Ultrastructure of transganglionic HRP transport in cat trigeminal system

The ultrastructure of transganglionic transport of horseradish peroxidase (HRP) from the inferior alveolar (IA) nerve to the brainstem is being studied in the cat. The IA nerve was soaked in an HRP solution and following a two-day survival the animal was perfused transcardially with a paraformaldehyde-glutaraldehyde solution. The tissue was immediately dissected and postfixed for 1-3 h in perfusate. Sections of 75 micron thickness were cut with a Vibratome and reacted utilizing tetramethyl benzidine (TMB) as the chromagen. Optimum results for electron microscopy were obtained by osmication in a pH 6.0, 1% osmium tetroxide solution for 45 min at 45 degrees C, followed by rapid dehydration and embedment in Epon. The resulting HRP-TMB reaction product was characterized and identified ultrastructurally in ganglion cells, peripheral and central axons and in brainstem terminals. The HRP-TMB reaction product varied in density but had consistent crystalline-like laminations of a repeating unit and characterized by a membrane 4-5 nm in diameter. Some of the HRP-TMB reaction product found in terminals and axons was below the limit of resolution of the light microscope.     

Somatotopic organization of the primary sensory trigeminal neurons in the hagfish, Eptatretus burgeri

Primary sensory trigeminal projections were investigated in the hagfish following application of horseradish peroxidase (HRP) to the sensory branches. In our control preparations we were able to distinguish five sensory ganglia and their respective nerves. HRP application confirmed the almost exclusive relation of each of these nerves to their respective ganglia, with very little overlap. In normal frontal sections of the medulla oblongata, five columns of fibers surrounded by neuronal cell bodies could be clearly distinguished, but the number is probably fortuitous, for there was no one-on-one relationship with the five trigeminal ganglia. From their peripheral connections, we surmised that columns 1 and 3 handle general cutaneous sensation, columns 2, 4, and 5 handle taste sensation, and column 5 handles general mucous cutaneous sensation conveyed by utricular ganglion cells. Dorsally located columns received projections from nerves with dorsal peripheral connections, and more ventrally located columns received projections from nerves with ventral peripheral connections. This relation is the reverse of that seen in other vertebrates.     

Transganglionic transport of wheat germ agglutinin-HRP and choleragenoid-HRP in rat trigeminal primary sensory neurons

Horseradish peroxidase conjugates of either the lectin wheat germ agglutinin (WGA-HRP) or choleragenoid (B-HRP) have been shown to be sensitive neuroanatomical tracers. In the present study a comparison was made between these two conjugates as transganglionic tracers in trigeminal primary sensory neurons following injection into the rat mystacial vibrissae skin. Differences between the two tracers were observed in the labeling of cell bodies in the trigeminal ganglion. Injection of WGA-HRP resulted in labeling of predominantly small cell bodies, whereas B-HRP gave rise to labeling of somewhat larger cell bodies. By increasing the concentration of the injected WGA-HRP solution the number of labeled cells increased substantially, while a corresponding increase in the concentration of B-HRP resulted in a relatively small increase in the number of labeled cells. WGA-HRP injection resulted in labeling of primary afferents mainly in the substantia gelatinosa of the trigeminal subnucleus caudalis. When the concentration of the injected WGA-HRP solution was increased, labeling was also observed in the marginal and magnocellular zones. Following B-HRP injection, labeling was only observed in the magnocellular zone and innermost part of the substantia gelatinosa. This general pattern of labeling was the same when the concentration of the B-HRP solution was increased.     

