Hindlimb Representation Research Articles

Thalamocortical and intracortical projections to the forelimb-stump SI representation of rats that sustained neonatal forelimb removal

We previously reported the abnormal expression of hindlimb receptive fields in the stump representation of the primary somatosensory cortex (SI) in rats that sustained neonatal forelimb removal when cortical gamma-aminobutyric acid (GABA) receptors were pharmacologically blocked (Lane et al. [1997] J. Neurophysiol. 77:2723-2735). In this study, we attempted to identify the substrate for this functional modification. Three potential substrates were examined: 1) changes in intracortical connections within SI; 2) alterations in the projection pattern of thalamocortical afferents from the ventroposterior lateral (VPL) nucleus to SI; and 3) changes in the receptive fields of thalamocortical neurons. We used biotinylated dextran amine and Phaseolus vulgaris leucoagglutinin to examine the intracortical projections associated with the stump and hindlimb representations of SI. True Blue and Diamidino Yellow were used to study the organization of the VPL projections to SI. Finally, single-unit recordings from VPL neurons were made to examine the functional organization of this nucleus in neonatally amputated adult rats. Tracer studies demonstrated no significant change in the intracortical connections or VPL projections associated with the stump and hindlimb SI in neonatally amputated rats. Recordings from VPL of neonatally manipulated rats revealed a small, but significant, population of cells (19.0%) within the stump representation that had dual stump and hindlimb receptive fields. Thus, the data suggest that the functional reorganization observed in SI of neonatally amputated rats may reflect functional alterations occurring in its thalamic inputs.

Input-output connections of the ?hindlimb? region of the inferior olive in cats

The aim of the present study was to establish whether gracile afferents to the inferior olive are topographically organized and whether such inputs coincide with the location of cells that output to the hindlimb C1 zone in the cerebellar posterior lobe. Small injections (n=15) of the anterograde tracers Fluoro-Ruby or biotinylated dextran amine were made into gracile, resulting in anterograde labelling often distributed in partially separate, rostrocaudally directed columns within the lateral half of the contralateral rostral dorsal accessory olive (rDAO). In 12 cases, anterograde labelling was also located within the caudolateral medial accessory olive. Evidence for a topographical organization was obtained, suggesting that medial gracile targets only the most lateral part of rDAO, whereas lateral gracile also targets more medial parts. Gracile injections centred dorsally also resulted in more extensive terminal fields in rDAO than similar sized injections centred ventrally which may relate to the representation of the distal hindlimb in dorsal gracile (e.g., Millar and Basbaum, 1975). Injections of retrograde tracer (green beads) into the C1 zone in the ipsilateral caudal paramedian lobule (7 cases), resulted in retrograde cell labelling in the middle one-third of contralateral DAO in two columns that fused caudally. The proportion of labelled cells that overlapped with anterograde labelled terminals was related to the dorsoventral locus of the gracile injection site: gracile injections centred dorsally produced the largest degree of overlap and therefore, potentially, the greatest functional convergence. In summary, the results suggest that a topographical organization is present within the gracilo-olivo-cerebellar projection which is discussed in relation to the functional organization of the olivocerebellar system.

Input‐output connections of the “hindlimb” region of the inferior olive in cats

The aim of the present study was to establish whether gracile afferents to the inferior olive are topographically organized and whether such inputs coincide with the location of cells that output to the hindlimb C1 zone in the cerebellar posterior lobe. Small injections (n = 15) of the anterograde tracers Fluoro-Ruby or biotinylated dextran amine were made into gracile, resulting in anterograde labelling often distributed in partially separate, rostrocaudally directed columns within the lateral half of the contralateral rostral dorsal accessory olive (rDAO). In 12 cases, anterograde labelling was also located within the caudolateral medial accessory olive. Evidence for a topographical organization was obtained, suggesting that medial gracile targets only the most lateral part of rDAO, whereas lateral gracile also targets more medial parts. Gracile injections centred dorsally also resulted in more extensive terminal fields in rDAO than similar sized injections centred ventrally which may relate to the representation of the distal hindlimb in dorsal gracile (e.g., Millar and Basbaum, 1975). Injections of retrograde tracer (green beads) into the C1 zone in the ipsilateral caudal paramedian lobule (7 cases), resulted in retrograde cell labelling in the middle one-third of contralateral DAO in two columns that fused caudally. The proportion of labelled cells that overlapped with anterograde labelled terminals was related to the dorsoventral locus of the gracile injection site: gracile injections centred dorsally produced the largest degree of overlap and therefore, potentially, the greatest functional convergence. In summary, the results suggest that a topographical organization is present within the gracilo-olivo-cerebellar projection which is discussed in relation to the functional organization of the olivocerebellar system. J. Comp. Neurol. 399:513–529, 1998. © 1998 Wiley-Liss, Inc.

