal motoneurons during sleep and wakefulness

In the present study we investigated the direct actions of ethanol on the membrane properties and excitatory and inhibitory postsynaptic potentials of trigeminal motoneurons in chronic cats. During states of sleep and wakefulness, extracellular and intracellular recordings were carried out together with juxtacellular (somatic and dendritic) and intracellular pressure injections of 0.05-2.5 M ethanol solutions in femtoliter quantities. Juxtacellularly applied ethanol induced: a sequence of excitatory-inhibitory alterations in firing activity which were accompanied by depolarizing-hyperpolarizing shifts in the resting membrane potential; a decrease in the amplitude of action potentials; and a depression in excitatory and inhibitory postsynaptic potentials. Intracellular ethanol injections resulted in depolarization of the membrane potential and a decrease in the amplitude of action potentials as well as a reduction in the amplitude of excitatory and inhibitory postsynaptic potentials. Both juxtacellularly and intracellularly applied ethanol affected the membrane potential and synaptic activity in a fashion that was not dependent upon the animal's behavioral state of sleep or wakefulness.

Activity during active sleep of bulbar reticular neurons firing rhythmically during mastication in cats

Unitary activity was recorded from 17 bulbar reticular neurons, which fired rhythmically during mastication, in unanesthetized, spontaneously respiring cats during sleep and wakefulness. All these neurons showed the highest mean firing rate during food ingestion, and none of them showed any tonic discharge during active sleep. The results are discussed in terms of a functional differentiation of bulbar reticular inhibitory neurons projecting to jaw-closer motoneurons in relation to phasic inhibition during mastication and tonic inhibition during active sleep of jaw-closer motoneurons.

Identification and assessment of factors contributing to variability of the jaw jerk

Unlike limb monosynaptic reflexes, the jaw jerk reflex ( JJR ) is extremely variable. We studied 35 healthy adults to determine the relative contributions of extrinsic and intrinsic factors underlying this variability. Each subject sat in a dental chair with his head and chin securely stabilized. Chin taps, delivered by a solenoid-driven plunger, were quantified with a piezo -transducer. The reflex response was recorded from surface electrodes over the right masseter muscle. A nasal thermistor signalled phases of respiration. Five of the 35 subjects had no reflex when relaxed, but during 15 degrees neck extension or voluntary contraction of the platysma muscle, a JJR appeared. The amplitude of the reflex varied considerably from trial to trial in all but one subject. A small component of this variability was due to minute changes in tap force despite head and chin stabilization and stimulus uniformity. Mean amplitudes of the reflex tended to increase with increases in tap force, but variability was large indicating intrinsic fluctuations in motoneuron excitability. Voluntary contraction of the platysma muscle and 15 degrees neck extension reliably enhanced the reflex. The JJR showed negligible respiratory modulation during quiet breathing. The reflex's variability in and among subjects precludes the use of the JJR as an index of masseteric motoneuron excitability. Our findings suggest that branchial motoneurons innervating the masticatory muscles receive far more diverse and fluctuating inputs than do somatic motoneurons innervating limb muscles.

EMG response to alteration of tooth contacts on occlusal splints during maximal clenching

Maximum clenching on an equilibrated occlusal splint yielded an increase of 17% in overall muscle activity over that of maximum intercuspation contributed mainly by masseter muscles. Maximum clenching on an anterior occlusal splint yielded a decrease of 13% in overall muscle activity compared with that of an equilibrated occlusal splint. When maximum clenching was performed with six left-sided teeth removed from contact while the left second molar remained in contact, there was no significant change in muscle activity when compared with that of an equilibrated occlusal splint. When left-sided muscles were compared with right-sided muscles for the same situation, there was no significant change. When maximum clenching was performed with all left-sided teeth removed from contact, there was a decrease of 21% in overall muscle activity compared with that of an equilibrated occlusal splint. When left-sided muscles were compared with right-sided muscles for the same situation, there was no significant change. Changes in the number of tooth contacts did not cause changes in the overall muscle activity during maximum clenching. Changes in the position of the tooth contacts altered the overall muscle activity during maximum clenching. Changes in occlusal contact symmetry did not cause changes in symmetry of muscle pairs during maximum clenching. Unilateral support produced the subjective response of pressure on the contralateral TMJ during maximum clenching.

Post-synaptic inhibition of bulbar inspiratory neurones in the cat

Stable intracellular recordings from thirty-six bulbar inspiratory neurones revealed three centrally originating, rhythmic patterns of synaptic inhibition (i.p.s.p.s). (i) A declining pattern of i.p.s.p.s accompanying the early stages of inspiration (early inspiratory inhibition) was identified in a total of twenty neurones representing examples of each of the functional classes of bulbar neurones examined, i.e. six R alpha- and two R beta-neurones of the dorsal respiratory group and twelve R alpha-neurones of the ventral respiratory group. (ii) A transient pattern of i.p.s.p.s just preceding or coinciding with the cessation of inspiration (late inspiratory inhibition) was present in the remaining sixteen neurones which were tested, representing six R alpha-neurones and three R beta-neurones of the dorsal respiratory group and seven R alpha-neurones of the ventral respiratory group. (iii) An augmenting pattern of expiratory i.p.s.p.s was present in all thirty-six neurones. Late inspiratory and expiratory i.p.s.p.s in the same neurones showed a similar time course of reversal when chloride was injected or allowed to diffuse into the cells and were associated with similar increases in input conductance. Both patterns of i.p.s.p.s appear to arise at or close to the cell soma. Early inspiratory i.p.s.p.s required a relatively longer period of chloride injection for reversal to be accomplished. Input conductance changes were either absent or smaller than those associated with late inspiratory or expiratory inhibition. These i.p.s.p.s appear to arise at more distal dendritic sites. These patterns of i.p.s.p.s are discussed in relation to the mechanisms shaping the growth of central inspiratory activity, bringing this activity to an end, and suppressing its redevelopment during expiration.

