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.     

Contralateral projections of trigeminal mandibular primary afferents in the guinea pig as seen by transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase (HRP) was used to investigate contralateral projections of trigeminal mandibular fibers in the guinea pig. After application of HRP to the buccal, lingual, auriculotemporal, mylohyoid, mental and inferior alveolar nerves, crossing fibers and contralateral endings were found in the caudal region of the nucleus of the solitary tract (most of these belonging to the buccal and lingual nerves), the dorsomedial region of the subnucleus caudalis of the trigeminal sensory nuclear complex (TSNC), and the dorsal horns of the first 5 cervical spinal cord segments (C1-C5). The greatest numbers of crossing fibers in the medullary and cervical dorsal horn segments belonged to the mental and mylohyoid nerves, though these nerves did not project contralaterally to C4-C5. Contralateral buccal and lingual endings were scattered sparsely from the subnucleus caudalis to C5, and only very few contralateral auriculotemporal terminals were observed. Though laminae I-V of the dorsomedial region of the medullary and cervical dorsal horns all exhibited contralateral endings of the mental and mylohyoid nerves, most such endings were found in laminae IIi-III, followed by lamina IV, which suggests their involvement in the reception of mechanical stimuli and in the sensory motor reflexes of the orofacial region. The contralateral buccal and lingual terminals were distributed somatotopically in the first 5 cervical cord segments, with the lingual endings rostral to the buccal terminals within each segment. In C4 and C5 lingual endings appeared exclusively in laminae I and IIo, suggesting that like the ipsilateral lingual projections at this level, which also terminate in these laminae, they may be involved in pain and temperature sensation.     

Difference in central projection of primary afferents innervating facial and intraoral structures in the rat

Transganglionic transport of horseradish peroxidase-wheat germ agglutinin conjugate was used to study the central projection of primary afferent neurons innervating facial and intraoral structures. The examined primary neurons innervating the facial structures were those comprising the frontal and zygomaticofacial nerves and those innervating the cornea, while the primary neurons innervating the intraoral structures included those innervating the mandibular incisor and molar tooth pulps and those comprising the palatine nerve. The primary afferents innervating the facial structures project to the lateral or ventral parts of the trigeminal principal, oral and interpolar subnuclei, and to the rostral cervical spinal dorsal horn across laminae I through V, with a greater proportion being directed to the spinal dorsal horn. The primary afferents innervating the intraoral structures terminate in the dorsomedial subdivisions of the trigeminal principal, oral and interpolar subnuclei, and in laminae I, II, and V of the medial medullary dorsal horn, with a much denser projection being distributed to the rostral subnuclei. In addition to the above brain stem trigeminal sensory nuclear complex, they project to the supratrigeminal nucleus, caudal solitary tract nucleus, and paratrigeminal nucleus. These observations agree with previously reported data that the central projection of trigeminal nerve is organized in different manners for the facial and intraoral structures. Furthermore, the present findings in conjunction with our previous studies clarify that the central projection of primary afferents from the facial skin is organized in a clear somatotopic fashion and that the terminal fields of primary afferents from the intraoral structures extensively overlap in the brain stem trigeminal nuclear complex particularly in its rostral subdivisions. The central mechanism of trigeminal nociception is discussed with particular respect to its difference between the facial and intraoral structures.     

The Central Distribution of Primary Afferents from the External Eyelids, Conjunctiva, and Cornea in the Rabbit, Studied Using WGA-HRP and B-HRP as Transganglionic Tracers