Excitability changes and glucose metabolism in experimentally induced focal cortical dysplasias.

Malformations of cortical development are increasingly recognized in association with severe epileptic syndromes, neuropsychological disorders and mental retardation. Several clinical and experimental studies suggest that functional consequences of cortical dysplasias are not restricted to the area of the dysplastic lesion but also involve remote brain regions. In the present study cortical malformations were induced in newborn rats at day of birth by intracerebral injection of the glutamatergic agonist ibotenate. The resulting cytoarchitectonic lesion associates neuronal depopulation of deep cortical layers, ectopic neurons in superficial layers and sulcus formation, mimicking human polymicrogyria and migration disorders. Electrophysiological recordings of evoked field potentials in slice preparations of adult animals reveal hyperexcitability in widespread cortical regions surrounding the dysplasia. Low-intensity stimulation induced epileptiform activity consisting of long-lasting, multiphasic and N-methyl-D-aspartate-dependent field responses. They appeared with high variability as all-or-none events. These widespread changes in excitability were not observed in sham-operated animals with small superficial ectopias but intact deep cortical layers, indicating that focal loss of these layers induces extended alterations in cortical connectivity and imbalance of excitation and inhibition. Restricted zones of increased excitability were also found in the forelimb and hindlimb representation cortex in sham-operated and control animals, demonstrating that this activity has to be considered as an intrinsic property of specific cortical areas. Deoxyglucose autoradiography showed that the widespread hyperexcitability in ibotenate-injected animals was not accompanied by alterations in glucose metabolism, although in the area of structural abnormality a typical metabolic pattern was found, revealing an increased glucose uptake in layer I. Hypometabolism as described for many types of human dysplastic lesions was not observed. This difference between the experimental and clinical data may be due to the absence of behavioral seizures in this model. However, it can be hypothesized that in patients with developmental malformations, additional pathogenic factors contribute to the manifestation of seizure disorders.

Hypergravity exposure decreases gamma-aminobutyric acid immunoreactivity in axon terminals contacting pyramidal cells in the rat somatosensory cortex: a quantitative immunocytochemical image analysis.

Quantitative evaluation of gamma-aminobutyric acid immunoreactivity (GABA-IR) in the hindlimb representation of the rat somatosensory cortex after 14 days of exposure to hypergravity (hyper-G) was conducted by using computer-assisted image processing. The area of GABA-IR axosomatic terminals apposed to pyramidal cells of cortical layer V was reduced in rats exposed to hyper-G compared with control rats, which were exposed either to rotation alone or to vivarium conditions. Based on previous immunocytochemical and behavioral studies, we suggest that this reduction is due to changes in sensory feedback information from muscle receptors. Consequently, priorities for muscle recruitment are altered at the cortical level, and a new pattern of muscle activity is thus generated. It is proposed that the reduction observed in GABA-IR of the terminal area around pyramidal neurons is the immunocytochemical expression of changes in the activity of GABAergic cells that participate in reprogramming motor outputs to achieve effective movement control in response to alterations in the afferent information.

Hypergravity exposure decreases γ‐aminobutyric acid immunoreactivity in axon terminals contacting pyramidal cells in the rat somatosensory cortex: A quantitative immunocytochemical image analysis

Quantitative evaluation of γ-aminobutyric acid immunoreactivity (GABA-IR) in the hindlimb representation of the rat somatosensory cortex after 14 days of exposure to hypergravity (hyper-G) was conducted by using computer-assisted image processing. The area of GABA-IR axosomatic terminals apposed to pyramidal cells of cortical layer V was reduced in rats exposed to hyper-G compared with control rats, which were exposed either to rotation alone or to vivarium conditions. Based on previous immunocytochemical and behavioral studies, we suggest that this reduction is due to changes in sensory feedback information from muscle receptors. Consequently, priorities for muscle recruitment are altered at the cortical level, and a new pattern of muscle activity is thus generated. It is proposed that the reduction observed in GABA-IR of the terminal area around pyramidal neurons is the immunocytochemical expression of changes in the activity of GABAergic cells that participate in reprogramming motor outputs to achieve effective movement control in response to alterations in the afferent information. J. Neurosci. Res. 53:135–142, 1998. © 1998 Wiley-Liss, Inc.

