Gamma-aminobutyric acid-immunoreactive neurons in the rat trigeminal nuclei

The distribution of GABAergic neurons in the rat trigeminal nuclei was studied using a highly specific monoclonal antibody (mAb3A12) to gamma-aminobutyric acid (GABA). Immunopositive cells were relatively abundant in the marginal and gelatinosa beds of the caudal part of the trigeminal spinal tract nucleus, and in the dorsomedial areas of the oral subnucleus and the principal nucleus. A high density of GABA-immunoreactive somata was also found in the rostral part of the oral subnucleus and in the adjacent parvicellular reticular formation as well as in the supratrigeminal and intertrigeminal regions. Thus, the distribution of the GABAergic cells showed a relatively high density in areas related to the convergence of sensory stimuli, and in zones that contain interneurons inhibiting masticatory motorneurons. The results suggest, therefore, that GABA might play an important role both in discriminative sensory processing and in reflex modulation of the orofacial region.Abbreviations RF reticular formation - FRp parvicellular reticular formation - Vc trigeminal nucleus of the spinal tract, subnucleus caudalis - Vmes mesencephalic nucleus - Vmo trigeminal motor nucleus - Vo trigeminal nucleus of the spinal tract, subnucleus oralis - Vp principal trigeminal nucleus - Vsp spinal trigeminal nucleus - Vsup supratrigeminal nucleus     

Arrangement and connections of mesencephalic trigeminal neurons in the rat

The morphology of the mesencephalic trigeminal nucleus was examined microscopically in serial frozen sections. The nucleus extends over a length of about 4.5 mm, and its cell number was calculated to range from 1,000 to 1,600. 60% of the cells were located in the caudal third of the nucleus. Clustering of large unipolar cells was seen throughout the nucleus. Small spindle-shaped multipolar cells were found in the pontine part of the nucleus. The efferent connections of the mesencephalic trigeminal neurons were investigated by means of iontophoretically delivered Phaseolus vulgaris leuco-agglutinin or horseradish peroxidase after electrophysiological identification of mesencephalic trigeminal neurons. All projections were found ipsilateral to the injection site; they were confined to the trigeminal motor nucleus, especially to its lateral part, and to the dorsolateral reticular formation. The latter projection area included the supratrigeminal nucleus, the nucleus of Probst, and the parvocellular reticular zone. There were no direct projections to the facial or hypoglossal motor nuclei. It is concluded that proprioceptive input from one side is mediated polysynaptically to the bilateral oral final common-path neurons, with the exception of the ipsilateral trigeminal motoneurons.     

Intramedullary projections of the rostral nucleus of the solitary tract in the rat: gustatory influences on autonomic output

The efferent connections of the rostral nucleus of the solitary tract (NTS) in the rat were studied by anterograde transport of Phaseolus vulgaris leucoagglutinin. Rostral to the injection site, fibers travel through the rostral parvocellular reticular formation and deflect medially or laterally around the motor trigeminal nucleus, giving off few terminals in these nuclei and terminate in the parabrachial nucleus. Moderate projections to the peritrigeminal zone, including the intertrigeminal nucleus and the dorsal subcoeruleus nucleus, were observed. Caudally to the injection site, dense innervations from the rostral nucleus of the solitary tract were detected in the parvocellular reticular formation ventral and caudal to the injection site and in the intermediate and ventral medullary reticular formation. The rostral central and ventral subdivisions of the NTS up to the level where the nucleus of the solitary tract abuts the fourth ventricle and the hypoglossal nucleus, receive moderate input from the rostral nucleus of the solitary tract. In general, the projections from the rostral nucleus of the solitary tract were bilateral with an ipsilateral predominance. The caudal part of the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the facial nucleus were not labeled. It is concluded that medullary rNTS projections participate in oral motor behavior and autonomic control of abdominal organs.     