Differential distribution of cell bodies and central axons of mesencephalic trigeminal nucleus neurons supplying the jaw-closing muscles and periodontal tissue: A transganglionic tracer study in the cat

Distribution of cell bodies and central axons of mesencephalic trigeminal nucleus (MTN) neurons were examined in the cat by the method of transganglionic transport of horseradish peroxidase (HRP). Jaw-closing muscle afferent MTN neurons were distributed throughout the whole rostrocaudal extent of the MTN, and sent their axons ipsilaterally to the supratrigeminal and intertrigeminal regions, dorsolateral division of the motor trigeminal nucleus, lateral part of the medullary reticular formation, lamina VI of C1-C3 cord segments, and cerebellum. On the other hand, periodontal receptor afferent MTN neurons were located mainly in the caudal part of the MTN, and sent their axons ipsilaterally to the supratrigeminal region and cerebellum. The existence of multipolar MTN neurons with 1-9 smooth dendrites was also confirmed; most of them were jaw-closing muscle afferent neurons.     

The central projections of tooth pulp afferent neurons in the rat as determined by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to map in detail the central projections of trigeminal primary afferent neurons that innervate the dental pulp organ of the rat. In each of ten animals, 0.5-2.0 microliters of enzyme solution was injected into the pulp chamber of the first maxillary molar tooth. Postmortem examination of the decalcified teeth in all cases showed that the HRP/HRP-WGA remained confined to the pulp chamber and pulp roots, with no spread of enzyme into periapical tissues. HRP-labeled tooth pulp afferent fibers projected to all four rostrocaudal subdivisions of the ipsilateral trigeminal brainstem nuclear complex (TBNC) and to the upper cervical spinal cord. The labeled terminal fields formed a column that stretched relatively uninterrupted from just caudal to the rostromedial tip of the trigeminal principal sensory nucleus to at least the C2 segment of the spinal cord. The density of the afferent projection varied markedly from one rostrocaudal level of the TBNC to the next but was heaviest in an area encompassing the caudal one-half of the principal sensory nucleus and the rostral two-thirds of pars oralis. Fibers projected only lightly to pars caudalis, where they terminated preferentially in laminae I, IIa, and the junctional zone between laminae IV and V. HRP-labeled terminals in C1 and C2 were located almost exclusively in laminae I. In the dorsoventral axis, the terminal fields in the TBNC were located in a surprisingly dorsal part of the complex, well within what has been shown by others to be largely an area of termination for mandibular division fibers. Most fibers ended in medial parts of the TBNC, with the exception of two modestly labeled terminal fields located in the lateral aspects of rostral pars oralis and rostral pars caudalis. No labeled fibers terminated in the contralateral TBNC or contralateral cervical spinal cord.     

Central distribution of primary afferent fibers in the Arnold's nerve (the auricular branch of the vagus nerve): A transganglionic HRP study in the cat

Central projections of the Arnold's nerve (the auricular branch of the vagus nerve; ABV) of the cat were examined by the transganglionic HRP method. After applying HRP to the central cut end of the ABV, HRP-labeled neuronal somata were seen in the superior ganglion of the vagus nerve. Main terminal labeling was seen ipsilaterally in the solitary nucleus, in the lateral portions of the ventral division of the principal sensory trigeminal nucleus, in the marginal regions of the interpolar subnucleus of the spinal trigeminal nucleus, in the marginal and magnocellular zones of the caudal subnucleus of the spinal trigeminal nucleus, in the ventrolateral portions of the cuneate nucleus, and in the dorsal horn of the C1–C3 cord segments. In the solitary nucleus, labeled terminals were seen in the interstitial, dorsal, dorsolateral and commissural subnuclei; some of these terminals may be connected monosynaptically with solitary nucleus neurons which send their axons to the somatomotor and/or visceromotor centers in the brainstem and spinal cord.     

Retrograde axonal transport of asialoglycoproteins in mouse trigeminal neurons in vivo and in rat dorsal root ganglia neurons in vitro

Different terminal sugars of the glycoprotein orosomucoid were exposed by sequential glycosidase digestions. The orosomucoid and its different derivatives were conjugated to horseradish peroxidase by a two-step glutaraldehyde coupling procedure, injected into the snout of 12-day-old mice or exposed to dorsal root ganglia neurons from embryonic rats, cultivated in a two chamber system. A marked increase in transport of the conjugates in the trigeminal and dorsal root ganglia neurons was observed histochemically after removal of sialic acid, exposing galactose as the terminal sugar. Quantitative hydrolysis of galactose residues resulted in reduced uptake. The data suggest the presence of a galactose-recognition molecule in the axon-terminal membrane, involved in retrograde axonal transport.     

