Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey

In 4 rhesus monkeys wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the mesencephalic tegmentum. In 3 cases with injections involving the red nucleus (RN), rubrospinal fibers descended mainly contralaterally to terminate in laminae V, VI and dorsal VII of the spinal cord and in the lateral motoneuronal cell groups at the level of the cervical and lumbosacral enlargements. In all 4 cases the area of the interstitial nucleus of Cajal (INC) was injected, which resulted in labeled interstitiospinal fibers in the medial part of the ipsilateral ventral funiculus of the spinal cord. The results indicate that there is no major qualitative difference between the mesencephalic (RN and INC) and motor cortical projections to the spinal cord.     

The possible role of collateral sprouting in the functional restitution of corticospinal connections after spinal hemisection

We investigated in monkeys whether the corticospinal fibers increase their connections with lumbosacral neurons after spinal hemisection, using the retrograde horseradish peroxidase (HRP) method. In three monkeys 3.5-38 months after spinal hemisection at the lower thoracic or upper lumbar cord, HRP was injected into the lumbosacral cord unilaterally on the hemisected side at a level caudal to the lesion. Control injections were performed in two intact monkeys and in two other monkeys immediately after hemisection. In all animals, corticospinal neurons in the precentral motor cortex were labeled bilaterally. However, in the chronically spinal hemisected monkeys, the number of the labeled neurons was significantly increased on the side ipsilateral to the lesion. These results suggest that corticospinal connections to lumbosacral motoneurons are newly formed on the side of spinal cord hemisection. This synapse formation may be due to collateral sprouting of intact corticospinal fibers, and it may underlie the mechanisms of motor recovery.     

Corticospinal tract fibers cross the ephrin-B3-negative part of the midline of the spinal cord after brain injury

The fibers of corticospinal tract (CST), which control fine motor function, predominantly project to the contralateral spinal cord, not recross to the ipsilateral side. Ephrin-B3, which is expressed in the midline of the spinal cord, and its receptor, EphA4, are crucial for preventing CST fibers from recrossing the midline in the developing spinal cord. However, these fibers can cross the midline to the denervated side after a unilateral CST or cortical injury. We determined the reason CST fibers can cross the midline after a cortical injury and the changes in ephrin-B3-EphA4 signaling associated with such a crossing. We first examined axonal sprouting from CST fibers after unilateral ablation of the motor cortex in postnatal and adult mice. CST fibers crossed the midline of the spinal cord after cortical ablation, especially when conducted during the early postnatal period. These fibers were well associated with functional recovery after the injury. We next assessed the mRNA expression of ephrin-B3 and EphA4 before and after the ablation. Surprisingly, no changes were detected in the expression patterns. We found, however, that ephrin-B3 expression in the ventral part of the midline disappeared after postnatal day 9 (P9), but was pronounced along the entire midline before P6. Most of the CST fibers crossed the midline through the ventral region, where ephrin-B3 expression was absent. Our results suggest that ephrin-B3 is not expressed along the entire midline of the spinal cord, and sprouting axons can cross the midline at ephrin-B3-negative areas.     

Fluorescent retrograde triple labeling of brainstem reticular neurons

The present study was aimed at the anatomical identification in the rat of neurons of the lower brainstem reticular formation which give off axonal branches ascending bilaterally to more rostral structures and descending unilaterally to the spinal cord. Three fluorescent tracers were injected in one and the same animal. Fast Blue was injected in the midbrain tegmentum, in the termination areas and fiber bundles of the ascending reticular efferents; Evans blue was injected in the midbrain tegmentum on the other side; either Nuclear Yellow or Diamidino Yellow was injected in the white and gray matter of the upper cervical cord. All three populations of single-labeled cells, as well as double labeled either from the midbrain injections or from the ipsilateral injections in the mesencephalon and spinal cord, were intermingled in the medial reticular formation. Very few cells double labeled from the contralateral mesencephalon and ipsilateral spinal cord were also seen. However, the main finding of the present study was the visualization of triple-labeled cells. The latter were mainly located ipsilaterally to the injections in the spinal cord. The present results indicate that reticular cells give off divergent multiple branches descending to the ipsilateral spinal cord and ascending bilaterally to rostral centers.     