Comparison of electrical thresholds of intradental nerves and jaw-opening reflex in the cat

In experimental animals the jaw-opening reflex in response to stimulation of pulp nerves has been used as a nociceptive reflex. However, there seems to be only scanty information about the amount and types of pulp nerve fibres that mediate the reflex. In the present work on 8 anesthetized cats electrical thresholds of single functional pulp nerve units were compared to the thresholds of jaw-opening reflex. Monopolar cathodal current pulses were applied to each canine tooth. Reflex responses of the digastric muscle were recorded. The inferior alveolar nerve of the left side in 3 cats was exposed for nerve dissection and responses of pulp nerve units coming from the lower left canine tooth were recorded. The mean threshold of the jaw-opening reflex with 10 ms pulses was 5.9 +/- 3.0 (SD) microA. Below or at the level only part of the fast conducting pulp nerve units could be activated. Thresholds of A- (n = 32) and C- (n = 24) fibres were 9.9 +/- 5.7 and 37.4 +/- 14.5 (SD) microA respectively. Nerves of the periodontal tissues (20 units recorded) were not activated with current pulses of up to 200 microA applied to the tooth. Consequently, at threshold level the jaw-opening reflex in response to the present type of stimulation is mediated by the fast conducting intradental nerve units.

Afferent projections to the oral motor nuclei in the rat

Projections to the trigeminal, facial, ambiguus, and hypoglossal motor nuclei were determined by using horseradish peroxidase histochemistry. Most of the afferent projections to these motor nuclei were from the brainstem reticular formation, frequently in areas adjacent to other synergetic motor nuclei. The reticular formation lateral to the hypoglossal nucleus and reticular structures surrounding the trigeminal motor nucleus projected to each of these other brainstem motor nuclei involved in oral-facial function. Afferent projections to these motor nuclei also were organized along the rostrocaudal axis. Within the reticular formation most of the afferent projections to the trigeminal motor nucleus originated rostral to the majority of neurons projecting to the hypoglossal and ambiguus nuclei, which in turn were rostral to the primary source of reticular afferents to the facial nucleus. In comparison, projections from the sensory trigeminal nuclei and nucleus of the solitary tract were sparse. The interneuron pools that project to the orofacial motoneurons provide one further link in understanding the brainstem substrates for integrating oral and ingestive behaviors.

Brainstem projections to the normal and noradrenergically hyperinnervated trigeminal motor nucleus

The noradrenergic innervation of the trigeminal motor nucleus of the rat can be increased severalfold by neonatal treatment with the neurotoxin, 6-hydroxydopamine. The brainstem projections to the nucleus were studied by injecting HRP into the nucleus of normal and noradrenergically hyperinnervated rats. In order to identify the source of the noradrenergic innervation, the fluorescent dye, True Blue, was used as a retrograde tracer in combination with the glyoxylic acid histofluorescence method for catecholamines. In both control and neonatally treated rats, the noradrenergic innervation of the motor nucleus was shown to arise from an ipsilateral group of cells located among the fibers of the lateral lemniscus just rostral to the motor nucleus. Our results confirmed the high degree of specificity of noradrenergic innervation, which arises exclusively from this lateral tegmental noradrenergic cell group. During the process of sprouting, this specificity is maintained since only those noradrenergic cells normally innervating the nucleus were retrogradely labeled in neonatally treated animals. Other noradrenergic projections which are also increased in these animals, such as the nearby locus ceruleus innervation of the main sensory trigeminal nucleus, do not spread to the motor trigeminal nucleus. HRP-labeled nonadrenergic cells were concentrated dorsally, with scattered cells surrounding the nucleus. A similar distribution was observed contralateral to the injection site. The mesencephalic trigeminal nucleus was labeled only ipsilateral to the injection. The motor nucleus also receives an extensive bilateral input from the pontine and medullary reticular formation. The medial reticular formation nuclei, including nucleus pontis caudalis, nucleus gigantocellularis, and nucleus reticularis ventralis contained large labeled cells, which were especially numerous in the retrotrigeminal area. Smaller, lateral reticular formation neurons were concentrated rostrally and ipsilaterally in the nucleus pontis lateralis. HRP retrograde labeling revealed no obvious change in the overall pattern of cells innervating the trigeminal motor nucleus following noradrenergic hyperinnervation.

Brainstem mechanisms underlying variations in the occurrence of experimentally elicited rhythmic oral movements in the rat

Mechanical stimulation of the oral cavity of the decerebrate rat induces rhythmic oral movements in some animals but not in others. Reasons for this difference were sought using electrical stimulation of the lip to elicit a variety of responses which were capable of accurate quantification. Neither the digastric reflex threshold, size of suprathreshold response nor the ability to generate rhythmic activity within the brainstem differed in the two groups. Depression of the digastric reflex by an electrical stimulus applied 30-150 ms previously was, however, significantly greater in those animals not showing rhythmic oral movements. It is suggested that the two groups differed in their ability to produce rhythmic oral movements because of differences in the level of stimulus related inhibition rather than any tonic effects on trigeminal neuronal activity.

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.