与咬肌运动神经元直接联系的运动前神经元在脑干的分布

目的为了定位向咬肌运动神经元投射的最后一级运动前神经元在脑干内的分布。方法注射麦芽凝集素结合的辣根过氧化物酶(WGA-HRP)至咬肌神经逆行跨突触追踪,然后通过免疫组织化学方法显示了该类神经元。结果这类神经元分布在双侧三叉上核(Vsup)、三叉神经感觉主核背侧部(Vpdm)、小细胞网状结构(PCR)和三叉神经脊束核吻侧亚核背侧部(Vodm),以及对侧三叉神经运动核(Vmo)。数量上,Vsup,特别是注射侧Vsup中,标记的神经元数量最多;其他核团内,双侧标记的神经元的数量无明显差别。结论一侧咬肌运动神经元直接接受脑干双侧多个区域调控。     

Supratrigeminal neurons mediate the shortest, disynaptic pathway from the central amygdaloid nucleus to the contralateral trigeminal motoneurons in the rat

Stimulation of the supratrigeminal area (STA) of the rat induced a monosynaptic EPSP in most mylohyoid-digastric motoneurons and a monosynaptic IPSP or EPSP in the majority of masseteric ones, contralaterally. Stimulation of the central amygdaloid nucleus induced the ipsilateral STA activity immediately followed by the contralateral mylohyoid nerve activities. The same amygdaloid stimulated excited 19 of 46 STA neurons, which were antidromically identified to project to the contralateral trigeminal motor nucleus. Nine of these were monosynaptically excited. The mean of the antidromic and monosynaptic latencies of these neurons explains the mean onset latencies of the amygdaloid influences on the contralateral trigeminal motoneurons. Therefore, the shortest crossing amygdalo-motoneuronal pathway is probably disynaptic and mediated by commissural STA neurons.     

Nociceptive afferents to the premotor neurons that send axons simultaneously to the facial and hypoglossal motoneurons by means of axon collaterals

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.     

Connections of brainstem respiratory nuclei studied by the retrograde horseradish peroxidase axon transport method

Intrabulbar connections of respiratory nuclei and the medullary reticular formation and also descending pathways from these structures in the spinal cord were studied by the retrograde horseradish peroxidase axonal transport method in cats. Neurons of the nucleus ambiguus and nucleus retroambigualis (ventral respiratory group) and of the ventrolateral part of the nucleus of the tractus solitarius (dorsal respiratory group) were shown to form direct two-way connections with each other and with the medial region of the medulla. Neurons of the pneumotaxic center send uncrossed axons to the nucleus ambiguus and to the medial medullary reticular formation. Neurons of the contralateral homonymous nucleus and neurons of the nucleus of the tractus solitarius are sources of projections of the locus coeruleus. A well developed system of direct connections was found between neurons of respiratory nuclei of the two halves of the brain. The possible role of these nuclear formations in genesis of the respiratory rhythm and regulation of the respiratory and other motor functions of the reticular formation is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 149–157, March–April, 1982.     

Course and termination of the pyramidal tract in the pig

To study the pyramidal tract in the pig, the motor cerebral cortex of one side was defined electrophysiologically and subsequently excised. The animals operated were killed after 7, 11 and 14 days, and the cerebral hemisphere of the operated side, brain stem and spinal cord were removed for histological examination. The pyramidal tract proved to run ipsilaterally as far as the oral extremity of the 12th cranial nerve nucleus. The decussation, which exhausted itself almost completely at the level of the rostral extremity of the 1st cervical metamere, started here. After the limit just mentioned only rare isolated fibres were visible. Along its course, the pyramidal tract sent a small number of axons to the ipsilateral and contralateral nucleus of the 7th cranial nerve, while the fibres running from the opposite side to the reticular formation and to the hypoglossal nerve nucleus, cuneatus, gracilis and trigeminal spinal tract nuclei were more numerous.     

