John H. Martin

Corticospinal tract (CST) connections to spinal interneurons are conserved across species. We identified spinal interneuronal populations targeted by the CST in the cervical enlargement of the cat during development. We focused on the periods before and after laminar refinement of the CST terminations, between weeks 5 and 7. We used immunohistochemistry of choline acetyltransferase (ChAT), calbindin, calretinin, and parvalbumin to mark interneurons. We first compared interneuron marker distribution before and after CST refinement. ChAT interneurons increased, while calbindin interneurons decreased during this period. No significant changes were noted in parvalbumin and calretinin. We next used anterograde labeling to determine whether the CST targets different interneuron populations before and after the refinement period. Before refinement, the CST terminated sparsely where calbindin interneurons were located and spared ChAT interneurons. After refinement, the CST no longer terminated in calbindin-expressing areas but did so where ChAT interneurons were located. Remarkably, early CST terminations were dense where ChAT interneurons later increased in numbers. Finally, we determined whether corticospinal system activity was necessary for the ChAT and calbindin changes. We unilaterally inactivated M1 between weeks 5 and 7 by muscimol infusion. Inactivation resulted in a distribution of calbindin and ChAT in spinal gray matter regions where the CST terminates that resembled the immature more than mature pattern. Our results show that the CST plays a crucial role in restructuring spinal motor circuits during development, possibly through trophic support, and provide strong evidence for the importance of connections with key spinal interneuron populations in development of motor control functions.

The corticospinal tract is a predominantly crossed pathway. Nevertheless, the primary motor cortex (M1) is activated bilaterally during unilateral movements and several animal studies showed that M1 has a bilateral motor representation. A better understanding of the uncrossed corticospinal system is especially important for elucidating its role in recovery of limb control after unilateral injury. We used intracortical microstimulation (ICMS) to determine the representation of contralateral and ipsilateral forelimb joints at single M1 sites in the rat. Most sites representing an ipsilateral joint corepresented the same joint contralaterally. We next determined whether ipsilateral responses evoked in one hemisphere depended on the function of M1 in the opposite hemisphere using reversible inactivation and pyramidal tract lesion. Ipsilateral responses were eliminated when the homotopic forelimb area of M1 in the opposite hemisphere was inactivated or when the pyramidal tract on the nonstimulated side was sectioned. To determine the role of transfer between M1 in each hemisphere we sectioned the corpus callosum, which produced a 33% increase in ipsilateral ICMS thresholds. Neither M1 inactivation nor callosal section changed contralateral response thresholds, indicating the absence of tonic excitatory or inhibitory drive to the opposite M1. Finally, ipsilateral responses following M1 inactivation and pyramidal tract lesion could be evoked after systemic administration of the K⁺channel blocker 4-aminopyridine, suggesting the presence of latent connections. Our findings show important interactions between the corticospinal systems from each side, especially at the spinal level. This has important implications for recruiting the ipsilateral corticospinal system after injury.

Motor cortex (M1) activity between postnatal weeks 5 and 7 is essential for normal development of the corticospinal tract (CST) and visually guided movements. Unilateral reversible inactivation of M1, by intracortical muscimol infusion, during this period permanently impairs development of the normal dorsoventral distribution of CST terminations and visually guided motor skills. These impairments are abrogated if this M1 inactivation is followed by inactivation of the contralateral, initially active M1, from weeks 7 to 11 (termed alternate inactivation). This later period is when the M1 motor representation normally develops. The purpose of this study was to determine the effects of alternate inactivation on the motor representation of the initially inactivated M1. We used intracortical microstimulation to map the left M1 1 to 2 mo after the end of left M1 muscimol infusion. We compared representations in the unilateral inactivation and alternate inactivation groups. Alternate inactivation converted the sparse proximal M1 motor representation produced by unilateral inactivation to a complete and high-resolution proximal–distal representation. The motor map was restored by week 11, the same age that our present and prior studies demonstrated that alternate inactivation restored CST spinal connectivity. Thus M1 motor map developmental plasticity closely parallels plasticity of CST spinal terminations. After alternate inactivation reestablished CST connections and the motor map, an additional 3 wk was required for motor skill recovery. Since motor map recovery preceded behavioral recovery, our findings suggest that the representation is necessary for recovering motor skills, but additional time, or experience, is needed to learn to take advantage of the restored CST connections and motor map.

