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

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

Modulation of respiratory activity by locomotion in lampreys.

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

Respiratory rhythms generated in the lamprey rhombencephalon.

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

Brainstem mechanisms underlying feeding behaviors.

The essential elements controlling trigeminal motoneurons during feeding lie between the trigeminal and facial motor nuclei. These include populations of neurons in the medial reticular formation and pre-motoneurons in the lateral brainstem that reorganize to generate various patterns. Orofacial sensory feedback, antidromic firing in spindle afferents and intrinsic properties of motoneurons also contribute to the final masticatory motor output.

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

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

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

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

Anatomical organization of efferent neurons innervating various regions of the rabbit masseter muscle.

Previous studies have shown that the masseter muscle is supplied by motoneurons located in the anterodorsal region of the trigeminal motor nucleus and by an additional group of efferent neurons located in cell group k. The present experiments were performed on nine rabbits and were designed to establish the locations of neurons innervating the different regions of this muscle. Retrograde labeling with two fluorescent tracers (FluoroGold and Fast Blue) was applied to the central ends of cut branches of the masseter nerve. Serial coronal sections of the brainstem were viewed with fluorescence microscopy. The labeled cells were counted in all animals, and three-dimensional reconstructions of their distribution were made in five cases. In each successful experiment, labeled neurons were seen in the anterodorsal region of the trigeminal motor nucleus and in the two dorsal cell columns of cell group k (k1 and k3). Within-animal comparisons of the median position of populations innervating two distinct muscle regions in five rabbits showed that there were no significant differences in either the dorsoventral or rostrocaudal axes. However, in each case, there was a small but significant difference (83-173 microm) in the mediolateral axis within the motor nucleus but not within cell group k. Even in this axis, there was a 94-99% overlap of the two populations. Comparisons of the neuronal cross-sectional area showed that the deep regions were innervated by a larger proportion of small neurons from both nuclei than were the superficial and intermediate regions. Our results suggest that there is no simple topographical arrangement of motoneurons that corresponds to the peripheral pattern of nerve supply to the different regions of the masseter muscle.

Modification of rhythmical jaw movements by noxious pressure applied to the periosteum of the zygoma in decerebrate rabbits.

The purpose of this study was to describe and quantify changes in the movement and EMG patterns caused by tonic noxious pressure on the periosteum of the zygoma during electrically induced rhythmic jaw movements in the decerebrate rabbit. Eight New Zealand rabbits were anesthetized with urethane. EMG electrodes were placed in the masseter and digastric muscles and a light was attached to the mandibular symphysis to track jaw movements. The animals were decerebrated and the anesthetic was discontinued. Rhythmic jaw movements were evoked by electrically stimulating the corticobulbar tracts (1-msec rectangular pulses, 50 Hz), in the absence and presence of tonic noxious pressure applied bilaterally to the zygomatic periosteum (range: 400-1500 kPa). The overall response to tonic noxious pressure was a statistically significant decrease in the frequency and amplitude of the rhythmic jaw movements and in the mandibular velocity during opening and closing. The slowing of the frequency was associated with a significant increase in the duration between muscle bursts. In those animals in which the jaw closing muscles were most active, there was a significant decrease in the area of the masseter muscle bursts during jaw closure. The changes in motor activity in response to the application of tonic noxious pressure are similar to those that have been reported for patients experiencing musculoskeletal pain, suggesting that pain modifies motor programs at the segmental level. Our data support the pain-adaptation model.

An assessment of the efficacy of physical therapy and physical modalities for the control of chronic musculoskeletal pain.

An analysis of review articles and controlled clinical trials for temporomandibular disorders and similar chronic musculoskeletal pain disorders was carried out. Although little evidence was found that any specific therapy had long-term efficacy greater than placebo, we did find strong evidence that symptoms improve during treatment with most forms of physical therapy, including placebo. When the frequency of significant between-group differences in trials that used placebo and waiting list control (i.e., no treatment) groups were compared, it was found that treatment was better than placebo in only 7/22 trials, whereas treatment was almost always better than no treatment (15/16). This difference was highly significant (P = 0.001). A similar analysis of trials that included more than one treatment group showed that while equal amounts of treatment were usually associated with equal outcome (9/10), unequal treatment regimes led to unequal outcome (10/15; P = 0.012). The group that received the most therapy appeared to do best. In conclusion, it seems that patients are helped during the period that they are being treated with most forms of physical therapy. However, most of these therapies have not been shown to be more efficacious than placebo.

