Medial, intralaminar, and lateral terminations of lumbar spinothalamic tract neurons: a fluorescent double-label study

A dorsolateral spinothalamic tract (DSTT), consisting primarily of lamina I neurons, was confirmed in the cat lumbar spinal cord by the use of thalamic injections of fluorescent dyes combined with selective thoracic spinal cord lesions. In addition, collateralization of spinothalamic tract (STT) terminations to medial, lateral, and intralaminar thalamic regions was investigated by injections of two different fluorescent dyes into pairs of these regions. The results of this study indicate that less than 15% of cat lumbar STT neurons collateralize to more than one of the thalamic regions evaluated. Lumbar lamina I cells project to the lateral and to the medial thalamus (13% collateralize to these two regions) and have only a scant projection to the intralaminar thalamus. Lumbar laminae IV-VI STT cells are very few in cat and demonstrate almost no collateralization to multiple thalamic areas. Neurons of laminae VII-X project equally to the three thalamic regions evaluated, and approximately 10-14% of cells from this laminar group collateralize to any two of the thalamic sites evaluated.

Excitatory pathways from Forel's field H to head elevator motoneurones in the cat

Projections of neurones in Forel's field H (FFH) to the upper cervical cord and to the lower brainstem were demonstrated by retrograde labelling of the neurones with horseradish peroxidase (HRP). Systematic threshold mapping for evoking antidromic spikes of FFH neurones revealed that they projected to the neck motor nuclei and to pontomedullary reticular formation (PMRF). Stimulation of FFH evoked large monosynaptic excitatory postsynaptic potentials (EPSPs) in reticulospinal neurones (RSNs) of the PMRF, and mono- and disynaptic EPSPs in the dorsal neck motoneurones. Above EPSPs were evoked from areas confined to FFH, thus indicating that they were elicited by stimulation of FFH neurones. Monosynaptic EPSPs in motoneurones were small but disynaptic EPSPs were markedly facilitated following stimulation with train pulses, becoming several times larger than the monosynaptic EPSPs. Disynaptic EPSPs were supposed to be relayed by RSNs in the PMRF which are known to project to dorsal neck motoneurones. The mono- and disynaptic EPSPs were induced chiefly in motoneurones of the head elevator (m. biventer cervicis and complexus) and rarely of the neck lateral flexor (m. splenius). It was suggested that FFH neurones are involved in the control of vertical head movements.

Organization of the spinocervicothalamic pathway in the rat

We used silver degeneration techniques to examine the termination of the spinocervical and cervicothalamic tracts in rats. Lesions of the dorsal portion of the lateral funiculus (DLF) of the spinal cord produced degeneration of a relatively small number of ascending fibers that were seen within the most lateral portion of the DLF rostral to the lesion. Within the lateral cervical nucleus, the degeneration was more extensive mediolaterally and of a finer caliber. Such labeling is attributable to the degeneration of fine fibers and terminals. Degenerating processes could be seen in apposition to neurons in the lateral cervical nucleus. At all levels of the cord, the lateral spinal nucleus was devoid of terminal labeling following lesions of the DLF. No terminal degeneration could be seen within the DLF at levels rostral to the lateral cervical nucleus. Lesions of the DLF at either midcervical or lower thoracic levels produced degeneration throughout the lateral cervical nucleus. This finding suggests that the lateral cervical nucleus of the rat is not somatotopically organized. Lesions of the lateral cervical nucleus produced degeneration of a small number of fibers within the contralateral midbrain and thalamus. Within the mesencephalon, degenerating fibers and terminals were seen primarily in the intercollicular region and the deep layers of the superior colliculus. Less degeneration was found in the lateral portion of the central gray. Within the diencephalon, a small area of termination was located in the ventromedial part of the rostral portion of the medial geniculate nucleus. A prominent termination was present in a restricted area within the caudal fourth of the ventrobasal complex.

