Morphological and electrophysiological determination of the projections of jaw-elevator muscle spindle afferents in rats

The Periodontium Damage Induces Neuronal Cell Death in the Trigeminal Mesencephalic Nucleus and Neurodegeneration in the Trigeminal Motor Nucleus in C57BL/6J Mice

Proprioception from masticatory apparatus and periodontal ligaments comes through the trigeminal mesencephalic nucleus (Vmes). We evaluated the effects of tooth loss on neurodegeneration of the Vmes and trigeminal motor nucleus (Vmo). Bilateral maxillary molars of 2-month-old C57BL/6J mice were extracted under anesthesia. Neural projections of the Vmes to the periodontium were confirmed by injecting Fluoro-Gold (FG) retrogradely into the extraction sockets, and for the anterograde labeling adeno-associated virus encoding green fluorescent protein (AAV-GFP) was applied. For immunohistochemistry, Piezo2, ATF3, Caspase 3, ChAT and TDP-43 antibodies were used. At 1 month after tooth extraction, the number of Piezo2-immunoreactive (IR) Vmes neurons were decreased significantly. ATF3-IR neurons were detected on day 5 after tooth extraction. Dead cleaved caspase-3-IR neurons were found among Vmes neurons on days 7 and 12. In the Vmo, neuronal cytoplasmic inclusions (NCIs) formation type of TDP-43 increased at 1 and 2 months after extraction. These indicate the existence of neural projections from the Vmes to the periodontium in mice and that tooth loss induces the death of Vmes neurons followed by TDP-43 pathology in the Vmo. Therefore, tooth loss induces Vmes neuronal cell death, causing Vmo neurodegeneration and presumably affecting masticatory function.

Suppression of the Swallowing Reflex during Rhythmic Jaw Movements Induced by Repetitive Electrical Stimulation of the Dorsomedial Part of the Central Amygdaloid Nucleus in Rats

A previous study indicated that the swallowing reflex is inhibited during rhythmic jaw movements induced by electrical stimulation of the anterior cortical masticatory area. Rhythmic jaw movements were induced by electrical stimulation of the central amygdaloid nucleus (CeA). The swallowing central pattern generator is the nucleus of the solitary tract (NTS) and the lateral reticular formation in the medulla. Morphological studies have reported that the CeA projects to the NTS and the lateral reticular formation. It is therefore likely that the CeA is related to the control of the swallowing reflex. The purpose of this study was to determine if rhythmic jaw movements driven by CeA had inhibitory roles in the swallowing reflex induced by electrical stimulation of the superior laryngeal nerve (SLN). Rats were anesthetised with urethane. The SLN was solely stimulated for 10 s, and the swallowing reflex was recorded (SLN stimulation before SLN + CeA stimulation). Next, the SLN and the CeA were electrically stimulated at the same time for 10 s, and the swallowing reflex was recorded during rhythmic jaw movements (SLN + CeA stimulation). Finally, the SLN was solely stimulated (SLN stimulation following SLN + CeA stimulation). The number of swallows was reduced during rhythmic jaw movements. The onset latency of the first swallow was significantly longer in the SLN + CeA stimulation than in the SLN stimulation before SLN + CeA stimulation and SLN stimulation following SLN + CeA stimulation. These results support the idea that the coordination of swallowing reflex with rhythmic jaw movements could be regulated by the CeA.

Acidic pH modulation of Na+ channels in trigeminal mesencephalic nucleus neurons

Cell bodies of trigeminal mesencephalic nucleus (Vmes) neurons are located within the central nervous system, and therefore, peripheral as well as central acidosis can modulate the excitability of Vmes neurons. Here, we report the effect of acidic pH on voltage-gated Na channels in acutely isolated rat Vmes neurons using a conventional whole-cell patch clamp technique. Acidic pH (pH 6.0) slightly but significantly shifted both the activation and steady-state fast inactivation relationships toward depolarized potentials. However, acidic pH (pH 6.0) had a minor effect on the inactivation kinetics of voltage-gated Na channels. Less sensitivity of voltage-gated Na channels to acidic pH may allow Vmes neurons to transduce the precise proprioceptive information even under acidic pH conditions.

Revisiting the supratrigeminal nucleus in the rat

The supratrigeminal nucleus (Vsup), originally proposed as a premotoneuron pool in the trigeminal reflex arc, is a key structure of jaw movement control. Surprisingly, however, the location of the rat Vsup has not precisely been defined. In light of our previous cat studies, we made two hypotheses regarding the rat Vsup: (1) the Vsup is cytoarchitectonically distinguishable from its surrounding structures; (2) the Vsup receives central axon terminals of the trigeminal mesencephalic nucleus (Vmes) neurons which are primary afferents innervating muscle spindles of jaw-closing muscles and periodontal ligaments around the teeth. To test the first hypothesis, we examined the cytoarchitecture of the rat Vsup. The Vsup was identified as an area medially adjacent to the dorsomedial part of trigeminal principal sensory nucleus (Vp), and extended from the level just rostral to the caudal two-thirds of the trigeminal motor nucleus (Vmo) to the level approximately 150 μm caudal to the Vmo. Our rat Vsup was much smaller and its location was considerably different in comparison to the Vsup reported previously. To evaluate the second hypothesis, we tested the distribution patterns of Vmes primary afferent terminals in the cytoarchitectonically identified Vsup. After transganglionic tracer applications to the masseter, deep temporal, and medial pterygoid nerves, a large number of axon terminals were observed in all parts of Vsup (especially in its medial part). After applications to the inferior alveolar, infraorbital, and lingual nerves, a small number of axon terminals were labeled in the caudolateral Vsup. The Vsup could also be identified electrophysiologically. After electrical stimulation of the masseter nerve, evoked potentials with slow negative component were isolated only in the Vsup. The present findings suggest that the rat Vsup can be cytoarchitectonically and electrophysiologically identified, receives somatotopic termination of the trigeminal primary afferents, and principally receives strong termination of the spindle Vmes primary afferents.

Evidence for a trigeminal mesencephalic-hypoglossal nuclei loop involved in controlling vibrissae movements in the rat

Previous studies performed in rats showed that the whisker-pad motor innervation involves not only the facial nerve, but also some hypoglossal neurons whose axons travel within the trigeminal infraorbital nerve (ION) and target the extrinsic muscles surrounding the whisker-pad macrovibrissae. Furthermore, the electrical stimulation of the ION induced an increase in the EMG activity of these muscles, while the hypoglossal nucleus stimulation elicited evoked potentials and single motor unit responses. However, the existence of a neural network able to involve the XIIth nucleus in macrovibrissae whisking control was totally unknown until now. Since other recent experiments demonstrated that: (1) the mesencephalic trigeminal nucleus (Me5) neurons respond to both spontaneous and artificial movements of macrovibrissae, and (2) the Me5 peripheral terminals provide a monosynaptic sensory innervation to the macrovibrissae, the present study was aimed at analyzing a possible role of the Me5 nucleus as a relay station in the sensory-motor loop that involves the XIIth nucleus neurons in rhythmic whisking control. Two tracers were used in the same animal: Fluoro Gold, which was injected into the whisker pad to retrogradely label the hypoglossal whisker-pad projection neurons, and Dil, which was instead injected into the Me5 to label its projections to these hypoglossal neurons. Results demonstrated that terminals of the Me5 neurons monosynaptically target the hypoglossal whisker-pad projection neurons. The functional role of this sensory-motor connection is discussed, with particular regard to a hypothesized proprioceptive reflex in whisker-pad extrinsic muscles that can be elicited by the activation of the Me5 macrovibrissae receptors.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

Neuronal circuitry and synaptic organization of trigeminal proprioceptive afferents mediating tongue movement and jaw-tongue coordination via hypoglossal premotor neurons

