Postnatal development of connectional specificity of corticospinal terminals in the cat

Inter-axonal molecular crosstalk via Lumican proteoglycan sculpts murine cervical corticospinal innervation by distinct subpopulations

How CNS circuits sculpt their axonal arbors into spatially and functionally organized domains is not well understood. Segmental specificity of corticospinal connectivity is an exemplar for such regional specificity of many axon projections. Corticospinal neurons (CSN) innervate spinal and brainstem targets with segmental precision, controlling voluntary movement. Multiple molecularly distinct CSN subpopulations innervate the cervical cord for evolutionarily enhanced precision of forelimb movement. Evolutionarily newer CSN_BC-lat exclusively innervate bulbar-cervical targets, while CSN_medial are heterogeneous; distinct subpopulations extend axons to either bulbar-cervical or thoraco-lumbar segments. We identify that Lumican controls balance of cervical innervation between CSN_BC-lat and CSN_medial axons during development, which is maintained into maturity. Lumican, an extracellular proteoglycan expressed by CSN_BC-lat, non-cell-autonomously suppresses cervical collateralization by multiple CSN_medial subpopulations. This inter-axonal molecular crosstalk between CSN subpopulations controls murine corticospinal circuitry refinement and forelimb dexterity. Such crosstalk is generalizable beyond the corticospinal system for evolutionary incorporation of new neuron populations into preexisting circuitry.

Does the cerebellum shape the spatiotemporal organization of muscle patterns? Insights from subjects with cerebellar ataxias

The role of the cerebellum in motor control has been investigated extensively, but its contribution to the muscle pattern organization underlying goal-directed movements is still not fully understood. Muscle synergies may be used to characterize multimuscle pattern organization irrespective of time (spatial synergies), in time irrespective of the muscles (temporal synergies), and both across muscles and in time (spatiotemporal synergies). The decomposition of muscle patterns as combinations of different types of muscle synergies offers the possibility to identify specific changes due to neurological lesions. In this study, we recorded electromyographic activity from 13 shoulder and arm muscles in subjects with cerebellar ataxias (CA) and in age-matched healthy subjects (HS) while they performed reaching movements in multiple directions. We assessed whether cerebellar damage affects the organization of muscle patterns by extracting different types of muscle synergies from the muscle patterns of each HS and using these synergies to reconstruct the muscle patterns of all other participants. We found that CA muscle patterns could be accurately captured only by spatial muscle synergies of HS. In contrast, there were significant differences in the reconstruction R² values for both spatiotemporal and temporal synergies, with an interaction between the two synergy types indicating a larger difference for spatiotemporal synergies. Moreover, the reconstruction quality using spatiotemporal synergies correlated with the severity of impairment. These results indicate that cerebellar damage affects the temporal and spatiotemporal organization, but not the spatial organization, of the muscle patterns, suggesting that the cerebellum plays a key role in shaping their spatiotemporal organization.NEW & NOTEWORTHY In recent studies, the decomposition of muscle activity patterns has revealed a modular organization of the motor commands. We show, for the first time, that muscle patterns of subjects with cerebellar damage share with healthy controls spatial, but not temporal and spatiotemporal, modules. Moreover, changes in spatiotemporal organization characterize the severity of the subject's impairment. These results suggest that the cerebellum has a specific role in shaping the spatiotemporal organization of the muscle patterns.

Muscle Synergies Obtained from Comprehensive Mapping of the Cortical Forelimb Representation Using Stimulus Triggered Averaging of EMG Activity

Neuromuscular control of voluntary movement may be simplified using muscle synergies similar to those found using non-negative matrix factorization. We recently identified synergies in electromyography (EMG) recordings associated with both voluntary movement and movement evoked by high-frequency long-duration intracortical microstimulation applied to the forelimb representation of the primary motor cortex (M1). The goal of this study was to use stimulus-triggered averaging (StTA) of EMG activity to investigate the synergy profiles and weighting coefficients associated with poststimulus facilitation, as synergies may be hard-wired into elemental cortical output modules and revealed by StTA. We applied StTA at low (LOW, ∼15 μA) and high intensities (HIGH, ∼110 μA) to 247 cortical locations of the M1 forelimb region in two male rhesus macaques while recording the EMG of 24 forelimb muscles. Our results show that 10-11 synergies accounted for 90% of the variation in poststimulus EMG facilitation peaks from the LOW-intensity StTA dataset while only 4-5 synergies were needed for the HIGH-intensity dataset. Synergies were similar across monkeys and current intensities. Most synergy profiles strongly activated only one or two muscles; all joints were represented and most, but not all, joint directions of motion were represented. Cortical maps of the synergy weighting coefficients suggest only a weak organization. StTA of M1 resulted in highly diverse muscle activations, suggestive of the limiting condition of requiring a synergy for each muscle to account for the patterns observed.SIGNIFICANCE STATEMENT Coordination of muscle activity and the neural origin of potential muscle synergies remains a fundamental question of neuroscience. We previously demonstrated that high-frequency long-duration intracortical microstimulation-evoked synergies were unrelated to voluntary movement synergies and were not clearly organized in the cortex. Here we present stimulus-triggered averaging facilitation-related muscle synergies, suggesting that when fundamental cortical output modules are activated, synergies approach the limit of single-muscle control. Thus, we conclude that if the CNS controls movement via linear synergies, those synergies are unlikely to be called from M1. This information is critical for understanding neural control of movement and the development of brain-machine interfaces.

