Modulation of alpha and gamma trigeminal motoneurons by various peripheral stimuli

The reciprocal jaw-muscle reflexes elicited by anterior- and back-tooth-contacts-a perspective to explain the control of the masticatory muscles

AIMS

Tooth-contact sensations are considered essential to boost jaw adductor muscles during mastication. However, no previous studies have explained the importance of the inhibitory reflex of human anterior-tooth (ANT)-contacts in mastication. Here I present the "reciprocal reflex-control-hypothesis" of mammalian mastication.

SUBJECTS AND SETTING OF THE STUDY

I demonstrate the hypothesis with the live kinematics of free jaw-closures as inferred from T-Scan recordings of dental patients.

RESULTS

The jaw-closures started with negligible force, predominantly with ANT-contacts (the AF-bites). The first ANT-contact inhibited the first kinematic tilt of the mandible, whereas the bites starting from a back-tooth (BAT)-contact (the BF-bites) accelerated the first tilt. The second tilt established a low-force static tripod of the ANT- and bilateral BAT-contacts for a fixed mandible-maxilla relation. Thereafter, semi-static bite force increased rapidly, relatively more in the BAT-area.

DISCUSSION AND CONCLUSIONS

In the vertical-closure phase of chewing, the primate joint-fulcrum (class 3 lever) conflicts with the food-bolus-fulcrum in the BAT-area (class 1 lever). The resilient class 3 and 1 lever systems are superseded by an almost static mechanically more advantageous class 2 lever with a more rigid fulcrum at the most anterior ANT-contact. For humans, the class 2 levered delivery of force also enables forceful horizontal food grinding to be extended widely to the BAT-area.

Vibrissa Self-Motion and Touch Are Reliably Encoded along the Same Somatosensory Pathway from Brainstem through Thalamus

Active sensing involves the fusion of internally generated motor events with external sensation. For rodents, active somatosensation includes scanning the immediate environment with the mystacial vibrissae. In doing so, the vibrissae may touch an object at any angle in the whisk cycle. The representation of touch and vibrissa self-motion may in principle be encoded along separate pathways, or share a single pathway, from the periphery to cortex. Past studies established that the spike rates in neurons along the lemniscal pathway from receptors to cortex, which includes the principal trigeminal and ventral-posterior-medial thalamic nuclei, are substantially modulated by touch. In contrast, spike rates along the paralemniscal pathway, which includes the rostral spinal trigeminal interpolaris, posteromedial thalamic, and ventral zona incerta nuclei, are only weakly modulated by touch. Here we find that neurons along the lemniscal pathway robustly encode rhythmic whisking on a cycle-by-cycle basis, while encoding along the paralemniscal pathway is relatively poor. Thus, the representations of both touch and self-motion share one pathway. In fact, some individual neurons carry both signals, so that upstream neurons with a supralinear gain function could, in principle, demodulate these signals to recover the known decoding of touch as a function of vibrissa position in the whisk cycle.

Jaw-opening reflex and corticobulbar motor excitability changes during quiet sleep in non-human primates

STUDY OBJECTIVE

To test the hypothesis that the reflex and corticobulbar motor excitability of jaw muscles is reduced during sleep.

DESIGN

Polysomnographic recordings in the electrophysiological study.

SETTING

University sleep research laboratories.

PARTICIPANTS AND INTERVENTIONS

The reflex and corticobulbar motor excitability of jaw muscles was determined during the quiet awake state (QW) and quiet sleep (QS) in monkeys (n = 4).

MEASUREMENTS AND RESULTS

During QS sleep, compared to QW periods, both tongue stimulation-evoked jaw-opening reflex peak and root mean square amplitudes were significantly decreased with stimulations at 2-3.5 × thresholds (P < 0.001). The jaw-opening reflex latency during sleep was also significantly longer than during QW. Intracortical microstimulation (ICMS) within the cortical masticatory area induced rhythmic jaw movements at a stable threshold (≤ 60 μA) during QW; but during QS, ICMS failed to induce any rhythmic jaw movements at the maximum ICMS intensity used, although sustained jaw-opening movements were evoked at significantly increased threshold (P < 0.001) in one of the monkeys. Similarly, during QW, ICMS within face primary motor cortex induced orofacial twitches at a stable threshold (≤ 35 μA), but the ICMS thresholds were elevated during QS. Soon after the animal awoke, rhythmic jaw movements and orofacial twitches could be evoked at thresholds similar to those before QS.

CONCLUSIONS

The results suggest that the excitability of reflex and corticobulbar-evoked activity in the jaw motor system is depressed during QS.