The neural framework and synaptic organization of trigeminal proprioceptive afferent-mediated jaw-tongue coordination were studied in rats using multiple electrophysiological and neuroanatomical approaches. Electrostimulation of the masseter nerve evoked short-latency responses (5.86 +/- 2.59 ms) in hypoglossal premotor pools including the parvocellular (PCRt) and intermediate (IRt) reticular nuclei and the dorsomedial part of the spinal trigeminal nucleus oralis (Vodm) and interpolaris (Vidm). Biocytin-labelled axon terminals from these areas traveled into the hypoglossal nucleus (XII) and contacted motoneurons. Double labelling of biotinylated dextran amine (BDA) tracing and cholera toxin B (CTB) transport demonstrated that labelled axons and terminals from the mesencephalic trigeminal nucleus (Vme) overlapped with XII premotor neurons in the alpha division and in PCRt, IRt, Vodm and Vidm. Confocal microscopic observations revealed that Vme terminals closely contacted XII premotor neurons. Dual labelling of intracellular neurobiotin staining of jaw-muscle spindle afferents (JMSAs) combined with horseradish peroxidase (HRP) retrograde transport revealed that 498 JMSA boutons apposed to 146 HRP-labelled premotor neurons. Electron microscopic observations demonstrated that 127 JMSA boutons made both axodendritic (68%) and axosomatic (32%) synapses with XII premotor neurons. Eighty-three per cent of synapses were asymmetric and the rest (17%) were symmetric. Thirty-nine per cent of JMSA boutons received presynaptic contacts from P-type terminals. Varieties of synaptic organizations were found. These results provide evidence that trigeminal proprioceptive afferents mediate jaw-tongue coordination through XII premotor neurons. Ultrastructural findings demonstrated that synapses between JMSA boutons and XII premotor neurons are predominantly excitatory, and synaptic transmission to XII motoneurons is modified on XII premotor neurons by presynaptic mechanisms. These frameworks and synaptic organizations are most probably the neural substrate for trigeminal proprioceptive afferent-mediated jaw-tongue coordination.

Is the human masticatory system devoid of recurrent inhibition?

The aim of the present study was to investigate the existence or otherwise of a functional recurrent inhibitory system (Renshaw cell system) in the motoneurons that innervate human masticatory muscles. In a previous study, L: -acetylcarnitine (L: -Ac), a substance known to potentiate recurrent inhibition in humans was found to alter, in a specific way, the discharge variability, and the synchronous activity of motor units depending on the presence or absence of recurrent inhibition in the corresponding motoneuron pool. Using a similar paradigm, we have recorded the tonic discharge activity of motor unit pairs from the masseter muscle during voluntary isometric contraction while subjects were undergoing continuous intravenous saline (SAL, NaCl 0.9%) perfusion. Following a brief baseline-recording period, the subjects were given a test injection of either L: -Ac or isotonic saline (SAL) in a double blind manner. The variability, synchronization, and coherence between the motor unit discharges were analysed during three successive periods: pre-injection, during injection, and post-injection, each lasting 2-3 min. Neither L: -Ac nor SAL injection induced a significant change in the inter-spike interval (ISI) or the coefficient of variation of the ISIs in the motor units tested. There were also no significant changes in the pattern of synchronous activity or in the coherence, which reflects the common frequency content of the unit discharges. Reminiscent of what had been observed previously with motoneurons without recurrent inhibition in the Abductor Digitorum Minimi muscle, the lack of effects of L: -Ac injection on the firing behaviour of masseter motoneurons may suggest that classical Renshaw cell inhibition is lacking in this motoneuron pool.

Participation of sodium currents in burst generation and control of membrane excitability in mesencephalic trigeminal neurons

Subthreshold sodium currents are important in sculpting neuronal discharge and have been implicated in production and/or maintenance of subthreshold membrane oscillations and burst generation in mesencephalic trigeminal neurons (Mes V). Moreover, recent data suggest that, in some CNS neurons, resurgent sodium currents contribute to production of high-frequency burst discharge. In the present study, we sought to determine more directly the participation of these currents during Mes V electrogenesis using the action potential-clamp method. In postnatal day 8-14 rats, the whole-cell patch-clamp method was used to record sodium currents by subtraction in response to application of TTX in voltage-clamp mode using the action potential waveform as the command protocol. We found that TTX-sensitive sodium current is the main inward current flowing during the interspike interval, compared with the h-current (Ih) and calcium currents. Furthermore, in addition to the transient sodium current that flows during the upstroke of action potential, we show that resurgent sodium current flows at the peak of afterhyperpolarization and persistent sodium current flows in the middle of the interspike interval to drive high-frequency firing. Additionally, transient, resurgent, and persistent sodium current components showed voltage- and time-dependent slow inactivation, suggesting that slow inactivation of these currents can contribute to burst termination. The data suggest an important role for these components of the sodium current in Mes V neuron electrogenesis.

Expression of vesicular glutamate transporter 1 immunoreactivity in peripheral and central endings of trigeminal mesencephalic nucleus neurons in the rat

The major neuronal components of the trigeminal mesencephalic nucleus (Vmes) are primary afferent neurons that convey proprioceptive information from the cranioorofacial regions. In the present study, we examined expression of vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, in the primary afferent neurons of the Vmes (Vmes neurons) in neonatal and adult rats. VGLUT1 immunoreactivity was detected in the cell bodies of Vmes neurons in neonatal rats younger than 11 days old, but not in older rats. However, in situ hybridization signals for VGLUT1 mRNA were detected in both neonatal and adult rats. No VGLUT2 immunoreactivity was detected in Vmes neurons of neonatal or adult rats. VGLUT1 immunoreactivity was also seen in the peripheral sensory endings on the equatorial regions of intrafusal fibers of muscle spindles in the masseter muscles in both neonatal and adult rats. In adult rats injected with cholera toxin B subunit (CTb) into the masseter nerve, central axon terminals of Vmes neurons were identified on masseter motoneurons within the trigeminal motor nucleus (Vm) by transganglionically and retrogradely transported CTb. VGLUT1-immunopositive axon terminals in close apposition to CTb-labeled Vm motoneurons were also detected by dual-immunofluorescence histochemistry for VGLUT1/CTb. Electron microscopy after dual immunolabeling for VGLUT1/CTb by the VGLUT1/immunoperoxidase and CTb/immunogold-silver methods further revealed synaptic contact of VGLUT1- and CTb-immunopositive axon terminals upon CTb-labeled neuronal profiles within the Vm. These data indicate that VGLUT1 is expressed in both the central axon terminals and the peripheral sensory endings of Vmes neurons, although no VGLUT1 immunoreactivity was detectable in the cell bodies of Vmes neurons in adult rats.

Convergence of selected inputs from sensory afferents to trigeminal premotor neurons with possible projections to masseter motoneurons in the rabbit

Peripheral input convergence on trigeminal premotor neurons in the vicinity of trigeminal motor nucleus has been investigated. Thirty neurons were identified by their antidromic responses to microstimulation of the masseteric subnucleus of trigeminal motor nucleus (NVmot-mass). Peripheral receptive fields were found in the buccal mucosae, periodontal ligaments, palate, tongue and vibrissae for 16 neurons located in the intertrigeminal area (NVint), supratrigeminal area (NVs), main sensory trigeminal nucleus (NVsnpr) and subnucleus gamma of the oral nucleus of the spinal trigeminal tract (NVspo-gamma). Eleven neurons in the NVint, NVs and NVspo-gamma responded to passive jaw opening: nine neurons were activated and two were inhibited. None of the neurons responded to both the orofacial mechanical stimulation and passive jaw opening. Forty-six percent of neurons (13 out of 28 tested) received inputs from the inferior alveolar nerve (IAN) and 53% of neurons (8 out of 15 tested) received inputs from the infraorbital nerve (ION). Out of 15 neurons tested for inputs from the IAN and ION, 7 neurons in the NVsnpr and NVspo-gamma received input from both. Sixteen percent of neurons (4 out of 25) received inputs from the masseteric nerve (MassN). None of the neurons with inputs from IAN and/or ION also received inputs from the MassN. We suggest that trigeminal premotor interneurons with projections to the NVmot-mass fall into two broad categories, those with inputs from the IAN and/or ION and those with inputs from the MassN, possibly muscle spindle afferents, and no neuron receiving inputs from both.

