Differential distribution of V2b interneuron subdivisions along the rostrocaudal axis of the spinal cord.
Cédric FranciusAudrey HarrisVincent RucchinTimothy J. HendricksFloor J. StamMelissa BarberDorota KurekFrank GrosveldAlessandra PieraniMartyn GouldingFrédéric Clotman
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Interneuron
Using differential retrograde axonal tracing, we identified motoneurons (MNs) and projection-specific interneuron (IN) classes in lumbar segment D9 of the adult red-eared turtle spinal cord. We characterized the distribution of these neurons in the transverse plane, and estimated their numbers and proportions. Different labeling paradigms allowed us to distinguish ipsilateral INs (IINs) from commissural INs (CINs), and to identify IINs and CINs with either ascending (a) axons, descending (d) axons, or axons that bifurcate to both ascend and descend (ad). Local interneurons with axons shorter than 1 segment in length were not studied. We show that most retrogradely labeled INs are located dorsal to the MNs, in the ventral horn, the intermediate zone and the dorsal horn. IINs predominate in the dorsal horn. CINs are located on average more medially than the IINs in the ventral horn and intermediate zone. Within the IIN and CIN populations, aINs and dINs overlap extensively. The adIINs and adCINs make up only a small fraction of the total number of INs and are scattered throughout much of the respective IIN and CIN domains. The proportions of IINs and CINs are about equal, as are the proportions of aIINs versus dIINs, of aCINs versus dCINs, and of adIINs versus adCINs. The findings are compared to the organization of lumbar spinal INs in other vertebrate species.
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Lumbar Spinal Cord
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There is strong evidence that neural circuits underlying certain rhythmic motor behaviors are located in the spinal cord. Such local central pattern generators are thought to coordinate the activity of motoneurons through specific sets of last-order premotor interneurons that establish monosynaptic contacts with motoneurons. After injections of biotinylated dextran amine into the lateral and medial motor columns as well as the ventrolateral white matter at the level of the upper and lower segments of the lumbar spinal cord, we intended to identify and localize retrogradely labelled spinal interneurons that can likely be regarded as last-order premotor interneurons in rats. Regardless of the location of the injection site, labelled interneurons were revealed in laminae V–VIII along a three- or four-segment-long section of the spinal gray matter. Although most of the stained cells were confined to laminae V–VIII in all cases, the distribution of neurons within the confines of this area varied according to the site of injection. After injections into the lateral motor column at the level of the L4–L5 segments, the labelled neurons were located almost exclusively in laminae V–VII ipsilateral to the injection site, and the perikarya were distributed throughout the entire mediolateral extent of this area. Interneurons projecting to the lateral motor column at the level of the L1–L2 segments were also located in laminae V–VII, but most of them were concentrated in the middle one-third or in the lateral half of this area. Following injections into the medial motor column at the level of the L1–L2 segments, the majority of labelled neurons were confined to the medial aspect of laminae V–VII and lamina VIII, and the proportion of neurons that were found contralateral to the injection site was strikingly higher than in the other experimental groups. The results suggest that the organization of last-order premotor interneurons projecting to motoneurons, which are located at different areas of the lateral and medial motor columns and innervate different muscle groups, may present distinct features in the rat spinal cord. J. Comp. Neurol. 389:377–389, 1997. © 1997 Wiley-Liss, Inc.
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The motor cortex represents muscle and joint control and projects to spinal cord interneurons and–in many primates, including humans–motoneurons, via the corticospinal tract (CST). To examine these spinal CST anatomical mechanisms, we determined if motor cortex sites controlling individual forelimb joints project differentially to distinct cervical spinal cord territories, defined regionally and by the locations of putative last-order interneurons that were transneuronally labeled by intramuscular injection of pseudorabies virus. Motor cortex joint-specific sites were identified using intracortical-microstimulation. CST segmental termination fields from joint-specific sites, determined using anterograde tracers, comprised a high density core of terminations that was consistent between animals and a surrounding lower density projection that was more variable. Core terminations from shoulder, elbow, and wrist control sites overlapped in the medial dorsal horn and intermediate zone at C5/C6 but were separated at C7/C8. Shoulder sites preferentially terminated dorsally, in the dorsal horn; wrist/digit sites, more ventrally in the intermediate zone; and elbow sites, medially in the dorsal horn and intermediate zone. Pseudorabies virus injected in shoulder, elbow, or wrist muscles labeled overlapping populations of predominantly muscle-specific putative premotor interneurons, at a survival time for disynaptic transfer from muscle. At C5/C6, CST core projections from all joint zones were located medial to regions of densely labeled last-order interneurons, irrespective of injected muscle. At C7/C8 wrist CST core projections overlapped the densest interneuron territory, which was located in the lateral intermediate zone. In contrast, elbow CST core projections were located medial to the densest interneuron territories, and shoulder CST core projections were located dorsally and only partially overlapped the densest interneuron territory. Our findings show a surprising fractionation of CST terminations in the caudal cervical enlargement that may be organized to engage different spinal premotor circuits for distal and proximal joint control.
