Representation of the masticatory muscles in the motor trigeminal nucleus of the macaque monkey

Forebrain projections to brainstem nuclei involved in the control of mandibular movements in rats

Mandibular movements occur through the triggering of trigeminal motoneurons. Aberrant movements by orofacial muscles are characteristic of orofacial motor disorders, such as nocturnal bruxism (clenching or grinding of the dentition during sleep). Previous studies have suggested that autonomic changes occur during bruxism episodes. Although it is known that emotional responses increase jaw movement, the brain pathways linking forebrain limbic nuclei and the trigeminal motor nucleus remain unclear. Here we show that neurons in the lateral hypothalamic area, in the central nucleus of the amygdala, and in the parasubthalamic nucleus, project to the trigeminal motor nucleus or to reticular regions around the motor nucleus (Regio h) and in the mesencephalic trigeminal nucleus. We observed orexin co-expression in neurons projecting from the lateral hypothalamic area to the trigeminal motor nucleus. In the central nucleus of the amygdala, neurons projecting to the trigeminal motor nucleus are innervated by corticotrophin-releasing factor immunoreactive fibers. We also observed that the mesencephalic trigeminal nucleus receives dense innervation from orexin and corticotrophin-releasing factor immunoreactive fibers. Therefore, forebrain nuclei related to autonomic control and stress responses might influence the activity of trigeminal motor neurons and consequently play a role in the physiopathology of nocturnal bruxism.

On the role of the pontine brainstem in vocal pattern generation: a telemetric single-unit recording study in the squirrel monkey

In a recent study, we localized a discrete area in the ventrolateral pontine brainstem of squirrel monkeys, which seems to play a role in vocal pattern generation of frequency-modulated vocalizations. The present study compares the neuronal activity of this area with that of three motoneuron pools involved in phonation, namely the trigeminal motor nucleus, facial nucleus, and nucleus ambiguous. The experiments were performed in freely moving squirrel monkeys (Saimiri sciureus) during spontaneous vocal communication, using a telemetric single-unit recording technique. We found vocalization-related activity in all motoneuron pools recorded. Each of them, however, showed a specific profile of activity properties with respect to call types uttered, syllable structure, and pre-onset time. Different activity profiles were also found for neurons showing purely vocalization-correlated activity, vocalization- and mastication-correlated activity, and vocalization- and respiration-correlated activity. By comparing the activity properties of the proposed vocal pattern generator with the three motoneuron pools, we show that the pontine vocalization area is, in fact, able to control each of the three motoneuron pools during frequency-modulated vocalizations. The present study thus supports the existence of a vocal pattern generator for frequency-modulated call types in the ventrolateral pontine brainstem.

Cytoarchitecture and musculotopic organization of the facial motor nucleus in Cebus apella monkey

The architecture and musculotopic organization of the facial motor nucleus in the Cebus apella monkey (a New World primate) were investigated using histological techniques and a multiple labelling strategy, in which horseradish peroxidase-conjugated neuroanatomical tracers (CTB-HRP and WGA-HRP) and fluorescent tracers were injected into individual facial muscles. The facial motor nucleus was formed by multipolar motoneurons and had an ovoid shape, with its rostrocaudal axis measuring on average 1875 micro m. We divided the nucleus into four different subnuclei: medial, intermediate, dorsal and lateral. Retrograde labelling patterns revealed that individual muscles were innervated by longitudinal functional columns of motoneurons. The columns of the orbicularis oculi, zygomaticus, orbicularis oris, auricularis superior, buccinator and platysma muscles were located in the dorsal, intermediate, lateral, medial, lateral and intermediate subnuclei, respectively. However, the motoneuron columns of the levator labii superioris alaeque nasi muscle and frontalis muscle could not be associated with a specific subnucleus. The present results confirm previous studies regarding the musculotopic organization of the facial motor nucleus. However, we observed some particularities in terms of the relative size of each column in C. apella, which might be related to the functional and behavioral importance of each muscle in the particular context of this primate.