Growth-associated protein-43 immunohistochemical and ultrastructural changes in jaw muscle spindles of the rat following loss of occlusion

The effects of complete loss of occlusion on the structural and functional status of these muscle spindles were investigated by immunohistochemistry either for protein gene product 9.5 (PGP 9.5) or growth-associated protein-43 (GAP-43) by light and electron microscopy. All the upper molars of 4-week-old Wistar rats were extracted and the erupted portions of the upper and lower incisors of the same animals were cut-off at the level of the interdental papilla every other day. In a control group, immunoreactivity for GAP-43 was positive in the developing annulospiral endings of 2-week-old rats, but was not detected in any of the muscle spindles after 3 weeks of age. At 4 weeks of age, the PGP 9.5 immunostained spindles had well-differentiated annulospiral endings. Ultrastructurally, these afferent endings showed lenticular or circular profiles in cross-sections, and were differentially indented into the intrafusal-fibres. The inner surfaces of the terminals formed rather smooth myoneural junctions, while the outer surfaces were covered only by basal lamina continuous with that of the underlying intrafusal muscle fibres. After the experimental elimination of occlusal contact, GAP-43 immunoreactivity reappeared in some nerve endings of muscle spindles by 3 days, and persisted for at least 28 days. During this period, the afferent-terminals exhibited various fine structural abnormalities such as irregular outlines and invaginated neuromuscular interfaces. Some sensory-terminal (ST) profiles were completely engulfed by intrafusal-fibres. However, GAP-43 expression and ultrastructural alterations became undetectable within a week of the end of incisal cutting and the recovery of incisal-contact. These data indicate that remodelling of nerve terminals in muscle spindles, as assessed by GAP-43 expression and ultrastructural changes, occurs soon after a loss of occlusion, and ceases if incisal-contact is restored. It is concluded that possible changes in jaw muscle function, as well as a sudden loss of proprioceptive sensory input from the periodontal mechanoreceptors of molars and incisors, induce the structural reorganisation of nerve terminations in jaw muscle spindles that is associated with the appearance and disappearance of GAP-43 immunoreactivity.

Evidence for functional compartmentalization of trigeminal muscle spindle afferents during fictive mastication in the rabbit

Primary afferent neurons innervating muscle spindles in jaw-closing muscles have cell bodies in the trigeminal mesencephalic nucleus (NVmes) that are electrically coupled and receive synapses. Each stem axon gives rise to a peripheral branch and a descending central branch. It was previously shown that some spikes generated by constant muscle stretch fail to enter the soma during fictive mastication. The present study examines whether the central axon is similarly controlled. These axons were functionally identified in anaesthetized and paralysed rabbits, and tonic afferent firing was elicited by muscle stretch. For the purpose of comparison, responses were recorded extracellularly both from the somatic region and from the central axon in the lateral brainstem. Two types of fictive masticatory movement patterns were induced by repetitive stimulation of the masticatory cortex and monitored from the trigeminal motor nucleus. Field potentials generated by spike-triggered averaging of action potentials from the spindle afferents were employed to determine their postsynaptic effects on jaw-closing motoneurons. Tonic firing of 32% NVmes units was inhibited during the jaw-opening phase, but spike frequency during closing was almost equal to the control rate during both types of fictive mastication. A similar inhibition occurred during opening in 83% of the units recorded along the central branch. However, firing frequency in these was significantly increased during closing in 94%, probably because of the addition of antidromic action potentials generated by presynaptic depolarization of terminals of the central branch. These additional spikes do not reach the soma, but do appear to excite motoneurons. The data also show that the duration and/or frequency of firing during the bursts varied from one pattern of fictive mastication to another. We conclude that the central axons of trigeminal muscle spindle afferents are functionally decoupled from their stem axons during the jaw-closing phase of mastication. During this phase, it appears that antidromic impulses in the central axons provide one of the inputs from the masticatory central pattern generator (CPG) to trigeminal motoneurons.

Oscillatory membrane potential activity in the soma of a primary afferent neuron

In the present report, we provide evidence that mesencephalic trigeminal (Mes-V) sensory neurons, a peculiar type of primary afferent cell with its cell body located within the CNS, may operate in different functional modes depending on the degree of their membrane polarization. Using intracellular recording techniques in the slice preparation of the adult rat brain stem, we demonstrate that when these neurons are depolarized, they exhibit sustained, high-frequency, amplitude-modulated membrane potential oscillations. Under these conditions, the cells discharge high-frequency trains of spikes. Oscillations occur at membrane potential levels more depolarized than -53 +/- 2.3 mV (mean +/- SD). The amplitude of these oscillations increases with increasing levels of membrane depolarization. The peak-to-peak amplitude of these waves is approximately 3 mV when the cells are depolarized to levels near threshold for repetitive firing. The frequency of oscillations is similar in different neurons (108.9 +/- 15.5 Hz) and was not modified, in any individual neuron, by changes in the membrane potential level. These oscillations are abolished by hyperpolarization and by TTX, whereas blockers of voltage-dependent K+ currents slow the frequency of oscillations but do not abolish the activity. These data indicate that the oscillations are generated by the activation of inward Na+ current/s and shaped by voltage-dependent K+ outward currents. The oscillatory activity is not modified by perfusion with low-calcium, high-magnesium, or cobalt-containing solutions nor is it modified in the presence of cadmium or Apamin. These results indicate that a calcium-dependent K+ current does not play a significant role in this activity. We postulate that the membrane oscillatory activity in Mes-V neurons is synchronized in adjoining electrotonically coupled cells and that this activity may be modulated in the behaving animal by synaptic influences.

Primary- and secondary-like jaw-muscle spindle afferents have characteristic topographic distributions

Single jaw-muscle spindle afferent axons were characterized physiologically and intracellularly stained to determine whether particular physiological types of spindle afferent show distinctive morphologies. Microelectrodes filled with either horseradish peroxidase (HRP) or biotinamide (Neurobiotin) were advanced into the mesencephalic trigeminal nucleus (Vme) in anesthetized rats. Intracellular recordings then were characterized by their response: to palpation of the jaw muscles; when pressure was applied to the teeth and during passive ramp and hold and sinusoidal jaw movement. Seventy-one afferents were characterized physiologically and injected with HRP; an additional 61 afferents were typed and injected with biotinamide. The response of 43 stained neurons was recorded in the presence of suxamethonium. The major projection areas of these afferents were the: trigeminal motor nucleus (Vmo); region dorsal to Vmo; reticular formation, spinal trigeminal nucleus, superior cerebellar peduncle and Vme. One afferent type was modulated strongly during stretching of the jaw-elevator muscles. Based on their high sensitivity during stretching of the jaw muscles and/or their silencing during the release phase of muscle stretch, these afferents were classified as primary-like spindle afferents. These afferents projected most strongly to Vmo. A second type of afferent was modulated only modestly during stretching of the jaw-elevator muscles. These tonic afferents were classified as secondary-like spindle afferents because of their low dynamic sensitivity during ramp muscle stretch and their continued discharge during the release phase of muscle stretch. Secondary-like afferents projected most strongly to the region dorsal to Vmo. Boutons (n = 3,834) from 11 afferents were studied in detail. Secondary-like afferents had statistically larger boutons within Vmo. In both secondary- and primary-like spindle afferents, only a small number of boutons were associated closely with the somata and proximal dendrites of trigeminal motoneurons. In these cases, however, two to five boutons appeared to contact individual motoneurons, implying multiple monosynaptic inputs to a selective subset of jaw-elevator motoneurons. Some "giant" boutons were present dorsal to Vmo and in Vme. These results demonstrate that dynamically sensitive and nondynamically sensitive jaw-elevator muscle spindle afferents project preferentially to different regions. Primary-like spindle afferents are capable of providing feedback related to the dynamic phases of muscle stretch and project most heavily to Vmo. Secondary-like spindle afferents can transmit a feedback signal associated with muscle length and project most strongly to the supratrigeminal region. Both types of afferent have projections caudal to Vmo that may serve longer latency jaw-muscle stretch reflexes and/or the projection of proprioceptive information to the thalamus and cerebellum.

