Central projections of the mesencephalic nucleus of the fifth nerve: an autoradiographic study

Projections of cells of the mesencephalic nucleus of the fifth nerve (Mes V) to brainstem structures in the cat were studied by labelling Mes V cells with tritiated leucine. Before the leucine injections were made, however, large kainic acid lesions were produced in the vicinity of Mes V cells because these neurons are resistant to being killed or injured by this neurotoxin. Thus Mes V cells were selectively labelled by the leucine even though they are scattered among many other neurons. Leucine injections near Mes V cells located in the mesencephalon, which are primarily the somata of jaw muscle spindle afferent fibers, produced essentially the same pattern of terminal labelling as injections near a caudally located group of Mes V cells that includes the somata of many tooth mechanoreceptive afferents. Labelling was dense above the trigeminal motor nucleus in the nucleus supratrigeminalis and in the most medial portion of the principal trigeminal sensory nucleus. A scattering of labelled axons and diffuse label was seen along the length of the tract of Probst, which follows the medial border of the descending trigeminal sensory nucleus as far caudally as the dorsal motor nucleus of the vagus. Labelling within most of the trigeminal motor nucleus, which is known to receive direct synaptic input from Mes V cells, was very light. The only reasonably dense region of label was confined to a small dorsolateral portion of the motor nucleus. Although Mes V has generally been supposed to be involved with jaw control in a direct, reflexive manner, the extensive projections to nucleus supratrigeminalis and parts of the trigeminal sensory system draw attention to the potential proprioceptive sensory contribution of Mes V.     

Morphology of jaw-muscle spindle afferents in the rat

The morphology of jaw-muscle spindle afferents in the rat has been studied by intra-axonal injection of horseradish peroxidase. All stained axons were located in the motor root of the trigeminal nerve and could be traced dorsomedially to the vicinity of the trigeminal motor nucleus, where they divided into an ascending branch in the tract of the mesencephalic nucleus and a descending branch in the tract of Probst. Axon collaterals and swellings on fine collateral branches presumed to be synaptic boutons were located in the following regions: the trigeminal motor nucleus, the region dorsal to the trigeminal motor nucleus including the supratrigeminal nucleus, the parvicellular reticular formation immediately caudal to the trigeminal motor nucleus, the reticular formation at the level of the facial nucleus, and the caudal portion of the mesencephalic nucleus. No evidence of a projection to the cerebellum was observed. Boutons were most numerous in the region surrounding the trigeminal motor nucleus, especially dorsally. Here they were not demonstrated in close proximity to counterstained cells, and therefore it was not possible to determine how many of these contacts are located on cells in this region and how many are on the distal dendrites of trigeminal motorneurons. Boutons located within the trigeminal motor nucleus were always confined to a small portion of the nucleus and were significantly larger than those located dorsally. Some boutons were found in close apposition to trigeminal motorneurons and presumably make somatic contacts. These results suggest that jaw-muscle spindle afferents make somatic and proximal dendritic contacts with only a limited number of trigeminal motorneurons and also project to masticatory interneuronal regions dorsal and caudal to the motor nucleus.     

Morphology of single mesencephalic trigeminal neurons innervating masseter muscle of the cat

The morphology of functionally identified single axons of mesencephalic trigeminal neurons was studied in the cat by the method of intra-axonal injection of horseradish peroxidase (HRP). Each axon can be divided into united (U), peripheral (P) and central branches (C). The united axon (U) descends from its soma within the tract of the trigeminal mesencephalic nucleus to the dorsal aspect of the trigeminal motor nucleus (Vmo), where it splits into peripheral and descending central branches with a Y-shaped bifurcation. The peripheral axon (P) joins the motor root of the trigeminal nerve to exit the brainstem. The central axon (C) travels caudally within the juxtatrigeminal regions (or lateral reticular formation). All 3 branches issue axon collaterals that distribute terminal boutons within the dorsolateral subdivision of Vmo, supra- and intertrigeminal regions. Collaterals emanating from the central axon (C) except for its proximal segment travel ventrolaterally within the juxtatrigeminal regions, and send their terminal branches into the lateral boundaries adjacent to the spinal trigeminal nucleus. The trajectory of terminal branches distinguishes group Ia afferents from the possible group II afferents. The majority of terminal boutons are found to distribute in the supra- and intertrigeminal regions for group II afferent fibers and in the dorsolateral subdivision of Vmo for group Ia afferents.     

