Spinal Border Cells Research Articles

Laterality of Spinocerebellar Neurons in the Chicken Spinal Cord

The aim in this study is to elucidate the laterality of chicken spinocerebellar (SC) neurons that originate from the caudal cervical to caudal lumbosacral spinal cord. SC neurons in the spinal segment (SS) 17-20 consisted of a mixture of crossed and uncrossed axons. SC neurons in the more cranial and caudal SS than SS 17-20 (transitional zone) were generally uncrossed and crossed, respectively. In the transitional zone, SC neurons in spinal border cells and ventral border cells of the ventral horn changed dramatically from an uncrossed to a crossed type between SS 17 and SS 18. Chicken SC neurons are markedly different in laterality from mammalian SC neurons.

Are paragriseal cells in the avian lumbosacral spinal cord displaced ventral spinocerebellar tract neurons?

In lumbosacral segments the spinal cord of birds contains numerous paragriseal neurons lying in the white matter of lateral and ventral funiculus. These paragriseal cells project to the cerebellum. Neurons of the dorsal horn (mainly Clarke's column) make up a dorsal spinocerebellar tract and neurons of the ventral horn (mainly spinal border cells) are at the origin of a ventral spinocerebellar tract. It was the aim of this investigation to look for the distribution of spinocerebellar ventral horn neurons and paragriseal cells in the thoracolumbosacral spinal cord of pigeons and to compare this distribution with that of the cervical enlargement. Neuroanatomical tracers were injected into the anterior cerebellum of pigeons and labeled spinal neurons were counted throughout the length of the spinal cord. In the cervical enlargement the number of spinocerebellar ventral horn neurons increases more rostral than that of dorsal horn neurons but the number of both groups of neurons decreases simultaneously at the caudal end of the enlargement. In the ventral horn of thoracolumbosacral segments the number of spinocerebellar ventral horn neurons and paragriseal cells increases again more rostrally than that of dorsal horn cells. However, the number of ventral horn cells decreases whereas that of paragriseal cells and of dorsal horn cells is maintained. This shows that the number of ventral horn cells decreases in favor of paragriseal cells, which supports the suggestion that paragriseal cells are displaced ventral horn spinocerebellar neurons. It is discussed whether the paragriseal neurons migrate toward their input.

Laterality of the spinocerebellar axons and location of cells projecting to anterior or posterior cerebellum in the chicken spinal cord

In the cervical and lumbosacral enlargements of the chicken, there are seven spinocerebellar nuclei, the Clarke’s column, the spinal border cells, the ventral margin of the ventral horn of both enlargements, and the ventral marginal nucleus in the lumbosacral enlargement. In the present study, we investigated the laterality of spinocerebellar tract axons and the distribution of the spinocerebellar tract neurons projecting into the anterior or posterior part of the cerebellum in these seven nuclei by retrograde transport of wheat germ agglutinin-horseradish peroxidase. The spinocerebellar tract neurons with uncrossed axons were found in the cervical Clarke’s column and the cervical spinal border cells, and with crossed ones in the lumbar Clarke’s column, lumbar spinal border cells, lumbar lamina IX included in the ventral margin of the ventral horn of the lumbosacral enlargement, and the ventral marginal nucleus. The ventral margin of the ventral horn of the cervical enlargement and lumbar lamina VIII included in the ventral margin of the ventral horn of the lumbosacral enlargement issued spinocerebellar tract axons bilaterally. The spinocerebellar tract neurons of the lumbar spinal border cells and lumbar lamina IX projected to the anterior part of the cerebellum only. And those of the other nuclei projected to both the anterior and posterior parts.