We have analyzed the afferent limb of the eyeblink and nictitating membrane response of the rabbit by tracing the central distribution of primary afferents from the periorbital skin, conjunctiva, and cornea using horseradish peroxidase agglutinated to wheat germ (WGA-HRP) or conjugated to choleragenoid (B-HRP) as transganglionic tracers. Afferents in the periorbital skin and conjunctiva distribute most heavily to pars caudalis of the spinal trigeminal nucleus (Vc) and to the dorsal horn of spinal segment C1 (dhC1). These afferents terminate predominantly in laminae IIo and IIi and more weakly to the adjacent laminae I and III. There are much weaker projections to spinal segment C2, rostral Vc, and adjacent reticular formation (laminae IV and V) and to the lateral part of pars interpolaris of the spinal trigeminal nucleus (Vi). No conjunctival primary afferents were seen in the rostral divisions of the trigeminal system. Weak afferent inputs from the periorbital skin are present ventrally in pars oralis of the spinal trigeminal nucleus (Vo) and in the principal trigeminal nucleus (Vp). Corneal afferents distribute most densely in the ventral part of Vi and in islands of neuropil within the trigeminal tract at the level of Vi. They also project to caudal Vc and the adjacent dhC1 in laminae I, II, and III. There are sparse projections to the ventral and dorsal parts of Vp and to the ventral part of Vo. Reticular areas adjacent to ventral Vi also receive a few corneal afferents. WGA-HRP- and B-HRP-labeled terminals were distributed similarly in most areas, but lamina I of Vc received terminals labeled with WGA-HRP and Vp and Vo received cutaneous afferents labeled with B-HRP only. Since all subdivisions of the trigeminal system receive periocular and corneal afferent inputs, we suggest that all these subdivisions may be involved in reflex eyeblinks in the rabbit.     

Organization of HRP-labeled trigeminal mandibular primary afferent neurons in the rat

Horseradish peroxidase (HRP) applied to the transected mandibular division of the trigeminal (V) ganglion was transported anterogradely to pri-mary afferent terminal zones in the dorsal and dorsomedial trigeminal brain-stem nuclear complex (TBNC). Primary V afferents of ganglionic origin were also visible in the ipsilateral cerebellar cortex (crus I and II, paraflocculus) and the dentate, cuneate, solitary, supratrigeminal, and dorsal motor vagal nuclei, parvicellular reticular formation, area postrema and C1–C6 dorsal horn, laminae I–V. Contralateral subnucleus caudalis and C1–C2 dorsal horn were also innervated by primary afferents which crossed in the spinal gray to terminate medially, primarily in laminae I, II, and V. Almost all of these projections were also labeled in various combinations when HRP was applied to individual sensory branches of the mandibular nerve: lingual, infe-rior alveolar, mylohyoid, and auriculotemporal. Transganglionic transport of HRP in the latter four cases revealed strong evidence for mtradivisional somatotopy among the four branches in both the ganglion and TBNC. Cell bodies innervating posterior and/or lateral portions of the head and face (i.e., auriculotemporal and mylohyoid) were found with greater frequency in dor-sal mandibular ganglion regions, while somata supplying more rostral oral-perioral regions (i.e., lingual and inferior alveolar) were predominant ventrally. Components of the mandibular projection to the TBNC were organized topographically in at least some portion of all of its three dimen-sions. Subnuclear preferences were not clear-cut; all four nerves innervated at least some portion of principalis, oralis, interpolaris, and caudalis, save for mylohyoid, which did not project to caudalis. Lingual fibers were most prominent in principalis and oralis, occupied medial portions of the mandib-ular projection to the TBNC, and descended only to rostral caudalis, most notably laminae I-III. Inferior alveolar afferents were ubiquitous in the mandibular component of the TBNC and C1–C2, save for its far lateral bor-der. Mylohyoid terminals were sparse, most prominent in interpolaris, and occupied only dorsolateral TBNC regions and laminae III and IV of C1–C3. The auriculotemporal innervation of the mandibular TBNC was heaviest in interpolaris and was restricted to mostly ventrolateral regions. Its primary focus, however, was laminae III and IV of C1–C4. The clinical implications of this topographical organization are discussed, particularly with respect to the rostrocaudal intradivisional lamination in caudalis and the cervical dorsal horn.     