The corticospinal tract: Evolution, development, and human disorders

The corticospinal tract (CST) plays a major role in cortical control of spinal cord activity. In particular, it is the principal motor pathway for voluntary movements. Here, we discuss: (i) the anatomic evolution and development of the CST across mammalian species, focusing on its role in motor functions; (ii) the molecular mechanisms regulating corticospinal tract formation and guidance during mouse development; and (iii) human disorders associated with abnormal CST development. A comparison of CST anatomy and development across mammalian species first highlights important similarities. In particular, most CST axons cross the anatomical midline at the junction between the brainstem and spinal cord, forming the pyramidal decussation. Reorganization of the pattern of CST projections to the spinal cord during evolution led to improved motor skills. Studies of the molecular mechanisms involved in CST formation and guidance in mice have identified several factors that act synergistically to ensure proper formation of the CST at each step of development. Human CST developmental disorders can result in a reduction of the CST, or in guidance defects associated with abnormal CST anatomy. These latter disorders result in altered midline crossing at the pyramidal decussation or in the spinal cord, but spare the rest of the CST. Careful appraisal of clinical manifestations associated with CST malformations highlights the critical role of the CST in the lateralization of motor control. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 810-829, 2017.

The Corticospinal System: From Development to Motor Control

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.

Competition with Primary Sensory Afferents Drives Remodeling of Corticospinal Axons in Mature Spinal Motor Circuits

Injury to the mature motor system drives significant spontaneous axonal sprouting instead of axon regeneration. Knowing the circuit-level determinants of axonal sprouting is important for repairing motor circuits after injury to achieve functional rehabilitation. Competitive interactions are known to shape corticospinal tract axon outgrowth and withdrawal during development. Whether and how competition contributes to reorganization of mature spinal motor circuits is unclear. To study this question, we examined plastic changes in corticospinal axons in response to two complementary proprioceptive afferent manipulations: (1) enhancing proprioceptive afferents activity by electrical stimulation; or (2) diminishing their input by dorsal rootlet rhizotomy. Experiments were conducted in adult rats. Electrical stimulation produced proprioceptive afferent sprouting that was accompanied by significant corticospinal axon withdrawal and a decrease in corticospinal connections on cholinergic interneurons in the medial intermediate zone and C boutons on motoneurons. In contrast, dorsal rootlet rhizotomy led to a significant increase in corticospinal connections, including those on cholinergic interneurons; C bouton density increased correspondingly. Motor cortex-evoked muscle potentials showed parallel changes to those of corticospinal axons, suggesting that reciprocal corticospinal axon changes are functional. Using the two complementary models, we showed that competitive interactions between proprioceptive and corticospinal axons are an important determinant in the organization of mature corticospinal axons and spinal motor circuits. The activity- and synaptic space-dependent properties of the competition enables prediction of the remodeling of spared corticospinal connection and spinal motor circuits after injury and informs the target-specific control of corticospinal connections to promote functional recovery.

SIGNIFICANCE STATEMENT

Neuroplasticity is limited in maturity, but it is promoted after injury. Axons of the major descending motor pathway for motor skills, the corticospinal tract (CST), sprout after brain or spinal cord injury. This contributes to spontaneous spinal motor circuit repair and partial motor recovery. Knowing the determinants that enhance this plasticity is critical for functional rehabilitation. Here we examine the remodeling of CST axons directed by sensory fibers. We found that the CST projection is regulated dynamically in maturity by the competitive, activity-dependent actions of sensory fibers. Knowledge of the properties of this competition enables prediction of the remodeling of CST connections and spinal circuits after injury and informs ways to engineer target-specific control of CST connections to promote recovery.

Corticospinal tract development and spinal cord innervation differ between cervical and lumbar targets

The corticospinal (CS) tract is essential for voluntary movement, but what we know about the organization and development of the CS tract remains limited. To determine the total cortical area innervating the seventh cervical spinal cord segment (C7), which controls forelimb movement, we injected a retrograde tracer (fluorescent microspheres) into C7 such that it would spread widely within the unilateral gray matter (to >80%), but not to the CS tract. Subsequent detection of the tracer showed that, in both juvenile and adult mice, neurons distributed over an unexpectedly broad portion of the rostral two-thirds of the cerebral cortex converge to C7. This even included cortical areas controlling the hindlimbs (the fourth lumbar segment, L4). With aging, cell densities greatly declined, mainly due to axon branch elimination. Whole-cell recordings from spinal cord cells upon selective optogenetic stimulation of CS axons, and labeling of axons (DsRed) and presynaptic structures (synaptophysin) through cotransfection using exo utero electroporation, showed that overgrowing CS axons make synaptic connections with spinal cells in juveniles. This suggests that neuronal circuits involved in the CS tract to C7 are largely reorganized during development. By contrast, the cortical areas innervating L4 are limited to the conventional hindlimb area, and the cell distribution and density do not change during development. These findings call for an update of the traditional notion of somatotopic CS projection and imply that there are substantial developmental differences in the cortical control of forelimb and hindlimb movements, at least in rodents.

