Presynaptic inhibition of jaw-opening reflex by high threshold afferents from the masseter muscle of the cat

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Superior laryngeal and hypoglossal afferents tonically influence upper airway motor excitability in anesthetized rats

Upper airway (UA) muscle activity is stimulated by changes in UA transmural pressure and by asphyxia. These responses are reduced by muscle relaxation. We hypothesized that this is due to a change in afferent feedback in the ansa hypoglossi and/or superior laryngeal nerve (SLN). We examined 1) the glossopharyngeal motor responses to UA transmural pressure and asphyxia and 2) how these responses were changed by muscle relaxation in animals where one or both of these afferent pathways had been sectioned bilaterally. Experiments were performed in 24 anesthetized, thoracotomized, artificially ventilated rats. Baseline glossopharyngeal activity and its response to UA transmural pressure and asphyxia were moderately reduced after bilateral section of the ansa hypoglossi (P < 0.05). Conversely, bilateral SLN section increased baseline glossopharyngeal activity, augmented the response to asphyxia, and abolished the response to UA transmural pressure. Muscle relaxation reduced resting glossopharyngeal activity and the response to asphyxia (P < 0.001). This occurred whether or not the ansa hypoglossi, the SLN, or both afferent pathways had been interrupted. We conclude that ansa hypoglossi afferents tonically excite and SLN afferents tonically inhibit UA motor activity. Muscle relaxation depressed UA motor activity after section of the ansa hypoglossi and SLN. This suggests that some or all of the response to muscle relaxation is mediated by alterations in the activity of afferent fibers other than those in the ansa hypoglossi or SLN.

Effects of the inferior alveolar nerve stimulation on tongue muscle activity during mastication in freely behaving rabbits

Genioglossus (Gg) reflexes elicited by electrical stimulation of the inferior alveolar nerve were examined in naturally chewing rabbits. To eliminate possible contaminations of the digastric (Dig) activity in the Gg responses, the Dig nerve was denervated bilaterally. Masticatory and tongue muscles were well coordinated during chewing after the denervation; i.e., there were no significant differences in the phase durations between before and after denervation. The Gg reflex measured was divided into three categories depending on the chewing phase (i.e., jaw-opening, OP; fast-closing, FC; and slow-closing, SC) in which the stimulus was delivered. The reflex amplitude was phasically modulated for the phases, in that the amplitude in the OP phase was larger than that in any other phase (P<0.05). On the other hand, the amplitude in the FC and SC phases was not significantly different to each other and from the control value obtained when the animal was awake and resting. The pattern of the modulation in the reflex amplitude was different from the previous report as to the Dig reflex in that OP<FC approximately SC<control was obtained. The results suggest that the modulatory mode in the Gg and Dig reflexes may be different in the pattern of the modulation under the natural chewing behavior and the Gg reflex is independent of the masticatory muscles in the nature. The reflex could be more sensitive to control the tongue movements collecting food bolus in the OP phase during chewing than in the jaw-closing phase.

Jaw-tongue reflex: afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat

The tongue position is reflexively controlled by the jaw position (the jaw-tongue reflex). The purpose of this study was to clarify the mechanism of this reflex in terms of afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat. Intracellular recordings from hypoglossal motoneurons revealed that electrical stimulation of the temporalis muscle nerve evoked excitatory and inhibitory post-synaptic potentials in hypoglossal motoneurons. The threshold of temporalis muscle nerve stimulation for evoking the synaptic potentials was higher than 2.0 times the nerve threshold. The amplitude of the potentials increased with stimulus intensity up to 5.0 times the nerve threshold. Punctate light pressure applied to the temporalis muscle induced a tonic depolarizing potential in hypoglossal motoneurons on which action potentials as well as depolarizing synaptic activation noise were superimposed. On the other hand, electrical stimulation of the temporalis muscle during jaw-opening could slightly inhibit the electromyographic activities in the genioglossus and styloglossus muscles. Lesions including the Probst's tract at the level caudal to the trigeminal motor nucleus abolished both excitation and inhibition in hypoglossal motoneurons induced by tonic depression of the lower jaw, but exerted no effects on either the tonic stretch reflex or the trigemino-hypoglossal reflex. In contrast, lesions including the trigeminal spinal tract produced no changes in either excitation or inhibition of hypoglossal motoneurons induced by temporalis muscle afferents, whereas the excitation of hypoglossal motoneurons was abolished by the lesions. We conclude that the group II muscle spindle afferents from the temporalis muscle are primarily responsible for evoking the jaw-tongue reflex.