The organization of the spinocerebellar tract neurons in the chicken

The organization of spinocerebellar tract (SCT) neurons in the chicken was studied quantitatively using retrograde wheat germ agglutinin-horseradish peroxidase labelling. The chicken spinal cord was divided into five regions based on the distribution of SCT neurons: the cervical region (the spinal segments [SS], 1–12, which contained 32% of the total number of SCT neurons), the cervical enlargement (SS13–15, 13.4%), the thoracic region (SS16–20, 13%), the thoraco-lumbosacral region (SS21–26, 34.6%), and the posterior lumbosacral region (SS27–30, 7%). Clarke’s column was found in two regions, a cervical one (SS12–16) and a lumbar one (SS21–28). The spinal border cells were less numerous and also present in two parts, a cervical one (SS10–15) and a caudal one (SS20–24). SCT neurons of the ventral part of the ventral horn were found in the cervical enlargement and SS16 (lamina VIII), and in the thoraco- and posterior lumbosacral regions (laminae VIII and IX). The ventral marginal nucleus consisted exclusively of SCT neurons but the major and minor marginal nuclei did not contain SCT neurons. In the cervical region most of SCT neurons were observed in the ventral horn, while the central cervical nucleus, which consists of SCT neurons in mammals, was not identified in the chicken.

Developmental plasticity of selected spinocerebellar axons. Studies using the North American opossum, Didelphis virginiana

When the thoracic spinal cord of the opossum is hemisected at postnatal day 5 or 8, but not at day 12 or later ages, spinocerebellar axons which originate from spinal border cells, the sacral/coccygeal ventrolateral nucleus, and Stilling's nucleus grow through the lesion and reach the cerebellum. The critical period for such growth is comparable to that reported previously for spinocerebellar axons originating within Clarke's nucleus and for axons of the fasciculus gracilis, but shorter than that for most descending spinal axons. It appears, therefore, that differences exist in the ability of ascending and descending axons to traverse a lesion of their spinal pathway during development.

Spinal neurons projecting to anterior or posterior cerebellum in the pigeon.

Spinal afferent fibers have been shown to project both to lobules III-VI and lobule IX of the cerebellum in the pigeon. In the present investigation the cells of origin of these projections and the course of the axons at spinal levels have been studied by the retrograde transport of fluorescent dyes injected into both parts of the cerebellum. In the upper cervical segments labeled neurons are located predominantly in the ventral horn; the axons cross to the contralateral side. In the cervical enlargement labeled neurons concentrate in the avian cervical Clarke's column (ClC) and in cervical "spinal border cells" (SBC). The axons of ClC neurons project ipsilaterally into the dorsolateral funiculus and SBC project ipsilaterally into the ventrolateral funiculus. In caudal cervical and in thoracic segments dorsal horn neurons (laminae IV/V) are at the origin of an ipsilateral spinocerebellar pathway in the dorsalmost part of the lateral funiculus. In the lumbosacral enlargement there are mainly three spinocerebellar cell groups all of which project contralaterally into the ventral funiculus: ClC, SBC and paragriseal cells. During its ascent this pathway shifts to the lateral funiculus. In addition there is a crossed pathway from ventral horn cells throughout the spinal cord. Whereas approximately equal numbers of dorsal horn cells project to lobules III-VI and to lobule IX, the number of ClC neurons is strongly reduced after lobule IX injections and SBC neurons are nearly absent. Altogether lobule IX has a substantial input from dorsal horn neurons (cutaneous mechanoreception) whereas that to lobules III-VI is dominated by ClC and SBC (proprioreception).

Origin of spinal projections to the anterior and posterior lobes of the rat cerebellum

The present study was carried out to analyze the topography of spinal projections to the anterior and posterior lobes of the cerebellum and to investigate whether projections to the two lobes come from different spinocerebellar neurons or from branching axons of the same cells. We used orthograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) to identify the cerebellar areas where spinocerebellar axons terminate and retrograde double-labeling techniques to estimate the incidence of spinocerebellar neurons projecting to both anterior and posterior lobes via axon collaterals. Orthograde labeling confirmed that the rat, like other mammalian species, has spinocerebellar projections to two different regions of cerebellar cortex, i.e., lobules I-V of the anterior lobe and lobule VIII of the posterior lobe, with the highest incidence in lobules II, III, and VIII. We did not observe a clear difference in the distribution of afferents coming from different spinal segments to either of the two lobes. The double-labeled cells were located primarily in the lower thoracic and upper lumbar segments, almost exclusively in Clarke's column and in the dorso-lateral part of lamina 7 (in the region of the spinal border cells). It is likely that most or all of the spinocerebellar neurons in these structures project to both anterior and posterior lobes. Therefore, the two lobes of the cerebellum are likely to receive common information from these cells, but different information from the separate populations of spinocerebellar neurons that project only to one lobe or the other.