Segregation of lemniscal inputs and motor cortex outputs in cat ventral thalamic nuclei: application of a novel technique

A double labeling method that permits accurate delineation of the terminals of medial lemniscal fibers was used to determine whether thalamic neurons projecting to motor cortex in the cat are in a position to be contacted by such terminals. Thalamic neurons in the VL nucleus were retrogradely labeled by injections of fluorogold placed in the cytoarchitectonically defined area 4, while lemniscal axons and their terminal boutons were anterogradely labeled, in a Golgi-like manner, from injections of Fast Blue placed under physiological control in different parts of the contralateral dorsal column nuclei. In additional experiments, spinothalamic fibers were similarly labeled by injections of Fast Blue in the spinal cord. The results reveal that there is no significant overlap in the distributions of lemniscal terminals and motor cortex-projecting neurons and that no somata or proximal dendrites of motor cortex-projecting neurons are in a position to receive lemniscal terminals. Spinothalamic terminals, on the other hand, end in clusters around motor cortex-projecting neurons in the VL nucleus as well as in other nuclei and are a more likely route for short latency somatosensory inputs to the motor cortex.

[Extra-lemniscal afferent projections of dorsal spinal cord nuclei to the ventrobasal nuclear complex structure in the contralateral optic thalamus]

It has been found that section of half the midbrain tegmentum in cats failed to prevent the afferent somatosensory projections from the foreleg to the ventrobasal nuclear complex of the contralateral thalamus. Specific evoked responses to the stimulation of the contralateral foreleg were recorded in this structure. These specific EP have the same latency as "lemniscal responses" (4-5 ms) and diminish the amplitude and duration of both components of the responses. Simultaneously, we have observed terminal axonal degeneration into the ventrobasal nuclear complex of the thalamus 5-7 days after the section of the contralateral midbrain tegmentum, using the electron microscopy method. All the results obtained indicate that the dorsal column nuclei have extra-lemniscal afferent connections with ventrolateral nuclear complex of the contralateral thalamus. These connections ascend in the back parts of the brainstem ipsilaterally to the corresponding pair of the dorsal column nuclei and rostrally to the midbrain on the contralateral side.

Topographic organization of convergent projections to the thalamus from the inferior colliculus and spinal cord in the rat

The purpose of this study was to identify thalamic areas receiving convergent sensory inputs from acoustic and spinal projection systems in the rat. The topographic distribution of afferents to the thalamus from the inferior colliculus and spinal cord was examined by using WGA-HRP as an anterograde axonal tracer. Following injections in the inferior colliculus, terminal labeling was present in ventral, medial, and dorsal divisions of the medial genicuate body (MGB) and in adjacent areas of the posterior thalamus, including the posterior limitans nucleus, the posterior intralaminar nucleus, the marginal zone, the peripeduncular region, the lateral or parvicellular part of the subparafascicular nucleus, and a region intercalated between the posterior limitans nucleus and the suprageniculate nucleus. In the caudal thalamus spinal projections remained in the reticular formation medial to the collicular terminal field. At intermediate levels of the MG, however, the spinal projection began to overlap the collicular field, terminating in the medial division of the MG and in the posterior intralaminar nucleus, the marginal zone, the lateral subparafascicular nucleus, and the area between the suprageniculate and posterior limitans nuclei. More rostrally, the convergent field expanded to include aspects of the dorsal MG division. The extent to which afferent projections to the thalamus from the inferior colliculus and spinal cord converge is thus graded in the caudorostral plane, with the greatest overlap occurring at the level of the rostral third of the MGB. These observations identify potential areas of acoustic and somesthetic integration and may account for observations of neuronal plasticity in the thalamus in response to the pairing of acoustic and somesthetic inputs.

Organization of anterogradely labeled spinocervical tract terminations in the lateral cervical nucleus of the cat