Suppression of jaw-opening and trigemino-hypoglossal reflexes during swallowing in the cat

Jaw-opening and trigemino-hypoglossal reflexes can be evoked by innocuous as well as noxious afferents from intra-oral structures. It has been reported that the amplitude of the jaw-opening reflex evoked by weak electrical stimulation of the upper lip is subject not only to tonic suppression but also to phase-linked modulation during mastication. In this study, we investigated whether the jaw-opening and trigemino-hypoglossal reflexes are modulated during swallowing. Data were obtained from 8 chloralose-anesthetized cats. Reflexes were monitored by electromyographic activities recorded from the anterior digastric, genioglossus, and styloglossus muscles and, after paralysis, by the efferent discharge in the digastric and hypoglossal nerves. Swallowing was elicited either by water dropped on the tongue or by repetitive stimulation of the superior laryngeal nerve. Jaw-opening and trigemino-hypoglossal reflexes were evoked by stimulation of the lingual nerve, and the evoked afferent volley was recorded from the Gasserian ganglion so that the threshold of the lingual nerve could be determined. The following results were obtained: (1) The jaw-opening and trigemino-hypoglossal reflexes evoked by stimulation of the low-threshold, but not high-threshold, lingual afferents were remarkably suppressed during swallowing; and (2) both the jaw-opening and trigemino-hypoglossal reflexes evoked by low-threshold lingual afferents were suppressed during fictive swallowing after the animals were paralyzed. We conclude that the jaw-opening and trigemino-hypoglossal reflexes evoked by low-threshold lingual afferents are suppressed during swallowing by a central motor program.

Effects of food consistency on the modulatory mode of the digastric reflex during chewing in freely behaving rabbits

Effects of food consistency on the mode of the phase-linked modulation in the digastric reflex amplitude were examined in naturally chewing rabbits. Two test foods with different textures (bread as a soft food, pellet as a hard food) were used. The digastric reflex was elicited by electrical stimulation (10 train pulses at 2 kHz) of the inferior alveolar nerve. The amplitude of the digastric reflex measured was divided into three categories depending on the chewing phases in which the stimulus was delivered and each value was compared with the control response obtained when the animal was resting. The reflex was strongly inhibited in the jaw-opening phase and no difference was observed in the inhibitory effect between the foods. In the jaw-closing phase, larger digastric reflexes than those in the opening phase were elicited with both foods. This was the case in both the fast-closing and slow-closing phases. Reflex amplitude was significantly larger during chewing of the hard food than the soft food and, thereafter, inhibition of the reflex was observed only during chewing of the soft food in the closing phase. The results suggest the following: (1) food consistency may affect the central mechanism which regulates the digastric reflex and (2) the reflex may contribute to the regulation of masticatory force during chewing particularly hard food.

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.

An explanation for reflex blink hyperexcitability in Parkinson's disease. II. Nucleus raphe magnus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. The first step in the circuit linking the basal ganglia and brainstem reflex blink circuits is the inhibitory nigrostriatal pathway (Basso et al., 1996). The current study reports the circuits linking the superior colliculus (SC) to trigeminal reflex blink circuits. Microstimulation of the deep layers of the SC suppresses subsequent reflex blinks at a latency of 5.4 msec. This microstimulation does not activate periaqueductal gray antinociceptive circuits. The brainstem structure linking SC to reflex blink circuits must suppress reflex blinks at a shorter latency than the SC and produce the same effect on reflex blink circuits as SC stimulation, and removal of the structure must block SC modulation of reflex blinks. Only the nucleus raphe magnus (NRM) meets these requirements. NRM microstimulation suppresses reflex blinks with a latency of 4.4 msec. Like SC stimulation, NRM microstimulation reduces the responsiveness of the spinal trigeminal nucleus. Finally, blocking the receptors for the NRM transmitter serotonin eliminates SC modulation of reflex blinks, and muscimol inactivation of the NRM transiently prevents SC modulation of reflex blinks. Thus, the circuit through which the basal ganglia modulates reflex blinking is (1) the substantia nigra pars reticulata inhibits SC neurons, (2) the SC excites tonically active NRM neurons, and (3) NRM neurons inhibit spinal trigeminal neurons involved in reflex blink circuits.

Premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles: differential distribution in the lower brainstem of the rat

The distribution of premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles was examined in the lower brainstem of the rat by using retrograde and anterograde labeling techniques. First, Fluorogold, a fluorescent retrograde tracer, was injected into the dorsolateral or ventromedial division of the trigeminal motor nucleus, each of which contains motoneurons innervating the jaw-closing or jaw-opening muscles, respectively. Second, Phaseolus vulgaris-leucoagglutinin, an anterograde tracer, was injected into each of the lower brainstem sites, where clusters of retrogradely labeled premotor neurons had been seen in the first set of experiments. Third, after injection of the anterograde tracer into a lower brainstem site, followed by injection of the retrograde tracer cholera toxin B subunit into a masticatory muscle, termination of anterogradely labeled axons onto retrogradely labeled motoneurons was confirmed with the aid of a confocal laser-scanning microscope. It was found that the premotor neurons distributed in the mesencephalic trigeminal nucleus, medial part of the parabrachial region, supratrigeminal region, and dorsal parts of the principal sensory, oral spinal and interpolar spinal trigeminal nuclei project preferentially to the dorsolateral division of the trigeminal motor nucleus, whereas those in the lateral part of the parabrachial region, intermediate parts of the principal sensory, oral spinal and interpolar spinal trigeminal nuclei, and alpha part of the gigantocellular reticular nucleus project preferentially to the ventromedial division of the trigeminal motor nucleus. The dorsal and lateral parts of the medullary reticular formation and the medullary raphe nuclei contain premotor neurons of both types. Group k motoneurons, a cluster of trigeminal motoneurons that innervate the tensor tympani muscle, receive projection fibers predominantly from the dorsolateral part of the oral pontine reticular formation.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

Intracellular response properties of neurons in the spinal trigeminal nucleus to peripheral and cortical stimulation in the cat

The responses of the secondary neurons in the spinal trigeminal nucleus oralis (STNo) were recorded intracellularly to peripheral and cortical stimulation in chloralose-anesthetized cats. Electrical stimulation of the trigeminal sensory nerves (the frontal, infraorbital and inferior alveolar nerves) evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP in one group of STNo neurons (Type I), and a prolonged EPSP superimposed by a burst of spikes in the other group of STNo neurons (Type II). Nearly half of Type I neurons were trigeminothalamic neurons projecting to the contralateral ventral posteromedial nucleus, while the remaining Type I and all the Type II neurons were non-projection neurons. A majority of Type I neurons responded with spike potentials to stimulation of only one sensory nerve, while most Type II neurons responded to stimulation of more than one nerve. Stimulation of the contralateral primary somatosensory cortex evoked IPSPs in most Type I projection neurons, and EPSPs in all Type II as well as most Type I non-projection neurons. In Type I neurons touch or pressure applied to a circumscribed area in the facial skin evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP, and IPSPs were evoked from a wide surrounding area in the face by the same mechanical stimulation. In Type II neurons innocuous mechanical stimulation within a wide area evoked an EPSP, while IPSPs could not be induced from anywhere. The results indicate that postsynaptic inhibition is involved in the surround inhibition as well as corticofugal descending inhibition of sensory transmission in the trigeminal sensory nucleus.

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.

Spatio-temporal patterns of pre- and postsynaptic inhibition induced by primary afferent activation in the trigeminal sensory nucleus in cats

Spatio-temporal patterns of pre- and postsynaptic inhibition were studied in the trigeminal spinal nucleus oralis of cats by means of systematic electrical stimulation of the facial skin. Stimulation of the facial skin induced an EPSP-IPSP sequence in trigemino-thalamic relay cells (TRC). The IPSP was depressed by picrotoxin but was resistant to strychnine. The largest IPSP was evoked from the center of the excitatory area, where stimulation induced the largest EPSP and spike potentials at the lowest intensity in the same TRC. The amplitude of the IPSP decreased with increasing distance from the center in parallel with that of the EPSP. In the great majority of trigeminal primary afferent fibers, the largest primary afferent depolarization (PAD) was not evoked from the center of the excitatory area, where the threshold for spike generation was lowest, but from the adjacent points on the face. Spike activities in a trigeminal primary afferent fiber did not evoke any detectable PAD in itself. The duration of the PAD was definitely longer than the IPSP in TRC. However, the temporal distribution of the peak of PADs was very similar to that of the EPSP in TRC. Inhibition was evoked in glutamate-induced spike discharges of TRC by stimulation of the points on the face, which were located close to the center of the excitatory area of the TRC. However, the afferent inhibition of both spontaneous and peripherally induced spike discharges of TRC outlasted the postsynaptic inhibition. Thus, the late phase of the afferent inhibition is most probably due to presynaptic inhibition. Presynaptic inhibition, together with postsynaptic inhibition, would be involved also in the early phase of afferent inhibition through its mutual inhibitory organization.

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.