Neurochemical properties of the synapses in the pathways of orofacial nociceptive reflexes

The brainstem premotor neurons of the facial nucleus (VII) and hypoglossal (XII) nucleus can integrate orofacial nociceptive input from the caudal spinal trigeminal nucleus (Vc) and coordinate orofacial nociceptive reflex (ONR) responses. However, the synaptoarchitectures of the ONR pathways are still unknown. In the current study, we examined the distribution of GABAergic premotor neurons in the brainstem local ONR pathways, their connections with the Vc projections joining the brainstem ONR pathways and the neurochemical properties of these connections. Retrograde tracer fluoro-gold (FG) was injected into the VII or XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the Vc. Immunofluorescence histochemical labeling for inhibitory/excitatory neurotransmitters combined with BDA/FG tracing showed that GABAergic premotor neurons were mainly distributed bilaterally in the ponto-medullary reticular formation with an ipsilateral dominance. Some GABAergic premotor neurons made close appositions to the BDA-labeled fibers coming from the Vc, and these appostions were mainly distributed in the parvicellular reticular formation (PCRt), dorsal medullary reticular formation (MdD), and supratrigeminal nucleus (Vsup). We further examined the synaptic relationships between the Vc projecting fibers and premotor neurons in the VII or XII under the confocal laser-scanning microscope and electron microscope, and found that the BDA-labeled axonal terminals that made asymmetric synapses on premotor neurons showed vesicular glutamate transporter 2 (VGluT2) like immunoreactivity. These results indicate that the GABAergic premotor neurons receive excitatory neurotransmission from the Vc and may contribute to modulating the generation of the tonic ONR.     

Quantitation and characterization of thyrotropin-releasing hormone in vagal nuclei and other regions of the medulla oblongata of the rat

Abstract: Since evidence is now available to support a nonendocrine autonomic function for thyrotropin-releasing hormone (TRH), quantitative measurements of TRH were made in nuclei of the vagal complex and other areas of the caudal medulla oblongata of the rat. Regions containing the dorsal motor nucleus of the vagus (DMN), nucleus tractus solitarius (NTS), hypoglossal nucleus, dorsal column nuclei, descending nucleus V (DNV), nucleus ambiguus (NA), raphe nuclei (MR) dorsomedial and ventromedial reticular formation, and inferior olivary nuclei were isolated from 300-μm-thick frozen sections of medulla by the micropunch technique. Each region was pooled bilaterally, homogenized in 0.1 M HCl, and vacuum-dried. Extracts were assayed for TRH by specific radioimmunoassay (RIA). TRH levels varied 100-fold among medulla nuclei. Highest content (ng/mg protein ± SEM) was found in DMN (14 ± 1.38) and NTS (4.7 ± 0.68), whereas lowest levels occurred in the DNV and MR (0.13, 0.06). Nearly 65% of the total medullary TRH was localized in nuclei associated with vagal complex (DMN, NTS, NA). Characterization of tissue immunoreactivity (TRHi) in these regions suggests the presence of TRH, since (1) medullary tissue extracts competed with ¹²⁵I-TRH for antibody binding sites with the same affinity as authentic TRH; (2) TRHi in tissue extracts co-migrated with synthetic TRH when subjected to reverse-phase high performance liquid chromatography and Sephadex G-10 chromatography; and (3) rat serum TRH peptidases degraded TRHi and authentic TRH at similar rates. Another group of rats was subjected to unilateral (right side) vagotomy. At 33 weeks post-vagotomy, the vagal preganglionic cell population in the ipsilateral DMN was depleted 50–75%, while the contralateral side was unaffected. Interestingly, the content of TRH in the ipsilateral (right) DMN remained unchanged, whereas TRH in the contralateral DMN increased by 50%. In contrast, TRH was significantly elevated in the NA on the ipsilateral side of the lesion. TRH in both ipsi- and contralateral NTS was unchanged when compared with sham-operated controls. These results indicate that (1) TRH is present in several specific loci of the medulla; (2) very high levels are found in the vagal complex; and (3) vagotomy may alter TRH in the contralateral DMN and ipsilateral NA.     