Spatial sharpening by second-order trigeminal neurons in crotaline infrared system

Neural responses in the nucleus of the lateral descending tract of the trigeminal nerve (LTTD) of the rattlesnake Crotalus viridis were recorded. Neurons in the LTTD respond phasically to infrared stimulation of the pit organ, in contrast to the tonic responses that have been reported for the primary afferents. The receptive field dimensions of LTTD neurons are smaller than those of the primary afferents; some LTTD neurons have inhibitory regions within their receptive fields. The smaller receptive fields of neurons in the LTTD, as well as the phasic responses of these cells, might be a result of this inhibition. This is an instance of spatial sharpening and possibly enhancement of responses to time-changing stimuli due to excitory and inhibitory neural interactions in a primary trigeminal nucleus.     

Oral and facial representation in the trigeminal principal and rostral spinal nuclei of the cat

Transganglionic transport of horseradish peroxidase (HRP) was used to study the patterns of termination of somatic afferent fibers innervating oral and facial structures within the principal nucleus (Vp), nucleus oralis (Vo), and nucleus interpolaris (Vi). The primary trigeminal afferent fibers that innervate the oral cavity supplied by the pterygopalatine, superior alveolar, lingual, buccal, and inferior alveolar branches, as well as the facial skin supplied by the frontal, corneal, zygomatic, infraorbital, auriculotemporal, mylohyoid, and mental branches, were traced in this experiment. The results show that trigeminal afferent nerves that innervate the oral cavity project mainly to the principal nucleus, the rostrodorsomedial part (Vo.r) and dorsomedial division (Vo.dm) of pars oralis, and the dorsomedial region of pars interpolaris, while an extensive overlap of projections is found in the Vo.r, Vo.dm, and rostral Vi. The central processes of fibers innervating the anterior face (i.e., mental, infraorbital, and frontal nerves) terminate in the ventral division of principalis (Vpv), caudal region pars oralis (Vo.c), and ventrolateral Vi, with the largest numbers of terminals being found in the Vpv and Vi. In contrast, the central projection patterns of the corneal, zygomatic, mylohyoid, and auriculotemporal afferents are different from those of other afferent nerves examined, and present a discrete projection to the trigeminal sensory nuclear complex (TSNC). The corneal, mylohyoid, and auriculotemporal afferents mainly project to the restricted regions of principalis and caudal Vi, while zygomatic afferent nerve fibers project to the caudal third of pars interpolaris. The typical somatotopic organization with the face of the mouth open inverted is represented in the rostrocaudal midlevels of the Vpv and caudal pars interpolaris. The Vpd receives topographical projection from primary afferent nerves that innervate the oral structure only, while this projection was organized in a complicated manner. The relationship between the functional segregation and the cytoarchitectonic differentiation of the TSNC is discussed, particularly with respect to this somatotopic organization, combined with the characteristics of projecting cells in the TSNC.     

Organization of sensory and motor nuclei of the trigeminal nerve in lampreys

Anterograde and retrograde HRP transport were used to elucidate the primary central projections of the trigeminal nerve in a lamprey, Lampetra japonica, by application to the ophthalmic, apical, basilar, suborbital, and mandibular branches of the trigeminal nerve. (1) Most of the trigeminal and a few facial ganglion cells were labeled. The ganglion cells of each nerve were distributed in separate areas within their respective ganglia. (2) Some ipsilateral medullary and spinal dorsal cells were labeled after HRP application to the ophthalmic and apical nerves, but there was no contralateral labeling. (3) Most of the neurons of the trigeminal motor nucleus were labeled, and when the apical or the basilar nerve was labeled, in each case a cluster of small motor neurons was found ventrolateral to the classic motor nucleus. (4) Miscellaneous neurons were found scattered along the course of the descending trigeminal tract and nucleus in all cases except after application to the mandibular branch. The shape, size, and distribution patterns of these neurons were varied, and several characteristics indicated that they were sensory in nature. (5) In the rostral part of the medulla, sensory fibers of each nerve showed restricted localization within the descending trigeminal tract and nucleus. When compared to the distribution of the same fibers in the hagfish Eptatretus burgeri, another member of the cyclostomes, the distribution pattern in the lampreys studied was closer to the type seen in gnathostomes.     