The sensory trigeminal complex and the organization of its primary afferents in the zebra finch (Taeniopygia guttata)

Our knowledge of the avian sensory trigeminal system has been largely restricted to the principal trigeminal nucleus (PrV) and its ascending projections to the forebrain. Studies addressing the cytoarchitecture and organization of afferent input to the sensory trigeminal complex, which includes both the PrV and the nuclei of the descending trigeminal tract (nTTD), have only been performed in pigeons and ducks. Here we extend such an analysis to a songbird, the zebra finch (Taeniopygia guttata). We describe the cytoarchitecture of the sensory trigeminal complex, the patterns of calbindin‐like and substance P‐like immunoreactivity, and the organization of afferents from the three branches of the trigeminal nerve and from the lingual branch of the hypoglossal nerve. On the basis of cytoarchitecture and immunohistochemistry, the sensory trigeminal column can be subdivided from caudal to rostral, as in other species, into cervical dorsal horn, subnucleus caudalis, subnucleus interpolaris, subnucleus oralis, and nucleus principalis. The relative positions of the terminal fields of the three trigeminal branches move from medial to lateral in the dorsal horn to dorsomedial to ventrolateral in nTTD, whereas in PrV there is considerable overlap of mandibular and ophthalmic terminal fields, with only a small maxillary input ventrally. The hypoglossal afferents, which terminate medially in the dorsal horn and dorsolaterally in nTTD, terminate in specific cell groups in the dorsolateral nTTDo and in PrV. This work sets the grounds for further analyses of the ascending connections of the nTTD and the afferents from the syrinx to the trigeminal sensory column.     

More a finger than a nose: The trigeminal motor and sensory innervation of the Schnauzenorgan in the elephant-nose Fish Gnathonemus petersii

The weakly electric fish Gnathonemus petersii uses its electric sense to actively probe the environment. Its highly mobile chin appendage, the Schnauzenorgan, is rich in electroreceptors. Physical measurements have demonstrated the importance of the position of the Schnauzenorgan in funneling the fish's self-generated electric field. The present study focuses on the trigeminal motor pathway that controls Schnauzenorgan movement and on its trigeminal sensory innervation and central representation. The nerves entering the Schnauzenorgan are very large and contain both motor and sensory trigeminal components as well as an electrosensory pathway. With the use of neurotracer techniques, labeled Schnauzenorgan motoneurons were found throughout the ventral main body of the trigeminal motor nucleus but not among the population of larger motoneurons in its rostrodorsal region. The Schnauzenorgan receives no motor or sensory innervation from the facial nerve. There are many anastomoses between the peripheral electrosensory and trigeminal nerves, but these senses remain separate in the sensory ganglia and in their first central relays. Schnauzenorgan trigeminal primary afferent projections extend throughout the descending trigeminal sensory nuclei, and a few fibers enter the facial lobe. Although no labeled neurons could be identified in the brain as the trigeminal mesencephalic root, some Schnauzenorgan trigeminal afferents terminated in the trigeminal motor nucleus, suggesting a monosynaptic, possibly proprioceptive, pathway. In this first step toward understanding multimodal central representation of the Schnauzenorgan, no direct interconnections were found between the trigeminal sensory and electromotor command system, or the electrosensory and trigeminal motor command. The pathways linking perception to action remain to be studied. J. Comp. Neurol. 523:769–789, 2015. © 2014 Wiley Periodicals, Inc.     