Can the period of postnatal codevelopment of the rubrospinal and corticospinal systems provide new insights into refinement of limb movement?

The corticospinal and the rubrospinal tracts are thought to synergistically contribute to the limb control during motor development. Williams et al. (J Neurosci 34: 4432-4441, 2014) demonstrate that the postnatal maturation of red nucleus motor map and the rubrospinal tract develops earlier than the corticospinal tract, to support early forelimb control. They have two distinct phases of maturation; a "precorticospinal" phase characterized by development of the rubrospinal system, and a "cocorticospinal" phase where they overlap with corticospinal development.

Postnatal maturation of the red nucleus motor map depends on rubrospinal connections with forelimb motor pools

The red nucleus (RN) and rubrospinal tract (RST) are important for forelimb motor control. Although the RST is present postnatally in cats, nothing is known about when rubrospinal projections could support motor functions or the relation between the development of the motor functions of the rubrospinal system and the corticospinal system, the other major system for limb control. Our hypothesis is that the RN motor map is present earlier in development than the motor cortex (M1) map, to support early forelimb control. We investigated RN motor map maturation with microstimulation and RST cervical enlargement projections using anterograde tracers between postnatal week 3 (PW3) and PW16. Microstimulation and tracer injection sites were verified histologically to be located within the RN. Microstimulation at PW4 evoked contralateral wrist, elbow, and shoulder movements. The number of sites producing limb movement increased and response thresholds decreased progressively through PW16. From the outset, all forelimb joints were represented. At PW3, RST projections were present within the cervical intermediate zone, with a mature density of putative synapses. In contrast, beginning at PW5 there was delayed and age-dependent development of forelimb motor pool projections and putative rubromotoneuronal synapses. The RN has a more complete forelimb map early in development than previous studies showed for M1, supporting our hypothesis of preferential rubrospinal rather than corticospinal control for early movements. Remarkably, development of the motor pool, not intermediate zone, RST projections paralleled RN motor map development. The RST may be critical for establishing the rudiments of motor skills that subsequently become refined with further CST development.

Effects of cathodal trans-spinal direct current stimulation on mouse spinal network and complex multijoint movements

Cathodal trans-spinal direct current (c-tsDC) stimulation is a powerful technique to modulate spinal excitability. However, the manner in which c-tsDC stimulation modulates cortically evoked simple single-joint and complex multijoint movements is unknown. To address this issue, anesthetized mice were suspended with the hindlimb allowed to move freely in space. Simple and complex multijoint movements were elicited with short and prolonged trains of electrical stimulation, respectively, delivered to the area of primary motor cortex representing the hindlimb. In addition, spinal cord burst generators are known to be involved in a variety of motor activities, including locomotion, postural control, and voluntary movements. Therefore, to shed light into the mechanisms underlying movements modulated by c-tsDC stimulation, spinal circuit activity was induced using GABA and glycine receptor blockers, which produced three rates of spinal bursting activity: fast, intermediate, and slow. Characteristics of bursting activity were assessed during c-tsDC stimulation. During c-tsDC stimulation, significant increases were observed in (1) ankle dorsiflexion amplitude and speed; (2) ankle plantarflexion amplitude, speed, and duration; and (3) complex multijoint movement amplitude, speed, and duration. However, complex multijoint movement tracing showed that c-tsDC did not change the form of movements. In addition, spinal bursting activity was significantly modulated during c-tsDC stimulation: (1) fast bursting activity showed increased rate, amplitude, and duration; (2) intermediate bursting activity showed increased rate and duration, but decreased amplitude; and (3) slow bursting activity showed increased rate, but decreased duration and amplitude. These results suggest that c-tsDC stimulation amplifies cortically evoked movements through spinal mechanisms.