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Abstract The somatotopic organization of cutaneous primary afferents projecting to the dorsal horn of the rat spinal cord was investigated. The fluorescent neurotracer, 1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) was applied to cutaneous incisions made along ventrodorsal axial lines (VDALs) or rostrocaudal axial lines (RCALs) of the trunk and hindlimb. DiI‐induced fluorescent zones appeared in laminae I‐III of the dorsal horn in the transverse section. Several fluorescent zones appeared at different mediolateral portions after tracer application to VDALs. After tracer was applied to RCALs, a single zone of fluorescence was observed. Serial transverse sections were used to reconstruct fluorescent zones in lamina II and to illustrate the rostrocaudally elongated band‐like projection fields in a horizontal plane. In the horizontal plane, the fluorescent zones of VDALs were reconstructed to band‐like projection fields. These fields were arranged mediolaterally and extended rostrocaudally for approximately the length of one spinal cord segment or less. The fluorescent zones of RCALs were reconstructed to one band‐like projection field. This field extended rostrocaudally over several spinal cord segments. Cutaneous afferents from the ventral median line of the trunk, tail, hindlimb, sole, and ventral side of the digits projected to the medial margin of the dorsal horn. Cutaneous afferents from the dorsal median lines projected to the lateral margin of the dorsal horn. By analyzing the pattern of the body surface regions and the VDALs and RCALs, the central projection fields of body surface regions could be hypothesized, based on the central projection fields of the individual VDAL and RCAL afferents. Thus, we established a detailed dorsal view map of the central projection fields of cutaneous primary afferents. J. Comp. Neurol. 445:133–144, 2002. © 2002 Wiley‐Liss, Inc.
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Abstract The ontogeny of intersegmental (propriospinal) projections was studied in the chick embryo spinal cord between embryonic day 2.5 and day 6. Our goals were (1) to determine the earliest projections of intersegmental interneurons between specific spinal regions and to establish the cell types involved; and (2) to follow the ontogeny of these projections during the early formative stages of spinal cord development. Studies were carried out in vitro by using an isolated spinal cord/brainstem preparation. Horseradish peroxidase injections were made either uni‐ or bilaterally at various levels of the spinal cord along the rostrocaudal axis of the embryo. HRP histochemistry was done on Vibratome sections with diaminobenzidine as the chromogen. Following unilateral injections at day 2.5, labelled commissural interneurons were found contralaterally and were confined to the injected segment. Subsequently, labelled cells were found progressively further away from the injected segment. By day 4.5 reciprocal projections extended between lumbar and brachial regions. Interneurons with intersegmental axonal projections were often undifferentiated, consisting of primitive unipolar or bipolar cells with little, if any, dendritic development. In some cases migrating interneurons could be retrogradely labelled from two or three segments away from the location of their translocating cell body. Anterograde Golgi‐like labelling of early undifferentiated cells revealed growing axons, axonal terminals, and growth cones. Five or six reasonably distinct classes of intersegmental interneurons were identified based on their location, axonal projections, and morphology of dendritic arbors. These appeared to be segmentally and bilaterally arranged along the rostrocaudal axis of the spinal cord. The axons of some of these types of interneurons exhibited preferences in their longitudinal projections within the ventral and ventrolateral marginal zone at the very onset of pathway formation. From the present observations it can be concluded that intersegmental connectivity precedes the development of ascending and descending supraspinal, as well as primary afferent connections in the chick embryo spinal cord.
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Lumbar Spinal Cord
Horseradish peroxidase
Interneuron
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