Spinocerebellar projections from spinal border cells in the cat as studied by anterograde transport of wheat germ agglutinin‐horseradish peroxidase

The cerebellar projections of spinal border cells, which give rise to crossed ascending axons, were studied in the lower lumbar segments by the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in the cat. Prior to the injections, the spinal cord was lesioned rostral and ipsilateral to the WGA-HRP injections to eliminate labeling of the ipsilaterally ascending axons. Following injections of WGA-HRP into the L4-L6 segments, labeled terminals were seen in sublobules Ib-Vf of the anterior lobe, sublobules VIf-VIe, lobule VIII, the paramedian lobule, and crus II. More than 90% of the total number of labeled terminals were in the anterior lobe; the projections were predominantly ipsilateral to their cells of origin (about 90% or more labeled terminals of the total number in each of sublobules IIb-Va). Many labeled terminals were seen ipsilaterally in sublobules IIb (11.1-17.5% of the total number), IIIa (11.7-12.9%), IIIb (16.1-17.6%), IVa (8.8-10.8%), IVb (6.2-10.5%), and Va (9.7-10.3%). In the posterior lobe, labeled terminals were numerous only in sublobule C of the ipsilateral paramedian lobule (2.7-4.9%). The projection fields in the horizontal plane of the lobules were reconstructed from a series of cross-sections through each lobule. In the anterior lobe, labeled terminals were distributed in four major areas. In sublobules IIa-IVa, area 1 was located between 1.0 and 2.0 mm lateral to the midline in zones B-C1 of Voogd; area 2, between 2.0 and 3.0 mm lateral to the midline in zones C1-C2; area 3, between 3.0 and 4.0 mm in zones C2-C3; and area 4, lateral to 4.0 mm from the midline in zone C3. These areas extended apicobasally in the apical to middle parts of the lobules. On the rostral side of sublobule C1 of the ipsilateral paramedian lobule, the projection areas extended in the entire apicobasal and the mediolateral axis of the sublobule, except in the most lateral part. The present results suggest that the SBCs project to specific areas in the cerebellar lobules.

Spinocerebellar tract neurons with axons passing through the inferior or superior cerebellar peduncles. A retrograde horseradish peroxidase study in rats.

Using the retrograde horseradish peroxidase (HRP) method, we determined whether axons of the spinocerebellar tract (SCT) neurons pass through the superior (SCP) or the inferior (ICP) cerebellar peduncle in rats. Following bilateral section of either the SCPs or the ICPs, HRP was injected into the cerebellar anterior lobe and lobule VI, and the resulting labeled neurons were quantitatively examined throughout the length of the spinal cord. Almost all SCT neurons in the central cervical nucleus, Clarke's column and lamina VII of the third cervical (C3) to third thoracic (T3) segments and the T11 to fifth lumbar (L5) segments, and the majority of SCT neurons in the ventrolateral part of the anterior horn of the L6 to caudal (Ca) segments and laminae V of the C8-L5 segments and VII of the L6-Ca segments project their axons through the ICPs. The majority of spinal border cells (T11-L5) and a large number of SCT neurons in lamina VII of the C3-T3, T11-L5 and L6-Ca segments project their axons through the SCPs. A nearly equal number of SCT neurons in lamina VIII (C1-L6) send axons through the SCPs or the ICPs. The proportion of SCT neurons projecting via the SCPs versus those projecting via the ICPs was approximately 1:5.

Development of spinocerebellar afferents in the clawed toad, Xenopus laevis.

