Axon ramification following unilateral cortical ablation in neonatal rats

Mechanism of Restoration of Forelimb Motor Function after Cervical Spinal Cord Hemisection in Rats: Electrophysiological Verification

The objective of this study was to electrophysiologically assess the corticospinal tracts of adult rats and the recovery of motor function of their forelimbs after cervical cord hemisection. Of 39 adult rats used, compound muscle action potentials (CMAPs) of the forelimbs of 15 rats were evaluated, before they received left C5 segmental hemisection of the spinal cord, by stimulating the pyramid of the medulla oblongata on one side using an exciting microelectrode. All 15 rats exhibited contralateral electrical activity, but their CMAPs disappeared after hemisection. The remaining 24 rats received hemisection first, and CMAPs of 12 rats were assessed over time to study their recovery time. All of them exhibited electrical activity of the forelimbs in 4 weeks after surgery. The remaining 12 rats received additional right C2 segmental hemisection, and variation of CMAPs between before and after surgery was examined. The right side of the 12 rats that received the additional hemisection exhibited no electrical activity in response to the stimulation of the pyramids on both sides. These results suggest that changes in path between the resected and healthy sides, activation of the ventral corticospinal tracts, and propriospinal neurons were involved in the recovery of motor function after cervical cord injury.

Differential contributions of rostral and caudal frontal forelimb areas to compensatory process after neonatal hemidecortication in rats

Following brain damage, especially in juvenile animals, large-scale reorganization is known to occur in the remaining brain structures to compensate for functional deficits. In rats with neonatal hemidecortication, corticospinal fibers originating from the undamaged side of the sensorimotor cortex issue collateral sprouts to the ipsilateral spinal gray matter that mediate cortical excitation to ipsilateral forelimb motoneurons and compensate for the deficit in forelimb movements. The present study was designed to investigate the origins of the ipsilateral corticospinal projection in neonatally hemidecorticated rats. Corticospinal neurons (CSNs) were labeled in adults by injecting retrograde neural tracers, cholera toxin subunit B with different fluorescent probes, into either side of the cervical spinal gray matter. In the undamaged cortex, double-labeled neurons were rarely found. CSNs with contralateral projections (contra-CSNs) and those with ipsilateral projections (ipsi-CSNs) were distributed both in the rostral forelimb motor area (RFA) and the caudal forelimb motor area (CFA). However, there was a difference in the distributions of the ipsi-CSNs between the two forelimb areas. Whereas the distribution of the ipsi-CSNs largely overlapped with that of the contra-CSNs in the RFA, the ipsi-CSNs tended to be segregated from the contra-CSNs in the CFA. The results suggested that the RFA and the CFA contribute to the compensatory process in different ways.

Formation of descending pathways mediating cortical command to forelimb motoneurons in neonatally hemidecorticated rats

Neonatally hemidecorticated rats show fairly normal reaching and grasping behaviors of the forelimb contralateral to the lesion at the adult stage. Previous experiments using an anterograde tracer showed that the corticospinal fibers originating from the sensorimotor cortex of the intact side projected aberrant collaterals to the spinal gray matter on the ipsilateral side. The present study used electrophysiological methods to investigate whether the aberrant projections of the corticospinal tract mediated the pyramidal excitation to the ipsilateral forelimb motoneurons and, if so, which pathways mediate the effect in the hemidecorticated rats. Electrical stimulation to the intact medullary pyramid elicited bilateral negative field potentials in the dorsal horn of the spinal cord. In intracellular recordings of forelimb motoneurons, oligosynaptic pyramidal excitation was detected on both sides of the spinal cord in the hemidecorticated rats, whereas pyramidal excitation of motoneurons on the side ipsilateral to the stimulation was much smaller in normal rats. By lesioning the dorsal funiculus at the upper cervical level, we clarified that the excitation was transmitted to the ipsilateral motoneurons by at least two pathways: one via the corticospinal tract and spinal interneurons and the other via the cortico-reticulo-spinal pathways. These results suggested that in the neonatally hemidecorticated rats, the forelimb movements on the side contralateral to the lesion were modulated by motor commands through the indirect ipsilateral descending pathways from the sensorimotor cortex of the intact side either via the spinal interneurons or reticulospinal neurons.

Large-scale reorganization of corticofugal fibers after neonatal hemidecortication for functional restoration of forelimb movements

As an experimental model to study the mechanism of large-scale network plasticity of the juvenile brain, functional compensation after neonatal brain damage was studied in rats that received unilateral decortication at postnatal day 5. These animals exhibited a marked ability in reaching and grasping movements in the contralesional side of the forelimb when tested at 10-14 weeks of age. Additional lesion of the sensorimotor cortex in the remaining contralesional hemisphere at this stage resulted in severe impairment of both forelimbs. It was suggested that the sensorimotor cortex on the contralesional side was controlling the movements of both forelimbs. Following the injection of an anterograde tracer into the remaining sensorimotor cortex, the corticofugal axons from the remaining sensorimotor cortex were found to issue aberrant projections to the contralateral red nucleus, contralateral superior colliculus, contralateral pontine nuclei, ipsilateral dorsal column nucleus and ipsilateral gray matter of the cervical spinal cord, all of which appeared to be necessary for the control of contralesional forelimb movements. These results suggest that the forelimb movements on the contralesional side were compensated by large-scale reorganization of the corticofugal axons from the remaining sensorimotor cortex.

Targeting myelin to optimize plasticity of spared spinal axons

Functional re-innervation of target neurons following neurological damage such as spinal cord injury is an essential requirement of potential therapies. There are at least two avenues by which this can be achieved: (a) through the regeneration of injured axons and (b) through promoting plasticity of those spared by the initial insult. There are several reasons why the latter approach may be more feasible, not the least of which are the inhibitory character of the glial scar, the often long distances over which injured axons must regrow, and the fact that spared axons are often already in the vicinity of denervated targets. The challenge is to unveil the well-recognized intrinsic plasticity of spared axons in a way that avoids complications, such as pain or autonomic dysfunction. One approach that we as well as others have taken is to target growth-suppressing signaling pathways initiated in spared axons by myelin-derived proteins. This article reviews models used for the study of spinal axon plasticity and describes the anatomical and behavioral effects of interfering with myelinderived proteins, their receptors, and components of their intracellular signaling cascades.

Development of corticospinal tract fibers and their plasticity. II. Neonatal unilateral cortical damage and subsequent development of the corticospinal tract in mice

In this study, the right cerebral cortices of mice on postnatal day 0 (P0) were cryocoagulated with dry ice. Subsequent development of the corticospinal tract (CST) was studied morphologically and quantitatively, and was compared with that in age-matched controls. When the pyramidal tract was traced anterogradely by injecting HRP into the sensorimotor area of the left cerebral cortex of adult operated mice, the right CST originating from the healthy left hemisphere showed remarkable hypertrophy. The number of axons in the CST at the C4-C6 level became maximum on P14 in the control mice and rapidly decreased thereafter. In the operated mice, the axonal number in the right CST also was maximal on P14 and then rapidly decreased. However, the decrease in axonal number after P21 was less in the operated mice than in the controls. Moreover, the number of axons showed a slight increase after P56. These results indicate that the physiological elimination of the parent axons and their collaterals is much lower in the operated mice than in the controls, and that the increase in axon collaterals from parent axons in the hypertrophic right CST persists a long time in the operated mice.