Dorsal border periaqueductal gray neurons project to the area directly adjacent to the central canal ependyma of the C4-T8 spinal cord in the cat

In a previous study horseradish peroxidase (HRP) injections in the upper thoracic and cervical spinal cord revealed some faintly labeled small neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study describes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WGA-HRP) injections into cervical and upper thoracic spinal segments resulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not found after injections in more caudal segments. WGA-HRP injections in the dorsal border PAG region surprisingly resulted in anterogradely labeled fibers terminating in the area dorsally and laterally adjoining the central canal ependyma of the C4-T8 spinal cord. No anterogradely labeled fibers were found more caudal in the spinal cord. The labeled fibers found in the upper cervical cord were not located in the area immediately adjoining the ependymal layer of the central canal, but in the lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border PAG-spinal neurons terminate in the neuropil of the subependymal area and in the vicinity of the basal membranes of capillaries located laterally to the central canal. The terminal profiles contain electron-lucent and densecored vesicles, suggesting a heterogeneity of possible transmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the termination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypothalamohypophysial system is discussed.     

Postsynaptic dorsal column pathway of the rat. I. Anatomical studies

As one of a series of studies of the ascending spinal cord pathways that might be involved in nociception in the rat, we have examined the projection to the dorsal column nuclei that originates in the spinal cord dorsal horn using the retrograde transport of horseradish peroxidase (HRP). This projection in other animals has been called the postsynaptic dorsal column (PSDC) pathway. Small iontophoretic injections of HRP into the cuneate nucleus (CN) labeled more than 350 neurons in alternate sections within the ipsilateral gray matter of segments C6-8. Fewer than 25 neurons were labeled in L4-6 by injections into CN. Injections of HRP confined to the gracile nucleus (GN) labeled more than 200 neurons within a narrow band extending across the ipsilateral dorsal horn subjacent to substantia gelatinosa of L4-6. Fewer than 10 cells were labeled in C6-8 by such injections. Labeling in lumbar neurons following injections into GN was prevented by transection of the dorsal columns at T10, T8, or C2. Thus, neurons labeled by such injections ascend entirely within the dorsal columns. Lesions of the dorsal columns in C2 reduced the number of labeled neurons in the cervical cord following CN injections by approximately 90%. Combined lesions of the dorsal columns and ipsilateral dorsal lateral funiculus (DLF) reduced the number of cells labeled in C6-8 by approximately 98%. Thus, the majority of labeled neurons in the cervical enlargement project to CN via the dorsal columns; a small secondary component of the cervical projection to CN appears to ascend within the DLF. To compare the relative sizes of the projections to the dorsal column nuclei from PSDC neurons and dorsal root ganglion cells (DRG), labeled neurons were counted in the gray matter of the cervical and lumbar enlargements and the corresponding DRG. In the four animals so examined, PSDC neurons constituted over 38% of the neurons that projected to CN and approximately 30% of the cells that projected to GN. These findings indicate that the PSDC projection of the rat is capable of providing a large somatotopically organized input to the dorsal column nuclei.     

Anatomical evidence for the re-crossing of lateral corticospinal fibers via the posterior gray commissure in the cat spinal cord

This study demonstrated the projections of the corticospinal tract (CST) by using the anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following injection of WGA-HRP into the pre- and post-cruciate cortices (somatosensory area) on one side, labeled fibers were found in the spinal cord both in the lateral and in the anterior CSTs; both crossed, and uncrossed. The most conspicuous labeling was found in the crossed lateral CST. In addition, labeled lateral CST fibers were seen to recross via the posterior gray commissure. These re-crossed lateral CST fibers were most frequently observed in the cervical spinal segments.     

Spinal projections from the presumptive midbrain locomotor region in the mouse

The mesencephalic locomotor region (MLR) plays an important role in the control of locomotion, but there is ongoing debate about the anatomy of its connections with the spinal cord. In this study, we have examined the spinal projections of the mouse precuneiform nucleus (PrCnF), which lies within the boundaries of the presumptive MLR. We used both retrograde and anterograde labeling techniques. Small clusters of labeled neurons were seen in the medial portion of the PrCnF following fluoro-gold injections in the upper cervical spinal cord. Fewer labeled neurons were seen in the PrCnF after upper thoracic injections. Following the injection of anterograde tracer (biotinylated dextran amine) into the PrCnF, labeled fibers were clearly observed in the spinal cord. These fibers traveled in the ventral and lateral funiculi, and terminated mainly in the medial portions of laminae 7, 8, and 9, as well as area 10, with an ipsilateral predominance. Our observations indicate that projections from the PrCnF to the spinal cord may provide an anatomical substrate for the role of the MLR in locomotion.     