The development of spinocerebellar projections in the clawed toad, Xenopus laevis, was studied with horseradish peroxidase as an anterograde and retrograde tracer. Early in development cells of origin of spinocerebellar projections were found, contralaterally, in or close to the medial motor column. In older tadpoles ipsilaterally projecting spinal neurons were also labeled from the cerebellum. These are virtually indistinguishable from the large primary motoneurons that occupy a very similar position in the spinal cord. Most of the labeled spinal cells were found in the thoracic spinal cord; they lie halfway between the brachial and lumbar secondary motor columns. Surprisingly, no primary spinocerebellar projection arising from dorsal root spinal ganglion cells could be demonstrated in X. laevis tadpoles and adult toads. Therefore, fibers in the cerebellum that were labeled anterogradely from the spinal cord can be expected to originate exclusively from the secondary spinocerebellar tract cells. These fibers appear to cross the cerebellum in or at the border of the granular layer. The present data suggest that in X. laevis early in the development of the cerebellum a distinct secondary spinocerebellar projection is already present, originating in neurons that can be compared with the "spinal border cells" in mammals. The relative sparseness of this secondary spinocerebellar projection and the apparent absence of primary spinocerebellar afferents probably indicate that spinocerebellar pathways are only of minor importance in X. laevis. The possibility remains, however, that the expansion of the secondary spinocerebellar pathway only starts when metamorphosis has been completed.

Spinocerebellar neurones in the guinea pig — a morphological study

The morphology and distribution of spinocerebellar neurones were examined in the guinea pig. Horseradish peroxidase was injected into the cerebellum, and after a survival time of 72 h retrogradely labelled cells were examined in the spinal cord. The distribution of spinocerebellar cells was similar to that previously demonstrated in the rat. Three major groups of neurones were distinguished: the central cervical nucleus (C 1–C 2), Clarke's column (T 2–L 3) and spinal border cells (L 3–L 6). Neurones in the central cervical nucleus were multipolar, had mean equivalent diameters of about 24 μm, and their axons ascended on the contralateral side of the spinal cord. Neurones in Clarke's column were spindle-shaped, approximately 25 × 35 μm, and their axons ascended on the ipsilateral side of the spinal cord. Spinal border cells were multipolar, with mean equivalent diameters of about 35 μm; their axons were predominantly crossed.

ラットの小脳前葉と後葉へ投射する脊髄小脳路細胞の定量的考察 ＨＲＰの逆行性標識法による研究

In the present study of the rat, in order to observe the distribution areas of neurons sending their axons into the anterior and posterior cerebellar lobes and to analyse quantitatively the number of spinocerebellar tract neurons, horseradish peroxidase (HRP)-labeled neurons were examined throughout the length of the spinal cord after HRP injection into the anterior or posterior cerebellar lobe.The distribution areas of labeled neurons sending their axons into the anterior and posterior cerebellar lobes were almost the same in all cases, which were described as follows: central cervical nucleus of C1-C4, Clarke's column (CC) of T1-L2, spinal border cells (SBCs) of T11-L5, ventrolateral part of the ventral horn (VL) of L6-Ca, lamina V of C1-Ca, lamina VI of C1-C8 and of L2-L5, lamina VII of C1-Ca, and lamina VIII of C 1-L6. The number of neurons sending their axons into the posterior lobe was about half the number of those sending them into the anterior lobe.In all areas except in lamina VI of C1-C8, the number of labeled neurons sending their axons into the anterior lobe was, at various ranges, more than that of the labeled neurons sending their axons into the posterior lobe. Particularly in VL and lamina VIII, the number of projection neurons into the anterior lobe greatly exceeded that projecting into the posterior lobe. In all cases, the number of labeled neurons of CC, which constitutes the dorsal spinocerebellar tract, was the largest as compared with that of labeled neurons in other areas. Namely, of the total number of neurons projecting into the anterior lobe and of the total number of those into the posterior lobe, 41% and 46% were labeled, respectively. The number of labeled neurons both in SBCs and lamina V of T11-L5 which constitute the ventral spinocerebellar tract and in laminae V, VI and VII of C5-C8 which constitute the rostral spinocerebellar tract was only 14% of the total number of labeled neurons after injection into the anterior lobe. The number of labeled neurons in all these areas constituting the ventral and rostral spinocerebellar tracts was only 21% of the total number of labeled neurons after injection into the posterior lobe. It should be emphasized that recently identified spinocerebellar tract neurons, (i.e. excluding dorsal, ventral and rostral spinocerebellar tract neurons), constituted 45% of the total number of projection neurons after injection into the anterior lobe and 33% of the total number of those after injection into the posterior lobe.