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

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

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1–P4 neonatal rats and inhibitory in P9–P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

The role of periodontal mechanoreceptors in mastication

The aim of this review is to discuss what is known about the reflex control of the human masticatory system by the periodontal mechanoreceptors and to put forward a method for standardised investigation. To deliver mechanical stimulus in a reproducible way, the following precautions are suggested: the stimulus should be brought into secure contact with the area of stimulation, and slack between the probe and the area to be stimulated should be taken up by the application of a preload. It is also important to ensure that there is minimal simultaneous activation of receptor systems other than the periodontal mechanoreceptors. It is also necessary to standardise the method for recording and analysing the response.

The influence of pain on masseter spindle afferent discharge

Muscle spindles provide proprioceptive feedback supporting normal patterns of motor activity and kinesthetic sensibility. During mastication, jaw muscle spindles play an important role in monitoring and regulating the chewing cycle and the bite forces generated during mastication. Both acute and chronic orofacial pain disorders are associated with changes in proprioceptive feedback and motor function. Experimental jaw muscle pain also alters the normal response of masseter spindle afferents to ramp and hold jaw movements. It has been proposed that altered motor function and proprioceptive input results from group III muscle afferent modulation of the fusimotor system which alters spindle afferent sensitivity in limb muscles. The response to nociceptive stimuli may enhance or reduce the response of spindle afferents to proprioceptive stimuli. Several experimental observations suggesting the possibility that a similar mechanism also functions in jaw muscles are presented in this report. First, evidence is provided to show that nociceptive stimulation of the masseter muscle primarily influences the amplitude sensitivity of spindle afferents with relatively little effect on the dynamic sensitivity. Second, reversible inactivation of the caudal trigeminal nuclei attenuates the nociceptive modulation of spindle afferents. Finally, functionally identified gamma-motoneurons in the trigeminal motor nucleus are modulated by intramuscular injection with algesic substances. Taken together, these results suggest that pain-induced modulation of spindle afferent responses are mediated by small diameter muscle afferents and that this modulation is dependent, in part, on the relay of muscle nociceptive information from trigeminal subnucleus caudalis onto trigeminal gamma-motoneurons. The implication of these results will be considered in light of current theories on the relationship between jaw muscle pain and oral motor function.

Physiological characterization, localization and synaptic inputs of bursting and nonbursting neurons in the trigeminal principal sensory nucleus of the rat

A population of neurons in the trigeminal principal sensory nucleus (NVsnpr) fire rhythmically during fictive mastication induced in the in vivo rabbit. To elucidate whether these neurons form part of the central pattern generator (CPG) for mastication, we performed intracellular recordings in brainstem slices taken from young rats. Two cell types were defined, nonbursting (63%) and bursting (37%). In response to membrane depolarization, bursting cells, which dominated in the dorsal part of the NVsnpr, fired an initial burst followed by single spikes or recurring bursts. Non-bursting neurons, scattered throughout the nucleus, fired single action potentials. Microstimulation applied to the trigeminal motor nucleus (NVmt), the reticular border zone surrounding the NVmt, the parvocellular reticular formation or the nucleus reticularis pontis caudalis (NPontc) elicited a postsynaptic potential in 81% of the neurons tested for synaptic inputs. Responses obtained were predominately excitatory and sensitive to glutamatergic antagonists DNQX and/or APV. Some inhibitory and biphasic responses were also evoked. Bicuculline methiodide or strychnine blocked the IPSPs indicating that they were mediated by GABA(A) or glycinergic receptors. About one-third of the stimulations activated both types of neurons antidromically, mostly from the masseteric motoneuron pool of NVmt and dorsal part of NPontc. In conclusion, our new findings show that some neurons in the dorsal NVsnpr display both firing properties and axonal connections which support the hypothesis that they may participate in masticatory pattern generation. Thus, the present data provide an extended basis for further studies on the organization of the masticatory CPG network.

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24-120 h later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-h postinoculation period; subsequent to 72-h survival times, the number of infected motoneurons increased, and at > or =96 h many of these cells showed signs of cytopathic changes. Following 72-h survival times, a few transynaptically labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96-120 h, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and nonsympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.

Stretching of the Mueller muscle results in involuntary contraction of the levator muscle

PURPOSE

Since the levator muscle involuntarily and tonically contracts against the weight and elastic resistance of the upper eyelid to maintain an adequate visual field, a mechanoreceptor such as a muscle spindle or a periodontal mechanoreceptor is thought to be essential for its functioning. It was surmised that the Mueller muscle might act as a serial kind of muscle spindle of the levator muscle.

METHODS

The response of the bilateral levator muscles evoked by stretching the Mueller muscle of each eyelid of 87 patients with dermatochalasis or aponeurotic blepharoptosis was electromyographically and photographically recorded.