The anterograde transport of horseradish peroxidase following injections into the cervical, thoracic, or lumbosacral spinal cord was used to examine the organization of spinocervical tract terminations in the lateral cervical nucleus (LCN) of the cat. A somatotopic organization of the labeling originating from different spinal levels was observed in the mediolateral dimension. Cervical labeling generally occurred in the ventromedial portion and lumbosacral labeling in the dorsolateral portion of the LCN. Thoracic labeling occurred both in the middle and the most lateral edge of the nucleus. In all cases, labeling was distributed over most of the rostrocaudal extent of the LCN. In addition, distinct patches of labeling were present in the medialmost portion of the nucleus, regardless of the spinal level injected. These observations corroborate the topographical organization of the LCN described previously on the basis of physiological and retrograde labeling data, and support the identification of the medialmost part of the LCN as a distinct portion of the nucleus. Terminal labeling in the LCN always occurred in multiple, longitudinally distributed fields. The afferent input to each terminal field coursed in separate, loose bundles of fibers that descended from the superficial dorsolateral funiculus. Large injections resulted in more extensive, overlapping terminal fields. These observations indicate that collateral projections result in several discrete representations of a given portion of the skin over the longitudinal extent of the LCN, but that topographical relationships are longitudinally maintained. It is suggested that these multiple terminal fields are the anatomical correlate of the functionally selective convergence of spinocervical tract terminations, that has previously been postulated on physiological grounds to explain the generation of LCN receptive fields with homogenous receptor input within a somatotopic framework.

The spinothalamic tract: an examination of the cells of origin of the dorsolateral and ventral spinothalamic pathways in cats

The locations of spinothalamic neurons and the funicular trajectories of their axons were studied in cats by retrograde transport of horseradish peroxidase (HRP). Five animals were used as controls to determine the cervical and lumbar laminar distributions of neurons contributing to the spinothalamic tract. An additional eight animals were used to determine the funicular trajectories of the spinothalamic axons of lumbar neurons by utilizing a series of thoracic spinal cord lesions in conjunction with retrograde transport of HRP from the sensory thalamus. Three of these animals underwent midthoracic ventral quadrant lesions, four animals underwent midthoracic dorsolateral funiculus lesions, and one animal underwent total spinal cord transection sparing the dorsal columns. The locations of the cells containing the HRP reaction product were then determined after a 3- to 5-day survival time, and the patterns of labeled cell locations of the lesion groups were compared to the control group patterns. In the lesioned animals, the cervical spinothalamic cell locations were used as a control to confirm the uniformity of the injection sites, transport and tissue processing. The major finding of this study is that there exist two distinct components of the spinothalamic tract. The dorsolateral spinothalamic tract (DSTT) is made up of axons originating in contralateral spinal cord lamina I and has negligible contribution from the deeper spinal cord laminae. The axons of lamina I cells cross segmentally and ascend exclusively in the dorsolateral funiculus (DLF). The DSTT comprises approximately 25% of the total spinothalamic input from the lumbar enlargement. The ventral spinothalamic tract (VSTT) is made up of axons originating in spinal cord laminae IV-V and VII-X. Very few lamina I cells contribute axons to the VSTT. This crossed pathway ascends in the ventrolateral and ventromedial portions of the spinal cord. No cells contributing to the spinothalamic tract were identified in spinal cord segments caudal to a dorsal column sparing lesion, indicating that there are no spinothalamic tract axons traveling in the dorsal columns. These results expand the classical concept of information processing by the spinothalamic tract. The DSTT is made up of lamina I cell axons. All lamina I spinothalamic cells respond exclusively to noxious peripheral stimuli. Hence the DSTT is a major nociceptive-specific ascending spinal pathway, yet lies outside the confines normally assigned to the spinothalamic tract.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical evidence for a spinothalamic pathway co-containing cholecystokinin- and galanin-like immunoreactivities in the rat

Using indirect immunofluorescence technique combined with retrograde tracing as well as surgical lesions, a system of spinothalamic neurons containing both galanin- and cholecystokinin-like immunoreactivity has been defined. The cell bodies are located in the lumbar segments L1-L5 with a preferential localization dorsal to the central canal at rostral levels and lateral to the canal at caudal levels. The cells project via the ventral part of the lateral funiculus to the most ventral and posterior parts of thalamus. Here a distinct, varicose terminal network was seen extending caudally from an area lateral to the medial lemniscus, running medially over the medial lemniscus, traversing the parafascicular nucleus and running dorsal to the fasciculus retroflexus into the periventricular gray matter. Transection of various parts of the spinal cord as well as retrograde tracing experiments indicate that the spinothalamic galanin cholecystokinin system represents a crossed pathway. The present results demonstrate that a spinothalamic system can be characterized by its content of galanin- and cholecystokinin-like peptides, two putative messenger molecules. It is only a minor component of the total spinothalamic projection.

