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.     

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.     

Spinal and trigeminal dorsal horn projections to the parabrachial nucleus in the rat

We studied afferents to the parabrachial nucleus (PB) from the spinal cord and the spinal trigeminal nucleus pars caudalis (SNVc) in the rat by using the anterograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into medial PB retrogradely labeled neurons in the promontorium and in lamina I of the dorsal rostral SNVc, while injections into lateral PB and the K?lliker-Fuse nucleus retrogradely labeled neurons in these areas as well as in lamina I throughout the caudal SNVc and spinal dorsal horn. Injections of WGA-HRP into the caudal SNVc and dorsal horn of the spinal cord resulted in terminal labeling in the dorsal, central, and external lateral subnuclei of PB and the K?lliker-Fuse nucleus, all of which are known to receive cardiovascular and respiratory afferent information. Injections of WGA-HRP into the promontorium and dorsal rostral SNVc resulted in terminal labeling in the same PB subnuclei, as well as in the medial and the ventral lateral PB subnuclei, which are sites of relay for gustatory information ascending from the medulla to the forebrain. The spinal and trigeminal projection to PB may mediate the convergence of pain, chemosensory, and temperature sensibilities with gustatory and cardiorespiratory systems in PB.     

Oral and facial representation within the medullary and upper cervical dorsal horns in the cat

Transganglionic transport of HRP was used to study the patterns of termination of somatic afferent fibers innervating oral and facial structures within the trigeminal nucleus caudalis and upper cervical dorsal horn of the cat. In separate animals, the superior alveolar, pterygopalatine, buccal, inferior alveolar, lingual, frontal, corneal, zygomatic, infraorbital, mental, mylohyoid, and auriculotemporal branches of the trigeminal nerve were traced in this experiment. The organization of the primary afferents innervating the oral structures is not uniform across laminae and at different rostrocaudal levels of the nucleus caudalis. The superior alveolar and pterygopalatine nerves mainly terminate in laminae I, II, and V at the level of the rostral one-third of the caudalis. By contrast, the lingual, inferior alveolar, and buccal nerve terminate in laminae I-V of, respectively, the rostral third, the entire length, and caudal two-thirds of the caudalis. In addition, the lingual, buccal, and pterygopalatine nerves terminate in the dorsal and middle parts of the interstitial islands or pockets of lamina I neuropil extending to the rostral levels parallel to the nucleus interpolaris. Mediolaterally, in laminae I, II, and V of the rostral third an extensive overlap of projections was found between the branches from each trigeminal division, and some overlap was observed between projections from the mandibular and maxillary divisions. On the other hand, the projections of primary afferents innervating the facial structures are arranged in a somatotopic fashion in rostrocaudal and mediolateral axes over the laminae (I-IV) through the nucleus caudalis and upper cervical dorsal horn. Fibers from the perioral and perinasal regions terminate most rostrally in caudalis, and fibers from progressively more posterior facial regions terminate at successively lower levels. A mediolateral somatotopic arrangement was observed, with fibers from the ventral parts of face ending in the medial regions and fibers from the progressively more dorsal parts of the face ending in successively more lateral regions of the medullary and upper cervical dorsal horns. Corneal afferent terminals are concentrated in the outer parts of lamina II at the levels of the rostral parts of the caudal two-thirds of the caudalis and the interstitial islands of lamina I. The maxillary division terminates first at the most caudal level of the caudalis, followed by the ophthalmic division descending as far as the C2 segment and the mandibular division reaching the most caudal level of the C2 segment.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

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.     

An HRP study of the central projections of primary trigeminal neurons which innervate tooth pulps in the cat

Trigeminal ganglia and brain stem of adult cats were studied following HRP injections into tooth pulps or after exposure of the cut end of the inferior alveolar nerve to HRP. Ipsilateral ganglion cells within a wide range of sizes were labeled in both experimental situations, whereas no labeled cells were observed in the contralateral ganglion in any animal. Labeled central branches of tooth pulp and inferior alveolar neurons were observed in all subdivisions of the ipsilateral trigeminal sensory complex. Terminal labeling in the tooth pulp experiments was confined to the dorsomedial parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was restricted to the medial halves of laminae I, IIa and V of the medullary dorsal horn. In the inferior alveolar nerve experiments dense terminal labeling was observed in the dorsal parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was located throughout laminae I to V in contrast to the tooth pulp experiments. Neither of the two experimental situations offers any evidence for a bilateral or contralateral brain stem projection of primary trigeminal neurons.     