RESULTS

Stretching of the unilateral Mueller muscle evoked contraction of the ipsilateral levator muscle in 18 and of the bilateral levator muscle in 69 of the 87 patients.

CONCLUSIONS

The Mueller muscle can be thought of as a large, serial kind of muscle spindle, so that stretching by voluntary phasic contraction of the levator muscle for initial eye opening may evoke an afferent impulse to the mesencephalic trigeminal nucleus. Subsequently, this nucleus may stimulate the central caudal nucleus of the oculomotor nuclear complex, leading to involuntary tonic contraction of the ipsilateral or bilateral levator muscles, in the form of a continuous stretch reflex, to maintain an adequate visual field.

Properties and interconnections of trigeminal interneurons of the lateral pontine reticular formation in the rat

Numerous evidence suggests that interneurons located in the lateral tegmentum at the level of the trigeminal motor nucleus contribute importantly to the circuitry involved in mastication. However, the question of whether these neurons participate actively to genesis of the rhythmic motor pattern or simply relay it to trigeminal motoneurons remains open. To answer this question, intracellular recordings were performed in an in vitro slice preparation comprising interneurons of the peritrigeminal area (PeriV) surrounding the trigeminal motor nucleus (NVmt) and the parvocellular reticular formation ventral and caudal to it (PCRt). Intracellular and extracellular injections of anterograde tracers were also used to examine the local connections established by these neurons. In 97% of recordings, electrical stimulation of adjacent areas evoked a postsynaptic potential (PSP). These PSPs were primarily excitatory, but inhibitory and biphasic responses were also induced. Most occurred at latencies longer than those required for monosynaptic transmission and were considered to involve oligosynaptic pathways. Both the anatomical and physiological findings show that all divisions of PeriV and PCRt are extensively interconnected. Most responses followed high-frequency stimulation (50 Hz) and showed little variability in latency indicating that the network reliably distributes inputs across all areas. In all neurons but one, excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) were also elicited by stimulation of NVmt, suggesting the existence of excitatory and inhibitory interneurons within the motor nucleus. In a number of cases, these PSPs were reproduced by local injection of glutamate in lieu of the electrical stimulation. All EPSPs induced by stimulation of PeriV, PCRt, or NVmt were sensitive to ionotropic glutamate receptor antagonists 6-cyano-7-dinitroquinoxaline and D,L-2-amino-5-phosphonovaleric acid, while IPSPs were blocked by bicuculline and strychnine, antagonists of GABA(A) and glycine receptors. Examination of PeriV and PCRt intrinsic properties indicate that they form a fairly uniform network. Three types of neurons were identified on the basis of their firing adaptation properties. These types were not associated with particular regions. Only 5% of all neurons showed bursting behavior. Our results do not support the hypothesis that neurons of PeriV and PCRt participate actively to rhythm generation, but suggest instead that they are driven by rhythmical synaptic inputs. The organization of the network allows for rapid distribution of this rhythmic input across premotoneuron groups.

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.

Functional changes of brainstem reflexes in Parkinson's disease. Conditioning of the blink reflex R2 component by paired and index finger stimulation

Recovery curves of the R2 component of the blink reflex have been studied in 10 control subjects and 13 parkinsonian patients both after ipsilateral paired stimulation of the supraorbital nerve and after index finger stimulation. In control subjects, both types of conditioning induced a comparable marked inhibition lasting more than 600 ms. In parkinsonian patients, inhibition was reduced after both conditionings. However, differences appeared in the magnitude of the changes: after paired stimulation, it was less significant (ANOVA and post-hoc Duncan's test: p = 0.04) than after index finger stimulation (p = 0.002). In that latter situation, the more marked reduction in inhibition is interpreted, in the light of current physiologic knowledge, by hypoactivity of the Nucleus Reticularis Giganto Cellularis (NRGC) which would make less efficient inhibitory interneurones in the trigemino-facial pathway. The results are thus compatible with the suggestion that NRGC is made indirectly less active in Parkinson's disease.

Orbicularis oculi responses to stimulation of nerve afferents from upper and lower limbs in normal humans

A brief mechanical or electrical stimulus to peripheral nerve afferents from the upper and lower limbs elicited a small and inconsistent EMG response of the orbicularis oculi muscles. This response was facilitated when the stimuli were delivered at fixed leading time intervals, of 45-300 ms, with respect to a supraorbital nerve electrical stimulus. Also, the peripheral nerve stimulus modified the conventional blink reflex responses, inducing facilitation of R1 and inhibition of R2. These results suggest a complex processing of sensory inputs from the face and the limbs at the brainstem, where they are probably integrated in a network of interneurons influencing the excitability of facial motoneurons.