Dopamine-immunoreactivity in the rat mesencephalic trigeminal nucleus: an ultrastructural analysis

The ultrastructure and distribution of dopaminergic boutons within the rat mesencephalic trigeminal (Me5) nucleus was examined with the use of electronmicroscopic immunocytochemistry. A total of 5102 boutons, comprising axosomatic and axodendritic synaptic terminals as well as non-synaptic boutons (or varicosities), located in the ventrocaudal portion of Me5 was analysed. Approximately 20% of these boutons were dopamine-immunoreactive. Morphological analysis showed that the dopaminergic synaptic terminals, axodendritic as well as axosomatic, were exclusively of the S- and G-bouton type; they contained, respectively, small spherical vesicles or small pleomorphic vesicles in combination with large granular dense-cored vesicles. All dopaminergic varicosities in the Me5 were of the G-bouton type. Quantitative analysis revealed that most of the dopaminergic synaptic terminals in the Me5 nucleus contacted dendrites, while only a minority (12%) contacted Me5 somata. This dopaminergic somatic input comprised about half (52%) of the total axosomatic input on Me5 neurons. The present results and previous findings with respect to the prominent serotonergic component of the axosomatic input to Me5 neurons indicate that dopamine and serotonin account for most of the axosomatic input in the ventrocaudal part of the Me5 nucleus. In fact, the present results seem to support previous observations regarding the existence of a population of afferent neurons in which dopamine and serotonin are colocalized.

Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the present study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Our observations are based on complete or near-complete reconstructions of 288 MS (six fibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All the labeled boutons contained spherical synaptic vesicles and were presynaptic to neuronal elements, and some were postsynaptic to axon terminals containing pleomorphic, synaptic vesicles (P-endings). In Vmo neuropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendritic profiles; PL afferents had a higher number of boutons (43%) contacting two or more dendritic profiles. A small but significant number of MS afferent boutons (12%) received contacts from P-endings, but PL afferent boutons (36%) received three times as many contacts from P-endings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) afferent boutons formed two contacts presynaptic to one dendrite and postsynaptic to one P-ending, and their participation in synaptic triads was much more frequent than in Vmo neuropil. The present study indicates that MS and PL afferent terminals have a distinct characteristic in synaptic arrangements in Vmo and Vsup and provides evidence that the synaptic organization of primary afferents differs between the neuropils containing motoneurons and their interneurons.

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.

Effects of afferent firing frequency on the amplitude of the monosynaptic EPSP elicited by trigeminal spindle afferents on trigeminal motoneurones

We have examined the effect of changes in afferent firing frequency on the monosynaptic EPSP elicited by trigeminal muscle spindle afferents in elevator motoneurones, using both paired pulse and repetitive activation of the masseter nerve. Both modes of stimulation resulted in significant facilitation of EPSP amplitude over intervals of 5-19 ms, but no significant change in EPSP amplitude at either longer or shorter intervals. The facilitation obtained stands in contrast to the predominant depression of EPSP amplitude reported in hindlimb motoneurones following stimulation at decreasing intervals.

Differences in monosynaptic EPSPs elicited by masseter and temporalis spindle afferents in anaesthetised rats

We have used the intracellular variant of the spike triggered averaging method to examine the monosynaptic connexions of masseter and temporalis spindle afferents on jaw-elevator motoneurones. Temporalis spindle afferents elicited larger averaged EPSPs in motoneurones than masseter spindle afferents, in part because transmission at synapses of temporalis afferents was associated with lower incidences of failures. We conclude that EPSP amplitude in this motor system in governed, at least in part, by the presynaptic neurone rather than the identity of the postsynaptic neurone.

Projection of jaw-muscle spindle afferents to the caudal brainstem in rats demonstrated using intracellular biotinamide

Intracellular staining with biotinamide was used to study the axonal projection and synaptic morphology of rat jaw-muscle spindle afferents. Intracellular recordings in the mesencephalic trigeminal nucleus (Vme) were identified as spindle afferent responses by their increased firing during stretching of the jaw-elevator muscles. Biotinamide-stained axon collaterals with boutons were found in the trigeminal motor nucleus (Vmo), Vme, the region dorsal to Vmo including the supratrigeminal region, the dorsomedial portion of the trigeminal principal sensory nucleus, and the dorsomedial part of the rostral spinal trigeminal subnucleus oralis. Additional, previously undescribed projections of jaw-muscle spindle afferents were found to the dorsomedial portion of the caudal spinal trigeminal subnucleus oralis (Vodm), the dorsomedial part of the spinal trigeminal subnucleus interpolaris (Vidm), the caudal parvicellular reticular formation, laminae IV and V of the spinal trigeminal subnucleus caudalis (Vc), and the dorsal division of the medullary reticular field. Labeled spindle boutons in Vodm formed predominately axodendritic synapses. Some of these boutons received presynaptic inputs from unlabeled P-type boutons containing clear, spherical, or flattened vesicles. In Vidm, labeled collaterals and boutons were densely clustered into glomerular-like structures. Labeled boutons in Vidm made axodendritic, axosomatic, and axoaxonic synapses and received synaptic contacts from unlabeled boutons containing clear, spherical, or flat and pleomorphic vesicles. Unlabeled presynaptic boutons in Vidm occasionally contained dense core vesicles. Labeled boutons in Vc mainly formed synaptic contacts with large diameter dendrites. This projection of jaw-muscle spindle afferents to caudal brainstem regions may play a significant role in masticatory-muscle stretch reflexes and in the integration of trigeminal proprioceptive information and its transmission to higher centers.

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

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

Inputs from identified jaw-muscle spindle afferents to trigeminothalamic neurons in the rat: a double-labeling study using retrograde HRP and intracellular biotinamide

Projections from physiologically identified jaw-muscle spindle afferents onto trigeminothalamic neurons were studied in the rat. Trigeminothalamic neurons were identified by means of retrograde transport of horseradish peroxidase from the ventroposteromedial nucleus of the thalamus. Labeled neurons were found contralaterally in the supratrigeminal region (Vsup), the trigeminal principal sensory nucleus, the ventrolateral part of the trigeminal subnucleus oralis, the spinal trigeminal subnuclei interpolaris and caudalis, the reticular formation, and an area ventral to the trigeminal motor nucleus (Vmo) and medial to the trigeminal principal sensory nucleus (AVM). Jaw-muscle spindle afferents were physiologically identified by their increased firing during stretching of the jaw muscles and intracellularly injected with biotinamide. Axon collaterals and boutons from jaw-muscle spindle afferents were found in Vmo; Vsup; the dorsomedial part of the trigeminal principal sensory nucleus (Vpdm); the dorsomedial part of the spinal trigeminal subnuclei oralis, interpolaris (Vidm) and caudalis; the parvicellular reticular formation (PCRt); and the mesencephalic trigeminal nucleus. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, PCRt, and AVM were associated with axon collaterals and boutons from intracellularly stained jaw-muscle spindle afferents. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, and PCRt were closely apposed by one to 14 intracellularly labeled boutons from jaw-muscle spindle afferents, suggesting a powerful input to some trigeminothalamic neurons. These data demonstrate that muscle length and velocity feedback from jaw-muscle spindle afferents is projected to the contralateral thalamus via multiple regions of the trigeminal system and implicates these pathways in the projection of trigeminal proprioceptive information to the cerebral cortex.