Observations on the development of cerebellar afferents in Xenopus laevis.

The development of cerebellar afferents has been studied in the clawed toad, Xenopus laevis, from stage 46 to 64, with the horseradish peroxidase retrograde tracer technique. Already in stage 48 tadpoles, i.e. before the formation of the limbs, a distinct set of cerebellar afferents was found. Vestibulocerebellar (mainly arising bilaterally in the nucleus vestibularis caudalis) and contralateral olivo-cerebellar projections dominate. Secondary trigeminocerebellar (from the descending nucleus of the trigeminal nerve) and reticulocerebellar connections were also found. At stage 50, spinocerebellar projections appear originating from cervical and lower thoracic/upper lumbar levels. The cells of origin of the spinocerebellar projection can be roughly divided in two neuronal types: ipsilaterally projecting large cells, which show a marked resemblance to primary motoneurones ('spinal border cells') and smaller contralaterally projecting neurons. Primary spinocerebellar projections from spinal ganglion cells could not be demonstrated. At stage 50, a possible anuran homologue of the mammalian nucleus prepositus hypoglossi was found to project to the cerebellum. In only one of the experiments labeled neurons were found in the contralateral mesencephalic tegmentum. At none of the studied stages a raphecerebellar projection could be demonstrated. It appears that already early in cerebellar development, before the formation of the limbs, most of the cerebellar afferents as found in adult Xenopus laevis are present.

ラット脊髄小脳路の脊髄および下部延髄における経路 逆行性軸索流法による研究

The spinocerebellar tract neurons and their fibers were observed throughout the spinal cord and lower medulla in the Wistar rat following injections of horseradish peroxidase (HRP) into the cerebellum.The fiber course before reaching into the periphery of the lateral funiculus was classified into three types. Type I fibers crossed contralaterally through the anterior white commissure, and then ascended obliquely in the periphery of the anterior funiculus to reach into the periphery of the lateral funiculus. These fibers mainly originated from Co-L6 (neurons in laminae V, VII, ventrolateral part of the ventral horn), L3-Tll (lamina VII neurons, spinal border cells) and C3-C1 (central cervical nucleus neurons). The fibers in Co-L6 and L3-Tll ascended obliquely for 2-4 segments to reach into the periphery of the lateral funiculus, while the fibers in C3-C1 ascended obliquely for 1-2 segments. Type II fibers entered the ipsilateral lateral funiculus through the dorsal part of the intermediate substance, and ascended obliquely for 2-9 segments in the medial part of the lateral funiculus to reach into the periphery of the lateral funiculus. The fibers of this type arose mainly from L2-T1 (neurons in lamina V, Clarke's column). Thpe III fibers entered the ipsilateral lateral funiculus from the lateral corner of the anterior horn, and ascended obliquely for 2-4 segments to reach into the periphery of the lateral funiculus. The fibers of this type mainly occurred in T3-C5 (lamina VII neurons).After reaching into the periphery of the lateral funiculus, the majority of type I fibers ascended in the ventral half of the periphery of the lateral funiculus, but the minority migrated dorsalward and ascended in the dorsal half. In the periphery of the lateral funiculus, most of type II fibers ascended in the dorsal half, but some of them ascended in the posterior part of the ventral half. Type III fibers ascended in the ventral half of the periphery of the lateral funiculus.There is a classical conception that the ventral spinocerebellar tract and dorsal spinocerebellar tract ascend through the ventral half of the lateral funiculus and through the dorsal half of it, respectively. The present results indicate that this should be modified.