Excitatory effects on neck and jaw muscle activity of inflammatory irritant applied to cervical paraspinal tissues

A study was carried out in 19 anaesthetized rats to determine if the electromyographic (EMG) activity of jaw and neck muscles could be influenced by injection of the inflammatory irritant mustard oil into deep paraspinal tissues surrounding the C1-3 vertebrae. The EMG activity was recorded ipsilaterally in the digastric, masseter and trapezius muscles and bilaterally in deep neck muscles (rectus capitis posterior). In comparison with control (vehicle) injections, mustard oil (20 microliters, 20%) injected into the deep paraspinal tissues induced significant increases in EMG activity in the neck muscles in all the animals and in the jaw muscles in the majority of the animals; the effects of mustard oil were more prominent in the former. The EMG response evoked by mustard oil injection was frequently reflected in two phases of enhanced activity. The early phase of the increase in EMG activity was usually initiated immediately following mustard oil injection (mean latency: 20.4 +/- 17.7 sec) and lasted 1.6 +/- 1.1 min. The second phase occurred 11.3 +/- 7.6 min later and lasted 11.0 +/- 8.1 min. Evans Blue extravasation was apparent in the deep paraspinal tissues surrounding the C1-3 vertebrae after mustard oil injection, and histological examination showed that mustard oil injection induced an inflammatory reaction in the rectus capitis posterior muscle. These results document that injection of the inflammatory irritant mustard oil into deep paraspinal tissues results in a sustained and reversible activation of both jaw and neck muscles. Such effects may be related to the reported clinical occurrence of increased muscle activity associated with trauma to deep tissues.

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.

Masseteric facilitation induced by electrical stimulation of rat orofacial tissues

The effects of a conditioning electrical shock applied to the periodontium of the lower incisor or the glabrous area of the lower lip on the jaw-closing reflex in the anesthetized, non-paralysed rat were studied. The masseteric reflex was triggered by stimulation of the mesencephalic nucleus as a test shock and was recorded from the masseter muscle. There was facilitation of the jaw-jerk reflex, which culminated at an interval of 10-15 ms between the conditioning and the test shocks. This facilitation was not suppressed by digastric excision or by blocking a possible rebound closing reflex evoked by jaw opening. No inhibitory influence was observed. This facilitatory effects relies on an A alpha input and on cell bodies making up the mesencephalic nucleus. The direct excitatory electrical events observed in the masseter muscle after periodontal or labial stimulation proved to be due to the diffusion of the bioelectrical activity generated in the neighbouring jaw-opening muscles.

Side asymmetry of the jaw jerk in human craniomandibular dysfunction

The jaw jerk elicited by tapping the chin with a reflex hammer was electromyographically recorded in 14 patients with craniomandibular dysfunction, who were selected because of their strictly unilateral symptoms. Mandibular deviation, as measured by means of a kinesiograph, was on the same side as the pain. Neurological and neurophysiological investigations, including the recording of masseter motor potentials evoked by transcranial stimulation, showed normal function of the sensory and motor trigeminal nerve fibres. Latency and amplitude of the jaw jerk recorded in postural position and intercuspal occlusion were, respectively, longer and smaller on the affected side. In some cases the latency difference exceeded 1 ms, the limit usually considered significant for trigeminal neuropathy or brainstem lesions. Jaw-jerk asymmetry is probably due to facilitation on the side contralateral to mandibular deviation. In intercuspal occlusion, contralateral facilitation might be produced by a stronger input from muscle spindles and periodontal mechanoreceptors. In postural position, other factors probably intervene.

The masseteric reflex evoked by tooth and denture tapping

The characteristics of the masseter reflex evoked by tapping a maxillary incisor were compared with the reflex pattern evoked by tapping a corresponding denture tooth after insertion of an immediate denture. Up to three inhibitory phases (I-1, I-2 and I-3), followed by excitation, were found on an averaged EMG. The tapping force threshold for the early inhibitory phase was lower than for the late phases. The pattern of the reflex was generally the same before and after insertion of the denture, but the threshold values increased. After insertion of the denture, the threshold for I-1 increased from 1 +/- 0.3N to 2.2 +/- 0.4N, the threshold for I-2 increased from 2.4 +/- 0.8N to 3.8 +/- 0.9N, and the threshold for I-3 increased from 5.1 +/- 0.6N to 8.3 +/- 0.9N. The latency period for I-1 also increased from 12.3 +/- 0.5 ms to 13.1 +/- 0.3 ms after insertion of the denture. After relining, the threshold for evoking I-1 decreased from 2.7 +/- 1.2N to 1.2 +/- 0.6N. It was assumed that the mechanoreceptors situated in the mucosa under the denture base could take over the functional role of the periodontal mechanoreceptors for evoking the masseter reflex during tapping, and that these afferents probably had connections to the same interneurones.

