Acute and chronic sectioning of fifth lumbar spinal nerve has equivalent effects on the primary afferents of sciatic nerve in rat spinal cord

The mechanism of neuropathic pain may be associated with sprouting of uninjured primary afferents of peripheral nerves into regions of the spinal cord denervated through peripheral injury. However, this remains controversial. Therefore, the purpose of the present investigation was, first, to determine in detail the central distributions of the unmyelinated primary afferents of each of the L4, L5, and L6 components of sciatic nerve, then to assess the distribution of afferent sciatic terminals following acute and chronic injury to (L5) nerve. First, we injected isolectin B4 (IB4), into the sciatic nerves in three groups of rats, each of which had two of the three L4, L5, or L6 components ligated and cut, and the one remaining, uninjured. Although the terminal labelling found in the L5 segment of the spinal cord originated from the L5 component, some terminal labelling remained in cases when either the L4 or L6 component was intact. Second, tracers transported in predominantly unmyelinated (IB4 and WGA‐HRP) or myelinated (cholera toxin subunit B) nerves were injected into the sciatic nerve following acute or chronic (21‐day) injury restricted to the L5 component. In each case, the central distribution of nerve terminals in the spinal dorsal horn was equivalent following either acute or chronic injury to the L5 component. Consequently, these data provide no support for the suggestion that neuropathic pain in spinal ligation model results from uninjured L4 and L6 components sprouting to occupy sites vacated by the injured L5 component of the sciatic nerve. J. Comp. Neurol. 517:481–492, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

Distribution and central projections of primary afferent neurons that innervate the masseter muscle and mandibular periodontium: a double-label study

A double-label strategy was used to determine the distribution and central projections of primary afferent neurons that innervate the periodontium and muscles of mastication in cats. Central injections of either Fast Blue (FB) or a mixture of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) and HRP were made into one of three cytoarchitectonically distinct regions of the spinal trigeminal nucleus. These regions included the subnucleus oralis (Vo), the subnucleus interpolaris (Vi), and the medullary dorsal horn (MDH). In each case, injections were also made into the periodontium of the ipsilateral mandibular teeth or into the ipsilateral masseter muscle. FB injections preceded the peroxidase injections by at least 48 hours and total survival time ranged from 72 to 96 hours. Animals were perfused with phosphate-buffered paraformaldehyde (4%; pH 7.2). Serial frozen sections were made through the brainstem and trigeminal ganglion. Tetramethylbenzidine was used as a chromagen to demonstrate HRP and sections were viewed with brightfield and epifluorescent illumination. Cells containing peripherally injected tracer were observed in the lateral portion of the ganglion and in the mesencephalic nucleus (Vmes). Double-labeled ganglion cells were observed in most cats that received periodontal injections in combination with central injections in the dorsal part of spinal trigeminal nucleus regardless of the rostrocaudal level of the central injection. In the animals that received intramuscular injections, double-labeled ganglion cells were observed only in the animals that received central injections caudal to the Vo. Double-labeled Vmes perikarya were observed in cats that received either intramuscular or periodontal injections in combination with central injections into the MDH and Vo but not in animals that received injections into the Vi. These results demonstrate that ganglion cell periodontal afferents project to the three major rostrocaudal subdivisions of the spinal trigeminal nucleus while ganglion cell muscle afferents have more limited central projections to caudal regions of the nucleus. Masseter and periodontal Vmes afferents also project ot the spinal trigeminal nucleus--specifically, to the Vo and MDH. These findings are consistent with physiological observations regarding the role of periodontal and masseteric afferents in oral and facial reflexes and somesthetic mechanisms.     

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.     

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 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.     