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.     

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.     

Primary afferent projections from the upper respiratory tract in the muskrat.

The central projections of the ethmoidal, glossopharyngeal, and superior laryngeal nerves were determined in the muskrat by use of the transganglionic transport of a mixture of horseradish peroxidase (HRP) and wheat germ agglutinin (WGA)-HRP. The ethmoidal nerve projected to discrete areas in all subdivisions of the ipsilateral trigeminal sensory complex. Reaction product was focused in ventromedial portions of the principal nucleus, subnucleus oralis, and subnucleus interpolaris. The subnucleus oralis also contained sparse reaction product in its dorsomedial part. Projections were dense to ventrolateral parts of laminae I and II of the rostral medullary dorsal horn, with sparser projections to lamina V. Label in laminae I and V extended into the cervical dorsal horn. A few labeled fibers were followed to the contralateral dorsal horn. The interstitial neuropil of the ventral paratrigeminal nucleus was densely labeled. Extratrigeminal primary afferent projections in ethmoidal nerve cases involved the K?lliker-Fuse nucleus and ventrolateral part of the parabrachial nucleus, the reticular formation surrounding the rostral ambiguous complex, and the dorsal reticular formation of the closed medulla. Retrograde labeling in the brain was observed in only the mesencephalic trigeminal nucleus in these cases. The cervical trunk of the glossopharyngeal and superior laryngeal nerves also projected to the trigeminal sensory complex, but almost exclusively to its caudal parts. These nerves terminated in the dorsal and ventral paratrigeminal nuclei as well as lamina I of the medullary and cervical dorsal horns. Lamina V received sparse projections. The glossopharyngeal and superior laryngeal nerves projected to the ipsilateral solitary complex at all levels extending from the caudal facial nucleus to the cervical spinal cord. At the level of the obex, these nerves projected densely to ipsilateral areas ventral and ventromedial to the solitary tract. Additional ipsilateral projections were observed along the dorsolateral border of the solitary complex. Near the obex and caudally, the commissural area was labeled bilaterally. Labeled fibers from the solitary tract projected into the caudal reticular formation bilaterally, especially when the cervical trunk of the glossopharyngeal nerve received tracer. Labeled fibers descending further in the solitary tract gradually shifted toward the base of the cervical dorsal horn. The labeled fibers left the solitary tract and entered the spinal trigeminal tract at these levels. Retrogradely labeled cells were observed in the ambiguous complex, especially rostrally, and in the rostral dorsal vagal nucleus after application of HRP and WGA-HRP to either the glossopharyngeal or superior laryngeal nerves. In glossopharyngeal nerve cases, retrogradely labeled neurons also were seen in the inferior salivatory nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

The central projection of masticatory afferent fibers to the trigeminal sensory nuclear complex and upper cervical spinal cord