Single motor unit and surface electromyogram analysis of human jaw-closing muscle reflexes after tapping an upper tooth

The mechanical stimulation of an upper tooth elicited reflex responses in masseter and temporalis motor units, which were recorded with both surface and needle electromyograms (EMGs) simultaneously. The subjects maintained one of the recorded motor units, which was the latest recruited unit, at a constant firing frequency, varying from 12 to 25 Hz. After a latency of 10 ms, all 19 motor units were inhibited for a period, the duration of which depended on the prestimulus firing frequency. A motor unit with a low firing frequency was inhibited for a longer time than a faster firing one. At the end of this inhibition there was an increased probability of firing recorded, in the form of a time-locked clustering of action potentials. Furthermore, in three motor units, all firing at a frequency of about 25 Hz, the first interspike interval after the inhibition was regularly half the duration of the mean prestimulus interspike interval. The timing of the last action potential before the stimulus influenced significantly the reflex responses in all motor units.

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)

Loci and characteristics of EMG silent periods during masticatory mandibular movements in rats

Frontal plane mandibular movements and the associated superficial masseter EMG signals of six 39-day-old rats were simultaneously recorded and digitized at a rate of 1 kHz by the optoelectronic method in order to investigate the loci and attributes of masseteric silent periods during mastication of hard (pellets) and soft (slurry) food items. The marked silent periods, defined as cessations of EMG activity during the slow-close (SC) phase of single chewing cycles, were analyzed for their (1) onset and offset durations relative to physiological centric occlusion (PCO), (2) frontal vertical (FV) and frontal horizontal (FH) loci relative to PCO, and (3) FV and FH velocities and accelerations of masticatory mandibular movements in relation to PCO. The start (SSP) and end (ESP) of silent period loci relative to PCO moved superiorly as sequences of pellet mastication progressed. All silent period attributes during slurry consumption were significantly different (p less than or equal to 0.01) from pellet attributes: Slurry SSP and ESP loci were closer to PCO than were pellet loci; durations of silent period loci during pellet mastication were more variable than were slurry durations. FV distance and velocity values for pellets were greater than with pellets. Although FV velocities during both pellet and slurry mastication decreased at ESP relative to SSP values, their FH velocities at ESP actually increased relative to SSP velocities. Loci attributes of EMG silent periods appeared largely dependent on the consistency of the food item being masticated.

The effect of prior jaw motion on the plot of electromyographic amplitude versus jaw position

Fabrication of interocclusal splint at a thickness determined by the vertical dimension at which the jaw muscle EMG amplitude is minimum has been recommended. However, the effect of prior jaw motion and the effect of the recording site on the EMG amplitudes and on the vertical dimension of minimum EMG activity have not been documented. IEMG amplitudes at various static jaw positions achieved during opening and during closing were analyzed in nine subjects. Surface IEMGs were recorded over the left anterior temporal muscle, left masseter and left suprahyoids muscles, and by nonspecific EMG recording as described by Rugh and Drago. The jaw position was recorded in 5 mm increments by a kinesiograph. After 30 seconds of relaxation, 10 successive IEMG reading at 4-second integration times were obtained at each recording site. These 10 recordings at each requested jaw position were averaged and analyzed. The IEMG activity changed with different jaw position. As the jaw opened from centric occlusion, the IEMG from jaw closing muscles decreased to a minimum and then increased with further opening. Moreover, the IEMG for a particular jaw position differed depending on the history of the jaw movement, that is, whether the position was achieved after an opening step or after a closing step. Two factors, the amount of jaw opening and the history of jaw movement to reach that position, seemed to influence the IEMG differently in each of the recorded muscles.

Interactive periodontal and acoustic influences on the masseteric post-stimulus electromyographic complex in man

Post-stimulus electromyogram (EMG) complexes (PSECs) were studied in the full-wave rectified and averaged EMGs of the masseter muscles in 15 subjects, who clenched at a controlled level. The PSECs, a series of downward- and upward-going waves reflecting inhibitory and excitatory influences upon the masseteric motoneurones, were elicited by mechanical stimulation of a tooth. The stimuli selectively activated mechanoreceptors in the periodontium and, by bone-conduction, acoustic receptors. Application of acoustic masking during the periods of stimulation revealed a series of inhibitory and excitatory acoustic influences in the PSEC, which were absent after local electrical stimulation of receptors in the periodontium or their afferents. By applying local anaesthesia to the periodontium of a mechanically stimulated tooth, the durations of the acoustic influences were on the average reduced by 76%. In subjects whose PSECs consistently included a second inhibitory period, the duration of the acoustic influences with respect to that of the PSEC (30%) was larger than otherwise (13%), suggesting a central gating of periodontal pathways which can block both periodontal and acoustic influences. The acoustic influences, of which the appearance in the PSEC largely depends upon activated periodontal pathways, represent a new finding of audio-motor reflexes.