Synaptic and functional linkages between spinal premotor interneurons and hand-muscle activity during precision grip

Grasping is a highly complex movement that requires the coordination of a number of hand joints and muscles. Previous studies showed that spinal premotor interneurons (PreM-INs) in the primate cervical spinal cord have divergent synaptic effects on hand motoneurons and that they might contribute to hand-muscle synergies. However, the extent to which these PreM-IN synaptic connections functionally contribute to modulating hand-muscle activity is not clear. In this paper, we explored the contribution of spinal PreM-INs to hand-muscle activation by quantifying the synaptic linkage (SL) and functional linkage (FL) of the PreM-INs with hand-muscle activities. The activity of 23 PreM-INs was recorded from the cervical spinal cord (C6–T1), with EMG signals measured simultaneously from hand and arm muscles in two macaque monkeys performing a precision grip task. Spike-triggered averages (STAs) of rectified EMGs were compiled for 456 neuron–muscle pairs; 63 pairs showed significant post-spike effects (PSEs; i.e., SL). Conversely, 231 of 456 pairs showed significant cross-correlations between the IN firing rate and rectified EMG (i.e., FL). Importantly, a greater proportion of the neuron–muscle pairs with SL showed FL (43/63 pairs, 68%) compared with the pairs without SL (203/393, 52%), and the presence of SL was significantly associated with that of FL. However, a significant number of pairs had SL without FL (SL∩!FL, n = 20) or FL without SL (!SL∩FL, n = 203), and the proportions of these incongruities exceeded the number expected by chance. These results suggested that spinal PreM-INs function to significantly modulate hand-muscle activity during precision grip, but the contribution of other neural structures is also needed to recruit an adequate combination of hand-muscle motoneurons.

Robustness of muscle synergies underlying three-dimensional force generation at the hand in healthy humans

Previous studies using advanced matrix factorization techniques have shown that the coordination of human voluntary limb movements may be accomplished using combinations of a small number of intermuscular coordination patterns, or muscle synergies. However, the potential use of muscle synergies for isometric force generation has been evaluated mostly using correlational methods. The results of such studies suggest that fixed relationships between the activations of pairs of muscles are relatively rare. There is also emerging evidence that the nervous system uses independent strategies to control movement and force generation, which suggests that one cannot conclude a priori that isometric force generation is accomplished by combining muscle synergies, as shown in movement control. In this study, we used non-negative matrix factorization to evaluate the ability of a few muscle synergies to reconstruct the activation patterns of human arm muscles underlying the generation of three-dimensional (3-D) isometric forces at the hand. Surface electromyographic (EMG) data were recorded from eight key elbow and shoulder muscles during 3-D force target-matching protocols performed across a range of load levels and hand positions. Four synergies were sufficient to explain, on average, 95% of the variance in EMG datasets. Furthermore, we found that muscle synergy composition was conserved across biomechanical task conditions, experimental protocols, and subjects. Our findings are consistent with the view that the nervous system can generate isometric forces by assembling a combination of a small number of muscle synergies, differentially weighted according to task constraints.

Single collateral reconstructions reveal distinct phases of corticospinal remodeling after spinal cord injury

Background

Injuries to the spinal cord often result in severe functional deficits that, in case of incomplete injuries, can be partially compensated by axonal remodeling. The corticospinal tract (CST), for example, responds to a thoracic transection with the formation of an intraspinal detour circuit. The key step for the formation of the detour circuit is the sprouting of new CST collaterals in the cervical spinal cord that contact local interneurons. How individual collaterals are formed and refined over time is incompletely understood.

Methodology/Principal Findings

We traced the hindlimb corticospinal tract at different timepoints after lesion to show that cervical collateral formation is initiated in the first 10 days. These collaterals can then persist for at least 24 weeks. Interestingly, both major and minor CST components contribute to the formation of persistent CST collaterals. We then developed an approach to label single CST collaterals based on viral gene transfer of the Cre recombinase to a small number of cortical projection neurons in Thy1-STP-YFP or Thy1-Brainbow mice. Reconstruction and analysis of single collaterals for up to 12 weeks after lesion revealed that CST remodeling evolves in 3 phases. Collateral growth is initiated in the first 10 days after lesion. Between 10 days and 3–4 weeks after lesion elongated and highly branched collaterals form in the gray matter, the complexity of which depends on the CST component they originate from. Finally, between 3–4 weeks and 12 weeks after lesion the size of CST collaterals remains largely unchanged, while the pattern of their contacts onto interneurons matures.

Conclusions/Significance

This study provides a comprehensive anatomical analysis of CST reorganization after injury and reveals that CST remodeling occurs in distinct phases. Our results and techniques should facilitate future efforts to unravel the mechanisms that govern CST remodeling and to promote functional recovery after spinal cord injury.

Modules in the brain stem and spinal cord underlying motor behaviors

Previous studies using intact and spinalized animals have suggested that coordinated movements can be generated by appropriate combinations of muscle synergies controlled by the central nervous system (CNS). However, which CNS regions are responsible for expressing muscle synergies remains an open question. We address whether the brain stem and spinal cord are involved in expressing muscle synergies used for executing a range of natural movements. We analyzed the electromyographic (EMG) data recorded from frog leg muscles before and after transection at different levels of the neuraxis-rostral midbrain (brain stem preparations), rostral medulla (medullary preparations), and the spinal-medullary junction (spinal preparations). Brain stem frogs could jump, swim, kick, and step, while medullary frogs could perform only a partial repertoire of movements. In spinal frogs, cutaneous reflexes could be elicited. Systematic EMG analysis found two different synergy types: 1) synergies shared between pre- and posttransection states and 2) synergies specific to individual states. Almost all synergies found in natural movements persisted after transection at rostral midbrain or medulla but not at the spinal-medullary junction for swim and step. Some pretransection- and posttransection-specific synergies for a certain behavior appeared as shared synergies for other motor behaviors of the same animal. These results suggest that the medulla and spinal cord are sufficient for the expression of most muscle synergies in frog behaviors. Overall, this study provides further evidence supporting the idea that motor behaviors may be constructed by muscle synergies organized within the brain stem and spinal cord and activated by descending commands from supraspinal areas.