Retrograde and anterograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) conjugate was used to study the organization of primary afferent neurons innervating the masticatory muscles. HRP applied to the nerves of jaw-closing muscles--the deep temporal (DT), masseter (Ma), and medial pterygoid (MP)--labeled cells in the trigeminal ganglion and the mesencephalic trigeminal nucleus (Vmes), whereas HRP applied to nerves of the jaw-opening muscles--anterior digastric (AD) and mylohyoid (My)--labeled cells only in the trigeminal ganglion. Cell bodies innervating the jaw-closing muscles were found with greater frequency in the intermediate region of the mandibular subdivision, while somata supplying the jaw-opening muscles were predominant posterolaterally. The distribution of their somatic sizes was unimodal and limited to a subpopulation of smaller cells. Projections of the muscle afferents of ganglionic origin to the trigeminal sensory nuclear complex (TSNC) were confined primarily to the caudal half of pars interpolaris (Vi), and the medullary and upper cervical dorsal horns. In the Vi, Ma, MP, AD, and My nerves terminated in the lateral-most part of the nucleus with an extensive overlap in projections, save for the DT nerve, which projected to the interstitial nucleus or paratrigeminal nucleus. In the medullary and upper cervical dorsal horns, the main terminal fields of individual branches were confined to laminae I/V, but the density of the terminals in lamina V was very sparse. The rostrocaudal extent of the terminal field in lamina I differed among the muscle afferents of origin, whereas in the mediolateral or dorsoventral axis, a remarkable overlap in projections was noted between or among muscle afferents. The terminals of DT afferents were most broadly extended from the rostral level of the pars caudalis to the C3 segment, whereas the MP nerve showed limited projection to the middle one-third of the pars caudalis. Terminal fields of the Ma, AD, and My nerves appeared in the caudal two-thirds of the pars caudalis including the first two cervical segments, the caudal half of the pars caudalis and the C1 segment, and in the caudal part of the pars caudalis including the rostral C1 segment, respectively. This rostrocaudal arrangement in the projections of muscle nerves, which corresponds to the anteroposterior length of the muscles and their positions, indicates that representation of the masticatory muscles in lamina I reflects an onion-skin organization. These results suggest that primary muscle afferent neurons of ganglionic origin primarily mediate muscle pain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Central projections of primary sensory neurons innervating different parts of the vibrissae follicles and intervibrissal skin on the mystacial pad of the rat.

The cell bodies and central projections of neurons innervating the vibrissae follicles and adjacent skin in the rat were investigated by retrograde and transganglionic transport of HRP. The cell bodies of neurons innervating the vibrissa follicle via the deep vibrissa nerve (DVN) were the largest, followed by those innervating the follicle via the superficial vibrissa nerve (SVN). The smallest cell bodies were those innervating the intervibrissal skin. The DVN neurons terminated centrally as an almost uninterrupted column through the trigeminal sensory nuclear complex. The DVN projections to nucleus caudalis and C1 dorsal horn were entirely restricted to laminae III, IV, and V. Besides the projections to lamina V, the DVN projections were strictly localized somatotopically at all levels replicating the peripheral organization of the vibrissae. The SVNs projected sparsely to midlevels of the main sensory nucleus but not to nuclei oralis and interpolaris. The main SVN projections appeared in laminae I-III of nucleus caudalis. In addition, a small projection to lamina V was observed. The projections to laminae II and III were organized mediolaterally in a similar way as the DVN projections; those to laminae I and V were less restricted. The intervibrissal skin neurons projected sparsely to the caudal main sensory nucleus and to the border between nuclei oralis and interpolaris. The projections to nucleus caudalis were restricted to laminae I-III and V and were organized in a similar way as the SVN projections.     

Morphology of central terminations of intra-axonally stained, large, myelinated primary afferent fibers from facial skin in the rat

Horseradish peroxidase was intra-axonally injected into functionally identified primary afferent fibers within the rat spinal trigeminal tract in order to study the morphology of their central terminations. They were physiologically determined to be large, myelinated, cutaneous primary afferents by means of electrical and mechanical stimulation of their receptive fields. Ninety-three axons that innervated vibrissa follicles, guard hair follicles, and slowly adapting receptors were stained for distances of 4-12 mm at the levels of the main sensory nucleus, spinal trigeminal nucleus, and rostral cervical spinal cord. The collaterals of single axons from these receptors formed terminal arbors in the outer part of the spinal trigeminal nucleus rostral to and near the level of the obex (rostral type collaterals). In the rostral part of the subnucleus caudalis (Vc) they were confined to lamina V (caudalis type collaterals) and in the caudal part of Vc and in cervical segments they were confined to lamina III/IV (spinal-dorsal-horn-type collaterals). There were no transitional forms between the rostral and caudalis types, but there was a transitional form between the caudalis and spinal dorsal horn types. This transitional form was distributed in laminae III/IV and V. The terminal arbors of the rostral type of collaterals formed an interrupted, rostrocaudally oriented column like those seen in the lumbar dorsal horn, but the column shifted down to lamina V near the obex, and more caudally, gradually shifted upward to lamina III. Major morphological differences were not observed among the three different functional types of collaterals with respect to the rostrocaudal distribution of collaterals, and the shape and location of collaterals. The differential laminar distribution of collateral arbors of single axons along the rostrocaudal axis distinguishes the spinal trigeminal nucleus from the spinal dorsal horn where functional types of mechanoreceptive afferents form continuous or interrupted sagittal columns of terminal arbors that do not shift dorsoventrally within segments.     