Localization of trigeminal, spinal, and reticular neurons involved in the rat blink reflex

Electrical stimulation of the supraorbital nerve (SO) induces eyelid closure by activation of orbicularis oculi muscle motoneurons located in the facial motor nucleus (VII). Neurons involved in brainstem central pathways implicated in rat blink reflex were localized by analyzing c-Fos protein expression after SO stimulation in conjunction with tracing experiments. A retrograde tracer (gold-horseradish peroxidase [HRP]) was injected into the VII. The distribution patterns of activated c-Fos-immunoreactive neurons and of neurons exhibiting both c-Fos immunoreactivity and gold-HRP labeling were determined in the sensory trigeminal complex (STC), the cervical spinal cord (C1), and the pontomedullary reticular formation. Within the STC, c-Fos immunoreactivity labeled neurons in the ipsilateral ventral part of the principal nucleus, the pars oralis and interpolaris, and bilaterally in the pars caudalis. Colocalization of gold-HRP and c-Fos immunoreactivity was observed in neurons of ventral pars caudalis layers I-IV and ventral pars interpolaris. In C1, SO stimulation revealed c-Fos neurons in laminae I-V. After additional injections in VII, the double-labeled c-Fos/gold-HRP neurons were concentrated in laminae IV and V. Although c-Fos neurons were found throughout the pontomedullary reticular formation, most appeared rostrally around the motor trigeminal nucleus and in the ventral parvocellular reticular nucleus medial to the fiber bundles of the seventh nerve. Caudally, c-Fos neurons were in the lateral portion of the dorsal medullary reticular field. In addition, these reticular areas contained double-labeled neurons in electrically stimulated rats that had received gold-HRP injections in the VII. The presence of double-labeled neurons in the STC, C1, and the reticular formation implies that these neurons receive sensory information from eyelids and project to the VII. These double-labeled neurons could then be involved in di- or trisynaptic pathways contributing to the blink reflex.     

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.     

Central distribution of synaptic contacts of primary and secondary jaw muscle spindle afferents in the trigeminal motor nucleus of the cat

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The present study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological characteristics of the physiologically classified afferents at the light and electron microscopic levels through the use of the intra-axonal horseradish peroxidase (HRP) injection technique. The effects of SCh on stretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty-eight and 61 afferents were classified as group Ia and group II afferents, respectively. The central projections of 17 intra-axonally stained afferents (10 group Ia and 7 group II afferents) were examined. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II afferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, the proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II afferents. The ultrastructure of serially sectioned afferent boutons (63 group Ia and 72 group II boutons) also was examined. The boutons from the two groups were distributed widely from the soma to small-diameter dendrites, but the frequency of synaptic contacts on proximal dendrites was higher in group Ia than group II afferents. The present study provides evidence that the two groups of jaw muscle spindle afferents differ in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons. J. Comp. Neurol. 391:50–63, 1998. © 1998 Wiley-Liss, Inc.     

Central projection of unmyelinated (C) primary afferent fibers from gastrocnemius muscle in the guinea pig

We have demonstrated the central projections of muscle C or group IV afferent fibers in the guinea pig by tracing arborizations in the spinal cord. C afferent fibers from the gastrocnemius muscle (GCM) were electrophysiologically identified by conduction velocity (less than 1 m/second). A single neuron in the lumbar 5 dorsal root ganglion (L5 DRG) was intracellularly labeled with Phaseolus vulgaris leucoagglutinin (PHA-L). After iontophoretic injection of PHA-L, we processed the lumbar cord and L5 DRG for PHA-L immunohistochemistry. Six muscle C afferent fibers from 40 animals were labeled, and whole trajectories were recovered. Labeled fibers were reconstructed by tracing of the arbor in serial parasagittal sections. The GCM C afferents projected rostrocaudally for two or three segments and ran at the surface of the dorsal funiculus, giving off collaterals into laminae I and II and sometimes into parts of lamina III. We determined, based on the branching pattern and form of the terminal plexus, that the branching of muscle C afferent fibers showed an intermediate pattern that fell morphologically between the terminal patterns of somatic and visceral afferents. The numbers and sizes of fiber swellings and terminal swellings were measured on all collateral branches. We found that the area of distribution of the terminal swellings of muscle C afferent fibers is larger than that of somatic terminals but that the density of terminal swellings in the terminal area was lower than that of the somatic terminals.     