The supratrigeminal region of the rat sends GABA/glycine-cocontaining axon terminals to the motor trigeminal nucleus on the contralateral side

The supratrigeminal region (STR), a reticular zone capping the motor trigeminal nucleus (Tm), contains gamma-aminobutyric acid (GABA)ergic and glycinergic neurons which send axons to the contralateral Tm (J. Comp. Neurol. 373 (1996) 498). In the present study we observed that some single synaptic terminals upon Tm motoneurons showed immunoreactivities (IRs) for both glutamic acid decarboxylase (GAD) and glycine transporter 2 (GlyT2). After injecting biotinylated dextran amine (BDA) into the STR, we further observed in the Tm contralateral to the BDA injection that some BDA-labeled axon terminals in close contact with Tm motoneurons showed both GAD- and GlyT2-IRs. Thus, the STR was indicated to send GABAergic/glycinergic axon terminals contralaterally to Tm motoneurons.

Neural pathways underlying vocal control

Vocalization is a complex behaviour pattern, consisting of essentially three components: laryngeal activity, respiratory movements and supralaryngeal (articulatory) activity. The motoneurones controlling this behaviour are located in various nuclei in the pons (trigeminal motor nucleus), medulla (facial nucleus, nucl. ambiguus, hypoglossal nucleus) and ventral horn of the spinal cord (cervical, thoracic and lumbar region). Coordination of the different motoneurone pools is carried out by an extensive network comprising the ventrolateral parabrachial area, lateral pontine reticular formation, anterolateral and caudal medullary reticular formation, and the nucl. retroambiguus. This network has a direct access to the phonatory motoneurone pools and receives proprioceptive input from laryngeal, pulmonary and oral mechanoreceptors via the solitary tract nucleus and principal as well as spinal trigeminal nuclei. The motor-coordinating network needs a facilitatory input from the periaqueductal grey of the midbrain and laterally bordering tegmentum in order to be able to produce vocalizations. Voluntary control of vocalization, in contrast to completely innate vocal reactions, such as pain shrieking, needs the intactness of the forebrain. Voluntary control over the initiation and suppression of vocal utterances is carried out by the mediofrontal cortex (including anterior cingulate gyrus and supplementary as well as pre-supplementary motor area). Voluntary control over the acoustic structure of vocalizations is carried out by the motor cortex via pyramidal/corticobulbar as well as extrapyramidal pathways. The most important extrapyramidal pathway seems to be the connection motor cortex-putamen-substantia nigra-parvocellular reticular formation-phonatory motoneurones. The motor cortex depends upon a number of inputs for fulfilling its task. It needs a cerebellar input via the ventrolateral thalamus for allowing a smooth transition between consecutive vocal elements. It needs a proprioceptive input from the phonatory organs via nucl. ventralis posterior medialis thalami, somatosensory cortex and inferior parietal cortex. It needs an input from the ventral premotor and prefrontal cortex, including Broca's area, for motor planning of longer purposeful utterances. And it needs an input from the supplementary and pre-supplementary motor area which give rise to the motor commands executed by the motor cortex.

Branchiogenic motoneurons innervating facial, masticatory, and esophageal muscles show aberrant distribution in the reeler-phenotype mutant rat, Shaking Rat Kawasaki

Shaking Rat Kawasaki (SRK) is an autosomal recessive mutant rat that is characterized by cerebellar ataxia. Although previous studies indicated many points of similarity between this mutant rat and the reeler mouse, nonlaminated structures such as the facial nucleus have not been studied in this mutant rat. Nissl-stained sections through the brainstem showed that the cytoarchitecture of the facial, motor trigeminal, and ambiguus nuclei was abnormal in SRK, especially in the lateral cell group of the facial nucleus and the compact formation of the ambiguus nucleus. To examine whether orofacial motoneurons are also malpositioned in the SRK rat, horseradish peroxidase (HRP) was injected into the facial, masticatory, and abdominal esophageal muscles of the SRK rats and normal controls to label facial, trigeminal, and ambiguus motoneurons, respectively. HRP-labeled facial, trigeminal, and ambiguus motoneurons of the SRK rat were distributed more widely than those of their normal counterparts, as in the case of the reeler mouse, with the one exception that labeled facial motoneurons innervating the nasolabial muscle were distributed more widely in the ventrolateral-to-dorsomedial direction in comparison with those of the reeler mutant. These data demonstrate that nonlaminated structures in the brainstem of the SRK rat are affected severely, as is the case in the reeler mutant mouse.