A new tectal afferent nucleus of the infrared sensory system in the medulla oblongata of crotaline snakes

The existence of an infrared sensory neuron group with ascending fibers which directly reach the optic tectum in Crotaline snakes was confirmed with three methods. (1) With the retrograde horseradish peroxidase (HRP) method, labeled neurons were not found within the nucleus descendens lateralis nervi trigemini (DLV), but in an unnamed cell group located immediately ventral to the DLV of the contralateral side at the transitional portion between the nucleus oralis (DVo) and the nucleus interpolaris (DVi). This unnamed cell group, which was seen only in the Crotalinae, was provisionally called the ‘new nucleus’. (2) Normal brain series of 15 species were stained by the methods of Bodian-Otsuka, Klüver-Barrera and Nissl staining to compare the cytoarchitecture of the medulla oblongata. The ‘new nucleus’ was found only in species belonging to the Crotalinae. This nucleus was situated in fiber tracts which appeared to correspond to the lemniscus spinalis and tractus spino-cerebellaris of the reptilian medulla oblongata, and contained medium-sized multipolar or fusiform neurons. (3) In an electrophysiological study 16 single units responding unimodally to an infrared stimulus were recorded. Three of these recording sites were determined with Pontamine sky blue marking to be near or within the ‘new nucleus’.     

Primary vagal nerve projections to the lateral descending trigeminal nucleus in boidae (Python molurus andBoa constrictor)

After injections of horseradish peroxidase into several areas of the neocortex in the macaque monkey longitudinal bands of labeled cells in the basal nucleus of Meynert related to areas of cortex in the frontal lobe have been found to overlap along their long axes with the bands related to widely separated but interconnected areas of the parieto-temporal cortex. The frontal and parietal lobes are related to the anterior and posterior halves respectively of the nucleus, the temporal cortex to the postero-lateral margin of the nucleus and the occipital lobe to its upturned posterior extension.     

Central projections from cat suboccipital muscles: a study using transganglionic transport of horseradish peroxidase

Central projections of suboccipital muscle nerves were examined following exposure of cut peripheral nerves to the tracer horseradish peroxidase. Labelled fibers entered the C1 and C2 dorsal roots and accumulated in the dorsolateral part of the dorsal funiculus. Many labelled fibers entered the grey matter of C1 to C3 in ventrally directed bundles which passed medially to the base of the dorsal horn. No terminal labelling was apparent in superficial layers of the dorsal horn. However, labelled fibers ramified extensively throughout medial parts of the intermediate laminae, in and around the central cervical nucleus. Labelled fibers also projected toward the ventral horn. In cats subjected to ventral root section at the time of peripheral nerve exposure, a modest distribution of reaction product was observed deep in the ventral horn. In cats which did not undergo ventral root section, anterograde projections in the ventral horn were obscured by the simultaneous retrograde filling of motoneurons both in the ventromedial nucleus and on the medial and lateral borders of the gray matter. Labelled axons also coursed rostrally into the medulla where they formed a circumscribed bundle between the main cuneate nucleus and the spinal nucleus of V. Three consistent regions of HRP deposition could be identified at medullary levels. Dense accumulations of reaction product were present in circumscribed regions of the external cuneate nucleus (ECN) throughout its rostrocaudal extent. A second zone of dense labelling occurred in the intermediate nucleus of Cajal, where it appeared to form a continuing column rostral to the central cervical nucleus in C1-C3. Sparse labelling was restricted to a third zone in the ventrolateral part of the main cuneate nucleus.     