Development and plasticity in hamster trigeminal primary afferent projections

At birth (gestational day 16), the hamster infraorbital nerve projects to the appropriate portion of the brainstem, though the projection lacks adult-like internal organization (patchiness). Infraorbital nerve damage at this time does not produce appreciable transganglionic atrophy in the central projections of the infraorbital nerve, but it does result in a failure to develop normal infraorbital primary afferent patches. Such damage also produces a more widespread central projection of spared mandibular afferents into regions occupied by 'regenerate' infraorbital terminals (J. Comp. Neurol., 235 (1985) 129-143). In the present study, transganglionic transport techniques were again used to show that, by postnatal day 5 (gestational day 21), rostrocaudally continuous aggregates of horseradish peroxidase-labelled infraorbital terminals are visible throughout the trigeminal brainstem nuclear complex. This aggregation pattern is nearly adult-like and isomorphic with the distribution of the mystacial vibrissae on the face. A similar infraorbital lesion performed on postnatal day 5, however, markedly decreased the density of the adult central projection of the infraorbital nerve to subnuclei principalis, oralis, interpolaris, and the magnocellular laminae of caudalis. The projection to superficial laminae of caudalis and the cervical dorsal horn was maintained. A postnatal-day-5 infraorbital lesion also failed to produce a more widespread central projection from spared mandibular primary afferents. These data suggest a relationship between the postnatal maturity of trigeminal primary afferents and the response of damaged and undamaged trigeminal afferents to infraorbital nerve transection in hamster. The similarity in the central primary afferent response to lesions at equivalent gestational times (postnatal days 5 and 0, respectively) in hamster and rat, suggests that this plasticity gradient may be a general characteristic of mammalian trigeminal primary afferents.     

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.     

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.     

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)     

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.     

Central projections of sensory innervation of the rat superficial temporal artery

Elucidating the central sensory projection pathways of extra- and intracranial vessels appears to be of fundamental importance for understanding the pathogenetic mechanisms of primary headaches. In this paper, two kinds of tracers, choleragenoid (cholera toxin subunit b, CTb) and wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP), were used to transganglionically label the central sensory projections of the innervation of the superficial temporal artery (STA). Following either of the tracers applied on the adventitia of the STA, labelled terminations were found mainly in the ipsilateral C1-C3 spinal dorsal horns. Sparse labelling was also found in the interpolar and caudal parts of the spinal trigeminal nucleus. In the spinal cord, CTb labelled profiles were mainly located in laminae III and IV, whereas WGA-HRP labelled profiles were mainly located in laminae I and II. In the medulla, CTb but not WGA-HRP labelled terminals were found in a small dorsolateral extension of the cuneate nucleus. The present results indicate that the primary sensory nervous center of the STA is located in the rostral cervical spinal dorsal horn. The caudal parts of the spinal trigeminal nucleus, which has been demonstrated as a center of pain and temperature sensations of the head and face, transmits limited information from the STA to higher nervous centers.     

An HRP study of the central projections from primary sensory neurons innervating the rat masseter muscle

Retrograde and transganglionic transport of horseradish peroxidase has been used to study the cell bodies of origin and the central projections of neurons innervating the rat masseter muscle. Labeled cell bodies were observed both in the trigeminal ganglion and in the mesencephalic trigeminal nucleus. Major central projections from mesencephalic trigeminal neurons were traced to the supratrigeminal nucleus and to the brainstem reticular formation. Smaller projections from these neurons could be followed to the borders of the solitary tract and hypoglossal nuclei as well as to lamina V of nucleus caudalis and corresponding areas in the dorsal horn at C₁−C₂ spinal cord segments. Labeling from trigeminal ganglion neurons was observed close to the trigeminal tract in all subdivisions of the trigeminal sensory nuclear complex and in the dorsal horn lamina I at C₁ and C₂ levels.     

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

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

Connections of the posterior region of the thalamus in rats]

Using retrograde and anterograde tracing methods we have studied in the posterior region of the thalamus of the rat the distribution of: (1) the terminal fields of the main afferents arising from somatosensory centers (dorsal column nuclei, interpolar trigeminal subnucleus, somatosensory cortex), motor centers (red nucleus, motor cortex) and multimodal structures (deep layers of the superior colliculus, zona incerta, cingular cortex) and of (2) the neurons giving rise to the main efferents towards the sensorimotor cortex, the red nucleus, the deep layers of the superior colliculus and the zone incerta. The overlap of the retrograde and anterograde labeling reveals a relatively homogeneous region. Considering however the cortical connections, three different subdivisions can be distinguished: a caudal pole completely devoid of cortical connections, a medial subdivision receiving cortical afferents from the sensorimotor and cingulate cortices and a rostral pole reciprocally connected with the sensorimotor cortex. Therefore the rostral pole would be the only part of this region which should be included in the thalamus.     