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15–27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia, whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [ 3H]dizocilpine, [ 3H] d, l- α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [ 3H]kainate and [ 3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [ 3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [ 3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [ 3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [ 3H] d, l- α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [ 3H]kainate binding sites did not show significant changes. The [ 3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [ 3H] d, l- α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprizes an up-regulation of the [ 3H]dizocilpine binding sites in the ion channels of N-methyl- d-aspartate receptors and a down-regulation of [ 3H]muscimol binding sites of the GABA A receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Effects of microgravity on muscle and cerebral cortex: A suggested interaction

The “slow” antigravity muscle adductor longus was studied in rats after 14 days of spaceflight (SF). The techniques employed included standard methods for light microscopy, neural cell adhesion molecule (N-CAM) immunocytochemistry and electron microscopy. Light and electron microscopy revealed myofiber atrophy, segmental necrosis and regenerative myofibers. Regenerative myofibers were N-CAM immunoreactive (N-CAM-IR). The neuromuscular junctions showed axon terminals with a decrease or absence of synaptic vesicles, degenerative changes, vacant axonal spaces and changes suggestive of axonal sprouting. No alterations of muscle spindles was seen either by light or electron microscopy. These observations suggest that muscle regeneration and denervation and synaptic remodeling at the level of the neuromuscular junction may take place during spaceflight. In a separate study, GABA immunoreactivity (GABA-IR) was evaluated at the level of the hindlimb representation of the rat somatosensory cortex after 14 days of hindlimb unloading by tail suspension (“simulated” microgravity). A reduction in number of GABA-immunoreactive cells with respect to the control animals was observed in layer Va and Vb. GABA-IR terminals were also reduced in the same layers, particularly those terminals surrounding the soma and apical dendrites of pyramidal cells in layer Vb. On the basis of previous morphological and behavioral studies of the neuromuscular system after spaceflight and hindlimb suspension it is suggested that after limb unloading there are alterations of afferent signaling and feedback information from intramuscular receptors to the cerebral cortex due to modifications in the reflex organization of hindlimb muscle groups. We propose that the changes observed in GABA immunoreactivity of cells and terminals is an expression of changes in their modulatory activity to compensate for the alterations in the afferent information.

Organization of the projections from the pericruciate cortex to the pontomedullary brainstem of the cat: A study using the anterograde tracerPhaseolus vulgaris-leucoagglutinin

The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was used to study the distribution and density of the projections that originate from four identified subdivisions of the pericruciate cortex (namely, the forelimb and hind limb representations of area 4, area 6a beta, and area 6a gamma) and that terminate in the pontomedullary brainstem in the cat. Injections of PHA-L in all areas of the pericruciate cortex labelled numerous fibers and their terminal swellings in the brainstem. The major target regions of all four cortical areas were the pontine nuclei and the pontomedullary reticular formation (PMRF). Injections into both the forelimb and hind limb representations of area 4 and into area 6a beta resulted in a dense pattern of terminal labelling in restricted regions of the medial and lateral parts of the ipsilateral pontine nuclei. The labelling following the area 6a beta injection was spatially distinct from that seen following the area 4 injections. Injections into the forelimb representation of area 4 as well as into area 6a beta and 6a gamma resulted in the labelling of numerous terminal swellings bilaterally in the PMRF; in contrast, there were few labelled terminal swellings in the PMRF following injections into the hind limb representation of area 4. Terminal swellings on individual corticoreticular fibers were far less densely aggregated than those in the pontine nuclei. The dense pattern of innervation to restricted regions of the pontine nuclei supports previous suggestions that the corticopontine projections retain a high degree of topographical specificity that could be used in the control of discrete voluntary movements. In contrast, the more diffuse pattern of the projections to the PMRF may facilitate the selection and activation of the complex postural patterns that accompany voluntary movement.