Developmental relationships between trigeminal ganglia and trigeminal motoneurons in chick embryos. I. Ganglion development is necessary for motoneuron migration

The migration and early development of trigeminal (V) motoneurons were studied in chick embryos in which two different populations of primary trigeminal sensory neurons had been removed prior to the birthdate of the V motoneurons. Ablation of mesencephalic neural crest cells, which eliminates monosynaptic sensory input, did not affect the migration, early development, or later differentiation of the V motoneurons. However, when the anlagen of the V ganglion were removed, the V motor root did not exit from the brainstem and the V motor nucleus did not develop. Although the neurons of the V ganglion do not innervate adult V motoneurons, these populations are related developmentally. In those embryos in which the V ganglion did not develop, medial column cells, which are midline, postmitotic, premigratory V motoneurons, and a few medial, elongated cells (possibly migratory) were present until days 5-6, but these cells did not complete their lateral migration to form the lateral nucleus of V. In cases where the ganglion anlagen were not completely removed, the number of postmigratory V motoneurons was positively correlated to the size of the ganglion remnant. There also was a correlation between the axial position of the postmigratory V motoneurons and the ganglion remnants. If a caudal remnant developed, only caudal V motoneurons, whose axons reached the ganglion, migrated; if a rostral remnant developed, only rostral V motoneurons, with axons reaching this remnant, migrated. Additionally, if the central axons of the ganglion remnant entered the metencephalon in either dorsal or ventral ectopic positions, the V motor nucleus was located in a corresponding aberrant position. Thus, some characteristic of the V ganglion cells appears to guide the motor axons and somas to their final brainstem position.     

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.     

Reevaluation of projections from the mesencephalic trigeminal nucleus to the medulla and spinal cord: New projections. A combined retrograde and anterograde horseradish peroxidase study

Microinjection of horseradish peroxidase (HRP) into the medullary parvocellular reticular formation (NPvc) resulted in retrograde labeling of neurons throughout the mesencephalic trigeminal nucleus (Mes V). Labeled cells were large and ovoid and were distributed primarily in the expanded pontine part of the nucleus. However, none of the small neurons in Mes V were labeled. Injections of HRP made into adjacent brainstem structures including the nucleus gigantocellularis, ventrolateral reticular formation, vestibular complex, and the spinal trigeminal nucleus failed to label neurons in Mes V. Injections made into the medullary raphe and into regions reported to receive inputs from Mes V–spinal cord, nucleus tractus solitarius, hypoglossal nucleus, and facial nucleus–were also not followed by transport to Mes V. Anterograde axonal transport of HRP from the region of reticular formation innervated by Mes V also labeled axons projecting to Mes V and to visceral and somatic sensorimotor nuclei in the lower brainstem. Recent reports of afferents from the amygdala to Mes V suggest that reflexes involving the mesencephlic trigeminal nucleus might be modulated by signals from limbic and autonomic as well as somatic centers in the brain.     

Anatomical regeneration and behavioral recovery following crush injury of the trigeminal root in lamprey