An anatomical demonstration of projections to the medullary dorsal horn (trigeminal nucleus caudalis) from rostral trigeminal nuclei and the contralateral caudal medulla

This study demonstrates that the medullary dorsal horn (MDH), the most caudal subdivision of the spinal trigeminal nucleus, receives input from neurons located in the trigeminal main sensory nucleus, the more rostral subdivisions of the spinal trigeminal nucleus, and the contralateral MDH. Using the retrograde transport of horseradish peroxidase (HRP), we show here that the MDH receives ipsilateral projections from rostral trigeminal nuclei but not from adjacent areas of the retricular formation. The rostral pole of spinal trigeminal nucleus oralis (nucleus oralis, pars beta) contains the highest density of MDH projection neurons. In addition, the MDH on one side receives projections from contralateral MDH neurons located in layers I, III, IV, V, VII and VIII but not from neurons in layers II and VI. We conclude that: (1) specific subdivisions of rostral trigeminal nuclei send projections to the MDH that could modulate the activity of MDH neurons; (2) projections from trigeminal nuclei to layers V and VI of the MDH, but not from adjacent areas of the reticular formation, provide further evidence that these deeper layers are related functionally to the MDH and trigeminal sensory processes; and (3) several populations of MDH neurons send axons across the midline into the contralateral MDH and may mediate contralateral inhibitory effects.     

The organization of the neonatal rat's brainstem trigeminal complex and its role in the formation of central trigeminal patterns

The present study delimits the relationship of primary trigeminal afferents to their targets, the brainstem trigeminal nuclei of the neonatal rat. Previously, the brainstem trigeminal complex of the rat has been subdivided on the basis of either cytoarchitectonics or patterns of succinic dehydrogenase activity into the principal sensory nucleus and the three subnuclei of the spinal trigeminal nucleus, oralis, interpolaris, and caudalis. In this paper, we demonstrate that each of these subdivisions can also be identified by its pattern of primary trigeminal afferents. In addition, we demonstrate that the terminations of these afferents are distributed in a punctate fashion which correlates with vibrissae-related patterns of histochemical staining. Further, vibrissae removal in the neonatal rat at any age studied results in a corresponding deafferentation of both the principal sensory nucleus and all subnuclei of the spinal trigeminal nucleus. This same procedure has a graded, age-dependent effect on the vibrissae-related pattern of cytochrome oxidase staining in somatosensory cortex. On this basis, we conclude that vibrissae-related pattern formation in the central trigeminal system can be best understood in terms of a single "sensitive" period for the entire system. We hypothesize that this is the period during which an interaction normally occurs between primary trigeminal afferents and target neurons of the principal sensory nucleus.     

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.     

Organization of cortical, basal forebrain, and hypothalamic afferents to the parabrachial nucleus in the rat.

In a previous study (Herbert et al., J. Comp. Neurol. [1990];293:540-580), we demonstrated that the ascending afferent projections from the medulla to the parabrachial nucleus (PB) mark out functionally specific terminal domains within the PB. In this study, we examine the organization of the forebrain afferents to the PB. The PB was found to receive afferents from the infralimbic, the lateral prefrontal, and the insular cortical areas; the dorsomedial, the ventromedial, the median preoptic, and the paraventricular hypothalamic nuclei; the dorsal, the retrochiasmatic, and the lateral hypothalamic areas; the central nucleus of the amygdala; the substantia innominata; and the bed nucleus of the stria terminalis. In general, forebrain areas tend to innervate the same PB subnuclei from which they receive their input. Three major patterns of afferent termination were noted in the PB; these corresponded to the three primary sources of forebrain input to the PB: the cerebral cortex, the hypothalamus, and the basal forebrain. Hypothalamic afferents innervate predominantly rostral portions of the PB, particularly the central lateral and dorsal lateral subnuclei. The basal forebrain projection to the PB ends densely in the external lateral and waist subnuclei. Cortical afferents terminate most heavily in the caudal half of the PB, particularly in the ventral lateral and medial subnuclei. In addition, considerable topography organization was found within the individual projections. For example, tuberal lateral hypothalamic neurons project heavily to the central lateral subnucleus and lightly to the waist area; in contrast, caudal lateral hypothalamic neurons send a moderately heavy projection to both the central lateral and waist subnuclei. Our results show that the forebrain afferents of the PB are topographically organized. These topographical differences may provide a substrate for the diversity of visceral functions associated with the PB.