The projection fields of spinal border cells in the cerebellar anterior lobe in the cat: an anterograde WGA-HRP study

The projection fields of spinal border cells (SBCs) in the cerebellar anterior lobe were studied using the anterograde transport of wheat germ agglutinin bound to horseradish peroxidase in the cat. Many anterogradely labeled terminals of the SBCs were seen in the apical to middle parts of sublobules IIb-Va on the side ipsilateral to the injection. In the mediolateral extent they were distributed densely in areas in the lateral part of the vermis (zone B) to the intermediate-lateral part of the lobules (zones C1-D). The present study suggests that the SBCs project, predominantly ipsilaterally, to specific parts of sublobules IIb-Va.

Spinocerebellar projections to lobules III to V of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase.

Spinocerebellar tract (SCT) neurons projecting to lobules III to V of the cerebellar anterior lobe were identified by the retrograde horseradish peroxidase technique. SCT neurons projecting to lobule III with crossed ascending axons were located mainly in the central cervical nucleus (CCN), the medial part of lamina VII of L6 to the caudal segments, and the dorsal horn (lamina V) and ventral horn (lamina VIII) of the sacral-caudal segments. Spinal border cells with crossed ascending axons also projected to lobule III. SCT neurons projecting to this lobule with uncrossed ascending axons were located in the medial part of lamina VI of the cervical segments and the middle part of lamina VII of C6 to T1, lamina V of the lower cervical, thoracic and the lumbar segments, Clarke's column including marginal neurons, and the medial part of lamina VI of L5 and L6. These neuronal groups also projected to lobule IV, except for those present caudal to L6 (in the medial part of lamina VII, and laminae V and VIII of the sacral-caudal segments). A far smaller number of similar neurons projected to lobule V. Injections of HRP restricted to the vermal region labeled mainly neurons in the CCN and Clarke's column while restricted injections to the intermediate-lateral regions labeled ipsilaterally spinal border cells, lamina V neurons, and Clarke column neurons, especially of the lumbar segments as well as marginal neurons of this column.

Spinocerebellar projections to lobules I and II of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase.

Spinocerebellar tract (SCT) neurons projecting to lobules I and II of the cerebellar anterior lobe were identified by the retrograde horseradish peroxidase technique in the cat. Instead of a conventional stereotaxic approach, we removed ventral parts of the vermis of the posterior lobe and approached the posterior aspect of lobule I through the fourth ventricle. Under direct visual guidance, discrete injections were made into lobule I or II with a glass micropipette. Neurons projecting to lobule I were located mainly in the central cervical nucleus (CCN), the medial part of lamina VII of L6 to the causal segments, and in lamina VIII of S2 to the caudal segments (with crossed ascending axons). The latter two groups correspond to medial lamina VII group of the lumbar to the caudal segments and the ventral horn group of the sacral-caudal segments of our previous studies. A small number of Clarke column neurons (with uncrossed ascending axons) also projected to lobule I. All of these neuronal groups projected to lobule II. In addition, large neurons in lamina V and the border between laminae IV and V from S2 to the caudal segments projected to sublobule IIA, and more numerously to sublobule IIB (with crossed ascending axons). They belong to the dorsal horn group of the sacral-caudal segments of our previous studies. Spinal border cells (with crossed ascending axons) projected to sublobule IIB, and a small number, to sublobule IIA. It was suggested that the CCN neurons project more densely to the median region whereas Clark column neurons project to the lateral part of these lobules.

Spinocerebellar projections to the vermis of the posterior lobe and the paramedian lobule in the cat, as studied by retrograde transport of horseradish peroxidase.