Topodiagnostic value of brain stem reflexes

The masseter and medial pterygoid stretch reflexes, the masseter inhibitory reflexes, and the blink reflexes are useful diagnostic tools for evaluation of brain stem disorders. The structures mediating these reflexes are largely known. Characteristic changes of the normal response patterns due to various lesions have been described. Distinct reflex abnormalities indicate lesions at specific sites. Multireflex testing improves the accuracy with which localization can be made. A number of lesions suspected on clinical data may be confirmed by reflex findings only and not by imaging studies. Reflex testing can be utilized to demonstrate multiple lesions evoked by a single vascular event and evaluate dissemination of central nervous involvement in multiple sclerosis patients.

Physiological identification of jaw-movement-related neurons in the trigeminal nucleus of cats

Although neurons responsive to jaw movements have been identified in most parts of the trigeminal brainstem nuclei, little is known about how this information is relayed to the thalamus and ultimately to the cortex for kinesthetic functions and sensorimotor integration. The present extracellular recording experiments showed that a substantial amount of movement-related information is relayed to the thalamus through the caudal part of subnucleus interpolaris (Vi) in adult cats. Vertical jaw displacements, natural mechanical stimuli, and electrical stimulation of the masseter nerve were used to determine the receptive fields and response properties of movement-related neurons. Movement-related responses were observed in 161 units. The receptive fields of these units were located in the masseter muscle, other deep structures, hairy skin, oral mucosa, or some combination of these structures (i.e., convergent). The latency of units responding to masseter nerve stimulation ranged from 1.0 msec to 20 msec, which suggested that some movement-related information was provided by smaller-diameter muscle afferents. Movement responses were either tonic or phasic. Tonic units showed continuous firing at some jaw position; some of these showed a "dynamic" response to jaw displacement. Phasic units were only active, or showed increased activity, when the jaw moved through a specific position. Seventy-one movement-related units were activated by stimulation from the contralateral ventroposteromedial nucleus (VPM) of the thalamus. Most of the brainstem recording sites were located in the dorsal part of Vi between the caudal pole of the facial motor nucleus and the obex. Neurons in caudal Vi may be important for facial kinesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptive field properties of trigeminothalamic neurons in the rostral trigeminal sensory nuclei of cats

This study described topographic and receptive field representation in the region of the rostral trigeminal nuclei, and evaluated whether thalamic neurons from the principal sensory nucleus relay muscle afferent information to the thalamus. Extracellular single-unit activity was recorded from anesthetized cats. Units were tested for responses to natural stimuli (i.e., air bursts, brushing, light pressure, and pinch) applied to the face and oral cavity, electrical stimulation of the masseter nerve, and ramp-and-hold movements of the jaw. The receptive fields and physiological properties for 110 units were studied; we were able to verify the recording site for 96 of these units. Most of the units had discrete receptive fields in the oral cavity, skin, hair, and masseter muscle. Only 2 units received convergent inputs. Stimulation of the ipsilateral and contralateral ventroposteromedial nucleus of the thalamus was performed to identify antidromically activated units. The results showed that the dorsal principal sensory nucleus received its input primarily from the oral cavity. Most of the units (85%) that were activated by antidromic stimulation from the ipsilateral thalamus were located in this nucleus. In contrast, 82% of the units that projected to the contralateral thalamus were located in the ventral principal sensory nucleus. A complete somatotopic representation of the ipsilateral face and oral cavity was observed in the ventral principal sensory nucleus. Although 24 units had muscle receptive fields, none were activated by stimulation of the ipsilateral thalamus, and only 1 responded to stimulation of the contralateral thalamus. Most of the units that were not antidromically driven were recorded outside of the cytoarchitectural boundaries of the principal sensory nucleus. Retrograde labeling of the rostral trigeminal nuclei indicated that most of the neurons in the dorsal principal sensory nucleus projected to the ipsilateral thalamus, whereas those in the ventral principal sensory nucleus projected to the contralateral thalamus. Taken together, these observations do not support the presence of a substantial relay for muscle afferent input from the dorsal principal sensory nucleus to the ventrobasal thalamus in cats.

Projection of cat jaw muscle spindle afferents related to intrafusal fibre influence

1. A method of classification of muscle spindle afferents using succinylcholine (SCh) and ramp stretches has recently been described, which appears to estimate separately the strength of influence of bag1 (b1) and of bag2 (b2) intrafusal fibres. Increase in dynamic difference (delta DD) indicates b1 influence whilst increase in initial frequency (delta IF) indicates b2 influence. The significance of this classification has now been examined by correlation with the strength of synaptic projection of jaw muscle spindle afferents to the fifth motor nucleus (MotV) and the supratrigeminal region (STR) in anaesthetized cats. 2. Projection strength was estimated by computing the extracellular focal synaptic potential (FSP) from spike-triggered averages of 1024 sweeps at 100 microns intervals along tracks through STR and MotV. Trigger pulses were derived from spindle afferent cell bodies of the jaw-closer muscles recorded in the mesencephalic trigeminal nucleus, and characterized by the effect of SCh on their responses to ramp-and-hold stretches. 3. The maximum size of FSPs in tracks traversing STR and MotV ranged from 2.08 to 36.99 microV with a mean of 7.55 microV. The amplitudes were bimodally distributed into roughly equal-sized groups with high and low amplitude FSPs. 4. Mean values of delta IF were significantly greater for the group with large FSPs than for those with small FSPs. There were no significant differences in delta DD. FSP amplitude was significantly positively correlated with delta IF, but not with delta DD. 5. Spindle afferents with high values of FSP amplitude in MotV had a wide range of values of delta DD (b1b2c and b2c groups), while units with large FSPs in STR were all in the b2c category. Some evidence is presented to indicate that this reflects a preferential projection of secondary afferents to the STR. 6. For those units with projection to both STR and to MotV, there was a significant positive correlation between FSP amplitude in the two nuclei. 7. These results indicate that the extent of the b2 influence on spindle afferents predicts the central projection strength better than does the b1 influence. This finding is discussed from the viewpoint of possible developmental and functional issues.

Gain and threshold of the jaw-jerk reflex in man during isometric contraction

The control of mandibular posture has been related to the activity of the anterior temporal muscles, whereas the masseter muscles have been viewed mainly as force producers. However, these groups of muscles, especially in the deep layers, are highly endowed with muscle spindles, so that a difference in function should imply a difference in the reflex sensitivity. By studying the jaw-jerk reflex by means of bipolar surface electromyogram, the reflex sensitivity was determined from relationships between reflex amplitude and jaw displacement from both groups of muscles in eight subjects. At a constant level of background muscle activity, and hence with a constant excitability of the alpha motoneurons, the reflex sensitivity can be determined from these relationships in terms of gain and threshold. In order to account for differences in thickness of the soft tissues overlying the various muscles studied, the reflex amplitude was normalized with respect to the level of maximal voluntary contraction (MVC). In experiments where the inter-electrode distance was 18 mm over both groups of muscles, the reflex gain of the anterior temporal muscles was larger than that of the masseter muscles (P < 0.05). The threshold value did not differ significantly from zero for either group of muscles. Normalization of the reflex amplitude with respect to MVC can be carried out correctly only if the reflexly activated muscle fibres are distributed uniformly within the muscle. In order to gain an insight into this distribution, control experiments were performed with three subjects in which the inter-electrode distance was varied, thus influencing the depth in the muscle from which active muscle fibres were recorded. The reflex gain of the masseter muscle with an inter-electrode distance of 22 mm, which records from deeper layers of the muscle as well as superficial ones, was larger than with a distance of 11 mm (P < 0.01-0.10 in the various subjects). No such differences were found for the anterior temporal muscle. It was concluded that the afferents of the spindles in the jaw-elevator muscles do not project uniformly upon the motoneurons, but involve mainly fibres in the deeper layers of the muscle. The difference in gain found between the masseter and the anterior temporal muscles with the same inter-electrode distance of 18 mm is likely to be due to a larger distance of the reflexly activated muscle fibres in the masseter muscle with respect to the electrodes rather than to a difference in reflex sensitivity between the muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Location, morphology, and central projections of mesencephalic trigeminal neurons innervating rat masticatory muscles studied by axonal transport of choleragenoid-horseradish peroxidase