Spatial localization and projection densities of brainstem mossy fibre afferents to the forelimb C1 zone of the rat cerebellum

The present study uses a double retrograde tracer technique in rats to examine the spatial localization and pattern of axonal branching in mossy fibres arising from three major sources in the medulla-the external cuneate nucleus, the sensory trigeminal nucleus and the reticular formation, to two electrophysiologically-identified parts of the cerebellar cortex that are linked by common climbing fibre input - the forelimb-receiving parts of the C1 zone in lobulus simplex and the paramedian lobule. In each experiment a small injection of rhodamine-tagged beads was injected into one cortical region and an injection of fluorescein-tagged beads was injected into the other region. The main findings were: (i) the proportion of double-labelled cells in each of the three precerebeller sources of mossy fibres was positively correlated with those in the inferior olive; and (ii) the C1 zone in lobulus simplex was found to receive a greater density of projections from all three sources of mossy fibres than the C1 zone in the paramedian lobule. These data suggest that two rostrocaudally separated but somatotopically corresponding parts of the C1 zone receive common mossy fibre and climbing fibre inputs. However, the differences in projection densities also suggest that the two parts of the zone differ in the extent to which they receive mossy fibre signals arising from the same precerebellar nuclei. This implies differences in function between somatotopically corresponding parts of the same cortical zone, and could enable a higher degree of parallel processing and integration of information within them.     

Expression of vesicular glutamate transporters,VGluT1 and VGluT2, in axon terminals of nociceptive primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns of the rat

We examined immunohistochemically whether the vesicular glutamate transporters (VGluTs), VGluT1 and VGluT2, might be expressed in synaptic terminals of nociceptive primary afferent fibers within laminae I and II of the medullary and spinal dorsal horns of the rat. VGluT1 immunoreactivity (IR) was intense in the inner part of lamina II but weak in lamina I and the outer part of lamina II. VGluT2-IR was most intense in lamina I and the outer part of lamina II. Expression of VGluTs in synaptic terminals was confirmed by dual immunofluorescence histochemistry for VGluTs and synaptophysin. Expression of VGluTs in axon terminals of primary afferent fibers terminating in laminae I and II was also confirmed immunohistochemically after unilateral dorsal rhizotomy. The dual immunofluorescence histochemistry indicated expression of VGluTs in substance P (SP)-containing axon terminals in lamina I and the outer part of lamina II. Electron microscopy confirmed the coexpression of VGluTs and SP in axon terminals within laminae I and II; VGluTs was associated with round synaptic vesicles at the asymmetric synapses. It was further observed that isolectin IB4, a marker for unmyelinated axons, often bound with VGluT2-immunopositive structures but rarely with VGluT1-immunopositive structures in lamina II. Thus, the results indicated in laminae I and II of the medullary and spinal dorsal horns that both VGluT1 and VGluT2 were expressed in axon terminals of primary afferent fibers, including SP-containing nociceptive fibers and that VGluT in unmyelinated primary afferent fibers terminating in lamina II was primarily VGluT2.     

Organization of the projections from the trigeminal brainstem complex to the superior colliculus in the rat and hamster: anterograde tracing with Phaseolus vulgaris leucoagglutinin and intra-axonal injection

Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and intra-axonal recording and injection techniques were employed to describe the projection from the trigeminal (V) brainstem complex to the deep laminae of the superior colliculus (SC) in the hamster and the rat. The organization of these projections was the same in the two species. Deposits of PHA-L into V nucleus principalis (PrV) produced labelled axons and boutonlike swellings in the lower stratum griseum intermediale (SGI) and upper stratum album intermedium (SAI) in the SC bilaterally. Plots of boutonlike swellings indicated that the terminals of this projection were arrayed in clusters. Nucleus principalis also projected to the stratum griseum profundum (SGP) and stratum album profundum (SAP). This deeper projection did not terminate in clusters and it was most prominent in the lateral SC. The ipsilateral PrV-SC projection appeared to arise mainly from axons that recrossed the midline at the level of the SC commissure. Reconstruction of individual PHA-L labelled fibers demonstrated that single axons gave rise to terminals on both sides of the midline. Deposits of PHA-L into V subnucleus interpolaris (SpI) yielded results that were identical to those obtained with PrV injections with one exception: none of these deposits produced any labelled terminals in the ipsilateral SC. Deposits of PHA-L into V subnucleus caudalis (SpC) produced only sparse labelling in SC. Most labelled swellings were located in the SGP and SAP and they were visible only in the SC contralateral to the PHA-L injection site. Single axons arising from cells in SpI were recorded and injected with horseradish peroxidase (HRP) in the hamster's SC. These fibers all responded to stimulation of multiple mystacial vibrissae and gave rise to 2-5 clusters of bouton-like swellings in the lower SGI and upper SAI.     

