Origin of corticospinal neurones evoking monosynaptic excitation in C3--C4 propriospinal neurones in the cat

Similar Motor Cortical Control Mechanisms for Precise Limb Control during Reaching and Locomotion

Throughout the course of evolution there has been a parallel development of the complexity and flexibility of the nervous system and the skeletomuscular system that it controls. This development is particularly evident for the cerebral cortical areas and the transformation of the use of the upper limbs from a purely locomotor function to one including, or restricted to, reaching and grasping. This study addresses the issue of whether the control of reaching has involved the development of new cortical circuits or whether the same neurons are used to control both locomotion and reaching. We recorded the activity of pyramidal tract neurons in the motor cortex of the cat both during voluntary gait modifications and during reaching. All cells showed generally similar patterns of activity in both tasks. More specifically, we showed that, in many cases, cells maintained a constant temporal relationship to the activity of synergistic muscle groups in each task. In addition, in some cells the relationship between the intensity of the cell discharge activity and the magnitude of the EMG activity was equally constant during gait modifications and reaching. As such, the results are compatible with the hypothesis that the corticospinal circuits used to control reaching evolved from those used to precisely modify gait.

Spinal interneurons and forelimb plasticity after incomplete cervical spinal cord injury in adult rats

Cervical spinal cord injury (cSCI) disrupts bulbospinal projections to motoneurons controlling the upper limbs, resulting in significant functional impairments. Ongoing clinical and experimental research has revealed several lines of evidence for functional neuroplasticity and recovery of upper extremity function after SCI. The underlying neural substrates, however, have not been thoroughly characterized. The goals of the present study were to map the intraspinal motor circuitry associated with a defined upper extremity muscle, and evaluate chronic changes in the distribution of this circuit following incomplete cSCI. Injured animals received a high cervical (C2) lateral hemisection (Hx), which compromises supraspinal input to ipsilateral spinal motoneurons controlling the upper extremities (forelimb) in the adult rat. A battery of behavioral tests was used to characterize the time course and extent of forelimb motor recovery over a 16 week period post-injury. A retrograde transneuronal tracer - pseudorabies virus - was used to define the motor and pre-motor circuitry controlling the extensor carpi radialis longus (ECRL) muscle in spinal intact and injured animals. In the spinal intact rat, labeling was observed unilaterally within the ECRL motoneuron pool and within spinal interneurons bilaterally distributed within the dorsal horn and intermediate gray matter. No changes in labeling were observed 16 weeks post-injury, despite a moderate degree of recovery of forelimb motor function. These results suggest that recovery of the forelimb function assessed following C2Hx injury does not involve recruitment of new interneurons into the ipsilateral ECRL motor pathway. However, the functional significance of these existing interneurons to motor recovery requires further exploration.

Activity-dependent codevelopment of the corticospinal system and target interneurons in the cervical spinal cord

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.

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.

The C3-C4 propriospinal system in the cat and monkey: a spinal pre-motoneuronal centre for voluntary motor control

This review deals with a spinal interneuronal system, denoted the C3-C4 propriospinal system, which is unique in the sense that it so far represents the only spinal interneuronal system for which it has been possible to demonstrate a command mediating role for voluntary movements. The C3-C4 propriospinal neurones govern target reaching and can update the descending cortical command when a fast correction is required of the movement trajectory and also integrate signals generated from the forelimb to control deceleration and termination of reaching.

Organization of the projections from the posterior parietal cortex to the rostral and caudal regions of the motor cortex of the cat

The posterior parietal cortex (PPC) is an important source of input to the motor cortex in both the primate and the cat. However, the available evidence from the cat suggests that the projection from the PPC to those rostral areas of the motor cortex that project to the intermediate and ventral parts of the spinal gray matter is relatively small. This leaves in question the importance of the contribution of the PPC to the initiation and modulation of voluntary movements in the cat. As this anatomical evidence is not entirely compatible with the physiological data, we reinvestigated the PPC projection to the motor cortex by injecting dextran amine tracers either into the proximal or distal representations of the forelimb in the rostral motor cortex, into the representation of the forelimb in the caudal motor cortex, or into the hindlimb representation. The results show strong projections from the PPC to each of these regions. However, projections to the rostral motor cortex were observed primarily from the caudal bank of the ansate sulcus and the adjacent gyrus, whereas those to the caudal motor cortex were generally located more rostrally. There was also evidence of some topographic organization with the distal limb being located progressively more laterally and rostrally in the PPC than the areas projecting to more proximal regions. In contrast to previous anatomical investigations, these results suggest that the PPC can potentially modulate motor activity via its strong projection to the more rostral regions of the motor cortex.

Bilateral Processing of Motor Commands in the Motor Cortex of the Cat During Target-Reaching

Single-unit activity of the motor cortex (area 4γ) was studied in cats performing reaching with the contra- versus ipsilateral forelimb. Reaching was initiated by a tone burst (Go cue), different limbs were used in separate blocks of trials. During reaching performed with the contralateral limb, three types of neurons were observed. The first type had biphasic pattern with an initial component locked to the Go cue followed by a component locked to the onset of reaching. The second type of neurons had monophasic discharges correlated both with the onset of the stimulus and with the movement. The third type showed responses related to the movement. Activity of the same cells investigated during reaching performed with the ipsilateral limb revealed that the cue-locked responses of the cells of the first type were effector independent, i.e., similar discharges locked to the Go cue were generated. The movement-related component of these cells was drastically reduced. The activity of some cells of the second type was suppressed during reaching with the ipsilateral limb. When performance was switched between limbs, a significant change of background discharge frequency was observed in 31% of the cells. The present results suggest that the sensory cue triggers elaboration of motor commands for reaching in both motor cortices, but further sensorimotor transformation is completed in only one hemisphere but is aborted actively in the other. It is also suggested that a certain pattern of background activity may serve a tuning function for elaboration of the command in the proper hemisphere.

Premotoneuronal and direct corticomotoneuronal control in the cat and macaque monkey

The literature on premotoneuronal and direct corticomotoneuronal (CM) control in the cat and macaque monkey is reviewed. The available experimental findings are not in accordance with a recently proposed hypothesis that direct CM connections have "replaced" the premotoneuronal pathways. Instead, we propose that premotoneuronal CM control plays an important role in motor control also in primates and that the direct CM connection has been added during phylogeny.

The medial pain system: neural representations of the motivational aspect of pain

In this article, we propose that the pathways mediating the motivational aspect of pain originate in laminae VII and VIII of the spinal cord, and in the deep layers of the spinal trigeminal complex, and projections from these areas reach three central structures where pain motivation is represented, the ventrolateral quadrant of the periaqueductal gray, posterior hypothalamic nucleus, and intralaminar thalamic nuclei. A final representation of the motivational aspect of pain is located within the anterior cingulate cortex, and this representation receives inputs from the intralaminar nuclei. Outputs from these representations reach premotor structures located in the medulla, striatum, and cingulate premotor cortex. We discuss pathways and structures that provide inputs to these representations, including those involved in producing involuntary (innate) and instrumental responses which occur in response to the recognition of stimuli associated with footshock and other nociceptive stimuli.