Doxorubicin: a fluorescent neurotoxin retrogradely transported in the central nervous system

Doxorubicin, a fluorescent cytotoxic antibiotic, was found to be both a retrograde neuron pathway tracer and neurotoxin to cells retrogradely labeled with it. Doxorubicin was injected into rat caudate-putamen and within 4 h the nuclei in the ipsilateral substantia nigra zona compacta (SNc) and ventral tegmental area (VTA) were stained with red fluorescent doxorubicin. After 2 weeks, portions, but not all of the ipsilateral SNc and VTA were depleted of neurons. Retrograde neurotoxicity was obvious following injections of 20%, 10%, 6%, 5% or 4% doxorubicin but not after 1% or 2%. Five months following doxorubicin treatment, the ipsilateral SNc and VTA were shrunken, distorted and nearly absent; the injected caudate was shrunken and replaced by ventricle. The ipsilateral thalamic parafasicular center median nucleus, a complex nucleus also known to project to the caudate, was depleted of large neurons 2 weeks following caudate injection. Doxorubicin can be transported over relatively long distances; lumbar spinal cord injections labeled cortical pyramidal neurons 3 days later. Doxorubicin's unique pathway-specific neurotoxicity may be useful in future neuroscientific studies.     

Demonstration of spinocerebellar projections in cat using anterograde transport of WGA-HRP,with some observations on spinomesencephalic and spinothalamic projections

Summary Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) conjugate was injected into the lumbar or cervical spinal cord of adult cats in order to examine its usefulness as an anterograde tracer of long ascending spinal tracts. Heavily labeled fibers and terminal-like structures were found in restricted regions of the granular layer of the cerebellar cortex in all animals. The terminal-like structures showed features characteristic of mossy fiber rosettes. Many labeled terminals were found in regions anatomically and physiologically defined as cerebellar hind limb regions after lumbar cord injections. Relatively few labeled terminals were found in both hind limb and in forelimb regions after cervical cord injections. Although the cervical spinal cord is known to project to both hind limb and forelimb regions, it cannot be entirely excluded that some of the labeling in the hind limb regions was derived from uptake by fibers of passage.In addition to labeling in the cerebellar cortex, labeled fibers and terminals were also observed in the cerebellar medial and interposed nuclei, regardless of whether the injections had been made into the lumbar or the cervical spinal cord.Several midbrain and thalamic regions were found to contain labeled fibers and terminal-like profiles in all cases. In the midbrain the periaqueductal gray, the nucleus of Darkschewitsch and the posterior pretectal nucleus contained fairly dense labeling. Labeling was more scattered in the cuneiform nucleus, the mesencephalic reticular formation, the superior colliculus and in the magnocellular part of the medial geniculate body. Four major regions in the thalamus, namely the central lateral nucleus, the zona incerta, the medial part of the posterior complex and the submedius nucleus contained labeling of variable density.The present findings show that the WGA-HRP conjugate can be used as a tracer for the study of long fiber tracts within the central nervous system.Supported by the Swedish Medical Research Council, Projects Nos. 553 and 3391     

The afferent connections of the inferior olivary complex in rats: A study using the retrograde transport of horseradish peroxidase