Central connections of the lingual-tonsillar branch of the glossopharyngeal nerve and the superior laryngeal nerve in lamb

Afferent and efferent central connections of the lingual-tonsillar branch of the glossopharyngeal nerve (LT-IX) and the superior laryngeal nerve (SLN) in the lamb were traced with horseradish peroxidase (HRP) histochemistry. After entering the brainstem, most LT-IX and SLN afferent fibers turned caudally in the solitary tract (ST). Some afferent fibers of LT-IX terminated in the medial nucleus of the solitary tract slightly caudal to their level of entry. The remaining fibers projected to the dorsolateral, ventrolateral, and interstitial areas of the nucleus of the solitary tract (NST) at the level of the area postrema. Superior laryngeal nerve afferent fibers terminated extensively in the medial and ventral NST at levels near the rostral pole of the area postrema. Further caudal, near the level of obex, SLN afferent terminations were concentrated in the region ventrolateral to the ST and in the interstitial NST. The caudal extent of LT-IX and the rostral extent of SLN terminals projected to similar levels of the NST, but only a relatively small proportion of the total projections overlapped. Lingual-tonsillar and SLN fibers also coursed rostrally to terminate in the caudal pons within and medial to the dorsomedial principal sensory trigeminal nucleus. Other labeled afferent fibers traveled caudally in the dorsal spinal trigeminal tract to terminate in the dorsal two-thirds of the spinal trigeminal nucleus at the level of obex. Large numbers of labeled cells with fibers in the LT-IX or SLN were located in the ipsilateral rostral nucleus ambiguus and surrounding reticular formation. Fewer labeled cells were observed in the inferior salivatory nucleus following HRP application to either the LT-IX or SLN. The LT-IX and SLN projections to areas of the NST associated with upper airway functions, like swallowing and respiration, suggest an important role for these two nerves in the initiation and control of airway reflexes.     

Corneal and periocular representation within the trigeminal sensory complex in the cat studied with transganglionic transport of horseradish peroxidase

The central projections of afferent fibers from the cornea, and the infraorbital, infratrochlear, frontal, lacrimal and auriculotemporal nerves were investigated by means of the transganglionic transport of horseradish peroxidase. Afferent projections to the dorsal horn of the medulla are organized along both the rostrocaudal axis and the ventrolateral to dorsomedial margin of the medullary dorsal horn. An inverted but discontinuous facial representation exists through the rostrocaudal axis of the dorsal horn of the medulla with perioral and nasal receptive fields innervated by the infraorbital and infratrochlear nerves represented rostral to the progressively more posterior receptive fields innervated by the frontal, lacrimal and auriculotemporal nerves, respectively. The organization of the primary afferents is not uniform over the laminae of the dorsal horn of the medulla; the projections from the different nerves show the least overlap in lamina II, while overlap is most extensive in laminae I and V. The sensory projection from the cornea to the medullary dorsal horn is most dense in laminae I and II. All nerves, including those innervating the cornea, project to the interpolar, oral and principal trigeminal nuclei and are somatotopically organized. Projections to the reticular formation and the contralateral trigeminal sensory complex were not found in this study. These results support the organization of the dorsal horn of the medulla proposed by Déjerine (1914) and show that this organization is most evident for the primary afferent projections to lamina II.     