Spinal neurons projecting to the posterior lobe of the cerebellum were identified with the retrograde horseradish peroxidase technique in the cat. In four cases with the injections, which were preceded by hemisections at cervical or thoracic levels, it was determined whether in the spinal cord the identified neurons give rise to crossed ascending axons or uncrossed ascending ones. The main groups of neurons projecting to sublobule VIIIB were located in the central cervical nucleus (with crossed ascending axons), Clarke's column (with uncrossed ascending axons), and the medial part of lamina VII of L6 to the caudal segments (with crossed ascending axons). Additional labeled neurons were found in the medial part of lamina VI between C2 and T1 and L5 and L6 (with uncrossed ascending axons), and in the ventral as well as dorsal horns of the sacral-caudal segments (with crossed ascending axons). On the other hand, neurons projecting to sublobule C (the copular part) of the paramedian lobule, which appeared always ipsilaterally to the side of the injections, were located in lamina V of C8 to L4 (with uncrossed ascending axons). Marginal neurons of Clarke's column (with uncrossed ascending axons) and spinal border cells (with crossed ascending axons that recross in the cerebellum) projected specifically to this part. At L1 and L2 or L2 and L3 labeled large and medium-sized neurons were also found within Clarke's column. The present study suggests that there are segregated projections of spinal neurons to the cortex of the cerebellar posterior lobe.

Spinothalamic and spinomedullary neurons in macaques: A single and double retrograde tracer study

The location and perikaryal size of spinothalamic cells and of spinal neurons projecting to the dorsal column nuclei has been compared in adult monkeys ( Macaco fascicularis) in which horseradish peroxidase has been injected in either the ventrobasal complex or the dorsal medulla. Spinothalamic neurons are found mainly in lamina I and in the lateral portions of laminae IV-VI throughout the side of the cord contralateral to the thalamic injection. Spinomedullary neurons are most ipsilateral to the medullary injection, in medial portions of laminae IV-VI throughout cervical levels but laterally, in the same laminae, in lumbar segments. At these levels, spinothalamic and spinomedullary neurons are also found along the lateral border of the ventral horn; neurons in this location, labelled by either injection, are large (40–65 μm in diameter), and are indistinguishable from ‘spinal border cells’ at the origin of the ventral spinocerebellar tract. Computer-assisted measurement of perikaryal size reveals that the mean values for dorsal horn neurons of the two systems differ significantly, although a sizeable fraction of both neuronal populations have identical cytological characteristics, including perikaryal size. A double-labelling strategy employing horseradish peroxidase and [ 3H]-apo-horseradish peroxidase has been applied (a) to identify simultaneously cells of origin of the two ascending systems and to appreciate better their topographical relationships, and (b) to answer the question of whether at least some of these spinal neurons have axons which, by way of collateral branching, reach both targets. The distribution of neurons at the origin of the two ascending tracts in animals with injection of the two tracers is identical to that revealed in animals in which only horseradish peroxidase has been injected. While primarily single-labelled neurons are found throughout the cord, some large dorsal horn neurons in both brachial and lumbosacral enlargements are labelled by both cytoplasmic horseradish peroxidasepositive granules and by reduced silver grains on the overlying emulsion. These double-labelled cells are interpreted as having an axon which gives origin, presumably at spinal cord levels, to collaterals ascending to both the ipsilateral medulla and the contralateral thalamus. Systematic evaluation of the extent to which ascending pathways to various supraspinal targets originate from common spinal neurons may provide new insights into the functional mechanisms of ascending spinal systems.

Spinal Neurons Project to the Dorsal Column Nuclei of Rhesus Monkeys

Cells of origin of ascending nonprimary afferents to the dorsal column nuclei of rhesus monkeys have been identified in the spinal cord by the retrograde transport of horseradish peroxidase. These neurons are mainly located in lamina IV and medially in more ventral laminae of the dorsal horn on the side ipsilateral to the medullary injection. Large neurons in the ventral horn ("spinal border cells") also appear to project to the ipsilateral dorsal medulla. The dorsal column nuclei of a primate thus are the recipient not only of ascending dorsal root fibers but also of a more complexly integrated spinal input.