Thalamic NMDA receptors modulate inflammation-produced hyperalgesia in the rat

The effects of inhibition of thalamic NMDA receptor function and synthesis on thermal and mechanical hyperalgesia induced by hindlimb intraplantar injection of carrageenan in the rat were studied in the `acute' phase (within 3–5 h) and the `subacute' phase (24 h) after carrageenan administration. Blockade of NMDA receptors was produced by intrathalamic injection of d,2-amino-5-phosphonovaleric acid (D-APV) and NMDA receptor synthesis was decreased (or not) by pretreatment of rats with intrathalamic (hindlimb representation area) injections of antisense, sense or missense oligodeoxynucleotides (ODNs) directed against the NR1 subunit of the NMDA receptor complex. Treatment with D-APV, but not saline, in the contralateral (but not ipsilateral) thalamus significantly reduced both the acute thermal and mechanical hyperalgesia in the injected paw; these same rats demonstrated significantly less thermal and mechanical hyperalgesia in the sub-acute phase than rats that had received saline or D-APV in the ipsilateral thalamus. None of the treatments had any effect on withdrawal responses of the uninjected hindpaw. Rats pretreated with NR1 sense or missense ODNs developed both thermal and mechanical hyperalgesia that was equivalent in magnitude and duration to rats that received intrathalamic saline injections. In contrast, rats pretreated with NR1 antisense ODN did not develop either acute or subacute thermal hyperalgesia; they developed less mechanical hyperalgesia than saline, sense or missense ODN-treated rats. Antisense ODN-treated rats also displayed a decrease in the number of thalamic NMDA receptors as determined by receptor binding assay. These results suggest an involvement of thalamic NMDA receptors in the development and maintenance of hyperalgesia associated with neurogenic inflammation in a model of tonic pain.

Trajectory of single primary afferents from the intercostal muscle spindle in the cat

Although the corticospinal tract is not essential for the production of the basic locomotor rhythm in cats, it does contribute to the regulation of locomotion, particularly in situations in which there is a requirement for precise control over paw placement or limb trajectory. Lesions of the dorsolateral funiculi at the low thoracic level (T13) that completely interrupted both the cortico- and rubrospinal pathways produced long-term deficits in locomotion on a level surface. These deficits included a paw-drag that was probably caused both by a loss of cortico- and rubrospinal input to motoneurones controlling distal muscles as well as by a change in the relative timing of muscles acting around the hip and knee. Smaller lesions produced similar deficits from which the cats recovered relatively quickly. Cats with the largest lesions of the dorsolateral funiculi were unable to modify their gait sufficiently to step over obstacles attached to the treadmill belt even 3–5 months postlesion. These results imply that the medial pathways, the reticulo- and vestibulospinal pathways, are unable to fully compensate for damage to the lateral pathways. Single unit recordings from identified pyramidal tract neurones (PTNs) within the hindlimb representation of the primary motor cortex (area 4) showed that a substantial proportion of neurones (67%) significantly increased their discharge frequency when the cats modified their gait to step over obstacles attached to the treadmill belt. Of those PTNs that showed increased activity during the swing phase, populations of neurones were activated at different times. A large proportion of PTNS discharged early in swing, in phase with knee flexors such as the semitendinosus. Others discharged slightly later, in phase with the activity of ankle flexors, such as tibialis anterior, while still others discharged at the end of swing, in phase with digit dorsiflexors, such as the extensor digitorum brevis. We suggest that different populations of cortical neurones may specifically modify the activity of selected groups of close synergistic muscles during different parts of the swing phase. We further suggest that these modifications are mediated, in part, by groups of interneurones that are involved in determining the base locomotor rhythm. This provides a means by which the changes specified by the descending signal from the motor cortex may be smoothly, and appropriately, incorporated into the locomotor cycle.