Spinal afferents and cortical efferents of the anterior intralaminar nuclei: an anterograde-retrograde tracing study

The topographical relations among the terminal field of spinothalamic fibers and the cells projecting upon areas 4 and 5 were studied in the anterior intralaminar nuclei of the cat. Terminals anterogradely labeled from the spinal cord and cell populations retrogradely labeled from the lateral pericruciate and anterior suprasylvian cortex were simultaneously observed by means of a multiple fluorescent tracing strategy. The present findings confirm that spinal afferents in the central lateral and paracentral nuclei overlap with the cells projecting to area 4. Further, the present data demonstrate that spinal terminals are largely segregated from the intralaminar cell population projecting to area 5.

The overlap of spinothalamic and dorsal column nuclei projections in the ventrobasal complex of the rat thalamus: a double anterograde labeling study using light microscopy analysis

Projections from the spinal cord and the dorsal column nuclei (DCN) to the ventrobasal complex of the thalamus (VB) were studied in the rat by using double anterograde labeling strategy. This strategy was based on the injection of 3H-leucine into the DCN and of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the spinal cord and their subsequent transport. Adjacent 30-micron-thick sections were then processed differentially for autoradiography or for HRP by using tetramethyl benzidine (TMB) as a chromogen. Similar areas of the ventrobasal complex were labeled, in adjacent sections, after a large injection of 3H-leucine into the DCN and when wheat germ agglutinin-HRP had been injected in any part of the spinal cord. If, however, a small injection of the radioactive tracer was centered in the gracile nucleus and compared with an injection of WGA-HRP placed in the lumbar enlargement of the cord, the rostral and dorsal portions of the lateral VB were labeled from both sources. On the other hand, if tritiated leucine was injected into the cuneate nucleus, and WGA-HRP placed in the cervical enlargement, then the caudal and ventral portions of the lateral VB demonstrated overlap of both labels. The present results show that, in the rat, areas of termination of both the spinothalamic tract and the lemniscal pathway originating from the DCN overlap in the lateral VB. This overlap is somatotopically organized, thus indicating that the same area of the VB receives somatic inputs from one particular part of the body through both pathways. These results are discussed in comparison to those of comparable studies performed in the cat and in the monkey and with reference to the electrophysiological data that have demonstrated that, in the rat VB, neurons responding to noxious stimulation are intermingled with neurons exclusively responding to non-noxious stimulation.

A dorsolateral spinothalamic pathway in cat

A spinothalamic tract that courses in the dorsolateral funiculus of the spinal cord and originates almost exclusively from spinal lamina I neurons has been demonstrated in the cat by retrograde transport of horseradish peroxidase. This tract is of special interest because the course of this predominantly lamina I, contralateral projection lies outside the classical course of the spinothalamic tract and because most lamina I cells contributing to the spinothalamic tract have been shown by other investigators to respond exclusively to somatic noxious stimuli. This newly described tract has important implications in the processing of noxious stimuli.

Nociceptive pathways: anatomy and physiology of nociceptive ascending pathways

In primates, the principal nociceptive pathways ascend in the anterolateral quadrant of the spinal cord. Among these, the spinothalamic tract (s.t.t.) is the best studied. Cells in Rexed's laminae I and V project to the ventro-posterolateral (v.p.l.) thalamic nucleus. Other cells in the same and deeper laminae terminate in the intralaminar complex. Spinothalamic tract cells may be nociceptive-specific or multireceptive. Those ending in v.p.l. have restricted, contralateral receptive fields, whereas those projecting to the intralaminar region often have large, bilateral receptive fields. Spinoreticular tract (s.r.t.) cells are concentrated in laminae VII and VIII and may be nociceptive. It is proposed that the s.t.t. contributes to sensory-discriminative processing of pain and that the s.t.t. and s.r.t. play a role in the motivational-affective components of pain. Alternative nociceptive pathways are the spinocervical and postsynaptic dorsal column tracts.