Retrograde transport of horseradish peroxidase (HRP) was used to define the origin of afferents to the inferior olivary complex (IOC) in rats. Using both ventral and dorsal surgical approaches to the brainstem, HRP was injected into the IOC through a micropipette affixed to the tip of a 1-μl Hamilton syringe. After a 2-day postoperative survival, animals were sacrificed by transcardiac perfusion with a 1% paraformaldehyde-1.25% gluteraldehyde solution, and brains were processed according to the DeOlmos protocol (1977), using o-dianisidine as the chromogen. Labeled cells were found at many levels of the nervous system extending from lumbar spinal cord to cerebral cortex. This wide-ranging input from numerous regions clearly underscores the complexity of the IOC and its apparent involvement in several functions. Within the spinal cord, labeled neurons were identified from cervical to lumbar but not at sacral levels. These neurons were found contralaterally in the neck region of the dorsal horn and in the medial portions of the intermediate gray. In the caudal brainstem, reactive cells in the dorsal column nuclei, the spinal trigeminal nucleus, and the subnucleus y of the vestibular complex were observed primarily contralateral to the injection sites. Labeling within the gigantocellular, magnocellular, ventral, and lateral reticular nuclei and the nucleus prepositus hypoglossi was primarily ipsilateral. Reactive neurons in the medial and inferior vestibular nuclei were predominantly ipsilateral or contralateral to HRP injections into the caudal or rostral IOC, respectively. The dentate and interposed nuclei of the cerebellum contained small, lightly labeled neurons primarily contralateral to the injection site, while the fastigial nuclei contained a few relatively large, heavily labeled cells bilateral to caudal olivary injections. Ipsilaterally labeled mesencephalic regions included the periaqueductal gray, interstitial nucleus of Cajal, rostromedial red nucleus, ventral tegmental area, medial terminal nucleus of the accessory optic tract, nucleus of the optic tract, and the lateral deep mesencephalic nucleus. The caudal part of the pretectum and small cells of the stratum profundum of the superior colliculus were labeled predominantly contralateral to the injection. In the caudal diencephalon labeled neurons were most numerous within the nucleus of Darkschewitsch and the subparafascicular nucleus, primarily ipsilateral to olivary injections. Scattered reactive neurons were also found within the ipsilateral zone incerta. With the exception of the zona incerta, all labeled mesencephalic and diencephalic nuclei had some bilateral representation of labeled cells. No labeled neurons were identified within the basal ganglia, while numerous reactive cells were found bilaterally within layer V of the frontal and parietal cerebral cortex.     

Individual corticorubral neurons project bilaterally during postnatal development and following early contralateral cortical lesions

The corticorubral projections in adult cats are primarily uncrossed. However, early in development and after early unilateral lesions of the sensorimotor cortex, crossed corticorubral projections are also observed. The present study was performed to disclose (1) whether the crossed projections originate from neuronal subpopulations different from those producing uncrossed ones and (2) how the neurons that give rise to the crossed projections in the lesioned animals are related to those occurring in normal development. We injected fluorescent latex microspheres into the red nucleus of two groups of animals: (1) intact kittens at postnatal week 3 and (2) kittens that had received unilateral ablation of the cerebral cortex at this stage and were then allowed to survive for at least 4 weeks. Red fluorescing microspheres were injected on one side and green ones on the other. In both normal and lesioned kittens, a number of cells in the cortex were labeled as a result of the contralateral as well as the ipsilateral injections, and no difference in size or distribution was found between the cells labeled from contralateral and ipsilateral injections. More than half of the cells labeled from contralateral injections were double-labeled in both groups of animals. These results indicate that individual corticorubral cells project bilaterally in normal development as well as following unilateral lesions of the cortex. With respect to the cells producing crossed projections, they were similar in both laminar and regional distributions between the intact and lesioned animal, suggesting that the crossed projections arise from the same neuronal subpopulation before and after cortical lesions. This view was supported by sequential injections of the tracers, which indicated that cells normally projecting contralaterally maintained the crossed projection after the lesions. Taking into account our previous observations that growth and proliferation of crossed corticorubral axons took place in the red nucleus (Murakami et al. 1991a), it is likely that growth and proliferation of the axons in denervated targets play a major role in lesion-induced establishment of aberrant projections.     

Projections from the lateral vestibular nucleus to the spinal cord in the mouse

The present study investigated the projections from the lateral vestibular nucleus (LVe) to the spinal cord using retrograde and anterograde tracers. Retrogradely labeled neurons were found after fluoro-gold injections into both the cervical and lumbar cord, with a smaller number of labeled neurons seen after lumbar cord injections. Labeled neurons in the LVe were found in clusters at caudal levels of the nucleus, and a small gap separated these clusters from labeled neurons in the spinal vestibular nucleus (SpVe). In the anterograde study, BDA-labeled fiber tracts were found in both the ventral and ventrolateral funiculi on the ipsilateral side. These fibers terminated in laminae 6–9. Some fibers were continuous with boutons in contact with motor neurons in both the medial and lateral motor neuron columns. In the lumbar and sacral segments, some collaterals from the ipsilateral vestibulospinal tracts were found on the contralateral side, and these fibers mainly terminated in laminae 6–8. The present study reveals for the first time the fiber terminations of the lateral vestibular nucleus in the mouse spinal cord and therefore enhances future functional studies of the vestibulospinal system.     