Cerebellar afferents in the frogs,Rana esculenta and Rana temporaria

Summary Afferents to the cerebellum in frogs (Rana esculenta, Rana temporaria) were studied by use of retrograde transport of horseradish peroxidase. Following injections restricted to the molecular layer of the cerebellum cell labelling was found in the contralateral inferior olive and the ventral portion of the caudal medullary raphe. Injections involving the granular layer resulted in labelling in the ventral horn of the cervical spinal cord, the caudal spinal trigeminal nucleus, the nucleus caudalis and the medial portion of the nucleus ventralis of the vestibular nerve, the inferior reticular nucleus and the nucleus of the fasciculus longitudinalis medialis. Following larger injections, which may have spread significantly into the cerebellar, secondary gustatory, trigeminal or vestibular nuclei, labelled cell bodies were also found in the nucleus ruber, nucleus solitarius, the rostral spinal trigeminal nucleus and the rostral rhombencephalic reticular formation. It is unclear whether the fibers from these latter areas innervate the cerebellum of the frog, as they do in mammals, or only reach the underlying areas. This situation emphasizes a limitation of the HRP technique when applied to small structures as is often the case in lower vertebrates.Supported by Grant Gr 276 to U. G.-C. from the Deutsche Forschungsgemeinschaft.     

Reciprocal connections between the red nucleus and the trigeminal nuclei: a retrograde and anterograde tracing study.

An anterograde biocytin and a retrograde WGA-colloidal gold study in the rat can provide information about reciprocal communication pathways between the red nucleus and the trigeminal sensory complex. No terminals were found within the trigeminal motor nucleus, in contrast with the facial motor nucleus. A dense terminal field was observed in the parvicellular reticular formation ventrally to the trigeminal motor nucleus. The parvicellular area may be important for the control of jaw movements by rubrotrigeminal inputs. On the other hand, the contralateral rostral parvicellular part of the red nucleus receives terminals from the same zone in the rostral part of the trigeminal sensory complex, where retrogradely labelled neurones were found after tracer injections into the red nucleus. Such relationships could be part of a control loop for somatosensory information from the orofacial area.     

Afferent projections of the trigeminal nerve in the goldfish,Carassius auratus

The horseradish peroxidase (HRP) histochemical technique was used to examine the peripheral distribution and afferent projections of the trigeminal nerve in the goldfish, Carassius auratus. Sensory fibers of the trigeminal nerve distribute over the head via four branches. The ophthalmic branch distributes fibers to the region above the eye and naris. The maxillary and mandibular branches innervate the regions of the upper and lower lip, respectively. A fourth branch of the trigeminal nerve was demonstrated to be present in the hyomandibular trunk. Upon entering the medulla the trigeminal afferent fibers divide into a rostromedially directed bundle and a caudally directed bundle. The rostromedially directed bundle terminates in the sensory trigeminal nucleus (STN) located within the rostral medulla. The majority of fibers turn caudally, forming the descending trigeminal tract. Fibers of the descending trigeminal tract terminate within three medullary nuclei: the nucleus of the descending trigeminal tract (NDTV), the spinal trigeminal nucleus (Spv), and the medial funicular nucleus (MFn). All projections, except for those to the MFn, are ipsilateral. Contralateral projections were observed at the level of the MFn following the labeling of the ophthalmic and maxillomandibular branches. All branches of the trigeminal nerve project to all four of the trigeminal medullary nuclei. Projections to the STN and MFn were found to be topographically organized such that the afferents of the ophthalmic branch project onto the ventral portion of these nuclei, while the afferents of the maxillo- and hyomandibular branches project to the dorsal portion of these nuclei. Cells of the mesencephalic trigeminal nucleus were retrogradely labeled following HRP application to the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve. In addition to demonstrating the ascending mesencephalic trigeminal root fibers, HRP application to the above-mentioned branches also revealed descending mesencephalic trigeminal fibers. The descending mesencephalic trigeminal fibers course caudally medial to the branchiomeric motor column and terminate in the ventromedial portion of the MFn.     

Afferent connections to the abducent nucleus in the cat.