Co-development of proprioceptive afferents and the corticospinal tract within the cervical spinal cord

In maturity, skilled movements depend on coordination of control signals by descending pathways, such as the corticospinal tract (CST), and proprioceptive afferents (PAs). An important locus for this coordination is the spinal cord intermediate zone. Convergence of CST and PA terminations onto common regions leads to interactions that may underlie afferent gating and modulation of descending control signals during movements. We determined establishment of CST and PA terminations within common spinal cord regions and development of synaptic interactions in 4-week-old cats, which is before major spinal motor circuit refinement, and two ages after refinement (weeks 8, 11). We examined the influence of one or the other system on monosynaptic responses, on the spinal cord surface and locally in the intermediate zone, evoked by either CST or deep radial nerve (DRN) stimulation. DRN stimulation suppressed CST monosynaptic responses at 4 weeks, but this converted to facilitation by 8 weeks. This may reflect a strategy to limit CST movement control when it has aberrant immature connections, and could produce errant movements. CST stimulation showed delayed development of mixed suppression and facilitation of DRN responses. We found development of age-dependent overlap of PA and CST terminations where interactions were recorded in the intermediate zone. Our findings reveal a novel co-development of different inputs onto common spinal circuits and suggest a logic to CST-PA interactions at an age before the CST has established connectional specificity with spinal circuits.

Postnatal refinement of proprioceptive afferents in the cat cervical spinal cord

Proprioceptive afferent (PA) information is integrated with signals from descending pathways, including the corticospinal tract (CST), by spinal interneurons in the dorsal horn and intermediate zone for controlling movements. PA spinal projections, and the reflexes that they evoke, develop prenatally. The CST projects to the spinal cord postnatally, and its connections are subsequently refined. Consequently, the tract becomes effective in transmitting control signals from motor cortex to muscle. This suggests sequential development of PAs and the CST rather than co-development. In this study we determined if there was also late postnatal refinement of PA spinal connections, which would support PA-CST co-development. We examined changes in PA spinal connections at 4 weeks of age, when CST terminations are immature, at 8 weeks, after CST refinement, and at 11 weeks, when CST terminations are mature. We electrically stimulated PA afferents in the deep radial nerve. Evoked PA responses were small and not localized at 4 weeks. By 8 and 11 weeks, responses were substantially larger and maximal in laminae VI and dorsal VII. We used intramuscular injection of cholera toxin β subunit to determine the distribution of PAs from the extensor carpii radialis muscle in the cervical enlargement at the same ages as in the electrophysiological studies. We found a reduction of the distribution of PAs with age that paralleled the physiological changes. This age-related sharpening of PA spinal connections also paralleled CST development, suggesting coordinated PA-CST co-development rather than sequential development. This is likely to be important for the development of adaptive motor control.

Activity-dependent plasticity improves M1 motor representation and corticospinal tract connectivity

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.

Pyramidal tract stimulation restores normal corticospinal tract connections and visuomotor skill after early postnatal motor cortex activity blockade

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.

Propriospinal neurons are sufficient for bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord

We recently showed that propriospinal neurons contribute to bulbospinal activation of locomotor networks in the in vitro neonatal rat brainstem-spinal cord preparation. In the present study, we examined whether propriospinal neurons alone, in the absence of long direct bulbospinal transmission to the lumbar cord, can successfully mediate brainstem activation of the locomotor network. In the presence of staggered bilateral spinal cord hemisections, the brainstem was stimulated electrically while recording from lumbar ventral roots. The rostral hemisection was located between C1 and T3 and the contralateral caudal hemisection was located between T5 and mid-L1. Locomotor-like activity was evoked in 27% of the preparations, which included experiments with staggered hemisections placed only two segments apart. There was no relation between the likelihood of developing locomotor-like activity and the distance separating the two hemisections or specific level of the hemisections. In some experiments, where brainstem stimulation alone was ineffective, neurochemical excitation of propriospinal neurons (using 5-HT and NMDA) at concentrations subthreshold for producing locomotor-like activity, promoted locomotor-like activity in conjunction with brainstem stimulation. In other experiments, involving neither brainstem stimulation nor cord hemisections, the excitability of propriospinal neurons in the cervical and/or thoracic region was selectively enhanced by bath application of 5-HT and NMDA or elevation of bath K(+) concentration. These manipulations produced locomotor-like activity in the lumbar region. In total, the results suggest that propriospinal neurons are sufficient for transmission of descending locomotor command signals. This observation has implications for regeneration strategies aimed at restoration of locomotor function after spinal cord injury.