Retrograde and transganglionic transport of horseradish peroxidase conjugated to the B-fragment of cholera toxin (B-HRP) was used to study the location, morphology, and central projections of mesencephalic trigeminal (Me5) neurons innervating rat masticatory muscles. Labeled Me5 cell bodies were found throughout the Me5 nucleus from a level slightly caudal to the trigeminal motor nucleus to the level of the superior colliculus 5 mm further rostrally. Occasionally, labeled Me5 cells were observed in the anterior medullary velum, in the cerebellum, and in the brainstem contralateral to the B-HRP injection. The vast majority of the labeled Me5 cells were pseudounipolar, but multipolar cells were also found. Extensive central projections from labeled Me5 cells could be seen extending from the nucleus of Darkschewitsch rostrally to the C2 segment caudally. Small but consistent projections from Me5 neurons were observed in nuclear islands among the incoming Me5 root fibers. Trigeminal and hypoglossal motor nuclei received direct projections from Me5 cells, but not the facial motor nucleus. The most prominent Me5 projections appeared in the brainstem reticular formation, including the supratrigeminal nucleus. Smaller projections also extended into the main sensory trigeminal nucleus, trigeminal subnucleus oralis, and the nucleus of the solitary tract.

The pterygoid reflex in man and its clinical application

A technique for eliciting and recording the stretch reflex (R) of the medial pterygoid muscle (Pter) is described. The latency was 6.9 +/- 0.43 ms in 23 healthy volunteers (mean age 23.7 years) showing a side-to-side difference of 0.29 +/- 0.21 ms. The PterR latencies were little shorter and side-to-side differences little greater than of the masseter reflex. Observations in 5 selected patients with small brainstem lesions suggest that the neurons of the PterR afferents form a cluster within the caudal portion of the trigeminal mesencephalic nucleus. Testing the masseter and pterygoid reflexes provides a more precise localization of small ponto-mesencephalic lesions.

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.

Monosynaptic EPSPs elicited by single interneurones and spindle afferents in trigeminal motoneurones of anaesthetized rats

1. Our aim has been to quantify the monosynaptic connections of trigeminal interneurones and spindle afferents onto jaw-elevator motoneurones as a step towards identifying common features in organization of monosynaptic inputs onto motoneurones. We have used the intracellular variant of the spike-triggered averaging method to examine the connections of single identified trigeminal interneurones and jaw-elevator muscle spindle afferents onto single jaw-elevator motoneurones. The interneurones examined lay in the region immediately caudal to the trigeminal motor nucleus. The experiments were performed on rats anaesthetized with pentobarbitone, paralysed and artificially ventilated. 2. Ten EPSPs and eight IPSPs were obtained from examining the connections of seventeen interneurones to thirty-six motoneurones, suggesting a functional connectivity of 50% for individual interneurones onto elevator motoneurones. Fourteen EPSPs were obtained from examining the connections of thirteen spindle afferents onto twenty-seven motoneurones, giving a functional connectivity of 52% for individual spindle afferents onto elevator motoneurones. The amplitudes of the EPSPs elicited by interneurones ranged from 7-48 microV (mean = 17, S.D. = 12.5, n = 10) and from 7 to 289 microV (mean = 64, S.D. = 76.0, n = 14) for the spindle-mediated EPSPs; the difference in the two means was not significant (P = 0.07). 3. However, the amplitude of averaged responses obtained by signal averaging methods are dependent on the assumption that the postsynaptic response occurs following every impulse in the presynaptic neurone. We therefore estimated the percentage of sweeps which contained EPSPs triggered by the presynaptic neurone under study. In essence the method used consisted of visual inspection of the individual sweeps comprising an average in order to assess the occurrence of EPSPs within six separate time windows, each of duration +/- 0.3 ms. Five windows were placed at randomly selected times on average and were used to provide an estimate of the frequency of occurrence of randomly triggered EPSPs. The sixth window was centred on the start of the averaged EPSP and the frequency of occurrence of randomly triggered EPSPs was subtracted from the frequency of occurrence of EPSPs in this window to produce an estimate of the incidence of EPSPs triggered by the presynaptic neurone under study. 4. Values of the incidence of occurrence of EPSPs triggered by the presynaptic neurones ranged from 4.3 to 92% for the fifteen averaged EPSPs which could be analysed in this manner (two elicited by interneurones and thirteen by spindle afferents).(ABSTRACT TRUNCATED AT 400 WORDS)

Monosynaptic connections between neurons of trigeminal mesencephalic nucleus and jaw-closing motoneurons in the rat: an intracellular horseradish peroxidase labelling study

In order to confirm the monosynaptic connections of muscle spindle-mediated jaw stretch reflexes, 8 neurons of trigeminal mesencephalic nucleus innervating masseteric muscle spindles were identified electrophysiologically and stained intracellularly with horseradish peroxidase. These axon terminals projected to ipsilateral dorsal and dorsolateral divisions of trigeminal motor nucleus and extensive premotor areas. Under electron microscope, labeled terminals made monosynaptic contacts predominantly with dendrites in the jaw-closing motoneuron pools. One labeled and many non-labeled terminals were frequently observed to converge simultaneously on one dendrite in the area. However, it was of particular interest that 28% of the labeled terminals constituted the intermediate component of axo-axodendritic synaptic triads. The present study confirmed, for the first time, monosynaptic connections between jaw-closing muscle spindle afferents and jaw-closing motoneurons. These findings also provided ultrastructural evidence for the monosynaptic excitation of muscle spindle-mediated jaw stretch reflexes which received presynaptic and postsynaptic inhibitions of the premotor neurons from other sources.

Projections of the parvocellular reticular formation to the contralateral mesencephalic trigeminal nucleus in the rat

Projections of the parvocellular reticular nucleus (PCRt) to the contralateral mesencephalic trigeminal nucleus (Me5) were studied in the rat with neurophysiological and neuroanatomical techniques. Three types of responses (classified by latencies) were recorded extracellularly in the Me5 area after electrical stimulation of the PCRt: (1) R1 fast unitary reactions (latency 0.2-0.5 ms) found throughout the entire caudal Me5 area; (2) R2 slower unitary reactions (latency 0.7-1.2 ms) recorded ventral and lateral to the caudal Me5; and (3) R3 compound potentials (latency 1.0-2.5 ms) recorded within the ventrocaudal Me5. Relayed or synaptic fields were not observed. Intracellular recordings of identified cell bodies of periodontal afferents, muscle spindle afferents and interneurones revealed no short-term postsynaptic potentials following PCRt stimulation. In some experiments jaw muscle spindle afferent activity was increased by PCRt stimulation and jaw-opening and jaw-closing reflexes were facilitated in the contralateral musculature. Neuroanatomical experiments, using Phaseolus vulgaris leucoagglutinin as an anterograde tracer, showed direct projections with intensive collateralization of PCRt fibres into the Me5 area. Synaptic contacts of PCRt fibres with primary afferent cell bodies were observed in the ventrocaudal parts of the Me5 only. The electrophysiological results are discussed in relation to the neuroanatomical findings.