The afferent connections to the abducent nucleus in the cat were studied by means of retrograde transport of WGA-HRP after implantations of the tracer in crystalline form. Retrogradely labelled cells were found bilaterally in the medial and descending vestibular nuclei, mainly in their ventral and medial portions, in the rostral part of the ipsilateral gigantocellular reticular nucleus, in the medial part of the contralateral caudal pontine reticular nucleus and bilaterally in the oculomotor nucleus, mainly in its dorsolateral division. Some labelled cells were also found bilaterally in the mesencephalic reticular formation, the periaqueductal grey and the nucleus of the trapezoid body.     

Medullary reticular neurons in the Japanese toad: morphologies and excitatory inputs from the optic tectum

1. To elucidate the neural mechanisms that mediate visual responses of optic tectum (OT) to medullary and spinal motor systems, we analyzed medullary reticular neurons in paralyzed Japanese toads (Bufo japonicus). We examined their responses to electrical stimulation of OT, and stained some neurons intracellularly. Responses to stimulation of the glossopharyngeal nerve (IX) were also analyzed. 2. Extracellular single unit recording revealed excitatory responses of medullary neurons to OT and IX stimulation. Among 92 units encountered, 79 responded to OT stimuli, 10 to IX stimuli, and 3 to both. Some units responded to successive stimuli of short intervals with relatively stable lags. 3. Intracellular recording and staining experiments revealed morphologies of reticular neurons that received excitatory inputs from OT. Thirteen units were identified after complete reconstruction of somata and dendrites. Neurons in the nucleus reticularis medius received excitatory inputs from bilateral OT. They had wide dendrites in ventral, ventrolateral and lateral funiculi, and single axons descending in the ipsilateral ventral funiculus as far caudally as the cervical spinal cord. Some collaterals of these axons projected directly to the hypoglossal and spinal motor nuclei. Some neurons in other medullary nuclei (nuc. reticularis superior, pretrigeminal nucleus, nuc. reticularis inferior, and nuc. tractus spinalis nervi trigemini) also responded to the OT stimulation. 4. Activities in bilateral OT converge onto medullary reticular neurons, which may directly control medullary and spinal motor systems.     

A crossed projection from the optic tectum to craniocervical premotor areas in the brainstem reticular formation. An anterograde and retrograde tracing study in the mallard (Anas platyrhynchos L.)

The optic tectum in birds receives visual information from the contralateral retina. This information is passed through to other brain areas via the deep layers of the optic tectum. In the present study the crossed tectobulbar pathway is described in detail. This pathway forms the connection between the optic tectum and the premotor area of craniocervical muscles in the contralateral paramedian reticular formation. It originates predominantly from neurons in the ventromedial part of stratum griseum centrale and to a lesser extent from stratum album centrale. The fibers leave the tectum as a horizontal fiber bundle, and cross the midline through the caudal radix oculomotorius and rostral nucleus oculomotorius. On the contralateral side fibers turn to ventral and descend caudally in the contralateral paramedian reticular formation to the level of the obex. Labeled terminals are found in the ipsilateral medial mesencephalic reticular formation lateral to the radix and motor nucleus of the oculomotor nerve, and in the contralateral paramedian reticular formation, along the descending tract. Neurons in the medial mesencephalic reticular formation in turn project to the paramedian reticular formation. Through the crossed tectobulbar pathway visual information can influence the activity of craniocervical muscles via reticular premotor neurons.     

The neurons of origin of non-cortical afferent connections to the oculomotor nucleus. A retrograde labeling study in the rabbit.