Peripheral Muscle Targets and Central Projections of the Mesencephalic Trigeminal Nucleus in Macaque Monkeys

The mesencephalic trigeminal nucleus (MesV) contains the somata of primary afferent neurons that innervate muscle spindles in masticatory muscles and mechanoreceptors in the periodontal ligaments. There are conflicting reports about additional peripheral targets of MesV, such as the extraocular muscles, as well as about its central targets. In addition, only limited primate data are available. Consequently, we examined MesV projections in macaque monkeys. The retrograde tracer wheat germ agglutinin‐conjugated horseradish peroxidase (WGA‐HRP) was injected into masticatory or extraocular muscles to define the peripheral targets of the primate MesV. Numerous labeled neurons were found in ipsilateral MesV after masticatory muscle injections. The scattered distribution of labeled cells, and their presence among clusters of unlabeled cells, suggests the muscle representations overlap. Just a few MesV neurons were labeled after extraocular muscle injections. This correlates with the small number of muscle spindles present in macaque extraocular muscles, suggesting MesV cells supplying extraocular muscle spindles may contribute a minor component to oculomotor proprioception. To examine the central connections of MesV, biotinylated dextran amine (BDA) was injected into the spinal trigeminal nucleus (Vs). The presence of retrogradely labeled MesV cells indicated a projection to Vs from MesV. These injections also anterogradely labeled terminals that lay in close association with MesV cells, suggesting an ascending projection from Vs to MesV. Finally, a small number of MesV neurons were labeled after WGA‐HRP injections into the upper cervical spinal cord. This pattern of central connections indicates MesV and Vs information is combined to guide mastication. Anat Rec, 291:974–987, 2008. © 2008 Wiley‐Liss, Inc.     

On the development of the pyramidal tract in the rat

Summary An anterograde tracer study has been made of the developing corticospinal tract (CST) in the rat using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Analysis of normal Rager stained material revealed that corticospinal axons reach upper cervical spinal cord levels at the day of birth (PO). Postnatal rats ranging in age from one (P1) to fourteen (P14) days received multiple WGA-HRP injections into the cortex of their left hemisphere and were allowed to survive for 24 h. The first labeled CST fibers caudally extend into the third thoracic spinal cord segment at P1; into the eighth thoracic segment at P3; into the first or second lumbar segment at P7 and into the second to third sacral segment at Pg. Thus the outgrowth of the leading pioneer fibers of the CST is completed at P9 but later developing axons are continuously added even beyond P9. Quantitative analysis of the amount of label along the length of the outgrowing CST revealed a characteristic pattern of labeling varying with age. The most striking features of that pattern are: (1) the formation of two standing peaks at the level of the cervical and lumbar enlargements respectively and (2) the transient presence of a smaller running peak which moves caudally with the front of the outgrowing bundle. The standing peaks are ascribed to the branching of the axon terminals at both intumescences, whereas the running peak probably arises by the accumulation of tracer within the growth cones at the tips of the outgrowing CST axons. Factors such as the number of axons, the varying axon diameters, the branching collaterals, the presence of varicosities, the transport rate of the tracer, the uptake of the tracer at the injection site, which possibly may affect the amount of label present in both the entire bundle and in the individual axons are discussed. Current research is focused upon an analysis of the relation between the site of injection within the cortex and the pattern of labeling of the CST. A delay of two days was found between the arrival of the CST axons at a particular spinal cord level and their outgrowth into the adjacent spinal gray. However, combined HRP and electronmicroscopic experiments are necessary to determine the factors behind the maturation of the CST as well as the maturation of the spinal gray.     

Anatomical plasticity of the tectospinal tract after unilateral lesion of the superior colliculus in the neonatal rat

Summary After unilateral ablation of the superior colliculus (SC) in neonatal or adult rats, the reorganization of the tectospinal tract (TST) was examined using the technique of anterograde transport of horseradish peroxidase to which wheat germ agglutinin had been conjugated (WGA-HRP). In neonatally lesioned rats, aberrant labeled terminals of TST axons were found on the ipsilateral side of the spinal cord. Postnatal development of the TST was then studied by retrograde transport of HRP to determine whether the aberrant tectospinal projections resulted from normally transient ipsilateral projections that persisted in operated rats or were due to collateral sprouting of projections to the contralateral projection field. The results failed to show an ipsilateral projection from the SC to spinal cord in normal neonatal rats. However, in neonatally lesioned rats, aberrant labeled fibers were observed recrossing the midline of the cervical spinal cord. Therefore, the increase in labeled terminals on the ipsilateral side following unilateral SC ablation appeared to originate from collateral sprouting at the spinal cord level of TST fibers from the intact pathway.     