Muscle synergies during locomotion in the cat: a model for motor cortex control

It is well established that the motor cortex makes an important contribution to the control of visually guided gait modifications, such as those required to step over an obstacle. However, it is less clear how the descending cortical signal interacts with the interneuronal networks in the spinal cord to ensure that precise changes in limb trajectory are appropriately incorporated into the base locomotor rhythm. Here we suggest that subpopulations of motor cortical neurones, active sequentially during the step cycle, may regulate the activity of small groups of synergistic muscles, likewise active sequentially throughout the step cycle. These synergies, identified by a novel associative cluster analysis, are defined by periods of muscle activity that are coextensive with respect to the onset and offset of the EMG activity. Moreover, the synergies are sparse and are frequently composed of muscles acting around more than one joint. During gait modifications, we suggest that subpopulations of motor cortical neurones may modify the magnitude and phase of the EMG activity of all muscles contained within a given synergy. Different limb trajectories would be produced by differentially modifying the activity in each synergy thus providing a flexible substrate for the control of intralimb coordination during locomotion.

Bilateral activity-dependent interactions in the developing corticospinal system

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.

Activity- and use-dependent plasticity of the developing corticospinal system

The corticospinal (CS) system, critical for controlling skilled movements, develops during the late prenatal and early postnatal periods in all species examined. In the cat, there is a sequence of development of the mature pattern of terminations of CS tract axons in the spinal gray matter, followed by motor map development of the primary motor cortex. Skilled limb movements begin to be expressed as the map develops. Development of the proper connections between CS axons and spinal neurons in cats depends on CS neural activity and motor behavioral experience during a critical postnatal period. Reversible CS inactivation or preventing limb use produces an aberrant distribution of CS axon terminations and impairs visually guided movements. This altered pattern of CS connections after inactivation in cats resembles the aberrant pattern of motor responses evoked by transcranial magnetic stimulation in hemiplegic cerebral palsy patients. Left untreated in the cat, these impairments do not resolve. We have found that activity-dependent processes can be harnessed in cats to reestablish normal CS connections and function. This finding suggests that aspects of normal CS connectivity and function might some day be restored in hemiplegic cerebral palsy.

Differential activity-dependent development of corticospinal control of movement and final limb position during visually guided locomotion

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.

Uncrossed actions of feline corticospinal tract neurones on hindlimb motoneurones evoked via ipsilaterally descending pathways

Despite numerous investigations on the corticospinal system there is only scant information on neuronal networks mediating actions of corticospinal neurones on ipsilateral motoneurones. We have previously demonstrated double crossed pathways through which pyramidal tract neurones can influence ipsilateral motoneurones, via contralaterally descending reticulospinal neurones and spinal commissural interneurones. The aim of the present study was to examine the effects of stimulation of pyramidal tract (PT) fibres mediated via ipsilaterally descending pathways and to find out which neurones relay these effects. This was done by using intracellular recordings from 96 lumbar motoneurones in deeply anaesthetized cats. To eliminate actions of fibres descending on the side contralateral to the location of the motoneurones, the spinal cords were hemisected on this side at a low-thoracic level. Stimuli that selectively activated ipsilateral PT fibres evoked EPSPs and/or IPSPs in 34/47 motoneurones tested. These PSPs were evoked at latencies indicating that the most direct coupling between PT neurones and motoneurones in uncrossed pathways is disynaptic. Occlusion and spatial facilitation between actions evoked by stimulation of ipsilateral PT and of reticulospinal tract fibres in the ipsilateral medial longitudinal fascicle (MLF) indicated that PT actions are mediated by reticulospinal neurones with axons in the MLF. However, after transection of the MLF in the caudal medulla, stimulation of the ipsilateral PT continued to evoke EPSPs and IPSPs with characteristics similar to when the MLF was intact (in 15/49 motoneurones) suggesting the existence of parallel disynaptic pathways via other relay neurones.

Synapse elimination in the corticospinal projection during the early postnatal period