Motor development depends on forming specific connections between the corticospinal tract (CST) and the spinal cord. Blocking CST activity in kittens during the critical period for establishing connections with spinal motor circuits results in permanent impairments in connectivity and function. The changes in connections are consistent with the hypothesis that the inactive tract is less competitive in developing spinal connections than the active tract. In this study, we tested the competition hypothesis by determining whether activating CST axons, after previous silencing during the critical period, abrogated development of aberrant corticospinal connections and motor impairments. In kittens, we inactivated motor cortex by muscimol infusion between postnatal weeks 5 and 7. Next, we electrically stimulated CST axons in the medullary pyramid 2.5 h daily, between weeks 7 and 10. In controls (n= 3), CST terminations were densest within the contralateral deeper, premotor, spinal layers. After previous inactivation (n= 3), CST terminations were densest within the dorsal, somatic sensory, layers. There were more ipsilateral terminations from the active tract. During visually guided locomotion, there was a movement endpoint impairment. Stimulation after inactivation (n= 6) resulted in significantly fewer terminations in the sensory layers and more in the premotor layers, and fewer ipsilateral connections from active cortex. Chronic stimulation reduced the current threshold for evoking contralateral movements by pyramidal stimulation, suggesting strengthening of connections. Importantly, stimulation significantly improved stepping accuracy. These findings show the importance of activity-dependent processes in specifying CST connections. They also provide a strategy for harnessing activity to rescue CST axons at risk of developing aberrant connections after CNS injury.

To restore motor control after spinal cord injury requires reconnecting the brain with spinal motor circuits below the lesion. A bridge around the injury is an important alternative to promoting axon regeneration through the injury. Previously, we reported a novel motor bridge in rats. The thirteenth thoracic nerve was detached from the muscle it innervates and the cut end implanted caudally into the lumbar gray matter where motor bridge axons regenerate. In this study, we first determined that regenerating bridge axons project to spinal motor circuits. Stable projections were present in ventral motor laminae of the cord, including putative synapses directly on motoneurons, 2 months after insertion in the intact cord. At this time, earlier‐forming dorsal horn projections were mostly eliminated. Regenerating axons were effective in evoking leg motor activity as early as 2 weeks. We next determined that bridge axons could regenerate caudal to a chronic injury. We hemisected the spinal cord at L2 and inserted the bridge nerve 1 month later at L5 and found ventral laminae projections similar to those in intact animals, including onto motoneurons directly. Finally, we determined that the bridge circuit could be activated by neural pathways rostral to its origin. For spinally hemisected animals, we electrically stimulated the rostral spinal cord and recorded evoked potentials from the bridge and, in turn, motor responses in the sciatic nerve. Our findings suggests that bridge motoneurons could be used by descending motor pathways as premotor interneurons to transmit neural signals to bypass a chronic spinal injury. J. Comp. Neurol. 506:838–850, 2008.

Activity-dependent competition shapes corticospinal (CS) axon outgrowth in the spinal cord during development. An important question in neural repair is whether activity can be used to promote outgrowth of CS axons in maturity. After injury, spared CS axons sprout and make new connections, but often not enough to restore function. We propose that electrically stimulating spared axons after injury will enhance sprouting and strengthen connections with spinal motor circuits. To study the effects of activity, we electrically stimulated CS tract axons in the medullary pyramid. To study the effects of injury, one pyramid was lesioned. We studied sparse ipsilateral CS projections of the intact pyramid as a model of the sparse connections preserved after CNS injury. We determined the capacity of CS axons to activate ipsilateral spinal motor circuits and traced their spinal projections. To understand the separate and combined contributions of injury and activity, we examined animals receiving stimulation only, injury only, and injury plus stimulation. Both stimulation and injury alone strengthened CS connectivity and increased outgrowth into the ipsilateral gray matter. Stimulation of spared axons after injury promoted outgrowth that reflected the sum of effects attributable to activity and injury alone. CS terminations were densest within the ventral motor territories of the cord, and connections in these animals were significantly stronger than after injury alone, indicating that activity augments injury-induced plasticity. We demonstrate that activity promotes plasticity in the mature CS system and that the interplay between activity and injury preferentially promotes connections with ventral spinal motor circuits.

