Chapter 3 Development of the corticospinal system and spinal motor circuits

Selective injury of thalamocortical tract in neonatal rats impairs forelimb use: model validation and behavioral effects

Unilateral brain injury in neonates results in largely contralateral hand function in children. Most research investigating neurorehabilitation targets for movement recovery has focused on the effects of brain injury on descending motor systems, especially the corticospinal tract. However, a recent human study demonstrated that sensory tract injury may have larger effects on dexterity than motor tract injury. To validate that the sensory tract injury impairs dexterity, we modeled the most common site of sensory tract injury in neonates by targeting the thalamocortical tract. In the postnatal day 7 rats, we used three types of lesions to the thalamocortical tract: periventricular blood injection, photothrombotic lesion, and electrolytic lesion. To test the sensitivity and specificity of these techniques, viral tracers were injected into the primary sensory or motor cortex immediately after injury. Electrolytic lesions were the most specific and reproducible for inducing a lesion compared to the other two methods. Electrolytic lesions disrupted 63% of the thalamocortical tract, while sparing the adjacent corticospinal tract in the internal capsule. To measure the impact on dexterity, the cylinder exploration and pasta handling tests were used to test the changes of forelimb use at 8 weeks after injury when the rats reached maturity. Lesions to the thalamocortical tract were associated with a significant decrease in the use of the contralateral forelimb in the cylinder task, and the degree of impairment positively correlated with the degree of injury. Overall, specific sensory system lesions of the thalamocortical tract impair forelimb use, suggesting a key role for skilled movement.

Rodent Models of Developmental Ischemic Stroke for Translational Research: Strengths and Weaknesses

Cerebral ischemia can occur at any stage in life, but clinical consequences greatly differ depending on the developmental stage of the affected brain structures. Timing of the lesion occurrence seems to be critical, as it strongly interferes with neuronal circuit development and determines the way spontaneous plasticity takes place. Translational stroke research requires the use of animal models as they represent a reliable tool to understand the pathogenic mechanisms underlying the generation, progression, and pathological consequences of a stroke. Moreover, in vivo experiments are instrumental to investigate new therapeutic strategies and the best temporal window of intervention. Differently from adults, very few models of the human developmental stroke have been characterized, and most of them have been established in rodents. The models currently used provide a better understanding of the molecular factors involved in the effects of ischemia; however, they still hold many limitations due to matching developmental stages across different species and the complexity of the human disorder that hardly can be described by segregated variables. In this review, we summarize the key factors contributing to neonatal brain vulnerability to ischemic strokes and we provide an overview of the advantages and limitations of the currently available models to recapitulate different aspects of the human developmental stroke.

Plasticity in One Hemisphere, Control From Two: Adaptation in Descending Motor Pathways After Unilateral Corticospinal Injury in Neonatal Rats

After injury to the corticospinal tract (CST) in early development there is large-scale adaptation of descending motor pathways. Some studies suggest the uninjured hemisphere controls the impaired forelimb, while others suggest that the injured hemisphere does; these pathways have never been compared directly. We tested the contribution of each motor cortex to the recovery forelimb function after neonatal injury of the CST. We cut the left pyramid (pyramidotomy) of postnatal day 7 rats, which caused a measurable impairment of the right forelimb. We used pharmacological inactivation of each motor cortex to test its contribution to a skilled reach and supination task. Rats with neonatal pyramidotomy were further impaired by inactivation of motor cortex in both the injured and the uninjured hemispheres, while the forelimb of uninjured rats was impaired only from the contralateral motor cortex. Thus, inactivation demonstrated motor control from each motor cortex. In contrast, physiological and anatomical interrogation of these pathways support adaptations only in the uninjured hemisphere. Intracortical microstimulation of motor cortex in the uninjured hemisphere of rats with neonatal pyramidotomy produced responses from both forelimbs, while stimulation of the injured hemisphere did not elicit responses from either forelimb. Both anterograde and retrograde tracers were used to label corticofugal pathways. There was no increased plasticity from the injured hemisphere, either from cortex to the red nucleus or the red nucleus to the spinal cord. In contrast, there were very strong CST connections to both halves of the spinal cord from the uninjured motor cortex. Retrograde tracing produced maps of each forelimb within the uninjured hemisphere, and these were partly segregated. This suggests that the uninjured hemisphere may encode separate control of the unimpaired and the impaired forelimbs of rats with neonatal pyramidotomy.

The Change of Intra-cerebral CST Location during Childhood and Adolescence; Diffusion Tensor Tractography Study

Objectives: Corticospinal tract (CST) is the most important tract in motor control. However, there was no study about the change of CST location with aging. In this study, using diffusion tensor tractography (DTT), we attempted to investigate the change of CST location at cortex, corona radiata (CR) and posterior limb of internal capsule (IC) level with aging in typically developing children. Methods: We recruited 76 healthy pediatric subjects (range; 0-19 years). According to the result of DTT, the location of CST at cortex level was classified as follows; prefrontal cortex (PFC), PFC with Premotor cortex (PMC), PMC, PMC with primary motor cortex (M1), M1, M1 with Primary sensory cortex (S1). Anterior-posterior location (%) of CSTs at CR and IC level was also assessed. Results: DTT results about CSTs of 152 hemispheres from 76 subjects were obtained. The most common location of CST projection was M1 area (58.6%) including PMC with M1 (25.7%), M1 (17.8%), and M1 with S1 (15.1%). The mean age of the projection of CST showed considerably younger at anterior cortex than posterior; (PFC; 4.12 years, PFC with PMC; 6.41 years, PMC; 6.72 years, PMC with M1; 9.75 years, M1; 9.85 years, M1 with S1; 12.99 years, S1; 13.75 years). Spearman correlation showed positive correlation between age and the location of CST from anterior to posterior brain cortex (r = 0.368). Conclusion: We demonstrated that the location of CST projection is different with aging. The result of this study can provide the scientific insight to the maturation study in human brain.