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.

The effect of sectioning the trigeminal sensory root on the periodontally-induced jaw-opening reflex

The experiment was designed to determine the pathway taken to the brain stem by periodontal afferents responsible for the digastric jaw-opening reflex induced by tooth-tapping. Cutting the trigeminal sensory roots of anesthetized decerebrate cats eliminated the ipsilateral periodontally-induced reflex, although the stretch reflexes of the jaw-closing muscles were undiminished. These results suggest that periodontal afferents causing the jaw-opening reflex reach the brainstem through the Vth sensory root, and confirm that muscle spindle afferents travel through the Vth motor root.

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.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

Neuronal responses in rostral trigeminal brain-stem nuclei of macaque monkeys after chronic trigeminal tractotomy

Unilateral trigeminal tractotomy was carried out at the level of the obex, just rostral to the subnucleus caudalis, in five young adult Macaca fascicularis monkeys. The animals had been trained previously to perform a behavioral shock avoidance task in response to electrical stimulation of dental pulp and facial skin. Tractotomy produced an elevation in the stimulus strength which elicited escape behavior when facial skin was stimulated but not when the tooth pulp was stimulated. Unit activity, evoked by electrical stimulation of the tooth pulp and facial skin as well as innocuous and noxious mechanical stimulation of orofacial regions, was recorded from neurons in the trigeminal main sensory nucleus and the subnuclei oralis and interpolaris of the spinal nucleus 8 to 12 weeks after tractotomy. Primary afferent input to these nuclei is unaffected by the tractotomy which is located more caudally. The tractotomy interrupts primary afferent input into the trigeminal nucleus caudalis and also intranuclear connections between caudalis and the more rostral nuclei. Forty-one units contralateral and 47 ipsilateral to the tractotomy were studied. Thirty-six of the units responded only to low-threshold mechanical or electrical stimulation of orofacial zones, 46 were responsive to innocuous mechanical and electrical stimulation of orofacial zones and also to electrical stimulation of the dental pulp. Six units responded only to dental pulp stimulation. No statistically significant differences between the populations of neurons ipsilateral and contralateral to the tractotomies were found relating to the size or location of the peripheral receptive fields, latencies, thresholds, mean firing densities, or responsiveness to the various forms of stimulation. The behavioral results suggest that trigeminal relay neurons rostral to the obex are able to signal dental pain sensation, and the physiological studies confirm that the firing of such neurons is unaffected by tractotomy. The physiological studies demonstrate that the firing patterns of relay neurons activated by natural and electrical cutaneous facial stimuli and which are located in trigeminal brain-stem nuclei rostral to the obex are also not affected by tractotomy. The cutaneous facial analgesia observed after tractotomy thus appears to be due to deafferentation of relay neurons in trigeminal nucleus caudalis rather than to alterations in coding patterns in rostrally located trigeminal neurons due to interruption of the intratrigeminal pathway between the caudal and rostral nuclear groups.

Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric poststimulus EMG complex following tapping of a tooth in man

Poststimulus EMG complexes (PSECs), consisting of a series of inhibitory and excitatory waves in full-wave rectified and averaged electromyogram (EMG), were elicited in the masseter muscles of 7 subjects following controlled tapping of a tooth, at a controlled clenching level. Applying local anaesthesia to this tooth decreased the total surface of the waves, on average by 89%. The excitatory and the inhibitory waves were similarly affected, indicating that mainly pressure receptors in the periodontium mediate the entire PSEC. In 4 subjects, who were exposed to acoustic noise to exclude a contribution of acoustic receptors, the recovery of the PSEC waves from local anaesthesia was tracked. In 3 subjects, one wave (the first inhibitory or the first excitatory one, respectively) recovered differently from the other waves, indicating that they are not necessarily mediated by one type of afferent axons. The evidence, nevertheless, suggests that the different PSEC waves in man reflect the projection of the periodontal afferents upon several brain structures, involved in the control of the activity of the masseteric motorneurones, as: inhibitory and excitatory control requires different groups of interneurones; and a mediation of the first inhibitory wave by slower conducting axons than the second inhibitory wave, or a mediation of both waves by axons of similar type, is not compatible with common interneurones.