In normal larval lamprey, bilateral application of horseradish peroxidase (HRP) to the dorsal part of the anterior oral hood labeled subpopulations of trigeminal components on both sides of the brain; peripherally projecting motoneurons, medullary dorsal cells (sensory), and spinal dorsal cells (sensory), as well as centrally projecting afferents in the trigeminal descending tracts. Following unilateral crush injury of the right trigeminal root, HRP labeling of sensory and motor trigeminal components on the right side gradually increased with increasing recovery time, between 2 weeks and 12 weeks postcrush (PC). Axons of trigeminal motoneurons appeared to exhibit robust regeneration, whereas restoration of projections in the descending trigeminal tract ipsilateral to the injury was incomplete. Control experiments indicated that motor and sensory axons from the intact side of the oral hood did not sprout across the midline to the denervated side. Several results suggested that regenerated trigeminal sensory fibers made synapses with brain neurons that have direct or indirect inputs to reticulospinal (RS) neurons. Following a unilateral crush injury of the right trigeminal root, escape behavior in response to stimulation of the right side of the oral hood gradually returned to normal. Muscle recordings at various recovery times confirmed that anatomical regeneration of trigeminal sensory axons was functional. In addition, at 8 or 12 weeks PC, brief stimulation of the oral hood ipsilateral or contralateral to the crush injury elicited synaptic responses in RS neurons on either side of the brain, similar to that in normal animals. In the lamprey, compensatory mechanisms probably allow recovery of behavioral function despite incomplete regeneration of trigeminal sensory axons within the central nervous system. J. Comp. Neurol. 396:322–337, 1998. © 1998 Wiley-Liss, Inc.     

Embryonic development of the chick primary trigeminal sensory-motor complex

The objective of this study is to define the development of all components in the chick embryonic trigeminal primary sensory-motor complex, from their first appearance through the formation of central and peripheral axonal projections up to stage 34 (8 days of incubation). This was accomplished by two labeling procedures: application of the monoclonal antibody HNK-1, which binds to the precursors of all these components except the placode-derived neurons, and application of HRP to axons cut immediately distal to the trigeminal ganglion. Single immunopositive motor neuron precursors are present at stage 12. These accumulate in the transient medial motor column, whose neurons initiate axon outgrowth by stage 13–14, concomitant with the onset of translocation of their somata to form the definitive trigeminal lateral motor column (LMC). Intiially these translocating somata accumulate on the medial margin of the LMC. Beginning on incubation day 5, axons growing from newly formed motor neurons pass beside the lateral margin of the LMC, and the nuclei of these cells subsequently follow this pathway. These events follow a rostral-to-caudal sequence, and this phase of motor nucleus formation is complete by day 8. The lateral translocation of some caudally located nuclei is arrested beginning on day 5. This cessation, which proceeds rostrally, demarcates neurons that form the dorsal motor nucleus of the trigeminal complex. Sensory neurite formation is intiated in ophthalmic placode-derived cells at stage 14.5, one stage later by maxillomandibular neurons, and from mesencephalic V cells at stage 15. Neural crest cells do not initiate axon formation until at least day 4 to 5. Following application of HRP distal to the condensing ganglion at stage 16, labeled ophthalmic nerve projections appear in contact with the wall of the hindbrain centrally and overlying the optic vesicle peripherally. Fibers forming the descending tract elongate rapidly, reaching the level of the VIIth nerve root (200 m?m caudal to the trigeminal root) by stage 18 and the cervical cord by stage 22. Labeled terminal arborizations of descending trigeminal afferents are first visible at stage 22 and are evident along the entire descending and proximal ascending tracts by stage 27. Later-developing descending axons grow in close association with existing trigeminal fibers, though a few growth cones are consistently evident superficial to the other fibers. No projections different from those reported in adult birds are seen, nor are there any contralateral afferent projections. Peripheral axons from neurons in the mesencephalic trigeminal nucleus emerge from the trigeminal ganglion beginning at stage 21. These cells are labeled only when tracer is applied to the mandibular nerve.     

Selective reinnervation of distal motor stumps by peripheral motor axons

Random matching of regenerating axons with Schwann tubes in the distal nerve stump is thought to contribute to the often poor results of peripheral nerve repair. Motor axons would be led to sensory end organs and sensory axons to motor end plates; both would remain functionless. However, the ability of regenerating axons to differentiate between sensory and motor environments has not been adequately examined. The experiments reported here evaluated the behavior of regenerating motor axons when given equal access to distal sensory and motor nerve stumps across an unstructured gap. "Y"-shape silicon chambers were implanted within the rat femoral nerve with the proximal motor branch as axon source in the base of the Y. The distal sensory and motor branches served as targets in the branches of the Y, and were placed 2 or 5 mm from the axon source. After 2 months for axon regeneration, horseradish peroxidase was used to label the motoneurons projecting axons into either the motor or the sensory stump. Equal numbers of motoneurons were labeled from the sensory and motor stumps at 2 mm, but significantly more motoneurons were labeled from the motor stump at 5 mm. (P = 0.016). This finding is consistent with selective reinnervation of the motor stump. Augmentation of this phenomenon to produce specific reunion of individual motor axons could dramatically improve the results of nerve suture.     