Midbrain vs. pontine medial longitudinal fasciculus lesions: the utilization of masseter and blink reflexes

Masseter (MR) and blink reflexes (BL) were investigated in 51 patients with internuclear ophthalmoplegia (INO) due to multiple sclerosis (28) and lacunar infarction (23). The MR was abnormal in 20 of 23 cases with bilateral INO and in 21 of 28 with unilateral INO. The R1 component of the BL (BL-R1) was abnormal in 7 of 23 patients with bilateral INO and 10 of 28 with unilateral INO. Combined MR and BL-R1 changes occurred in 8 of 28 cases with unilateral INO and 7 of 23 with bilateral INO. The findings provide evidence for a rostral/caudal localization of lesions within the medial longitudinal fasciculus causing INO on the basis of MR and BL-R1 abnormalities. An abnormality limited to MR suggests a midbrain location in 58.8% of patients while abnormal BL-R1 with or without an associated MR change suggests a rostral pontine location in 35.3%.

Integration in trigeminal premotor interneurones in the cat. 2. Functional characteristics of neurones in the subnucleus-gamma of the oral nucleus of the spinal trigeminal tract with a projection to the digastric motoneurone subnucleus

Seventy-one (n = 71) premotor interneurones have been localized by extracellular recordings within the subnucleus-gamma of the oral nucleus of the spinal trigeminal tract (NVspo-gamma) in nineteen chloralose anaesthetized cats. The neurons were antidromically activated by microstimulation (minimum = 3 microA) applied to the digastric motoneurone subnucleus of the trigeminal motor nucleus. Fifty-one (n = 51) of the interneurones were discharged from the ipsi- and nineteen (n = 19) from the contralateral digastric subnucleus. One neurone out of four tested was antidromically activated from both stimulation sites suggesting a bifurcated axon. The identified premotor neurones had a unique convergence profile of oral and perioral primary afferents. Latency calculations indicated that at least 55% of these interneurones were monosynaptically activated by low stimulus strength applied to the inferior alveolar (minimum = 1.0 T) and/or the lingual nerve (minimum = 1.0 T). The thresholds for evoking the neuronal discharges coincided statistically with those required to evoke a jaw opening reflex response by stimulation of the same nerves. It is suggested that the specific group of NVspo-gamma interneurones under different contexts mediates the disynaptic reflex and participates in the centrally and reflexly evoked "patterning" adjustments of the digastric jaw opening motoneurones during ongoing jaw movements. A companion paper reports the convergence of descending cortical, tectal and ascending cervical inputs, as well as of oro-facial and neck primary afferent inputs onto an unselected population of interneurones in the NVspo-gamma (Westberg and Olsson 1991).

Sensory neurons and motoneurons of the jaw-closing reflex pathway in rats: a combined morphological and physiological study using the intracellular horseradish peroxidase technique

Motoneurons and muscle spindle afferents of the rat masseter muscle were physiologically and morphologically characterized. Their soma-dendritic morphology and axonal course were investigated using the intracellular horseradish peroxidase method. Following electrical stimulation of the masseter nerve, individual motoneurons were identified by antidromic all-or-none action potentials and individual sensory neurons by orthodromic action potentials. Using threshold separation an excitatory input from muscle spindles to a masseter motoneuron was demonstrated. The short latency difference of 0.34 ms between the mean orthodromic response in the sensory neurons and the beginning of the synaptic potential in the masseter motoneuron suggests a monosynaptic connection between the spindle afferents and the motoneurons. Following intrasomatic horseradish peroxidase injection large multipolar cell bodies of masseter motoneurons were found within the motor nucleus. Their positions corresponded to the topographic organization of the motor trigeminal nucleus as described in retrograde tracing studies. Dendrites of masseter motoneurons were complex and could be found far beyond the nuclear borders. Distal dendrites extended to the mesencephalic trigeminal nucleus, the supratrigeminal nucleus, the lateral lemniscus and the reticular formation. Within the reticular formation dendrites were seen in the intertrigeminal nucleus and the peritrigeminal zone. Unipolar cell bodies of muscle spindle afferents were found in the mesencephalic trigeminal nucleus after intra-axonal injection of horseradish peroxidase. For all reconstructed sensory neurons a similar axonal course was found. Axonal terminals were found ipsilateral in the motor trigeminal nucleus, indicating a direct connection between sensory neurons and motoneurons. Further collaterals were found ipsilateral in the supratrigeminal nucleus and caudal to the motor trigeminal nucleus in the parvocellular reticular nucleus alpha. Since the latter termination areas are important for bilateral control of jaw-movements, the muscle spindle afferents are likely to participate not only in a monosynaptic motor reflex, but also in more complex neuronal circuits involved in jaw-movements.

The morphology and electrical geometry of rat jaw-elevator motoneurones

1. The aim of this work was to quantify both the morphology and electrical geometry of the dendritic trees of jaw-elevator motoneurones. To do this we have made intracellular recordings from identified motoneurones in anaesthetized rats, determined their membrane properties and then filled them with horseradish peroxidase by ionophoretic ejection. Four neurones were subsequently fully reconstructed and the lengths and diameters of all the dendritic segments measured. 2. The mean soma diameter was 25 microns and values of mean dendritic length for individual cells ranged from 514 to 773 microns. Dendrites branched on average 9.1 times to produce 10.2 end-terminations. Dendritic segments could be represented as constant diameter cylinders between branch points. Values of dendritic surface area ranged from 1.08 to 2.52 x 10(5) microns 2 and values of dendritic to total surface area from 98 to 99%. 3. At branch points the ratio of the summed diameters of the daughter dendrites to the 3/2 power against the parent dendrite to the 3/2 power was exactly 1.0. Therefore the individual branch points could be collapsed into a single cylinder. Furthermore for an individual dendrite the diameter of this cylinder remained constant with increasing electrical distance from the soma. Thus individual dendrites can be represented electrically as cylinders of constant diameter. 4. However dendrites of a given neurone terminated at different electrical distances from the soma. The equivalent-cylinder diameter of the combined dendritic tree remained constant over the proximal half and then showed a pronounced reduction over the distal half. The reduction in equivalent diameter could be ascribed to the termination of dendrites at differing electrical distances from the soma. Therefore the complete dendritic tree of these motoneurones is best represented as a cylinder over the proximal half of their electrical length but as a cone over the distal half.

Mastication and its control by the brain stem

This review describes the patterns of mandibular movements that make up the whole sequence from ingestion to swallowing food, including the basic types of cycles and their phases. The roles of epithelial, periodontal, articular, and muscular receptors in the control of the movements are discussed. This is followed by a summary of our knowledge of the brain stem neurons that generate the basic pattern of mastication. It is suggested that the production of the rhythm, and of the opener and closer motoneuron bursts, are independent processes that are carried out by different groups of cells. After commenting on the relevant properties of the trigeminal and hypoglossal motoneurons, and of internuerons on the cortico-bulbar and reflex pathways, the way in which the pattern generating neurons modify sensory feedback is discussed.

Contrasting effects of urethane and pentobarbitone anaesthesia on the electrical properties of rat jaw-elevator motoneurones

Our main finding is that elevator motoneurones do not show sustained firing to intracellular injections of depolarising current pulses in rats anaesthetised with urethane. In contrast, virtually all elevator motoneurones show sustained firing in pentobarbitone-anaesthetised rats. The differences in firing are not associated with significant differences in membrane potential, spike amplitudes, AHP amplitude or duration, input resistance, time constant or rheobase (P greater than 0.06 in all cases). However, there are clear differences in the extent of sag seen under the two anaesthetics and so we tentatively suggest that the anaesthetics may differ in their effects on the inward rectifier.

Contralateral projections of cells in the motor trigeminal nucleus of the rat

With horseradish peroxidase and Phaseolus vulgaris lectin as tracers, a direct connection between the jaw closing parts of the ipsi- and contralateral motor trigeminal (Mo5) nuclei of the rat is shown. The contralateral projecting cells in Mo5 were small (18 X 11 microns) ovoid and bipolar. It is speculated that these contralateral projecting cells in Mo5 are interneurons that are involved in the coordination of the bilateral activity of jaw closing motoneurons during orofacial motor behavior.