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

Retrograde and transganglionic transport of horseradish peroxidase conjugated to the B-fragment of cholera toxin (B-HRP) was used to study the location, morphology, and central projections of mesencephalic trigeminal (Me5) neurons innervating rat masticatory muscles. Labeled Me5 cell bodies were found throughout the Me5 nucleus from a level slightly caudal to the trigeminal motor nucleus to the level of the superior colliculus 5 mm further rostrally. Occasionally, labeled Me5 cells were observed in the anterior medullary velum, in the cerebellum, and in the brainstem contralateral to the B-HRP injection. The vast majority of the labeled Me5 cells were pseudounipolar, but multipolar cells were also found. Extensive central projections from labeled Me5 cells could be seen extending from the nucleus of Darkschewitsch rostrally to the C2 segment caudally. Small but consistent projections from Me5 neurons were observed in nuclear islands among the incoming Me5 root fibers. Trigeminal and hypoglossal motor nuclei received direct projections from Me5 cells, but not the facial motor nucleus. The most prominent Me5 projections appeared in the brainstem reticular formation, including the supratrigeminal nucleus. Smaller projections also extended into the main sensory trigeminal nucleus, trigeminal subnucleus oralis, and the nucleus of the solitary tract. © 1993 Wiley-Liss, Inc.     

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.     

Cell loss in lumbar dorsal root ganglia and transganglionic degeneration after sciatic nerve resection in the rat

The effects of sciatic nerve resection on lumbar dorsal root ganglion cells and their central branches have been studied in the adult rat. A quantitative analysis of the lumbar dorsal root ganglia indicated a 15–30% cell loss on the operated side. Argyrophilia indicating transganglionic degeneration was observed in Fink-Heimer stained sections from the lumbar spinal cord and the brainstem. The areas of degeneration argyrophilia were mainly located in the medial part of the ipsilateral L2–L6 dorsal horn laminae I–IV, the tract of Lissauer, the dorsal funiculus and the gracile nucleus. A few degenerating fibers could also be observed in the ipsilateral dorsal horn laminae V and VI, and in the ipsilateral ventral horn as well as in the contralateral dorsal and the gracile nucleus. The results confirm and extend previous findings at other levels and in other species. This suggests that cell loss and transganglionic degeneration may be general phenomena affecting a substantial proportion of primary sensory neurons following peripheral nerve injury.     

Central projections of the normal and ‘regenerate’ infraorbital nerve in adult rats subjected to neonatal unilateral infraorbital lesions: a transganglionic horseradish peroxidase study

The visuotopic organization of the primary visual cortex (area 17) and the extrastriate visual regions surrounding it (areas 18a and 18) has been studied in gray rats using standard microelectrode mapping techniques. The results confirm and extend previous observations in the rat. Apart from the representation of the contralateral visual field (VF) in area 17, in which the upper VF is represented caudally and the nasal VF laterally, there are additional representations of the VF in the extrastriate cortex. In lateral extrastriate cortex (area 18a) there are at least 4 such representations, namely lateromedial (LM), anterolateral (AL), laterointermediate (LI) and laterolateral (LL). In LM (second visual area) the upper VF is represented caudally and the nasal VF medially, being thus a mirror image of V1. In AL (third visual area) the upper VF is represented rostrally and the nasal VF, medially, being thus a mirror image of LM. In LI, the upper VF is medial and the nasal VF, lateral, being thus a mirror image of LM, or a reduced copy of V1. In medial extrastriate cortex (area 18) there are two representations of the temporal VF, labeled anteromedial (AM) and posteromedial (PM). In AM, the upper temporal VF is medial and the lower temporal VF, lateral, the extreme temporal field being rostral. The 30 degrees azimuth provides the boundary between AM and PM. Thus, AM is organized as a counter-clockwise rotation by 90 degrees of the V1 representation. In PM, the upper lower VF topography is like in AM, but the extreme temporal VF is caudal, being thus a mirror image of AM.     