Afferent projections to the oral motor nuclei in the rat

Projections to the trigeminal, facial, ambiguus, and hypoglossal motor nuclei were determined by using horseradish peroxidase histochemistry. Most of the afferent projections to these motor nuclei were from the brainstem reticular formation, frequently in areas adjacent to other synergetic motor nuclei. The reticular formation lateral to the hypoglossal nucleus and reticular structures surrounding the trigeminal motor nucleus projected to each of these other brainstem motor nuclei involved in oral-facial function. Afferent projections to these motor nuclei also were organized along the rostrocaudal axis. Within the reticular formation most of the afferent projections to the trigeminal motor nucleus originated rostral to the majority of neurons projecting to the hypoglossal and ambiguus nuclei, which in turn were rostral to the primary source of reticular afferents to the facial nucleus. In comparison, projections from the sensory trigeminal nuclei and nucleus of the solitary tract were sparse. The interneuron pools that project to the orofacial motoneurons provide one further link in understanding the brainstem substrates for integrating oral and ingestive behaviors.     

Evidence for excitatory amino acid transmission between mesencephalic nucleus of V afferents and jaw-closer motoneurons in the guinea pig

Previous studies have suggested that monosynaptic transmission between spinal primary afferent fibers and motoneurons is mediated by an excitatory amino acid, most likely glutamate or aspartate. No such comparable studies have been carried out in the trigeminal system. In an attempt to elucidate the neurotransmitter(s) mediating monosynaptic transmission between mesencephalic of V nucleus afferents (Mes V) and trigeminal jaw-closer motoneurons, the effect of iontophoretic application of excitatory amino acid antagonists on the Mes V-induced field potential, recorded in the trigeminal motor nucleus (Mot V), was examined. Application of DL-2-amino-4-phosphonobutyrate (APB) and the broad spectrum amino acid antagonists, kynurenic acid (KYN) and gamma-D-glutamylglycine (DGG), for 3-4 min reversibly reduced the amplitude of the Mes V induced field potential. The effect of APB was much greater than any of the other compounds tested. On the other hand, the specific N-methyl-D-aspartate (NMDA) receptor blocker DL-2-amino-5-phosphonovaleric acid (APV), was without effect on the field potential. Based on current-response curves for each antagonist tested, the order of potency was determined to be APB greater than KYN greater than DGG greater than APV. These antagonists were also compared with respect to their efficacy in blocking individual jaw-closer motoneuron activity induced by iontophoretic application of amino acid receptor excitants glutamate (Glut), aspartate (Asp), kainate (K), and quisqualate (Q). NMDA application was without effects on these motoneurons. The profile of activity of these antagonists on these amino acid excitants was similar to that found in other areas of the CNS by other investigators. KYN and DGG both significantly reduced responses induced by all excitants tested, whereas APB had more modest effects on K and Q excitation and was without effect on Glut and Asp excitations in most cells tested. The data suggest that an excitatory amino acid, activating non-NMDA receptors, mediates some component of synaptic transmission between Mes V afferents and jaw-closer motoneurons. The data is also consistent with the proposal made in other systems that APB blocks synaptic transmission by a mechanism other than postsynaptic receptor blockade.     