Superficial dorsal horn of the adult human spinal cord

Golgi studies in the adult human spinal cord reveal 10 cell types in the first three laminae. Five are Golgi Type II or ipsilateral proprioneurons of short or long range--the latter including Waldeyer cells. Several of the cells in this group have dendrites that help to form interlaminar boundaries on the gray-white boundary. Two of the four cell types in Lamina II have dendritic fields that correspond exactly to the primary afferent terminal axonal fields described in the cat by Rethelyi (1977). Three cell types, one in each lamina, can be tentatively homologized with monkey spinothalamic cells described by other authors. Our previously described classification method based on dendritic patterns suggests that the Golgi Type II interneurons and ipsilateral proprioneurons belong to two different cell families (and Waldeyer cells to a third), whereas the putative spinothalamic neurons are all different cell types.

Tracing of afferent and efferent pathways in the left inferior cardiac nerve of the cat using retrograde and transganglionic transport of horseradish peroxidase

Retrograde and transganglionic transport of horseradish peroxidase (HRP) was used to trace afferent and efferent pathways in the left inferior cardiac nerve of the cat. Cardiac efferent and afferent neurons were located, respectively, in the stellate ganglion (average cell count per experiment:2679) and in the ipsilateral dorsal root ganglia (DRG) from C8 to T9 (average cell count per experiment:213). Labeled cardiac afferent projections to the spinal cord were most dense in segments T2-T6 where they were located in Lissauer's tract and in lamina 1 on the lateral border of the dorsal horn. Labeled afferent axons extended ventrally through lamina 1 into lamina 5 and the dorsolateral region of lamina 7 in proximity to the intermediolateral nucleus. A weak projection was noted on the medial side of the dorsal horn. These sites of termination are similar to projections by other sympathetic afferent pathways (i.e. renal, hypogastric and splanchnic nerves) to the lower thoracic and lumbar spinal cord, indicating that visceral afferents may have a uniform pattern of termination at various segmental levels. This pattern of termination in regions of the gray matter containing spinothalamic tract neurons and neurons involved in autonomic mechanisms is consistent with the known functions of sympathetic afferent pathways in nociception and in the initiation of autonomic reflexes.

Diencephalic connections of the raphé nuclei of the rat brainstem: an anatomical study with reference to the somatosensory system

The present study was undertaken to analyse in detail the connections of the various raphé nuclei with thalamic structures. Micropipettes filled with an aqueous solution of wheat-germ agglutinin conjugated to horseradish peroxidase were used to produce small iontophoretic deposits restricted to the various raphé nuclei in male Sprague Dawley albino rats. Tetramethyl benzidine was used as a chromogen to reveal both fiber terminals anterogradely labelled and retrogradely filled neurons. A detailed discussion of the possible cases of artefactual labelling using this technique is given. The present study confirms the results obtained previously in the cat that indicate that the various raphé nuclei project to different areas of the diencephalon. Related to the somatosensory system, the B3 area (nucleus raphé magnus) projects to the nucleus submedius and anterior intralaminar nuclei known to receive spinothalamic inputs, but not to the ventrobasal complex. The distribution of afferents from this nucleus suggests an innervation primarily of thalamic structures involved in the somatosensory system. The nucleus raphé medianus projects to the ventrobasal complex and the nucleus submedius , but the fact that its projections are widespread, including all thalamic sensory "relay" nuclei and the entire nucleus reticularis thalami, suggests that it could participate in a "nonspecific" system of control of different sensory modalities. The nucleus raphé dorsalis generally does not project to the thalamic nuclei believed to be involved in the somatosensory system.

Collateralization in the spinothalamic tract: new methodology to support or deny phylogenetic theories

Components of the spinothalamic system that ascend in the anterolateral funiculus are reviewed. The presence of collateralization in this system in mammals is discussed with regard to theories of the phylogenetic development of pathways. The major theory investigated suggested that collateralization is an intermediate stage between a multisynaptic pathway and a direct non-collateralized lemniscus. The evidence and theories are reviewed. Methods for confirming or rejecting this theory are discussed. The literature reporting ascending spinal projections for non-mammalian vertebrates is reviewed. Certain reptiles have projections analogous to both the mammalian neospinothalamic and paleospinothalamic tracts. The presence of spinothalamic projections in elasmobranchs and amphibians is still controversial. Confirmation of earlier reports of projections in salamander and dogfish shark based on degeneration techniques have not been done. In addition, results from too few species of these classes have been reported. However, it is possible that paleospinothalamic connections are present in some species (e.g. salamander, nurse shark) and not in others (e.g. frog, dogfish shark) of the same class. Spinothalamic projections have not been reported for teleosts. A plea for new research in this area is made.