Diverse changes in fibre type composition of the human lateral pterygoid and digastric muscles during aging

The fibre type composition of the superior and inferior portions of the human lateral pterygoid and the anterior and posterior bellies of the digastric muscles of five elderly subjects (mean age 73 years) was studied by morphological and enzyme-histochemical methods. Both muscles showed significant age-related changes in fibre type composition as compared with previous data for young adults. In the lateral pterygoid we observed a large proportion of type IIA fibres, which are rare or absent in young adults, and muscle fibre atrophy and an increased variability in fibre diameter. The digastric muscle of elderly showed a decrease in the proportion of type IIB fibres. The only difference in age-related changes between muscle portions was found in the lateral pterygoid with fibre atrophy in its inferior portion. Both the lateral pterygoid and digastric muscles are known to be active in mandibular depression (jaw opening) and horizontal positioning of the mandible. The present results and previous data from young adults show that the lateral pterygoid and digastric muscles differ not only in fibre type composition, but also in muscular changes following aging. This suggests that, even if they are simultaneously active, they fulfill different, specific tasks in natural jaw function. The differences in age-related changes in fibre type composition between these two muscles indicate that mechanisms underlying their alterations during aging are muscle specific. The results indicate that, although nerve supply and developmental history are essential for fibre composition of skeletal muscles, functional tasks and demands are of major importance.

The trigeminal motonucleus in man

The constitutional elements of the mototrigeminal nucleus in man are described. Apart from the well-known alpha motoneurons, interneurons and gamma motoneurons can be discerned. Cortical projections to the mototrigeminal nucleus in man arise both ipsilateral and contralateral. The contralateral projection is dominant. Terminal cortical input is present on the alpha-motoneurons in man.

Organization of choline acetyltransferase-containing structures in the cranial nerve motor nuclei and spinal cord of the monkey

Cholinergic structures in the cranial nerve motor nuclei and ventral and lateral horns of the spinal cord of the monkey, Macaca fuscata, were investigated immunohistochemically with a monoclonal antibody against monkey choline acetyltransferase (ChAT). ChAT-immunoreactive perikarya and dendrites were present in the oculomotor, trochlear, abducent, trigeminal motor, facial and hypoglossal nuclei, nucleus of Edinger-Westphal, nucleus ambiguus, dorsal nucleus of the vagus, lamina IX of the cervical, thoracic and lumbar spinal cords, and intermediolateral nucleus of the thoracic spinal cord. The neuropil of the trigeminal motor, facial and hypoglossal nuclei, nucleus ambiguus and lamina IX of the cervical, thoracic and lumbar spinal cords contained many ChAT-positive bouton-like structures and they were seemingly in contact with perikarya and dendrites of motoneurons, suggesting that motoneurons in these nuclei are cholinoceptive as well as cholinergic. The oculomotor, trochlear and abducent nuclei, nucleus of Edinger-Westphal, dorsal nucleus of the vagus and intermediolateral nucleus of the thoracic spinal cord contained a small number of ChAT-immunoreactive bouton-like structures, but they did not contact with perikarya and dendrites of ChAT-positive neurons. These observations suggest that the organization of the motor nuclei is complex, at least regarding the cholinoceptivity.

GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat

The distribution of GABAergic and glycinergic premotor neurons projecting to the trigeminal motor nucleus (Vm) was examined in the lower brainstem of the rat by a double labeling method combining retrograde axonal tracing with immunofluorescence histochemistry. After injection of the fluorescent retrograde tracer, tetramethylrhodamine dextran amine (TRDA), into the Vm unilaterally, neurons labeled with TRDA were seen ipsilaterally in the mesencephalic trigeminal nucleus, and bilaterally in the parabrachial region, the supratrigeminal and intertrigeminal regions, the reticular formation just medial to the Vm, the principal sensory and spinal trigeminal nuclei, the pontine and medullary reticular formation, especially the parvicellular part of the medullary reticular formation, the alpha part of the gigantocellular reticular nucleus, and the medullary raphe nuclei. Some of these neurons labeled with TRDA were found to display glutamic acid decarboxylase (the enzyme involved in GABA synthesis)-like or glycine-like immunoreactivity. Such double-labeled neurons were seen mainly in the supratrigeminal region, the reticular region adjacent to the medial border of the Vm, and the dorsal part of the lateral reticular formation of the medulla oblongata; a number of them were further scattered in the intertrigeminal region, the alpha part of the gigantocellular reticular nucleus, the nucleus raphe magnus, the principal sensory trigeminal nucleus, and the interpolar subnucleus of the spinal trigeminal nucleus. These neurons were considered to be inhibitory (GABAergic or glycinergic) neurons sending their axons to motoneurons in the Vm, or to local interneurons within and around the Vm.