Central distribution of trigeminal and upper cervical primary afferents in the rat studied by anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin

Injections of WGA-HRP were made in the rat trigeminal ganglion and C1-3 dorsal root ganglia (DRGs) to study the central projection patterns and their relations to each other. Trigeminal ganglion injections resulted in heavy terminal labeling in all trigeminal sensory nuclei. Prominent labeling was also observed in the solitary tract nucleus and in the medial parts of the dorsal horn at C1-3 levels, but labeling could be followed caudally to the C7 segment. Contralateral trigeminal projections were found in the nucleus caudalis and in the dorsal horn at C1-3 levels. The C1 DRG was found to be inconstant in the rat. When it was present, small amounts of terminal labeling were found in the external cuneate nucleus (ECN) and the central cervical nucleus (CCN). No dorsal horn projections were seen from the C1 DRG. Injections in the C2 DRG resulted in heavy labeling in the ECN, nucleus X, CCN, and dorsal horn, where it was mainly located in lateral areas. Labeling could be followed caudally to the Th 7 segment. C2 DRG projections also appeared in the cuneate nucleus (Cun), in all the trigeminal sensory nuclei, and in the spinal, medial, and lateral vestibular nuclei. A small C2 DRG projection was observed in the ventral cochlear nucleus. C3 DRG injections resulted in heavy labeling in both medial middle and lateral parts of the dorsal horn, in the ECN, and in nucleus X, whereas the labeling in the CCN was somewhat weaker. Smaller projections were seen to trigeminal nuclei, Cun, and the column of Clarke. Comparisons of the central projection fields of trigeminal and upper cervical primary afferents indicated a somatotopic organization but with a certain degree of overlap.     

Substance P-containing trigeminal sensory neurons project to the nucleus of the solitary tract

Intense substance P-like immunoreactivity (SPLI) was identified in fiber bundles coursing between the spinal nucleus of the trigeminal nerve and the ventrolateral nucleus of the solitary tract at the level of the area postrema. These bundles were apparent only when tissue was stained for substance P immunoreactivity and were not visible in preparations treated with antisera to somatostatin or neurotensin. Following unilateral section of the trigeminal nerve, the SPLI-containing fiber bundles were absent ipsilateral to the nerve section. The fibers were absent bilaterally in rats which were previously injected with capsaicin. Unilateral removal of the nodose ganglion did not diminish the intensity or apparent number of SPLI fibers. These data indicate the presence of a trigeminosolitary projection which is composed of primary trigeminal sensory neurons containing substance P. The results provide an anatomical route by which substance P of trigeminal origin may modulate vagal or glossopharyngeal sensory information.     

An HRP study of the central course of sensory intermediate and vagal fibers in peripheral facial nerve branches in the cat

Previous studies have shown that sensory fibers of intermediate and vagal nerve origin are present in facial nerve branches to the mimetic muscles in the cat. In the present study the central course of these fibers has been examined by transganglionic transport of horseradish peroxidase (HRP). In some of these experiments the facial nerve proper was transected central to the site of HRP application. In this way, the central course of the vagal fibers could be studied separately. For comparison HRP-conjugated wheat germ agglutinin was injected into the geniculate ganglion, revealing the central course of the entire afferent component of the intermediate nerve. The results show that labeled sensory intermediate nerve fibers, at their level of entrance in the brainstem, form a tract at the dorsal margin of the spinal trigeminal tract (5T). While some fibers ascend from this level to terminate in the main sensory trigeminal nucleus, and a few fibers terminate in the rostral part of the solitary tract nucleus, the majority take a descending course. The main site of termination for these descending fibers is in the medial part of the C2 dorsal horn. Terminal labeling is also seen in the ventrolateral part of the cuneate nucleus (CUN) and in a small area of gray substance between CUN and trigeminal nucleus caudalis. After entering the brainstem some sensory vagal fibers project to the trigeminal nucleus interpolaris and to an interstitial nucleus within the 5T, but the larger part joins the descending tract of intermediate nerve fibers. These descending vagal fibers have a terminal distribution pattern similar to the intermediate nerve fibers.