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nuclei

Although the corticospinal tract is not essential for the production of the basic locomotor rhythm in cats, it does contribute to the regulation of locomotion, particularly in situations in which there is a requirement for precise control over paw placement or limb trajectory. Lesions of the dorsolateral funiculi at the low thoracic level (T13) that completely interrupted both the cortico- and rubrospinal pathways produced long-term deficits in locomotion on a level surface. These deficits included a paw-drag that was probably caused both by a loss of cortico- and rubrospinal input to motoneurones controlling distal muscles as well as by a change in the relative timing of muscles acting around the hip and knee. Smaller lesions produced similar deficits from which the cats recovered relatively quickly. Cats with the largest lesions of the dorsolateral funiculi were unable to modify their gait sufficiently to step over obstacles attached to the treadmill belt even 3–5 months postlesion. These results imply that the medial pathways, the reticulo- and vestibulospinal pathways, are unable to fully compensate for damage to the lateral pathways. Single unit recordings from identified pyramidal tract neurones (PTNs) within the hindlimb representation of the primary motor cortex (area 4) showed that a substantial proportion of neurones (67%) significantly increased their discharge frequency when the cats modified their gait to step over obstacles attached to the treadmill belt. Of those PTNs that showed increased activity during the swing phase, populations of neurones were activated at different times. A large proportion of PTNS discharged early in swing, in phase with knee flexors such as the semitendinosus. Others discharged slightly later, in phase with the activity of ankle flexors, such as tibialis anterior, while still others discharged at the end of swing, in phase with digit dorsiflexors, such as the extensor digitorum brevis. We suggest that different populations of cortical neurones may specifically modify the activity of selected groups of close synergistic muscles during different parts of the swing phase. We further suggest that these modifications are mediated, in part, by groups of interneurones that are involved in determining the base locomotor rhythm. This provides a means by which the changes specified by the descending signal from the motor cortex may be smoothly, and appropriately, incorporated into the locomotor cycle.

1561 Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nuclei

Although the corticospinal tract is not essential for the production of the basic locomotor rhythm in cats, it does contribute to the regulation of locomotion, particularly in situations in which there is a requirement for precise control over paw placement or limb trajectory. Lesions of the dorsolateral funiculi at the low thoracic level (T13) that completely interrupted both the cortico- and rubrospinal pathways produced long-term deficits in locomotion on a level surface. These deficits included a paw-drag that was probably caused both by a loss of cortico- and rubrospinal input to motoneurones controlling distal muscles as well as by a change in the relative timing of muscles acting around the hip and knee. Smaller lesions produced similar deficits from which the cats recovered relatively quickly. Cats with the largest lesions of the dorsolateral funiculi were unable to modify their gait sufficiently to step over obstacles attached to the treadmill belt even 3–5 months postlesion. These results imply that the medial pathways, the reticulo- and vestibulospinal pathways, are unable to fully compensate for damage to the lateral pathways. Single unit recordings from identified pyramidal tract neurones (PTNs) within the hindlimb representation of the primary motor cortex (area 4) showed that a substantial proportion of neurones (67%) significantly increased their discharge frequency when the cats modified their gait to step over obstacles attached to the treadmill belt. Of those PTNs that showed increased activity during the swing phase, populations of neurones were activated at different times. A large proportion of PTNS discharged early in swing, in phase with knee flexors such as the semitendinosus. Others discharged slightly later, in phase with the activity of ankle flexors, such as tibialis anterior, while still others discharged at the end of swing, in phase with digit dorsiflexors, such as the extensor digitorum brevis. We suggest that different populations of cortical neurones may specifically modify the activity of selected groups of close synergistic muscles during different parts of the swing phase. We further suggest that these modifications are mediated, in part, by groups of interneurones that are involved in determining the base locomotor rhythm. This provides a means by which the changes specified by the descending signal from the motor cortex may be smoothly, and appropriately, incorporated into the locomotor cycle.

Quantitative changes of GABA‐immunoreactive cells in the hindlimb representation of the rat somatosensory cortex after 14‐day hindlimb unloading by tail suspension

The present study was aimed at evaluating quantitatively γ-aminobutyric acid (GABA) immunoreactivity in the hindlimb representation of the rat somatosensory cortex after 14 days of hindlimb unloading by tail suspension. A reduction in the number of GABA-immunoreactive cells with respect to the control animals was observed in layer Va and Vb. GABA-containing terminals were also reduced in the same layers, particularly those terminals surrounding the soma and apical dendrites of pyramidal cells in layer Vb. On the basis of previous morphological and behavioral studies of the neuromuscular system of hindlimb-suspended animals, it is suggested that the unloading due to hindlimb suspension alters afferent signaling and feedback information from intramuscular receptors to the cerebral cortex due to modifications in the reflex organization of hindlimb muscle groups. We propose that the reduction in immunoreactivity of local circuit GABAergic neurons and terminals is an expression of changes in their modulatory activity to compensate for the alterations in the afferent information. © 1996 Wiley-Liss, Inc.