The digastric reflex evoked by tooth-pulp stimulation in the cat and its modulation by stimuli applied to the limbs

The digastric reflex evoked by electrical stimulation of tooth pulp in anaesthetized cats was studied together with the effects on this reflex of stimulating other parts of the body. The threshold for the digastric reflex generally lay in the range of stimulus intensities which would excite a large proportion of the pulpal afferent fibres which suggested that a large amount of central summation was required to evoke the reflex. During the course of 25/27 experiments, the threshold for the reflex increased. It was also found that repeated application of suprathreshold stimuli produced first an increase and then a decrease in the reflex response. The application of noxious but not of non-noxious mechanical conditioning stimuli to the limbs produced strong, long-lasting depressions of the digastric reflex. Electrical conditioning stimuli applied to the limbs also depressed the reflex; this depression had a latency of onset of 20-50 ms and lasted for up to 500 ms. When conditioning stimuli were applied to the saphenous nerve, the depression of the reflex occurred only when the stimuli were of an intensity sufficient to excite fibres conducting at less than 40 m X s-1; it may be assumed that some of these fibres would have been high threshold mechanoreceptors or nociceptors. These results show that noxious stimulation of anatomically remote structures can depress the activity of a population of trigeminal brainstem neurones. The opiate antagonist, naloxone, had no detectable effect on either the digastric reflex or the depression of the reflex produced by stimulating other parts of the body. The serotonin antagonists, methysergide and cinanserin, strongly depressed the digastric reflex but it was not clear whether these drugs also affected the depression of the reflex by the conditioning stimuli.

The effects of anaesthetics and remote noxious stimuli on the jaw-opening reflex evoked by tooth-pulp stimulation in the cat

Previous studies indicate that the threshold of the jaw opening reflex (JOR) evoked by tooth-pulp stimulation is much lower in cats subjected to minimal surgical trauma and a short period of anaesthesia than in animals prepared for stereotaxic recording from the brainstem. Experiments have been carried out to determine whether the higher JOR thresholds observed in the latter group of cats could be attributed to the duration of the anaesthesia or the greater surgical trauma to which they were subjected. The effects on the JOR evoked by tooth-pulp stimulation of brief episodes of noxious and high intensity electrical stimulation of other tissues have been studied in anaesthetized cats. In lightly anaesthetized, control animals, the reflex threshold was usually below 100 microA, 0.1 ms and maintained anaesthesia did not affect this. Alphaxalone/alphadolone, methohexitone and alpha-chloralose produced similar results. Noxious or high intensity electrical stimuli applied to a paw, a pinna or the scalp caused either no change or a decrease in the JOR threshold of cats lightly anaesthetized with alphaxalone/alphadolone. With deeper anaesthesia, these same conditioning stimuli caused a maintained increase in JOR threshold which could be reversed by decreasing the anaesthetic dose. The results suggest that the high threshold of the JOR observed in earlier experiments was not due to anaesthesia but may have been caused by trauma.

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.

Exteroceptive and proprioceptive afferents of the trigeminal and facial motor nuclei in the mallard (Anas platyrhynchos L.)

Central pathways converging upon the trigeminofacial motor nuclei of the mallard were studied in order to elucidate neuroanatomically the presumed influence of primary sensory trigeminal afferents upon jaw muscle activity. The techniques used included the Fink-Heimer I method after lesions, and axonal transport labeling following injections of 3H-leucine or of HRP for retrograde identification of the neurons of origin. A general description is given of the trigeminofacial motor complex. Jaw closer muscles are innervated by trigeminal motor neurons, and facial motor neurons innervate the jaw depressor muscles. Two afferents premotor systems, one including the mesencephalic trigeminal nucleus (MesV) and the other the rhombencephalic reticular formation, are distinguished. The proprioceptive neurons of the mesencephalic trigeminal nucleus project upon the ipsilateral trigeminal motor nucleus and upon the nucleus supratrigeminalis. The latter cell group bilaterally projects upon the dorsal and intermediate parts of the facial motor nucleus and upon the dorsal and intermediate parts of the facial motor nucleus and upon part of the trigeminal motor nucleus. Exteroceptive information, relayed through the primary sensory trigeminal column (PrV and nTTD), ultimately reaches the motor nuclei via the reticular formation. The reticular formation forms the final link of three separate circuits: a telencephalic one entered through the principal trigeminal sensory nucleus, a cerebellar one via subnucleus oralis of the descending trigeminal system, and a direct one via subnucleus interpolaris. No direct connections between the principal trigeminal sensory nucleus or subnuclei of the descending trigeminal system and the motor nuclei of the trigeminal (NV) and facial (NVII) nerves have been observed, nor are such direct projections present in the outflow of the presumed telencephalic and cerebellar circuits, viz. of the archistriatum and the central cerebellar nuclei, respectively. The archistriatum projects via the occipitomesencephalic tract upon the lateral rhombencephalic reticular formation as far down as the rostral cervical cord, as well as upon the subnucleus interpolaris of the descending trigeminal system. Similarly, efferents from the central cerebellar nuclei reach the reticular formation, which in turn projects bilaterally upon the motor nuclei. Finally, commissural intermotor connections apparently are mediated by reticular cells surrounding the motor nuclei of NV or NVII, rather than emanating from these nuclei directly.