A morphological evidence of direct connections from the cochlear nuclei to tensor tympani motoneurons in the cat: a possible afferent limb of the acoustic middle ear reflex pathways

The central circuitry of the acoustic middle ear reflex activating the tensor tympani muscle was studied in the cat by tracer methods using horseradish peroxidase (HRP), wheat germ agglutinin (WGA) or HRP conjugated with WGA (WGA-HRP). The results indicate that the dorsal and ventral cochlear nuclei send fibers to motoneurons innervating the tensor tympani muscle, bilaterally with an ipsilateral dominance.     

Morphology of central terminations of intra-axonally stained, low-threshold mechanoreceptive primary afferent fibers from oral mucosa and periodontium in the rat

Horseradish peroxidase (HRP) was injected intra-axonally 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 afferent fibers from periodontium or oral mucosa by means of electrical and mechanical stimulation of their receptive fields. Twenty-eight axons that innervated the periodontium of incisors and 21 axons that innervated the oral mucosa were stained for distances of 2-5 mm from the injection sites at the levels of the main sensory nucleus (Vms), spinal trigeminal nucleus and rostral cervical spinal cord. The collaterals of these primary afferent fibers formed terminal arbors in the medial or dorsomedial part of the Vms, and the oral and interpolar spinal trigeminal nuclei (Vo and Vi). In the caudal spinal trigeminal nucleus (Vc), the collaterals of one half of the periodontium afferent fibers terminated mainly in lamina V at the rostral and middle levels of Vc. On the other hand, the collaterals of the other half of the periodontium afferent fibers terminated mainly in lamina IV at the rostral level of Vc, and rostrally these terminal areas shifted to the most medial part of Vi. The collaterals of mucosa afferent fibers terminated in lamina V at the rostral level of Vc, and these terminal areas shifted gradually to laminae III and IV as the parent axons traveled more caudally. These shifts were staggered rostrocaudally according to the rostrocaudal locations of the receptive fields. The density of collaterals of periodontium afferent fibers in Vi was significantly larger than that of mucosa afferent fibers. The average size of the varicosities of periodontium afferent fibers was significantly larger than those of mucosa afferent fibers in Vo, Vi and Vc. The average number of varicosities belonging to single collaterals of slowly-adapting periodontium afferent fibers in Vi were significantly larger than those in Vo. In Vi, the average number of varicosities of single collaterals of slowly-adapting periodontium afferent fibers were significantly larger than those of rapidly-adapting periodontium afferent fibers.     

The primary somatosensory cortex and the insula contribute differently to the processing of transient and sustained nociceptive and non‐nociceptive somatosensory inputs

Transient nociceptive stimuli elicit consistent brain responses in the primary and secondary somatosensory cortices (S1, S2), the insula and the anterior and mid‐cingulate cortex (ACC/MCC). However, the functional significance of these responses, especially their relationship with sustained pain perception, remains largely unknown. Here, using functional magnetic resonance imaging, we characterize the differential involvement of these brain regions in the processing of sustained nociceptive and non‐nociceptive somatosensory input. By comparing the spatial patterns of activity elicited by transient (0.5 ms) and long‐lasting (15 and 30 s) stimuli selectively activating nociceptive or non‐nociceptive afferents, we found that the contralateral S1 responded more strongly to the onset of non‐nociceptive stimulation as compared to the onset of nociceptive stimulation and the sustained phases of nociceptive and non‐nociceptive stimulation. Similarly, the anterior insula responded more strongly to the onset of nociceptive stimulation as compared to the onset of non‐nociceptive stimulation and the sustained phases of nociceptive and non‐nociceptive stimulation. This suggests that S1 is specifically sensitive to changes in incoming non‐nociceptive input, whereas the anterior insula is specifically sensitive to changes in incoming nociceptive input. Second, we found that the MCC responded more strongly to the onsets as compared to the sustained phases of both nociceptive and non‐nociceptive stimulation, suggesting that it could be involved in the detection of change regardless of sensory modality. Finally, the posterior insula and S2 responded maximally during the sustained phase of non‐nociceptive stimulation but not nociceptive stimulation, suggesting that these regions are preferentially involved in processing non‐nociceptive somatosensory input. Hum Brain Mapp 36:4346–4360, 2015. © 2015 Wiley Periodicals, Inc.     