Preferential motor reinnervation: a sequential double-labeling study

Previous experiments have shown that motor axons regenerating in mixed nerve will preferentially reinnervate a distal motor branch. The present experiments examine the mechanism through which this sensory-motor specificity is generated. An enclosed 0.5 mm gap was created in the proximal femoral nerves of juvenile rats. Two, three or eight weeks later the specificity of motor axon regeneration was evaluated by simultaneous application of horseradish peroxidase (HRP) to one distal femoral branch (sensory or motor) and Fluoro-Gold to the other. Motoneurons were then counted as projecting (i) correctly to the motor branch, (ii) incorrectly to the sensory branch, and (iii) simultaneously to both branches (double-labeled). Motor axon regeneration was random at 2 weeks, with equal numbers of motoneurons projecting to sensory and motor branches. However, the number of correct projections increased dramatically between 2 and 3 weeks. Twenty-six percent of neurons labeled at 2 weeks contained both tracers, indicating axon collateral projections to both sensory and motor branches. This number decreased significantly at each time period. Axon collaterals were thus 'pruned' from the sensory branch, increasing the number of correct projections at the expense of double-labeled neurons. These findings suggest random reinnervation of the distal stump, with specificity generated through trophic interaction between axons and the pathway and/or end organ.     

The motor column and sensory projections of the branchial cranial nerves in the frog.

Motor nuclei and primary afferent projections of branchial cranial nerves were shown with the CoCl₂ staining technique. Motor nuclei constitute a discontinuous column at the ventrolateral margin of the gray matter. The size of motoneurons decreases in the orocaudal direction within the column. The dendrites assemble in two main directions forming a dorsomedial and a ventrolateral dendritic array. The axons originate in the ventrolateral dendrites and run straight to the motor roots. Preganglionic vegetative neurons were found in a separate group at the level of the abducens nucleus, and intermingled with motoneurons at the level of the glossopharyngeus and vagus nuclei. Their axons join the visceral sensory roots of the respective nerve. The tractus spinalia trigemini has a short and weak ascending limb, and a strong descending limb which decussates in the first and second segments of the spinal cord. A few uncrossed large fibers continue as far as the midthoracic level, In addition to trigeminal fibers, a small contingent of facial fibers and a significant number of vagus fibers contribute to the tractus spinalis. Small caliber fibers of the tractus and fine collaterals of large caliber fibers terminate in a continuous column which lies, in its largest part, in the neuropil medial to the tractus. The pattern of fiber terminations in this area is identical with that in the substantia gelatinosa. This area is called the nucleus spinalis trigemini. Coarse collaterals terminate in a discontinuous column of large and medium-sized neurons, extending caudally to the obex region. This area is the largest at the level of the trigeminus nucleus and is called the nucleus principalis trigemini. The cells of origin of the mesencephalic tractus of the trigeminus lie in the second and fourth layers of the optic tectum. The descending fibers run ventral to the tractus spinalis in a separate bundle. They emit collaterals to the large fiber termination areas and to branchiomotor nuclei. Direct contacts are established with trigeminus motoneurons. In the obex region a significant number of collaterals invade the dorsomedial part of the gray matter. The fasciculus solitarius begins at the level of the trigeminal nucleus and terminates in the commissura infima at the medullospinal border. The nucleus solitarius begins at the oral pole of the glossopharyngeus nucleus. The fasciculus receives fibers from the facialis (intermedius), glossopharyngeus and vagus nerves. The fibers spread over the total extent of the fasciculus: the facialis fibers exhibit a preference for a dorsomedial position and the glossopharyngeusvagus fibers for a ventrolateral position. There are several interchanging fibers between the somatic sensory and visceral sensory projecting areas. The results indicate a close similarity between amphibian and mammalian brain stem motor nuclei and sensory projections, though some parts are present in a primordial form in the frog.     