Spinal neurons reaching the lateral reticular nucleus as studied in the rat by retrograde transport of horseradish peroxidase

An anatomical technique based on the retrograde transport of horseradish peroxidase (HRP) was used to investigate the projections of spinal cord neurons to the lateral reticular nucleus (LRN). Labeled cells were found at all spinal levels and in particular large numbers in cervical and lumbar segments. Various spinal areas gave rise to cells of origin of this tract, which appears to be more prominent than any other tract previously studied with a similar approach. Labeling common to all spinal segments was observed in (1) ventromedial parts of both intermediate zone and ventral horn (laminae VII, VIII and X), mainly contralaterally; (2) the reticular extension of the neck of the dorsal horn, partly bilateral; and (3) superficial layers of the dorsal horn and nucleus of the dorsolateral funiculus (NDLF), mainly contralateral and projecting essentially to the lateral zone of the LRN. Additional labeling was observed at cervical and lumbar levels, each with specific qualities: (1) the cervical enlargement, which displayed labeling in the central part of the ipsilateral intermediate zone (lamina VII); (2) the rostral lumbar levels, which had projections from the contralateral median portion of the neck of the dorsal horn. These latter projections appear to be specific to pathways reaching the lateral reticular nucleus and the inferior olive. Control injections in neighboring structures demonstrated the similarity between the afferents to the lateral reticular nucleus and the inferior olive. Control injections in neighboring structures demonstrated the similarity between the afferents to the lateral reticular nucleus and the inferior olive (except lamina I and NDLF projections) and the differences between these afferents and those projecting to the dorsal reticular formation, i.e., the nucleus reticularis ventralis.

Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

Previous studies have shown that ascending somatic sensory pathways arising from the dorsal column nuclei, lateral cervical nucleus and spinothalamic tract terminate in parts of the thalamus adjacent to those which receive cerebellar terminations. This termination pattern creates a border between the ventroposterolateral nucleus (VPL) and the ventrolateral nucleus (VL) in the cat and between the caudal and oral parts of VPL (VPLc and VPLo, respectively) in the monkey. Since it is not clear how sharp these borders are, a double orthograde labeling strategy was used in the present study to make direct comparisons of the projections to the thalamus from these sources of input. It was found that there was a change in the sources of afferent input to the different target areas that paralleled changes in cytoarchitecture. Moving caudally to rostrally, VPL in the cat and VPLc in the monkey received projections predominantly from the middle, dorsal (clusters) portion of the dorsal column nuclei. These projections were gradually replaced near the VPL-VL border in the cat and VPLc-VPLo border in the monkey first by input from the lateral cervical nucleus (cat only) and the rostral and ventral portions of the dorsal column nuclei and then by spinothalamic projections. Towards VL in the cat and the rostral parts of VPLo in the monkey (referred to as Vim by Hassler, '59 and Mehler, '71), these projections were in turn replaced by those from the cerebellum. This sequence resulted in a complex pattern (summarized in Fig. 10) where some thalamic territories received input predominantly from one source and others received converging input from several sources. The major region receiving converging ascending somatic sensory and cerebellar terminations was located at the border between VPL and VL in the cat and in the caudal parts of Olszewski's ('52) VPLo in the monkey (that is, between VPLc and Vim). In general, the results in the cat were similar to those in the monkey. One notable difference was that the domain containing terminals from the cerebellum and the rostral-ventral parts of the dorsal column nuclei was located medially between VPLc and Vim in the monkey, whereas it extended across the entire mediolateral border between VPL and VL in the cat. In both species, thalamic neurons received input predominantly from one afferent source and only minor input, if any, from other sources.(ABSTRACT TRUNCATED AT 400 WORDS)