The cellular origin of the brainstem projections to the oculomotor nucleus in the rabbit has been investigated by using free (HRP) and lectin-conjugated horseradish peroxidase (WGA-HRP). Following injections of these tracers into the somatic oculomotor nucleus (OMC), retrogradely labeled cells have been observed in numerous brainstem structures. In particular, bilateral labeling has been found in the four main subdivisions of the vestibular complex, predominantly in the superior and medial vestibular nuclei and the interstitial nucleus of Cajal, while ipsilateral labeling was found in the rostral interstitial nucleus of the medial longitudinal fascicle (Ri-MLF), the Darkschewitsch and the praepositus nuclei. Neurons labeled only contralaterally have been identified in the following structures: mesencephalic reticular formation dorsolateral to the red nucleus, abducens internuclear neurons, group Y, several areas of the lateral and medial regions of the pontine and medullary reticular formation, ventral region of the lateral cerebellar nucleus and caudal anterior interpositus nucleus. This study provides also information regarding differential projections of some centers to rostral and caudal portions of the OMC. Thus, the rostral one-third appears to receive predominant afferents from the superior and medial vestibular nuclei, while the caudal two-thirds receive afferents from all the four vestibular nuclei. Finally, the group Y sends afferents to the middle and caudal, but not to the rostral OMC.     

Brainstem areas involved in the aspiration reflex: c-Fos study in anesthetized cats

Expression of the immediate-early gene c-fos, a marker of neuronal activation was employed in adult anesthetized non-decerebrate cats, in order to localize the brainstem neuronal populations functionally related to sniff-like (gasp-like) aspiration reflex (AR). Tissues were immunoprocessed using an antibody raised against amino acids of Fos and the avidin-biotin peroxidase complex method. The level of Fos-like immunoreactivity (FLI) was identified and counted in particular brainstem sections under light microscopy using PC software evaluations in control, unstimulated cats and in cats where the AR was elicited by repeated mechanical stimulation of the nasopharyngeal region. Fourteen brainstem regions with FLI labeling, including thirty-seven nuclei were compared for the number of labeled cells. Compared to the control, a significantly enhanced FLI was determined bilaterally in animals with the AR, at various medullary levels. The areas included the nuclei of the solitary tract (especially the dorsal, interstitial and ventrolateral subnuclei), the ventromedial part of the parvocellular tegmental field (FTL -- lateral nuclei of reticular formation), the lateral reticular nucleus, the ambigual and para-ambigual regions, and the retrofacial nucleus. FLI was also observed in the gigantocellular tegmental field (FTG -- medial nuclei of reticular formation), the spinal trigeminal nucleus, in the medullar raphe nuclei (ncl. raphealis magnus and parvus), and in the medial and lateral vestibular nuclei. Within the pons, a significant FLI was observed bilaterally in the parabrachial nucleus (especially in its lateral subnucleus), the Kolliker-Fuse nucleus, the nucleus coeruleus, within the medial region of brachium conjunctivum, in the ventrolateral part of the pontine FTG and the FTL. Within the mesencephalon a significantly enhanced FLI was found at the central tegmental field (area ventralis tegmenti Tsai), bilaterally. Positive FLI found in columns extending from the caudal medulla oblongata, through the pons up to the mid-mesencephalon suggests that the aspiration reflex is thus co-ordinated by a long loop of medullary-pontine-mesencephalic control circuit rather than by a unique "center".     

Termination areas of primary afferent fibers of the trigeminal nerve in the rat

The cobalt-labelling technique was used to investigate the termination areas of trigeminal primary afferent fibers. The familiar somatotopic arrangement of fibers and terminals of the three divisions of the trigeminal nerve was recognized both in the spinal tract and in the nuclear complex of the trigeminus. The spinal tract could be traced as far as the 3rd cervical segment of the spinal cord where fibers crossed to the contralateral side. The different divisions of the nuclear complex could be unambiguously defined on the basis of the pattern of fiber terminations. The nucleus principalis was characterized by the even distribution of terminals in the nucleus. The nucleus spinalis was characterized by small bundles of fibers of intranuclear origin, which broke up the even distribution pattern of terminals. The presence of mesencephalic trigeminal fibers in the nucleus oralis distinguished this nucleus from the nucleus interpolaris. The nucleus caudalis was recognized on the ground of its striated structure. Primary trigeminal afferent fibers were located in the following sites: in the solitary nucleus, in the lateral part of the reticular formation, in the dorsal-column nuclei and in the superior vestibular nuclei. Primary fiber terminations could not be observed in the cerebellum.