In corticospinal synapses reconstructed in vitro by slice co-culture, we previously showed that the synapses were distributed across the gray matter at 6-7 days in vitro (DIV). Thereafter, they began to be eliminated from the ventral side, and dorsal-dominant distribution was nearly complete at 11-12 DIV. The synapse elimination is associated with retraction of the corticospinal (CS) terminals. We studied whether this specific type of synapse elimination is a physiological phenomenon rather than in vitro artifact. The rat corticospinal tract was stimulated at the medullary pyramid, and field potentials were recorded at the cervical cord along an 200-microm interval lattice on the axial plane. Clearly defined negative field potential were identified as field excitatory postsynaptic potentials (fEPSPs) generated by corticospinal synapses. They were recorded from the entire spinal gray matter at postnatal day 7 (P7). These negative fEPSPs reversed to positive in the most ventrolateral part at P8. Reversal extended to the more mediodorsal area at P10, indicative of progressive synapse elimination in the ventrolateral area. To verify that regression of the axons in vivo paralleled the changes in spatial distribution of fEPSPs as observed in vitro, corticospinal axons were anterogradely labeled. Redistribution of the labeled terminals closely paralleled the fEPSP distribution, being present in the ventrolateral spinal cord at P7, decreased at P8, further deceased at P10, but unchanged at P11. Furthermore, double immunostaining for labeled terminals and synaptophysin observed under a confocal microscope suggests that corticospinal fibers at P7 possess presynaptic structures in the ventrolateral area as well as the dorsomedial area. These findings suggest that corticospinal synapses are widely formed in the spinal gray matter at P7, are rapidly eliminated from the ventrolateral side from P8 to P10, a time-course very similar to that observed in vitro, and are associated with axonal regression.

Motor but not sensory representation in motor cortex depends on postsynaptic activity during development and in maturity

The movement representation in the primary motor cortex (M1) of the cat develops between postnatal weeks 7-12. The somatosensory representation in motor cortex is present by the age that the motor map begins to develop. In this study we examined the role of neural activity in development and maintenance of the M1 movement and somatosensory representations. We blocked activity of M1 neurons unilaterally for one month by intracortical infusion of the GABA agonist muscimol during the motor map development period in kittens and in mature cats. After the drug effects were no longer present, we used microstimulation and multiunit recording in the forelimb areas of M1 to determine the motor and somatosensory representations in the infused and noninfused sides. In both kittens and adults, there was a severe reduction or elimination of sites where microstimulation evoked a motor response in the inactivated compared with the control side. In contrast, there was no difference in the percentage, topography or receptive field modality of sites receiving somatosensory inputs on the inactivated and control sides. Moreover, the pattern of somatosensory input to M1 was similar before and after inactivation. This suggests that somatosensory input to M1 is stable after the connections initially develop. Since activity blockade had the same effects on the motor representation of kittens and adult cats, M1 neuronal activity, while possibly important in map development, is equally necessary for map maintenance.

Effect of Forelimb Use on Postnatal Development of the Forelimb Motor Representation in Primary Motor Cortex of the Cat

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.

Rescuing transient corticospinal terminations and promoting growth with corticospinal stimulation in kittens

Development of corticospinal (CS) terminations is activity dependent. In the cat, activity-dependent refinement of termination topography occurs between weeks 3 and 6. Initially, sparse terminals are present in the gray matter bilaterally, including the motor nuclei. By week 6, virtually all motor nuclear terminations are eliminated, as are most ipsilateral terminations. In this study, we determined whether electrical stimulation of CS axons could be used to rescue transient terminations and promote their growth. We implanted microwires in the pyramid or spinal white matter to stimulate CS axons (2 hr/d, 330 Hz, 45 msec burst, 2 sec intervals) for 2-3 weeks during the refinement period. CS terminations were traced using wheat germ agglutinin conjugated to horseradish peroxidase. Animals were killed after week 6. Stimulation produced dense terminations bilaterally, including within the motor nuclei. Termination density was least in lamina 1 and ventral lamina 9. Reticular formation stimulation produced a control (i.e., nonstimulated) termination pattern. To determine whether CS stimulation affected development of the nonstimulated CS system, we traced terminations from the contralateral cortex using biotinylated dextran amine. Compared with controls or after reticular formation stimulation, there was a shift in the distribution of terminations of the nonstimulated side to more dorsal laminas, which is where the stimulated CS system had fewer terminals. This distribution shift is consistent with competition for termination space between the CS systems on both sides. Our findings indicate that activity can be harnessed to bias CS axon terminal development. This has important implications for using activity to modify motor system organization after perinatal CNS trauma.

Role of sensory-motor cortex activity in postnatal development of corticospinal axon terminals in the cat

The initial pattern of corticospinal (CS) terminations, as axons grow into the spinal gray matter, bears little resemblance to the pattern later in development and in maturity. This is because of extensive axon pruning and local axon terminal growth during early postnatal development. Pruning is driven by activity-dependent competition between the CS systems on each side during postnatal weeks (PW) 3-7. It is not known whether CS axon terminal growth and final topography are activity dependent. We examined the activity dependence of CS axon terminal growth and topography at different postnatal times. We inactivated sensory-motor cortex by infusion of the gamma-aminobutyric acid type A (GABA(A)) agonist muscimol and traced CS axons from the inactivated side. Inactivation between PW5 and PW7 produced permanent changes in projection topography, reduced local axon branching, and prevented development of dense clusters of presynaptic sites, which are normally characteristic of CS terminals. Inactivation at younger (PW3-5) and older (PW8-12) ages did not affect projection topography but impeded development of local axon branching and presynaptic site clusters. These effects were not due to increased cortical cell death during inactivation. Neural activity plays an important role in determining the morphology of CS terminals during the entire period of development, but, for the projection topography, the role of activity is exercised during a very brief period. This points to a complex, and possibly independent, regulation of termination topography and terminal morphology. Surprisingly, when a CS neuron's activity is blocked during early development, it does not recover lost connections later in development once activity resumes.