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.

Musculotopic organization in the motor trigeminal nucleus of the reeler mutant mouse

We examined the musculotopic organization in the motor trigeminal nucleus and the somatotopical arrangement in the trigeminal ganglion of the normal and reeler mice. To determine whether or not masticatory motoneurons are malpositioned in the reeler mouse, we injected horseradish peroxidase (HRP) into the masticatory muscles of normal and reeler mice. Injections of HRP into the jaw-closing muscles, i.e., the masseter and temporalis muscles, labeled large multipolar neurons in the dorsolateral division of the motor trigeminal nucleus of both normal and reeler mice. Similar injections into the jaw-opening muscles, i.e., the anterior belly of the digastric muscle and mylohyoid muscle, labeled large multipolar neurons in the ventromedial division of the motor trigeminal nucleus of both mouse strains. Thus, the normal myotopical arrangement of the masticatory muscles on the motor trigeminal nucleus is preserved in the reeler mouse. However, detailed analysis revealed that jaw-opening motoneurons were more widely scattered in the reeler mouse than in the normal control. To examine the somatotopical arrangement of the first-order sensory neurons in the trigeminal ganglion of the normal and reeler mice, we subcutaneously injected Fast blue (FB) into the mental region and Diamidino yellow (DY) into the vibrissal region of the same animals. No differences in distribution patterns of FB-labeled and DY-labeled neurons in the whole-mounted trigeminal ganglion could been seen between these two strains, suggesting that migration of trigeminal ganglion cells, which are derived from the neural crest and placode, is not deranged by the reeler genetic locus.

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

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

An HRP study of the location of the motoneurons supplying the tensor veli palatini muscle of the rabbit

Location of the motoneurons supplying the tensor veli palatini muscle of the rabbit was examined with the retrograde labeling technique following intramuscular injection of HRP. Labeled motoneurons were ipsilaterally located in the ventral or ventromedial portion of the rostral two-thirds of the motor trigeminal nucleus at the level of about 6.0 to 8.5 mm rostral to the obex. The location of the labeled motoneurons was ventromedial to the region supplying the masseter, the temporalis, and the medial pterygoid muscles and ventral to the region supplying the anterior digastric and the mylohyoid muscles, the location which coincided with the lateral pterygoid region. The labeled motoneurons were scattered around or in this region.

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

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

Location of the motoneurons innervating the transverse mandibular muscle in the guinea pig

Motoneurons supplying the transverse mandibular muscle (TMM) in the guinea pig have been traced by injecting wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the TMM, and after applying HRP to the mylohyoid nerve. The TMM is bilaterally innervated by 22-36 motoneurons in each trigeminal motor nucleus, forming a column located ventrolaterally along the entire length of the superficial masseter motoneuron group. The axons are incorporated to the mylohyoid nerve. The location, the axon pathways in the brainstem and the pattern of the dentritic tree suggest that in the guinea pig the TMM motoneurons are involved in the masticatory movements in coordination with other jaw-closing muscles.

Representation of the tensor veli palatini muscle in the trigeminal motor nucleus of the Japanese monkey (Macaca fuscata)

Distribution of motoneurons supplying the tensor veli palatini (TVP) muscle was examined in the Japanese monkey (Macaca fuscata) by the retrograde horseradish peroxidase (HRP) method. Neurons labeled with HRP which was injected into the tensor veli palatini muscle were seen in the ventromedial aspects of the dorsolateral division of the trigeminal motor nucleus, at all rostrocaudal levels of the trigeminal motor nucleus. The vast majority of these TVP motoneurons were distributed around the margin, especially the dorsal margin, of the cluster of motoneurons which innervate the lateral pterygoid muscle.