Topographic organization of central terminal region of different sensory branches of the rat mandibular nerve

The central projection of primary neurons comprising the auriculotemporal nerve, cutaneous branch of the mylohyoid nerve, inferior alveolar nerve, mental nerve, lingual nerve, and buccal nerve was investigated using transganglionic transport of HRP in young rats. In view of the topographic organization of central projection fields, the nerves were divided into two groups; i.e., those projecting to the dorsolateral margin of the trigeminal nucleus principalis, subnucleus oralis, and interpolaris (the auriculotemporal, mylohyoid, and mental nerves) and those projecting more medially (the inferior alveolar, lingual, and buccal nerves). The former group of nerves projected more caudally than the latter in the medullary and spinal dorsal horn complex rostral to the 3rd cervical segment, in general. Furthermore, the latter group projected to the nucleus of the solitary tract and the supratrigeminal and paratrigeminal nuclei, whereas the other nerves did not. The data indicate the following points: Primary neurons innervating the intraoral structures terminate medial (in trigeminal nucleus principalis and subnucleus oralis) and ventral (in subnucleus interpolaris) to the terminal fields of those innervating the facial skin. Primary neurons innervating the intraoral structures project to the nucleus of the solitary tract and the supra- and paratrigeminal nuclei, whereas those innervating the facial skin do not. Primary neurons innervating the periphery of the face project to the spinal dorsal horn and those innervating the intra/perioral region project to medullary dorsal horn, though this segregation from the medulla to the 3rd cervical segment is relatively loose. Only those trigeminal primary neurons, whose receptive fields extend to or beyond the midline, project to the contralateral dorsal horn from the medulla to the 3rd cervical segment.     

A horseradish peroxidase study of the central projections of primary trigeminal neurons innervating the hard palate in the cat

The central projections of primary sensory neurons innervating the hard palate in the cat were studied by transganglionic transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP). Following injection of WGA-HRP in the incisive papilla terminal labeling was observed in all subdivisions of the sensory trigeminal nuclear complex. In the main sensory nucleus labeling was located in the dorsal part, especially in its most rostral portion. At the levels of nuclei oralis and interpolaris labeling was observed along the medial borders of the nuclei. In addition, at these levels distinct terminal labeling was located in patches within the trigeminal tract. In nucleus caudalis terminal labeling was confined to laminae I, II and V of the most rostral part of the nucleus. Some terminal labeling was observed also in the mid part of the solitary tract nucleus. After WGA-HRP injection in the posterior part of the hard palate a similar labeling pattern was found, but no labeling was observed in the solitary tract nucleus. The results in general indicate a relatively diffuse somatotopic organization of primary afferents innervating the palate. However, the somatotopic organization of palatine afferents within nucleus caudalis is at least partly consistent with the view that the central representation of the oral cavity is rotated 90 degrees to that of extraoral areas.     

Corneal sensory pathway in the rat: a horseradish peroxidase tracing study

The methods of transganglionic transport of horseradish peroxidase (HRP) and horseradish peroxidase--wheat germ agglutinin (HRP-WGA) were used to determine the location within the trigeminal ganglion of the primary afferent neurons that innervate the rat central cornea, and the brainstem and spinal cord termination sites of these cells. In each of 18 animals, solutions of HRP or HRP-WGA were applied to the scarified corneal surface and allowed to infiltrate into the corneal epithelium and stroma for 15 minutes. Postmortem examination of the corneal whole mounts from the experimental animals, and of corneas and neural tissues from several control animals, showed that the HRP/HRP-WGA remained confined to the central cornea with no spread into adjacent intra- or extraorbital tissues. HRP-labeled corneal afferent somata were located in the dorsal part of the ophthalmic region of the ipsilateral trigeminal ganglion. The central fibers of the corneal afferent neurons projected very heavily to interstitial nuclei of Cajal in the spinal tract of V at the level of caudal pars interpolaris and rostral pars caudalis, lightly to the pars caudalis/C1 transition zone, and sparsely to the dorsal horn of spinal cord segments C1-C3. The trigeminal main sensory nucleus, pars oralis, the rostral three-fourths of pars interpolaris, and an extensive midregion of pars caudalis were totally devoid of reaction product. Terminal fields in caudal pars caudalis and in the spinal cord dorsal horn were concentrated largely in the outer half of lamina II, with lesser accumulations in lamina I, the deeper half of lamina II, and in lamina III. The present study demonstrates for the first time by means of an anatomical tracing procedure the brainstem termination sites of corneal afferent neurons in the rat. The patchy, discontinuous nature of the corneal afferent projection to the caudal trigeminal brainstem nuclear complex (TBNC), and the total lack of corneal projections to rostral subdivisions of the TBNC, provide an exception to the general rule of trigeminal organization in which most areas of the head and face are represented as continuous columns throughout the rostrocaudal extent of the ipsilateral TBNC.     