Anatomy of the gustatory system in the hamster: central projections of the chorda tympani and the lingual nerve

The sensory modalities of taste and touch, for the anterior tongue, are relegated to separate cranial nerves. The lingual branch of the trigeminal nerve mediates touch: the chorda tympani branch of the facial nerve mediates taste. The chorda tympani also contains efferent axons which originate in the superior salivatory nucleus. The central projections of these two nerves have been visualized in the hamster by anterograde labelling with horseradish peroxidase (HRP). Afferent fibers of the chorda tympani distribute to all rostral-caudal levels of the solitary nucleus. They synapse heavily in the dorsal half of the nucleus at its rostral extreme; synaptic endings are sparser and located laterally in caudal regions. These taste afferents travel caudally in the solitary tract and reach different levels by a series of collateral branches which extend medially in the the solitary nucleus, where they exhibit preterminal and terminal swellings. Taste afferent axons range in diameter from 0.2 micrometer to 1.5 micrometers. The thickest axons project exclusively to the rostral and intermediate subdivisions of the solitary nucleus; the find ones may distribute predominantly to the caudal subdivision. Afferent fibers of the lingual nerve terminate heavily in the dorsal one-third of the spinal nucleus of the trigeminal nerve and also as a dense patch in the lateral solitary nucleus at the midpoint between its rostral and caudal poles. This latter projection overlaps that of the chorda tympani. Thus the two sensory nerves which subserve taste and touch from coincident peripheral fields on the tongue converge centrally on the intermediate subdivision of the solitary nucleus. Efferent neurons of the superior salivatory nucleus were labelled retrogradely following application of HRP to the chorda tympani. These cells are located ipsilaterally in the medullary reticular formation ventral to the rostral pole of the solitary nucleus; their dendrites are oriented dorsoventrally. The efferent axons course dorsally, form a genu lateral to the facial somatomotor genu, and course ventrolaterally through the spinal nucleus of the trigeminal nerve to exit the brain ventral to the entering facial afferents.     

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.     

Representation of the cecum in the lateral dorsal motor nucleus of the vagus nerve and commissural subnucleus of the nucleus tractus solitarii in rat.

Motor fibers of the accessory celiac and celiac vagal branches are derived from the lateral columns of the dorsal motor nucleus of the vagus nerve. These branches also contain sensory fibers that terminate within the nucleus of the tractus solitarii. This study traces the innervation of the intestines by using the tracer cholera toxin-horseradish peroxidase. In 53 rats, the tracer was injected into either the stomach, duodenum, jejunum, terminal ileum, cecum, or ascending colon. With all cecal injections, prominent retrograde labeling of cell bodies occurred bilaterally in the lateral columns of the dorsal motor nucleus of the vagus nerve above, at, and below the level of the area postrema. Dendrites of laterally positioned neurons projected medially and rostrocaudally within the dorsal motor nucleus of the vagus nerve and dorsomedially into both the medial subnucleus and parts of the commissural subnucleus of the nucleus of the tractus solitarii. Sensory terminal labeling occurred in the dorsolateral commissural subnucleus at the level of the rostral area postrema and the medial commissural subnucleus caudal to the area postrema. Additionally, there was sensory terminal labeling within a small confined area of the dorsomedial zone of the nucleus of the tractus solitarii immediately adjacent to the fourth ventricle at a level just anterior to the area postrema. Stomach injections labeled motoneurons of the medial column of the entire rostrocaudal extent of the dorsal motor nucleus of the vagus nerve and a sensory terminal field primarily in the subnucleus gelatinosus, with less intense labeling extending caudally into the medial and ventral commissural subnuclei. Dendrites of gastric motoneurons project rostrocaudally and mediolaterally within the dorsal motor nucleus of the vagus nerve and dorsolaterally within the nucleus of the tractus solitarii. They are most pronounced at the level of the rostral area postrema where many dendrites course dorsolaterally terminating primarily within the subnucleus gelatinosus. Injections of the duodenum labeled a small number of the cells within the medial aspects of the dorsal motor nucleus of the vagus nerve. Jejunal, ileal, and ascending colon injections labeled cells sparsely within the lateral aspects of the dorsal motor nucleus of the vagus nerve bilaterally. No afferent terminal labeling was evident after injection of these areas of the bowel.(ABSTRACT TRUNCATED AT 400 WORDS)     