The effect of experimental jaw muscle pain on postural muscle activity.

The purpose of this study was to examine whether tonic muscle pain of an intensity at least as great as that reported by the majority of chronic muscle pain patients causes an increase in postural electromyographic activity of the affected musculature. Twenty young adults volunteered for experiments, knowing that they involved experimental pain. We chose a controlled, two-period crossover and repeated measures design involving four experimental conditions: (1) baseline 1, (2) tonic experimental muscle pain, (3) sham pain, and (4) baseline 2. Subjects were randomly assigned to one of two sequential orders that differed with respect to whether tonic pain preceded sham pain or vice versa. At the within-subject level, the condition (baseline 1, sham pain, tonic pain, baseline 2) had a statistically significant effect on mean rms electromyographic activity at all four recording sites. We found that postural activities at all four recording sites, left/right masseter and left/right anterior temporalis were statistically different from baseline 1 and 2 during tonic pain (P < 0.004, s.; P < 0.024, s.). However, postural activities during tonic pain and sham pain were not significantly different from each other (P < 0.493, n.s.). We concluded that our data do not support the hyperactivity model which assumes a re-enforcing link between pain and muscle hyperactivity.

Differences between the sexes in post-surgical pain.

It has been shown that women have a lower pain threshold and lower tolerance to some forms of experimental pain then men. However, the evidence that clinical pain is perceived differently by the two sexes is not yet as strong. The placement of intraoral implants is a highly controlled surgical procedure that we have used to investigate this possibility. Forty-eight edentulous (without teeth) subjects (27 females), aged from 35 to 63 years, received two titanium implants in the anterior mandible under local anesthesia. After the surgery, subjects completed a pain diary three times each day, rating pain intensity and unpleasantness on 100 mm visual analog scales (VAS). Once a day, they chose verbal descriptors from the McGill Pain Questionnaire (MPQ). Age of subjects, duration of surgery, the amount of local anesthetic used and the amount of pain medication taken were not statistically different for the two groups (P>/=0.32). Results showed that the senior surgeon produced significantly less pain than a 4th year resident (P=0.04). Although there were no significant differences between sexes for mean daily ratings of intensity or unpleasantness over time (P>/=0.10), most women experienced the highest intensity of pain during the day, while most men had higher pain in the evening (P=0.025). Also, the relative unpleasantness (unpleasantness/intensity ratio) increased significantly with time for males, but not for females (P=0.016). Males and females did not differ in the total number of words chosen from the MPQ (P=0.61), or in the averaged Pain Rating Index (PRI) (P=0.53). However, women used significantly more evaluative words than men (P=0.04), suggesting that woman found the overall intensity greater. These results indicate that women find post-surgical pain more intense than males, but that men are more disturbed than women by low levels of pain that last several days.

A physiological study of brainstem and peripheral inputs to trigeminal motoneurons in lampreys.

The inputs to trigeminal motoneurons from sensory afferents and rhombencephalic premotor regions were studied in isolated brainstem preparations of adult lampreys (Petromyzon marinus). Stimulation of both trigeminal nerves, contralateral nucleus motorius nervi trigemini, nucleus sensibilis nervi trigemini and ipsilateral rostral reticular formation elicited large-amplitude excitatory postsynaptic potentials with short latencies. These were significantly attenuated by adding 6-cyano-7-nitroquinoxaline2,3-dione (10 microM) and 2-amino-5-phosphonopentanoate (200 microM) to the bath, suggesting participation of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and N-methyl-D-aspartate receptors. The inputs from ipsilateral trigeminal afferents included a di- or oligosynaptic glycinergic inhibition. Sustained rhythmical membrane potential oscillations were observed in 52% of the recorded cells upon stimulation of trigeminal afferents or the contralateral nucleus sensibilis nervi trigemini. Two types of rhythm were obtained: (i) low-frequency oscillations (0.1-0.5 Hz), with peak-to-peak amplitudes between 8.5 and 17 mV; and (ii) higher frequency oscillations (1.0-2.8 Hz) with smaller amplitudes (1.8-5.1 mV). The two types of trigeminal rhythm could occur independently of fictive locomotion and fictive breathing. In a decerebrate semi-intact preparation, slow rhythmical trigeminal motoneuron potential oscillations were also evoked by stimulation of the oral disc. This study shows that trigeminal motoneurons receive excitatory synaptic inputs from several brainstem sites, and that membrane potential oscillations can be triggered upon stimulation of trigeminal afferents or the nucleus sensibilis nervi trigemini. We suggest that these oscillations recorded in vitro may represent the centrally generated components that underlie rhythmical feeding in lampreys.