Quantitative changes of GABA-immunoreactive cells in the hindlimb representation of the rat somatosensory cortex after 14-day hindlimb unloading by tail suspension.

The present study was aimed at evaluating quantitatively gamma-aminobutyric acid (GABA) immunoreactivity in the hindlimb representation of the rat somatosensory cortex after 14 days of hindlimb unloading by tail suspension. A reduction in the number of GABA-immunoreactive cells with respect to the control animals was observed in layer Va and Vb. GABA-containing terminals were also reduced in the same layers, particularly those terminals surrounding the soma and apical dendrites of pyramidal cells in layer Vb. On the basis of previous morphological and behavioral studies of the neuromuscular system of hindlimb-suspended animals, it is suggested that the unloading due to hindlimb suspension alters afferent signaling and feedback information from intramuscular receptors to the cerebral cortex due to modifications in the reflex organization of hindlimb muscle groups. We propose that the reduction in immunoreactivity of local circuit GABAergic neurons and terminals is an expression of changes in their modulatory activity to compensate for the alterations in the afferent information.

Somatotopy of monkey premotor cortex examined with microstimulation

We reinvestigated the organization of the premotor cortex (PM) using intracortical microstimulation. Movements of forelimb, hindlimb, and orofacial structures were evoked from broad regions of PM that appeared to be contiguous with other motor areas. There were two principal findings: (1) the somatotopy of PM lies roughly parallel to that of the primary motor cortex (MI). Forelimb movements were evoked from sites deep in the caudal bank of the arcuate sulcus and throughout the adjacent cortex bounded by a face representation (laterally) and a hindlimb representation (medially and caudally); (2) unlike the MI, the PM forelimb representation overlaps significantly with its own face representation. PM hindlimb movement sites overlap only slightly with PM forelimb sites, in a manner similar to the MI. There was no obvious boundary between PM, Ml, or supplementary motor area hindlimb representations. The present findings are discussed in relation to recently identified subdivisions of the PM.

Lesion-induced reorganization in the brainstem is not completely expressed in somatosensory cortex.

Electrophysiological and neuroanatomical methods were used to determine the extent to which neonatal forelimb removal altered the organization of the cuneate nucleus and representations of the fore- and hindlimbs in the primary somatosensory cortex of adult rats. Neonatal forelimb removal resulted in invasion of the cuneate nucleus by sciatic nerve primary afferents and development of cuneothalamic projection neurons with split receptive fields that included both the hindlimb and forelimb stump. Mapping in the primary somatosensory cortex of the neonatally manipulated adult rats demonstrated abnormalities, but the major change observed in the cuneate nucleus was demonstrable at only a few (5%) cortical recording sites in the remaining stump representation and there were none at all in the hindlimb representation. These results suggest that lesion-induced brainstem reorganization may be functionally suppressed at either the thalamic or cortical level.

Evolution of cortical responsiveness subsequent to multiple forelimb nerve transections: an electrophysiological study in adult cat somatosensory cortex.

Multiunit recordings along mediolateral rows in the primary somatosensory cortex of the animals described by C. Avendaño, D. Umbriaco, R.W. Dykes, and L. Descarries (1995, J. Comp. Neurol. 354:321-332) provided information about the functional status of the regions in and near the deafferented cortex. Responses changed along this axis from normally organized receptive fields in the hindlimb representation through a transition zone of unusually small receptive fields into the clearly deafferented forelimb representation, where receptive fields were uncommon and often had unusual characteristics. The most abrupt change along this axis was the appearance of a repetitive, bursting discharge pattern in the multiunit activity near the border of the deprived cortex. The appearance of this pattern was used as a reference to describe differences between normal and deprived cortices. The nature of these differences evolved with time. Much of the deprived cortex lacked identifiable receptive fields for months after the nerve transections and, 1 year later, still only about half of the recording sites within the deprived region displayed organized receptive fields. Some sites within the deprived region lacking definable receptive fields could be excited at long latencies by somatic stimuli anywhere on the body. With time, regions of normal cortex near the border with the deprived zone became more involved in these processes. Spontaneous activity and thresholds also changed with time in both normal and deprived cortices.(ABSTRACT TRUNCATED AT 250 WORDS)

Limits on plasticity in somatosensory cortex of adult rats: hindlimb cortex is not reactivated after dorsal column section.