The formation of specific synaptic connections between muscle sensory and motor neurons in the absence of coordinated patterns of muscle activity

The influence of patterned neuronal activity on the formation of specific monosynaptic connections between muscle sensory and motor neurons was studied in the developing spinal cord of the bullfrog. Motor innervation of the forelimb was disrupted in tadpoles by resection of the brachial ventral root before these synaptic connections began to form in the spinal cord. In those frogs accepted for analysis, motor reinnervation of the forelimb was nonspecific and there was no coordinated movement of the limb. Synaptic connections therefore developed in the absence of temporal correlations of activity in muscle spindle afferents and motoneurons. Despite this disruption, afferent fibers supplying the triceps brachii muscles selectively innervated a restricted subpopulation of brachial motoneurons. Those motoneurons that received large synaptic inputs from afferents in one branch of the triceps nerve also received large inputs from afferents in the other triceps branches. Inputs from afferents supplying other muscles were not correlated with those from triceps afferents, suggesting the existence of a property common to all triceps afferents causing them to innervate a common subpopulation of motoneurons. These results show that in the absence of normal patterned sensory activity, sufficient cues still exist to permit the formation of specific sets of synaptic connections, and they argue indirectly for the existence of chemical labels that can determine the pattern of these connections.     

Morphological characteristics and terminating patterns of masseteric neurons of the mesencephalic trigeminal nucleus in the rat: an intracellular horseradish peroxidase labeling study

In order to study the morphological characteristics and terminating patterns of the neurons of the trigeminal mesencephalic nucleus (Vme), 55 masseteric neurons in Vme in the rat were stained by intracellular injection of horseradish peroxidase (HRP). Labeled cells were distributed throughout the nucleus. These neurons were divided into three types: uni- or pseudounipolar (type A, n = 43), bipolar (type B, n = 5), and multipolar cells (type C, n = 7). Each type was further divided into two subtypes according to the largest diameter of the perikarya (type a greater than or equal to 30 microns, type b less than 30 microns). The central processes of type Aa neurons projected to the following three groups of target nuclei: 1) nuclei functioning as interneurons, including supratrigeminal nucleus (Vsup), intertrigeminal nucleus (Vint), juxta-trigeminal region (Vjux), and parvicellular nucleus of the pontomedullary reticular formation (PcRF); 2) motor nuclei, including the trigeminal motor nucleus (Vmo), accessory facial nucleus (NVIIacs), accessory abducens nucleus (NVIacs), and a small number of labeled axons in the oculomotor nucleus and trochlear nucleus; 3) sensory nuclei, including the dorsomedial part of the principal trigeminal sensory nucleus (Vpdm) and the dorsomedial part of subnucleus oralis of the trigeminal spinal nucleus (Vodm). Labeled processes were dense in the Vsup, Vmo, and Vpdm. The proprioceptive pathway of the fifth nerve is discussed. Direct projections from type Aa neurons of Vme to the Vpdm and dorsolateral part of the Vsup contribute to conduction of the proprioceptive information from spindles of masticatory muscle to the contralateral thalamus in the rat. Different axon morphology, distribution, terminal branch density, and terminating patterns of type Aa neurons were noted in different functional groups of the projecting nuclei, especially in the Vsup, Vmo, and Vpdm. The highest terminal branching density, the most extensive distribution, and two different types of branching patterns (claw-like and comb-like) were observed in Vsup. Selective distribution and single-beaded or "Y"-shaped terminal branches were observed in Vmo. In the Vppdm the axonal branches were sparser than in the Vsup or Vmo, and had an arrangement like the branches of a weeping willow tree. These characteristics of anatomical organization might be related to the function of each projecting nucleus.