Corticospinal system development depends on motor experience

Early motor experiences have been shown to be important for the development of motor skills in humans and animals. However, little is known about the role of motor experience in motor system development. In this study, we address the question of whether early motor experience is important in shaping the development of the corticospinal (CS) tract. We prevented limb use by the intramuscular injection of botulinum toxin A into selected forelimb muscles to produce muscle paralysis during the period of development of CS connection specificity, which is between postnatal weeks 3 and 7. CS terminations were examined using an anterograde tracer. Preventing normal forelimb use during CS axon development produced defective development of CS terminations at week 8 and in maturity. There were reductions in the topographic distribution of axon terminals, in terminal and preterminal branching, and in varicosity density. This suggests that limb use is needed to refine CS terminals into topographically specific clusters of dense terminal branches and varicosities. To determine correlated effects on motor behavior, cats were tested in a prehension task, to reach and grasp a piece of food from a narrow food well, when the neuromuscular blockade dissipated (by week 10) and in maturity (week 16). Preventing normal limb use also produced a prehension deficit later in development and in maturity, in which there was a loss of the supination component of grasping. This component of prehension in the cat depends on CS projections from the paw representation of rostral motor cortex to the cervical enlargement. Our findings show that motor experiences are necessary for normal development of CS terminations and function.

The transition from development to motor control function in the corticospinal system

During early postnatal development, corticospinal (CS) system stimulation, electrical or transcranial magnetic, is minimally effective in producing muscle contraction, despite having axon terminals that excite spinal neurons. Later, after stimulation becomes more effective, the cortical motor representation develops, and movements the system controls in maturity are expressed. We determined whether development of temporal facilitation (response enhancement produced by the second of a pair of pyramidal tract stimuli, or a higher stimulus multiple of a train of stimuli) correlated with these changes. Facilitation of the monosynaptic CS response was larger in older kittens and adults than younger kittens. When facilitation was strong, strong motor responses were evoked by pyramidal stimulation with small currents and few pulses. With strong facilitation in older kittens, corticospinal axon varicosities colocalize synaptophysin like adults, suggesting a presynaptic mechanism. With effective facilitation, control signals from the cortex can be sufficiently effective to provoke muscle contraction for guiding movements.

Firing properties of spinal interneurons during voluntary movement. II. Interactions between spinal neurons

The relationship between the activity of pairs of simultaneously recorded spinal interneurons (INs) in the cervical enlargement was studied in five monkeys performing voluntary wrist movements. The tendency for INs to exhibit similar response properties and synchronized firing was tested as a function of physical distance between the cells and their correlational linkages with forearm muscles. Nearby INs tended to have more similar torque and direction turning (signal correlation) and more similar response profiles (e.g., tonic vs phasic firing) than INs that were far apart. This suggests that nearby cells receive common synaptic input. In contrast, the trial-to-trial covariation of rate around the mean rate for all trials (noise correlation) was independent of the distance between the neurons. Furthermore, signal and noise correlation were independent, suggesting different underlying mechanisms. Surprisingly, spike-to-spike correlation between INs was relatively infrequent and weak, as measured by cross-correlation histograms. In contrast, single motor units (SMUs) in forearm muscles fired more synchronously, particularly for SMUs in single extensor muscles. Either common drive to INs is too weak to induce synchronized firing, or there is an active decorrelation mechanism within IN networks.

Postnatal development of corticospinal postsynaptic action

The corticospinal system has a delayed and prolonged postnatal development. In the cat, lesion, inactivation, or stimulation of the system influence motor output minimally when corticospinal (CS) terminals have an immature topographic pattern but produce robust effects immediately after developing the mature pattern by weeks 6-7. In this study, we directly tested if the delay in expression of cortical motor functions is due to the inability of the corticospinal synapse to activate spinal neurons. We stimulated corticospinal axons in the pyramid and recorded evoked field potentials from the surface of the cervical spinal cord and locally from within the gray matter in anesthetized cats during development and in adults. Pyramidal stimulation in animals between week 4 and maturity evoked an initial corticospinal surface volley followed by a postsynaptic field response. Depth recordings from the superficial dorsal horn to the ventral white matter showed that local pre- and postsynaptic field potentials could be recorded over the full extent of the gray matter in 4- to 5-wk animals but were restricted to the intermediate zone in older animals and adults. Dorsoventral refinement of CS field potentials parallels anatomical refinement of individual CS axon terminals shown in our earlier studies. Our present findings indicate that the developing corticospinal system could influence the excitability of virtually the entire contralateral gray matter before cortical motor functions are expressed. Given the importance of activity-dependent axon terminal refinement, this capacity for activating spinal neurons during early postnatal life could play an important role in development of CS circuit connectivity.