Non-reciprocal facilitation of trigeminal motoneurons innervating jaw-closing and jaw-opening muscles induced by iontophoretic application of serotonin in the guinea pig

Effects of iontophoretically applied serotonin (5-HT) and its antagonist, methysergide (MS), on masseter (a jaw-closer, MA.MNs) and anterior digastric motoneurons (a jaw-opener, AD.MNs) were studied in paralyzed guinea pigs, chloralose-anesthetized or decerebrate. Unitary activity was recorded with multibarrel capillary electrodes from MA.MNs and AD.MNs identified by antidromic spikes evoked by stimulation of the masseter and anterior digastric nerves, respectively. Under chloralose anesthesia, both MA.MNs and AD.MNs were almost quiescent, and application of 5-HT alone induced no changes in discharge of either of them. However, iontophoretically applied 5-HT increased the frequency of discharge induced by iontophoretic application of glutamate in 26 of 34 MA.MNs (76%) and 17 of 30 AD.MNs (59%) tested. MS depressed the glutamate-induced activity in 17 MA.MNs and 3 AD.MNs, respectively, in which 5-HT exerted a facilitatory effect on the glutamate-induced activity. In decerebrate preparations, the firing index of spikes of MA.MNs monosynaptically evoked by stimulation of the trigeminal mesencephalic nucleus was increased by 5-HT and decreased by MS. 5-HT also enhanced the discharge of MA.MNs induced by a ramp-and-hold stretch of the masseter muscle. We conclude that 5-HT alone does not excite either MA.MNs or AD.MNs, but potentiates the effect of excitatory inputs to them: 5-HT exerts a modulatory facilitatory action on trigeminal motoneurons.

A specialized innervation of the tensor tympani muscle in Macaca fascicularis

The innervation of the tensor tympani muscle of the middle ear in Macaca fascicularis (cynomolgus monkey) was studied using the horseradish peroxidase (HRP) neural tracing technique. A compact column of small trigeminal motoneurons was labeled ipsilaterally following intramuscular application of HRP to the tensor tympani muscle. This column is located ventral and lateral to the dorsolateral division of the trigeminal motor nucleus, and just medial to the descending trigeminal nerve rootlets. No labeled neurons were present in the trigeminal mesencephalic nucleus or any other brainstem nucleus. Results are compared with those previously reported in several non-primate mammalian species, and in detail with that of the cat. A possible differential role of the tensor tympani muscle in acoustic modulation/middle ear aeration between primate and non-primate mammals is discussed.

Enzyme-histochemical differences in fibre-type between the human major and minor zygomatic and the first dorsal interosseus muscles

Human masticatory muscles, innervated by the trigeminal nerve, differ in fibre-type composition from limb and trunk muscles, but the anterior and the posterior belly of the human digastric muscle, innervated by the trigeminal and facial nerves, respectively, do not. The major and minor zygomatic muscles from adult males, which originate from the second branchial arch and are supplied by the facial nerve, were analysed enzyme-histochemically and compared with the first dorsal interosseus hand muscle, which has spinal innervation and, like the masticatory and facial muscles, a large cortical representation. Both zygomatic muscles had a marked predominance of type II fibres, the minor one having the largest proportion (89.1 per cent) ever reported in human skeletal muscle. Besides type I, IIA, IIB, and a few type IIC fibres, there was a large group with an ATPase reaction at pH 4.6, between that of type IIA and type IIB, and termed IIAB. This fibre-type profile may reflect a special isomyosin composition. Type I and II fibres were of about equal diameter, corresponding to that of type I fibres in the masticatory muscles. Individual and intra-muscular variability in fibre size and shape was considerable. The unusually high frequency of type II fibres in the zygomatic muscles suggest that they have fast-contraction properties and relatively large motor units, and therefore are poorly adapted to finely-graded movements. The absence of muscle spindles supports this view. The hand muscle had a chequer-board pattern of type I, IIA and IIB fibres, similar to that of large limb and trunk muscles, with no difference between its two heads.(ABSTRACT TRUNCATED AT 250 WORDS)

The motor complex and primary projections of the trigeminal nerve in the monitor lizard, Varanus exanthematicus

The sensory projections and the motor complex of the trigeminal nerve of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The primary afferent fibers diverge in the brainstem into a short ascending and a long descending tract. The former distributes its fibers to the principal sensory trigeminal nucleus, where nerves V1, V2, and V3 are represented along a lateromedial axis. The fibers of the descending tract enter the nucleus of this tract and the reticular formation. Both in the tract and its nucleus, nerves V1, V2 and V3 occupy successively more dorsal positions. A small contingent of nerve V1 fibers course to the accessory abducens nucleus. The descending tract extends caudally into the first and second cervical segments of the spinal cord. The trigeminal motor complex consists of dorsal, ventral, and dorsomedial nuclei. The m. adductor mandibulae externus (the main jaw closer) is represented in the dorsal nucleus, predominantly in its rostral part. The muscles innervated by nerve V3 are represented in the ventral nucleus, mainly in its caudal part. All three divisions of the trigeminal nerve contain peripheral branches of the mesencephalic trigeminal system. Collaterals of the central branches of this system were traced to the ventral motor and the principal sensory trigeminal nuclei.