Two types of jaw-muscle spindle afferents in the cat as demonstrated by intra-axonal staining with HRP

Intra-axonal records and horseradish peroxidase (HRP) injection techniques were employed to define the response properties of the jaw-closing muscle spindle afferents in the trigeminal mesencephalic nucleus (Vmes) and their morphological characteristics. The axonal trajectories of 9 spindle afferents from the masseter and 4 afferents from the temporalis were recovered for detailed analyses. Of 13 afferents, 6 cell bodies were stained and they were located at the rostrocaudal mid-levels of the Vmes. The central courses of the stem fibers were organized in a similar manner to the Vmes periodontal afferent nerves with the exception that peripheral (P) fibers of all spindle afferents passed through the trigeminal motor tract and root. On the basis of collateral terminal arborizations, the Vmes spindle afferents could be classified into two types: type I (n = 6) and type II (n = 7). Type I afferents sent their collaterals into the trigeminal motor nucleus (Vmo), intertrigeminal region (Vint) and juxtatrigeminal region (Vjux), but collaterals from the two neurons also projected to Vmes and the nucleus oralis (Vo). The collaterals from type II afferents formed their terminal arbors in the supratrigeminal nucleus (Vsup) in addition to the Vmo, Vint and Vjux, but collaterals from one neuron also projected to the Vo. In type I afferents, terminal arbors encompassed the whole Vmo including jaw-closing motoneurons. In contrast, boutons from type II afferents were restricted to a few small portions within the Vmo in proximity to its lateral and dorsal boundaries. The diameters of the united (U), central (C) and peripheral (P), fibers were larger in type I than type II afferents; those of the U fibers were statistically significant. Any differences between the two distinct types were not found in the response pattern to the sustained jaw opening. These results suggest that the difference of primary and secondary muscle-spindle afferent nerves is reflected in a distinctive morphology in the terminal arborizations and in the diameters of united fibers rather than the response patterns in deeply anesthetized cats.

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

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

Neurotransmitters and neuropeptides within the mesencephalic trigeminal nucleus of the rat: an immunohistochemical analysis

In order to determine which neurotransmitters and neuropeptides are utilized by the neurons of the mesencephalic trigeminal nucleus and by the fibres making synaptic contact with these primary sensory cells, we have set up an immunohistochemical study using antibodies against 17 major neurotransmitters and neuropeptides in the rat. Apart from some intracellular immunostaining for glutamate, no immunoreactivity to any of the tested neurotransmitters and neuropeptides could be detected inside mesencephalic nucleus of the trigeminal nerve neurons. Our immunohistochemical observations indicate that mesencephalic nucleus of the trigeminal nerve neurons receive input from various nerve fibres that appear to utilize serotonin, GABA, dopamine, noradrenaline (and likely glutamate) as transmitters. The innervation appeared randomly distributed over all mesencephalic nucleus of the trigeminal nerve neurons. The presence of substance P, cholecystokinin, vasoactive intestinal polypeptide, bombesin/gastrin releasing peptide, [Leu]enkephalin and neuropeptide Y observed in some fibres that contact with mesencephalic nucleus of the trigeminal nerve neurons, presumably reflect the co-existence of these peptides with one of the neurotransmitters.

Intersubnuclear connections within the rat trigeminal brainstem complex

Prior intracellular recording and labeling experiments have documented local-circuit and projection neurons in the spinal trigeminal (V) nucleus with axons that arborize in more rostral and caudal spinal trigeminal subnuclei and nucleus principalis. Anterograde tracing studies were therefore carried out to assess the origin, extent, distribution, and morphology of such intersubnuclear axons in the rat trigeminal brainstem nuclear complex (TBNC). Phaseolus vulgaris leucoagglutinin (PHA-L) was used as the anterograde marker because of its high sensitivity and the morphological detail provided. Injections restricted to TBNC subnucleus caudalis resulted in dense terminal labeling in each of the more rostral ipsilateral subnuclei. Subnucleus interpolaris projected ipsilaterally and heavily to magnocellular portions of subnucleus caudalis, as well as subnucleus oralis and nucleus principalis. Nucleus principalis, on the other hand, had only a sparse projection to each of the caudal ipsilateral subnuclei. Intersubnuclear axons most frequently traveled in the deep bundles within the TBNC, the V spinal tract, and the reticular formation. They gave rise to a number of circumscribed, highly branched arbors with many boutons of the terminal and en passant types. Retrograde single- or multiple-labeling experiments assessed the cells giving rise to TBNC intersubnuclear collaterals. Horseradish peroxidase (HRP) and/or fluorescent tracer injections into the thalamus, colliculus, cerebellum, nucleus principalis, and/or subnucleus caudalis revealed large numbers of neurons in subnuclei caudalis, interpolaris, and oralis projecting to the region of nucleus principalis. Cells projecting to more caudal spinal trigeminal regions were most numerous in subnuclei interpolaris and oralis. Some cells in lamina V of subnucleus caudalis and in subnuclei interpolaris and oralis projected to thalamus and/or colliculus, as well as other TBNC subnuclei. Such collateral projections were rare in nucleus principalis and more superficial laminae of subnucleus caudalis. TBNC cells labeled by cerebellar injections were not double-labeled by tracer injections into the thalamus, colliculus, or TBNC. These findings lend generality to currently available data obtained with intracellular recording and HRP labeling methods, and suggest that most intersubnuclear axons originate in TBNC local-circuit neurons, though some originate in cells that project to midbrain and/or diencephalon.

Physiological and morphological characteristics of periodontal mesencephalic trigeminal neurons in the cat--intra-axonal staining with HRP

Intra-axonal recording and horseradish peroxidase (HRP) injection techniques were employed to define the response properties of periodontal mechanoreceptive afferents originating from the trigeminal mesencephalic nucleus (Vmes) and their morphological characteristics. The periodontal Vmes neurons were classified into two types: slowly adapting (SA) and fast adapting (FA) types. The central terminals of 7 SA and 4 FA afferents were recovered for detailed analyses. The whole profile of SA and FA neurons were unipolar in shape and their cell bodies were located in the dorsomedial parts of the Vmes. The united (U) fiber traveled caudally from the soma to the dorsolateral aspect of the trigeminal motor nucleus (Vmo), where it split into the peripheral (P) and C fibers with a T- or Y-shaped appearance. The P fiber joined the trigeminal sensory or motor tract. The C fiber descended caudally within Probst's tract. All 3 stem fibers issued main collaterals. The main collaterals of all neurons examined formed terminal arbors in the supratrigeminal nucleus (Vsup) and all but two SA neurons projected to the intertrigeminal region (Vint), while the projections to other nuclei of the trigeminal motor nucleus (Vmo), juxtatrigeminal region (Vjux), main sensory nucleus (Vp) and oral nucleus (Vo.r) differed between SA and FA afferents and between neurons of the same type. The SA and FA neurons were classified into three and two subgroups, respectively. The major differences in central projections between the two types were that all the FA neurons projected to the Vp or Vo.r but none of SA type and this relation was reversed in the projection to the Vjux, and that more than half of SA neurons projected to Vmo but only one FA neuron to the Vmo. The Vmes neurons which sent their collaterals into the Vmo had the P fiber passing through the tract of the trigeminal motor nerve. The average size of somata and mean diameters of U fibers and main collaterals from C fiber were significantly larger in SA neurons than FA neurons. The average size of fiber varicosities became smaller in the following nuclei, Vmo, Vsup, Vp, Vint and Vo.r, but not significant between the two functional types. The functional role of the periodontal Vmes afferents to jaw reflexes was discussed particularly with respect to their central projection sites in the brainstem nuclei.