1. To better understand the limits and extents of plasticity in sensory systems of adult mammals, we unilaterally sectioned the dorsal funiculus at thoracic levels in nine adult rats to deactivate ascending afferents from the hindpaw and lower body. After postsurgical recovery periods of 3 h to 3 mo, the region of primary somatosensory cortex (S1) representing the limbs and trunk was extensively mapped with microelectrodes. 2. Recording sites were later identified as being within the hindlimb representation and other parts of S1 by relating locations of microlesions to the cytochrome oxidase pattern in sections of cortex cut tangential to the pial surface. The extent and effectiveness of spinal cord lesions were evaluated by injecting cholera toxin B subunit conjugated with horseradish peroxidase (B-HRP) at various sites in the deafferented hindpaw. 3. In five animals with complete section of the dorsal funiculus, we failed to detect any response to cutaneous stimulation of any part of the body in the deafferented hindlimb cortex. In four other animals with incomplete lesions, neurons in some penetrations could be activated by hindlimb stimulation, but not by stimulating other body parts. In those cases without activation of hindlimb cortex, B-HRP was detected in the spinal cord only caudal to the lesion, and it was not transported to the nucleus gracilis. Limited transport past the lesion to nucleus gracilis was detected in cases with incomplete lesions. 4. The results indicate that forelimb inputs do not substitute for missing hindlimb inputs in primary somatosensory cortex in rats and that the potential for somatotopic reorganization is more limited than previously thought.

Thalamic connections of the primary motor cortex (M1) of owl monkeys

To determine the relative contributions of transthalamic cerebellar and pallidal projections to the primary motor cortex (M1) of owl monkeys, we examined the thalamic labeling resulting from injections of fluorescent tracers and wheatgerm agglutinin/horseradish peroxidase conjugate (WGA-HRP) into regions of M1 identified by intracortical microstimulation. Injections were placed in the major somatotopic divisions of M1 (the hindlimb, trunk, forelimb, and face representations) and in the caudal and rostral M1 subareas. In most cases, we injected several differentiable tracers into different parts of M1. Our results indicate that the strongest connections of M1 are with subdivisions of the ventral lateral thalamus (VL); other connections are mainly with intralaminar nuclei (the central lateral, paracentral, and center median nuclei) and the reticular nucleus. Most projections are reciprocal and topographically organized. M1 is strongly connected with the principal (VLp), medial (VLx), and anterior (VLa) subdivisions of the VL complex but has at most weak connections with the dorsal division (VLd). Each of the major somatotopic divisions of M1 is connected with an anteroposteriorly elongated territory within the VL complex. The connections are somatotopically organized such that the M1 hindlimb representation is connected with a band of cells in the lateral and anterior portions of the VL complex (spanning VLa and VLp), whereas the trunk, forelimb, and face representations are connected with successively more medially and posteriorly situated cell bands (spanning VLa, VLp, and VLx). There is some degree of overlap between the somatotopic territories within VL, although the absence of double-labeled cells in cases with injections of adjacent somatotopic divisions of M1 suggests that individual thalamic neurons project to single somatotopic regions. In addition to somatotopic differences, the connections of the caudal and rostral subdivisions of M1 differ to some extent. Caudal M1 is connected most strongly with VLp, a target of cerebellar projections, but it is also connected with VLa, which receives pallidal inputs. In complementary fashion, rostral M1 is most strongly connected with VLa, but it is also connected with VLp. VLx, a target of cerebellar projections, has significant connections with both caudal and rostral M1. These results indicate that all parts of M1 are influenced by both the cerebellum and globus pallidus in owl monkeys, as has been suggested in some recent studies of macaque monkeys.