Neural mechanisms of tardive dyskinesia

Tardive dyskinesia is a disabling movement disorder, caused by antipsychotic medications, that occurs frequently and is not responsive to treatment. It is not known how the brain damage underlying tardive dyskinesia produces abnormal movement. We propose that altered sensory flow to motor systems results in this syndrome. Verification of such a mechanism could lead to early detection and improved treatment of tardive dyskinesia.

Raphe-induced suppression of the jaw-opening reflex and single neurons in trigeminal subnucleus oralis, and influence of naloxone and subnucleus caudalis

(1) The effects of stimulation of the nucleus raphe magnus (NRM) and the periaqueductal gray (PAG) were tested on the digastric (jaw-opening) reflex and on the activity of functionally identified single neurons recorded in trigeminal (V) subnucleus oralis in the brain stem. Reflex and neuronal responses evoked by tooth pulp stimulation could be readily suppressed for 250--1000 msec by PAG and NRM conditioning stimuli. The effects were not specific for tooth pulp afferent inputs, however, since suppression was also apparent in jaw-opening reflex responses evoked by low-intensity electrical or tactile stimulation of oral-facial sites, and in the mechanically or electrically evoked responses of oralis neurons with localized low-threshold mechanoreceptive fields. (2) The modulatory effects on the jaw-opening reflex and oralis neuron activity were not altered by reversible cold block of synaptic transmission in V subnucleus caudalis. Thus it appears that the PAG- and NRM-induced effects on the reflex and oralis neurons are not dependent on relays via caudalis. (3) Some of the suppressive influences on responses to oral-facial stimuli could be reversed by the administration of the opiate antagonist naloxone. This suggests that some of the modulatory influences involve endogenous opiate-related mechanisms. (4) Many of the oralis neurons were identified as trigeminothalamic relay neurons on the basis of their antidromic response to ventrobasal thalamic stimulation; PAG and NRM conditioning produced not only a suppression of their orthodromic responses to oral-facial stimuli but also caused a decrease in the antidromic excitability of the relay neurons. This decrease may be indicative of raphe-induced postsynaptic inhibition of oralis neurons, and/or presynaptic facilitation of their thalamic endings.

The synaptic organization of the motor nucleus of the trigeminal nerve in the opossum

The motor nucleus of the opossum trigeminal nerve consists of a main body and a small dorsomedial cell cluster. The cell bodies form a unimodal population with areas that range from 150-2700 mum2. Golgi impregnations reveal that each neuron has three to six primary dendrites which radiate in all planes from the cell body. Within 300 mum from the soma, the primary dendrites divide into secondary branches and these, in turn, bifurcate into thinner distal dendrites. The overall diameter of the dendritic tree often extends as much as 1 mm, with a rare branch leaving the confines of the nucleus to enter the neighboring reticular formation. Somatic and dendritic spines are often present and are either sessile or complex appendage forms. The perikarya and initial dendritic trunks of trigeminal neurons are contacted by four types of presynaptic terminals which cover more than 40% of the membrane. Most endings are 1-3 mum long and contain either spherical (S) or pleomorphic (P) synaptic vesicles. Another, less common, type of bouton is marked by large dense-core (DC) vesicles. Approximately 8% of the terminals on trigeminal cell bodies are large (2-5 mum) with spherical synaptic vesicles and are always associated with a subsynaptic cistern (C-boutons). These terminals very often interdigitate with adjacent synaptic endings. S-, P-, and C-boutons synapse on the dendritic tree of trigeminal neurons in the following characteristic pattern: proximal dendrites (greater than 5 mum in diameter) are contacted by all three types of terminals; intermediate-sized dendrites (between 2.5 and 5.0 mum in diameter) are most often contacted by S-boutons although P-boutons are also present; and small, distal dendrites (less than 2.5 mum in diameter) are almost always contacted by S- boutons. Both S- and P-boutons contact spines. In order to determine the ultrastructural identity of some of the major afferent systems to the trigemina motor nucleus, adult opossums were subjected to two different types of lesions. Three and 5 days subsequent to lesions which destroyed most of the trigeminal mesencephalic nucleus, degenerating terminals containing spherical vesicles were found. These endings were S-boutons on more distal parts of the dendritic tree while on the cell body and proximal dendrites they were C-boutons. Seven days after a mesencephalic lesion, expanded glial processes approximated the trigeminal cell membrane. Two days subsequent to lesions which transected commissural fibers from the contralateral trigeminal complex, degenerating S- and P-boutons were found in contact with intermediate and distal parts of the trigeminal dendritic tree.