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

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

Identification of motoneurons innervating the tensor tympani and tensor veli palatini muscles in the cat

The somatotopic arrangement of the motoneurons associated with the two non-masticatory muscles innervated by the trigeminal motor nerve, tensor tympani (TT) and tensor veli palatini (TVP), was determined in the cat using retrograde transport of horseradish peroxidase. The motoneurons of the TT are distinct and separate, ventral and ventral-lateral to the rostral two-thirds of the trigeminal motor nucleus. The cells are smaller than those of the motor nucleus and constitute a parvocellular division. Based on functional and morphological criteria, TT motoneurons may be considered as an accessory trigeminal nucleus. The somatotopic arrangement of TVP motoneurons has been described for the first time. These motoneurons are located in the rostral two-thirds of the ventromedial division of the cat trigeminal motor nucleus. The location of motoneurons associated with TT and TVP does not fit the parcellation of the cat trigeminal motor nucleus as described by previous investigators. The motoneurons of these muscles can now be assigned to areas either within (TVP) or adjacent to (TT) the rostral two-thirds of the motor nucleus.

Axonal trajectories of masticatory motoneurons: a genu formation of axons of jaw-opening motoneurons in the cat

Axonal courses of jaw-opening motoneurons were examined in the cat by applying horseradish peroxidase to the central cut end of the nerve branch which supplies the mylohyoid and/or the anterior digastric muscle. The axons of mylohyoid and anterior digastric motoneurons, which are located in the ventromedial division of the trigeminal motor nucleus, initially swing dorsally before running ventrolaterally to leave the pons; axons proceeding in the more rostral levels make turns in the more dorsomedial tegmental regions, and those running in the most rostral levels form a small genu in the region near the midline under the floor of the fourth ventricle.

Identification of motoneurons supplying the tensor veli palatini muscle in the guinea pig and cat: an horseradish peroxidase study

Identification of motoneurons supplying the tensor veli palatini muscle was attempted by the horseradish peroxidase (HRP) method in the guinea pig and cat. After HRP injection into the tensor veli palatini muscle, HRP-labeled neurons were seen in the regions closely medial to the cluster of the lateral pterygoid motoneurons in the dorsolateral division of the trigeminal motor nucleus. In the guinea pig the tensor veli palatini motoneurons were observed at the level of the whole rostrocaudal extent of the trigeminal motor nucleus, while in the cat they were seen at the level of the rostral two-thirds of the nucleus.

Localization of motoneurons innervating the tensor tympani muscles: an horseradish peroxidase study in the guinea pig and cat

Motoneurons innervating the tensor tympani muscle were identified in the adult guinea pig and cat by the horseradish peroxidase (HRP) method. After HRP injection into the tensor tympani muscle, HRP-labeled neurons were seen in the regions outside the cytoarchitectonically-defined confines of the trigeminal motor nucleus; in the regions rostral to the rostral pole of the nucleus, as well as in the regions ventral and ventrolateral to the nucleus at the levels of the rostral half (guinea pig) or the rostral two-thirds (cat) of the nucleus. The tensor tympani motoneurons were generally smaller than the masticatory motoneurons.

Localization of masticatory motoneurons in the trigeminal motor nucleus of the guinea pig

The myotopical arrangement of masticatory motoneurons was examined in the guinea pig by the horseradish peroxidase method. The trigeminal motor nucleus was composed of dorsolateral and ventromedial divisions; the former was seen in the whole rostrocaudal extent of the nucleus, while the latter was present at levels of the caudal two thirds of the nucleus. The jaw-closer motoneurons were located in the dorsolateral division, and the jaw-opener motoneurons (mylohyoid and anterior digastric motoneurons) were seen in the ventromedial division.