An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit.

The results of a previous experiment suggest that the cell bodies of many jaw closing muscle spindle afferents in the trigeminal mesencephalic nucleus of the rabbit are phasically inhibited during fictive mastication. The aim of this study was to investigate one possible neurotransmitter system that could be involved in this modulation, serotonin, by use of receptor autoradiography techniques and immunofluorescence combined with retrograde labelling of masseteric spindle afferents and motoneurons. A second objective was to compare the serotonin innervation of neurons in the trigeminal mesencephalic nucleus with that of masseteric motoneurons. Serotoninergic fibres were seen surrounding labelled masseteric spindle afferents, as well as unlabelled neurons, in the trigeminal mesencephalic nucleus. These fibres were close to the cell bodies and sometimes to the axon hillocks of the neurons. Although it has been reported that many neurons of the trigeminal nucleus are multipolar in some species, none of the labelled spindle afferent in this study had more than one process. Throughout the motor trigeminal nucleus, serotonin fibres were found in close proximity with cell bodies and with the proximal portions of axons and dendrites of labelled and unlabelled motoneurons. Serotonin fibres were also seen adjacent to cell bodies and processes of efferent neurons in cell group k. Autoradiography with several tritiated ligands was used to reveal the presence of receptors for serotonin as well as its uptake sites. Only serotonin2 receptors were found to be abundant in the trigeminal mesencephalic nucleus. The motor nucleus and cell group k contained serotonin2 and serotonin3 receptors, as well as serotonin uptake sites. Serotonin1A receptors appear to be absent from both nuclei. The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors. In the motor nucleus and cell group k, serotonin may alter neuronal properties through actions on at least two receptor subtypes (serotonin2 and serotonin3).

Evidence that the masticatory muscles receive a direct innervation from cell group k in the rabbit.

These experiments have shown that a group of neurons lateral to the trigeminal motor nucleus innervates the muscles of mastication. The work began to describe the location of digastric last-order interneurons, using the technique of transneuronal labeling with wheatgerm agglutinin-conjugated horseradish peroxide injected into the left digastric muscle of rabbits under general anaesthesia. Four to eight days later, the animals were killed with an overdose of anaesthetic and perfused. Coronal sections of the frozen brainstem were cut at 20 microns thickness and processed for peroxidase activity. Motoneurons in the ventral and caudal divisions of the trigeminal motor nucleus were labeled in all animals as expected. An additional population of neurons located ventrolaterally to the motor nucleus in cell group k were also found to be labeled if the survival time was five days or more. In an attempt to determine whether cell group k neurons were labeled transynaptically, two series of control experiments were carried out. In the first, crystals of fluorescein- and rhodamine-conjugated dextran amines and horseradish peroxidase were applied directly to central ends of cut digastric nerves. In the second, central ends of cut digastric nerves were enclosed in cuffs containing 40-60% horseradish peroxidase solutions. Again, neurons in both the trigeminal motor nucleus and cell group k were labeled suggesting that neurons within cell group k project to the digastric muscle. Similar experiments using dextran amines and wheatgerm peroxidase were carried out on the masseter muscle. Motoneurons in the dorsomedial and rostral half of the trigeminal motor nucleus, as well as primary afferent cell bodies in the mesencephalic nucleus of the trigeminal nerve, were labeled in all experiments. In addition, a population of neurons in cell group k, dorsal to those associated with the digastric muscle, were found to contain each one of the reaction products. Since it is thought that only the wheatgerm agglutinin-conjugated horseradish peroxidase transferred from one neuron to another, we conclude that cell group k neurons provide an additional innervation to the digastric and masseter muscles.