Activity-dependent competition between the corticospinal (CS) systems in each hemisphere drives postnatal development of motor skills and stable CS tract connections with contralateral spinal motor circuits. Unilateral restriction of motor cortex (M1) activity during an early postnatal critical period impairs contralateral visually guided movements later in development and in maturity. Silenced M1 develops aberrant connections with the contralateral spinal cord whereas the initially active M1, in the other hemisphere, develops bilateral connections. In this study, we determined whether the aberrant pattern of CS tract terminations and motor impairments produced by early postnatal M1 activity restriction could be abrogated by reducing activity-dependent synaptic competition from the initially active M1 later in development. We first inactivated M1 unilaterally between postnatal weeks 5–7. We next inactivated M1 on the other side from weeks 7–11 (alternate inactivation), to reduce the competitive advantage that this side may have over the initially inactivated side. Alternate inactivation redirected aberrant contralateral CS tract terminations from the initially silenced M1 to their normal spinal territories and reduced the density of aberrant ipsilateral terminations from the initially active side. Normal movement endpoint control during visually guided locomotion was fully restored. This reorganization of CS terminals reveals an unsuspected late plasticity after the critical period for establishing the pattern of CS terminations in the spinal cord. Our findings show that robust bilateral interactions between the developing CS systems on each side are important for achieving balance between contralateral and ipsilateral CS tract connections and visuomotor control.

Although we understand that activity- and use-dependent processes are important in determining corticospinal axon terminal development in the spinal cord, little is known about the role of these processes in development of skilled control of limb movements. In the present study we determined the effects of unilateral motor cortex activity blockade produced by muscimol infusion during the corticospinal axon terminal refinement period, between postnatal weeks 5–7, on visually guided locomotion. We examined stepping and forepaw placement on the rungs of a horizontal ladder and gait modifications as animals stepped over obstacles during treadmill walking. When cats traversed the horizontal ladder, the limb contralateral to inactivation was placed significantly farther forward on the rungs than the ipsilateral limb, indicating defective endpoint control. Similarly, when animals stepped over obstacles on a treadmill, the contralateral limb was placed farther in front of the obstacle, but only when it was the first (i.e., leading) limb to step over the obstacle, not when it was the second (i.e., trailing) limb. This is also indicative of an endpoint control deficit. In contrast, neither during ladder walking, nor when stepping over obstacles on the treadmill, was there any consistent evidence for a major impairment in limb trajectory. These results point to distinct and possibility independent corticospinal mechanisms for movement trajectory control and endpoint control. Although corticospinal activity during early postnatal development is needed to refine circuits for accurate endpoint control, this activity-dependent refinement is not needed for movement trajectory control.

In the cat, the motor representation in motor cortex develops between wk 8 and wk 13. Motor map development is accompanied by a decrease in the current thresholds for evoking movements with a concomitant increase in the number of effective sites, an increase in the distal representation, and the representation of multijoint synergies. In this study we used intracortical microstimulation in anesthetized cats to examine how forelimb motor experiences influence development of map characteristics. To promote skilled movements during wks 8–13, animals were engaged in daily performance of a prehension task. Forelimb movements were prevented by intramuscular botulinum toxin injection or restraint. To determine whether experience-dependent changes were permanent, we examined the map in different animals between 1 wk and 1 yr after cessation of testing. Promoting forelimb use resulted in an increase in the number of sites from which multiple joint effects were produced by stimulation and the number of joints represented at those sites. The effect was maximal at 1 wk after cessation of testing, and became progressively less at 1 mo and at 4 mo. Preventing limb use resulted in a decreased number of effective sites, an increase in current thresholds for evoking responses, and a decreased representation of joints at multijoint sites. Our findings show that the motor map can respond to novel motor demands as it is forming during development but that it reverts back to one with the properties of a map in a control animal if those demands are not maintained in the animal's behavioral repertoire.

The corticospinal system is the principal motor system for controlling movements that require the greatest skill and flexibility. It is the last motor system to develop. The pattern of termination of corticospinal axons, as they grow into the spinal gray matter, bears little resemblance to the pattern later in development and in maturity. Refinement of corticospinal terminations occurs during a protracted postnatal period and includes both elimination of transient terminations and growth to new targets. This refinement is driven by neural activity in the motor cortical areas and by limb motor experience. Developing corticospinal terminals compete with each other for synaptic space on spinal neurons. More active terminals are more competitive and are able to secure more synaptic space than their less active counterparts. Corticospinal terminals can activate spinal neurons from very early in development. The importance of this early synaptic activity appears to be more for refining corticospinal connections than for transmitting signals to spinal motor circuits for movement control. The motor control functions of the corticospinal system are not expressed until development of connectional specificity with spinal cord neurons, a strong capacity for corticospinal synapses to facilitate spinal motor circuits, and the formation of the cortical motor map.