Topographic representation of lower and upper teeth within the trigeminal sensory nuclei of adult cat as demonstrated by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the lower and upper tooth pulps within the trigeminal sensory nuclear complex (TSNC). HRP injections were made into the inferior and superior alveolar nerves in order to compare the central projections of the whole nerve with those from tooth pulps. In addition, the relationship between the distribution of the trigeminothalamic tract cells and the projection sites of the tooth pulp afferents was investigated by injecting HRP into the posterior ventral thalamus. HRP-labeled tooth pulp afferent fibers innervating the lower and upper teeth projected to the subnucleus dorsalis (Vpd) of pars principalis, the rostrodorsomedial part (Vo.r) and nucleus dorsomedialis (Vo.dm) of pars oralis, the medial regions of pars interpolaris, and laminae I, II, and V of pars caudalis. Terminal fields of the lower tooth pulp afferents formed a rostrocaudally running, uninterrupted column from the midlevel of Vpd to the caudal tip of caudalis. In contrast, the column of termination of upper tooth pulp afferents was discontinuous at the Vpd/Vo.r transition, and ended at the more rostral level of the caudalis than that of the lower tooth pulp afferents. The representation of the lower and upper teeth in the TSNC was organized in a somatotopic fashion which varied from one subdivision to the next, although terminal zones of the inferior and superior alveolar nerves overlapped within the Vo.r, Vo.dm, and dorsomedial part of rostral pars interpolaris. The lower and upper teeth were represented in the Vpd, Vo.r, Vo.dm, medial region of pars interpolaris, and laminae I, II, and V, in a ventrodorsal or caudorostral, dorsoventral, lateromedial, dorsoventral, and mediolateral or dorsomedial-ventrolateral sequence, respectively. The smaller, more focal terminal areas of the teeth contrasted sharply with more extensive terminal fields of the alveolar nerves. The HRP injections within the thalamus indicated that neurons in Vpd, the caudal pars interpolaris, and laminae I/V of caudalis, which are subdivisions of TSNC that receive pulpal projections, sent their axons to the ipsilateral and contralateral posterior ventral thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)     

The corticotrigeminal projection in the cat. A study of the organization of cortical projections to the spinal trigeminal nucleus

The projection from the cerebral cortex to the spinal trigeminal nucleus has been studied light microscopically in adult cats. Both orthograde degeneration and orthograde intra-axonal labeling techniques have been applied. Our results indicate that the projection from the coronal gyrus (face area of primary somatosensory cortex) to the spinal trigeminal complex is somatotopically organized. In subnucleus caudalis this somatotopy is organized dorsoventrally and appears to match the somatotopic distribution of the divisional trigeminal afferents. Hence cortical fibers originating from the posterior coronal gyrus (upper representation) project ventrolaterally into caudalis where division I trigeminal afferents terminate. Likewise cortical fibers from the anterior coronal gyrus (jaw and tongue representation) terminate dorsomedially in caudalis to overlap with division III trigeminal afferents. In contrast, the distribution of corticofugal afferents to the rostral spinal trigeminal subnuclei (pars interpolaris and oralis) is organized mediolaterally. Therefore in these subnuclei the cortical projection does not appear to overlap the dorsoventral lamination of the divisional trigeminal afferents. In addition, our results suggest that the cortical projection to subnucleus caudalis includes fibers which terminate in the marginal zone (lamina I) and its extensions into the spinal trigeminal tract (the interstitial cells of Cajal). We have been unable to document a projection from the proreate gyrus to the spinal trigeminal complex.     