大鼠前庭神经核群向腰髓投射特征——BDA法逆行研究

目的　研究大鼠前庭神经核群向脊髓的投射纤维特征。方法　在 7例SD大鼠采用结合生物素的葡聚糖胺(BDA)逆行法观察大鼠前庭核群向脊髓的投射。结果　除前庭神经上核 (SVN)外的其余各前庭核均有向大鼠腰髓的投射 ,单侧注射的实验动物中 ,前庭神经内侧核 (MVN)、外侧核 (LVN)和降核 (DVN)的标记神经元可见于双侧 ,其中MVN和LVN的标记神经元以注射同侧占优势 ,而DVN标记神经元两侧数量基本一致。结论　大鼠前庭脊髓尾侧束发出纤维投向脊髓腰段     

Ipsilateral, ventral corticospinal tract of the adult rat: ultrastructure, myelination and synaptic connections

The corticospinal tract (CST) of the rat is a widely used model system in developmental, physiological, and regeneration studies. The CST of the rat consists of a main tract, that runs in the dorsomedial funiculus and several minor components. We have shown earlier that one of the minor components, the ipsilateral, ventral CST, projects all the way down the spinal cord in the adult rat and single fibers form large terminal arbors in their spinal target areas. Here we investigated its ultrastructure and compared it to that of CST fibers of the main tract. By the use of anterograde axonal tracing with biotin dextran-amine (BDA) and pre-embedding avidin-peroxidase histochemistry we investigated axon diameters and myelination using electron microscopy. Ipsilateral, ventral CST fibers were found to run in the ventral funiculus close to the midline. They were intermingled with heavily myelinated large diameter axons, presumably reticulospinal, vestibulospinal, or tectospinal fibers. Ipsilateral, ventral CST fibers were of small diameter (0.68 m, ±0.04) and about [frac34] of them were moderately myelinated (9.64 ± 0.7 layers of myelin). Co-localization of a rhodamine-dextrane anterograde tracer with the presynaptic marker synaptophysin using confocal microscopy and electron microscopy revealed varicosities on terminal arborisations to be presynaptic boutons and clearly demonstrated contacts to neurons in intermediate laminae of the spinal cord at lumbar spinal levels. This study extends our earlier work indicating that the ipsilateral, ventral CST component of the adult rat is a morphologically complete CST component and may perform similar functions to the main CST component.     

Pericruciate cortical neurons projecting to brain stem reticular formation,dorsal column nuclei and spinal cord in the cat

Cells of origin of the pericruciate cortical fibers to the bulbar medial reticular formation, the dorsal column nuclei and the pinal cord in the cat were identified by means of the retrograde axonal transport of horseradish peroxidase. After injection of the enzyme in the dorsal column nuclei or the spinal cord many layer V pyramidal neurons were labeled retrogradely in areas 2, 3 and 4 (see ref. 6), but area 4 giant Betz cells were only labeled after spinal cord injections. Medial bulbar reticular formation injections resulted in the labeling of pyramidal neurons mainly in area 6.     

Contralateral corticorubral fibers induced by neonatal lesions are not collaterals of the normal ipsilateral projection

Unilateral neonatal cortical ablation induces the development of a bilateral corticorubral projection from the remaining sensorimotor cortex. The retrograde fluorescent tracers Fast blue (FB) and Nuclear yellow (NY) were used to determine if the aberrant contralateral projection arises from axon collaterals of the normal uncrossed projection. Six to 8 weeks after unilateral cortical ablation in neonatal rats, the red nuclei were injected with FB on one side and NY on the other to study the source of the normal and aberrant afferents from the cerebral cortex. In control animals, many neurons in layer V of the sensorimotor cortex were retrogradely labeled with the tracer that had been injected into the ipsilateral red nucleus. In animals with unilateral ablations, many neurons throughout the remaining sensorimotor cortex were retrogradely labeled with FB or NY. No cortical neurons were doubly labeled. In addition to demonstrating the bilaterality of the corticorubral projection in animals which had received neonatal lesions, these results indicate that the aberrant contralateral corticorubral projection does not consist of axon collaterals of the normal ipsilateral fibers.