The spinothalamic tract in the primate: a re-examination using wheatgerm agglutinin conjugated to horseradish peroxidase

The sites of termination of the primate spinothalamic tract have been reinvestigated using the anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase. Monkeys which received an injection of the conjugate at the spinal cervical level (C7-C8) displayed a "patchy" pattern of labelling in the coronal plane in the ventral posterior lateral and caudal ventrolateral nucleus. In three dimensional reconstructions this labelling appeared to be rod-like in shape. A more homogeneous pattern of labelling was present in parts of the central lateral, posterior, suprageniculate, limitans, submedius, medial dorsal, paracentral, central medial, reuniens and periventricular nucleus. Lumbar injections (L2-L3) produced a similar although less intense pattern of labelling with only the ventral posterior lateral and ventrolateral nuclei displaying an obvious topological organization. Comparison of these results with previous physiological and pharmacological reports suggests several morphological-functional correlations: first, that both the discriminative and motivational/arousal aspects of spinothalamic tract function, associated with the lateral and medial thalamic nuclei, respectively, may be conveyed by direct spinothalamic tract projections. In support of this hypothesis medial spinothalamic tract termination sites receive a homogeneous input which does not have an obvious topographical organization, whereas lateral spinothalamic tract termination sites receive a "patchy" pattern of terminals which are topographically organized; second, that the patchy pattern of labelling observed in the coronal plane in the lateral thalamus corresponds to a "rodlike" pattern of labelling in three dimensions. This "rodlike" pattern of labelling has previously been observed for medial lemniscal projections to the thalamus and has been postulated to be the thalamic equivalent of cortical "columns"; third, that there appears to be a tight overlap between spinothalamic tract terminals and opiate receptor binding in some medial but not lateral thalamic nuclei. Such an overlap may be indicative of a pharmacological difference in the types of spinothalamic tract inputs which could be modulated by opiates at the thalamic level.

The terminations of the spinothalamic tract in the cat

The termination sites of the spinothalamic tract (STT) in the cat have been examined using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Cats which received an injection of WGA-HRP at the spinal cervical level (C7-C8) displayed anterograde labelling in the ventral posterior lateral (VPL), ventrolateral, lateral posterior, limitans, suprageniculate, medial geniculate pars magnocellularis, central lateral, paracentral, centrum medianum, submedius, central medial, reuniens, rhomboid and periventricular thalamic nuclei. Lumbar injection (L2-L3) produced a similar although less intense pattern of labelling, with only the VPL displaying an obvious topological organization. Although the STT terminations in the cat appear to be less dense than those observed in the rat or monkey the present results demonstrate that in the cat a wide variety of thalamic nuclei receive a direct STT projection and that the cat VPL, like rat, monkey and man, receives a direct STT input.

Collaterals of spinothalamic cells in the rat

Spinothalamic (STT) cells were investigated in the rat to determine the distribution of subpopulations with terminals in both the lateral and medial thalamus, the thalamus bilaterally, or the thalamus and the medullary reticular formation. Two or more retrogradely transported substances (fluorescent dyes, and/or horseradish peroxidase) were injected in each animal. Three combinations of injections were most commonly used: (1) injections of the medullary reticular formation and thalamus, (2) separate injections into each side of the thalamus, and (3) separate injections into the medial and lateral thalamus. The distribution of single labeled cells after each injection was compared with previously published results for rats. The distribution of cells which contained both tracers, double-labeled (DL) cells, was the focus of this study. An average of 15% of STT cells and 8% of spinoreticular cells projected to both the reticular formation and thalamus. However, only a small component of STT cells (less than 2%) projected bilaterally into the thalamus. Most DL cells were found in upper cervical segments. The laminar distribution of all three groups of DL neurons were similar. These cells were most often located in the reticulated part of lamina V and the intermediate zone, lamina VII. STT cells that had terminals in both the medial and lateral thalamus and STT cells with collaterals in the reticular formation were concentrated on the side contralateral to their terminals. These DL neurons provide an anatomical substrate for noxious stimuli to stimuli to activate the reticular formation and thalamus and/or specific sensory and intralaminar thalamus simultaneously.