Organization and origin of the connection from the inferior to the superior colliculi in the rat

The inferior colliculus (IC) is the main ascending auditory relay station prior to the superior colliculus (SC). The morphology and origin of the connection from inferior to superior colliculus (I-SC) was analyzed both by anterograde and retrograde tracing. Irrespective of the subregion of the IC in which they originate, the terminal fields of these connections formed two main tiers in the SC. While the dorsal one primarily involved the stratum opticum and the stratum griseum intermediale, the ventral one innervated the deep strata, although some fibers did connect these tiers. While the dorsal tier occupied almost the whole extension of the SC, the ventral one was mostly confined to its caudomedial quadrant. The fiber density in these tiers decreased gradually in a rostral gradient and the terminal fields became denser as the anterograde tracer at the injection site was distributed more externally in the cortex of the IC. Retrograde tracing confirmed this result, although it did not reveal any topographic ordering for the I-SC pathway. Most presynaptic boutons of the I-SC terminal field were located either inside or close to the patches of acetylcholinesterase activity. Together with previous anatomical and physiological studies, our results indicate that the I-SC connection relays behaviorally relevant information for sensory-motor processing. Our observation that this pathway terminates in regions of the superior colliculus, where neurons involved in fear-like responses are located, reinforce previous suggestions of a role for the IC in generating motor stereotypes that occur during audiogenic seizures.     

Intraganglionic distribution of the primary afferent neurons in the frog glossopharyngeal nerve and its transganglionic projection to the rhombencephalon studied by HPR method

Anterograde transport of horseradish peroxidase (HRP) along the bullfrog IXth nerve was studied 6–16 days after application of HRP to the cut end of either the IXth nerve trunk or its distal 2 branches. The jugular and IXth nerve ganglia attached to the rhombencephalon were removed after fixation and serial sections of 50 υm in thickness were stained by the Graham and Karnovsky method. Of all the primary afferent neurons in the IXth nerve, 62% of the cell bodies were distributed within the IXth nerve ganglion, the remaining 38%, within the jugular ganglion. Similar distribution was found with the cells belonging to each of the IXth nerve branches. A part of the transganglionic IXth nerve fibers entering the medulla oblongata ascended to the cerebellar peduncle while the majority descended along the fasciculus solitarius. Some of the descending fibers in the fasciculus extended to the dorsal field of the spinal cord at the third spinal nerve, while some others run to join the descending tract of trigeminal nerve.     

Distribution of primary afferent fibers projecting from hindlimb cutaneous nerves to the medulla oblongata in the cat and rat

The dorsal column nuclear complex, one of the most important relays for tactile perception, has well been known to be somatotopically organized. Topographical arrangements of terminal sites of individual cutaneous nerves within the dorsal column nuclei, however, have not been examined systematically, although many studies have been done upon primary afferents to the medulla oblongata, including the dorsal column nuclear complex. Thus, in the present study, distribution of primary afferent fibers projecting from the hindlimb cutaneous nerves to the medulla oblongata was examined in the cat and rat by means of the transganglionic transport method with horseradish peroxidase. Cutaneous primary afferent fibers projecting from the hindlimb to the medulla oblongata were distributed mainly in the ipsilateral gracile nucleus. Terminal labeling in the gracile nucleus was seen at all rostrocaudal levels of the nucleus, occasionally including the nuclear part straddling the midline (the median or accessory nucleus). The labeled axon terminals in the gracile nucleus were more densely distributed in the middle and caudal parts of the nucleus than in the rostral part. Although the fields of termination of the hindlimb cutaneous nerves overlapped in the gracile nucleus, the foci of the terminal labeling of the nerves innervating the distal parts of the hindlimb were located more medially or dorsomedially than those of the nerves innervating the proximal parts. Terminal labeling was further found in a small zone immediately medial to the rostromedial border of the external cuneate nucleus. This hitherto undescribed zone (U zone) contained a small cluster of medium-sized neurons in the cat. Although no particular cell cluster was found in the U zone of the rat, convergence of the primary afferent fibers of the cutaneous nerve from the hindlimb appeared to occur as in the U zone of the cat.