Ascending and descending internuclear projections within the trigeminal sensory nuclear complex

The cells of origin of ascending and descending internuclear pathways in the trigeminal sensory nuclear complex were studied by the method of retrograde transport of horseradish peroxidase in the cat. The cells of origin of the ascending internuclear pathways are distributed in all laminae of the caudal part of the spinal trigeminal nucleus (Vc) except for lamina II and the caudal regions of the pars interpolaris of the spinal trigeminal nucleus (Vi). The cells arising from the Vc project to all rostral trigeminal nuclei except the caudal Vi and dorsal part of the principal trigeminal nucleus (Vpd), and neurons of the caudal Vi project to the dorsomedial (Vo.dm) and rostrodorsomedial (Vo.r) divisions of the spinal trigeminal nucleus and the ventral part of the principal trigeminal nucleus (Vpv), although the main ascending fibers from the Vc arise from laminae III-V and project to the rostral Vi and pars oralis. By contrast, the cells of origin of the descending internuclear pathways are distributed in all trigeminal nuclei, with chain-like connections between the neighboring nuclei, while the caudal regions of the Vi and laminae I-II do not receive any descending projections. The main ascending fibers from the paratrigeminal nucleus (or interstitial nucleus) at the caudal level of the Vi project to the parabrachial nucleus. These findings indicate that the internuclear pathways are differentially organized between the ascending and descending projections, and suggest that the internuclear trigeminal connections have a smaller influence on the trigeminothalamic tract cells in the Vpd, caudal Vi, and lamina I.     

Projections of the dorsal raphe nucleus to the brainstem: PHA-L analysis in the rat

Early studies that used older tracing techniques reported exceedingly few projections from the dorsal raphe nucleus (DR) to the brainstem. The present report examined DR projections to the brainstem by use of the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). DR fibers were found to terminate relatively substantially in several structures of the midbrain, pons, and medulla. The following pontine and midbrain nuclei receive moderate to dense projections from the DR: pontomesencephalic central gray, mesencephalic reticular formation, pedunculopontine tegmental nucleus, medial and lateral parabrachial nuclei, nucleus pontis oralis, nucleus pontis caudalis, locus coeruleus, laterodorsal tegmental nucleus, and raphe nuclei, including the central linear nucleus, median raphe nucleus, and raphe pontis. The following nuclei of the medulla receive moderately dense projections from the DR: nucleus gigantocellularis, nucleus raphe magnus, nucleus raphe obscurus, facial nucleus, nucleus gigantocellularis-pars alpha, and the rostral ventrolateral medullary area. DR fibers project lightly to nucleus cuneiformis, nucleus prepositus hypoglossi, nucleus paragigantocellularis, nucleus reticularis ventralis, and hypoglossal nucleus. Some differences were observed in projections from rostral and caudal parts of the DR. The major difference was that fibers from the rostral DR distribute more widely and heavily than do those from the caudal DR to structures of the medulla, including raphe magnus and obscurus, nucleus gigantocellularis-pars alpha, nucleus paragigantocellularis, facial nucleus, and the rostral ventrolateral medullary area. A role for the dorsal raphe nucleus in several brainstem controlled functions is discussed, including REM sleep and its events, nociception, and sensory motor control. © Wiley-Liss, Inc.