Anatomical and physiological study of respiratory motor innervation in lampreys.

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

Distribution of cholinergic neurons in cell group K of the rabbit brainstem.

The cell bodies of efferent neurons supplying the masseter and digastric muscles of the rabbit are located in two brainstem nuclei: the trigeminal motor nucleus and cell group k. The latter also contains neurons innervating muscles of the middle ear and Eustachian tube, as well as neurons that project to the cerebellum and the oculomotor complex. As part of an attempt to identify the functional subpopulations within the three cell divisions (kl-k3) that make up cell group k, we have investigated the distribution of neurons containing choline acetyltransferase, because these are likely to be motoneurons. Five rabbits anaesthetized with sodium pentobarbital (90 mg/kg, i.v.) were used in this study. They were perfused with 4% paraformaldehyde and 0.1% glutaraldehyde in phosphate buffer (0.1 M, pH 7.4). Two animals were used for preliminary studies. In the other three cases, serial Vibratome coronal sections of the brainstem were cut at 50 microm and two series of alternating sections were collected. The first was stained with a monoclonal antibody (code AB8, Incstar) directed against choline acetyltransferase, using the avidin-biotin-peroxidase method. The other was stained with Cresyl Violet. Cell counts and three-dimensional reconstructions were made for both series to determine positions and ratios of cholinergic and non-cholinergic neurons within the trigeminal motor nucleus and the subdivisions of cell group k. The results showed that the numbers of choline acetyltransferase- and Nissl-stained neurons within the trigeminal motor nucleus were almost identical. In cell group k, significantly fewer choline acetyltransferase-stained cells were counted in all three animals (ratios of choline acetyltransferase/Nissl=0.53-0.71). In addition, the distribution of cholinergic neurons was not uniform throughout cell group k. Subdivisions kl and k3 contained proportionately fewer choline acetyltransferase-positive cells (ratios of choline acetyltransferase/Nissl=0.23-0.64) than did k2 (ratios choline acetyltransferase/ Nissl=0.75-0.88). Within each subdivision, there were significant differences in the spatial coordinates of Nissl- and choline acetyltransferase-positive neurons. We conclude that cell group k contains at least two populations of neurons which are unevenly distributed between and within the three subdivisions. While the majority of neurons in subgroup k2 contain choline acetyltransferase and thus are likely to be motoneurons, more than half of the neurons in subgroups k1 and k3 are not cholinergic. It remains to be determined whether these are the neurons that project to the cerebellum and to other CNS regions.

An anatomical study of brainstem projections to the trigeminal motor nucleus of lampreys.

This study was undertaken to identify and describe populations of brainstem neurons that project to the area of the nucleus motorius nervi trigemini in lampreys as a first step in the study of neurons that control feeding behavior in this species. To identify these neurons, the retrograde tracer cobalt-lysine was injected into the nucleus motorius nervi trigemini on one side of the in vitro isolated brainstem preparation of seven spawning adult lampreys (Petromyzon marinus). Transport times ranged from 42 to 48 h. Retrogradely labeled neurons were found within the rostral spinal cord, the rhombencephalon, the mesencephalon and the caudal diencephalon. This study concentrates on the labeled neurons in the rhombencephalon, since the essential circuits for mastication and swallowing are confined to this region in higher vertebrates. Within the rhombencephalon, labeled cells were in the nucleus sensibilis nervi trigemini on both sides. A densely packed column of labeled neurons was found medial to the nucleus motorius nervi trigemini on the ipsilateral side, extending further rostrally in the isthmic region. Continuous columns of labeled cells were observed in the lateral reticular formation on each side in the basal plate ventral to rhombencephalic cranial motor nuclei. They extended from the rostral trigeminal region down into the rostral spinal cord. A comparison with data from cats and rats shows that the distribution of neurons that project to the nucleus motorius nervi trigemini is very similar in mammals and in agnathes. We conclude that the organization of the motor command network of the trigeminal system is well preserved throughout phylogeny and that the in vitro isolated brainstem of lampreys should be a useful model for the study of vertebrate feeding behavior.

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

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

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.