Neural bases of postural control

Use of galvanic vestibular feedback to control postural orientation in decerebrate rabbits

In quadrupeds, the dorsal-side-up body orientation during standing is maintained due to a postural system that is driven by feedback signals coming mainly from limb mechanoreceptors. In caudally decerebrated (postmammillary) rabbits, the efficacy of this system is considerably reduced. In this paper, we report that the efficacy of postural control in these animals can be restored with galvanic vestibular stimulation (GVS) applied transcutaneously to the labyrinths. In standing intact rabbits, GVS causes a lateral body sway towards the positive electrode. We used this GVS-caused sway to counteract the lateral body sway resulting from a mechanical perturbation of posture. Experiments were performed on postmammillary rabbits that stood on the tilting platform with their hindlimbs. To make the GVS value dependent on the postural perturbation (i.e., on the lateral body sway caused by tilt of the platform), an artificial feedback loop was formed in the following ways: 1) Information about the body sway was provided by a mechanical sensor; 2) The GVS current was applied when the sway exceeded a threshold value; the polarity of the current was determined by the sway direction. This simple algorithm allowed the "hybrid" postural system to maintain the dorsal-side-up orientation of the hindquarters when the platform was tilted by ± 20°. Thus, an important postural function, i.e., securing lateral stability during standing, can be restored in decerebrate rabbits with the GVS-based artificial feedback. We suggest that such a control system can compensate for the loss of lateral stability of various etiologies, and can be used for restoration of balance control in patients with impaired postural functions.

Facilitation of postural limb reflexes in spinal rabbits by serotonergic agonist administration, epidural electrical stimulation, and postural training

In quadrupeds, spinalization in the thoracic region severely impairs postural control in the hindquarters. The goal of this study was to improve postural functions in chronic spinal rabbits by regular application of different factors: intrathecal injection of the 5-HT2 agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), epidural electrical spinal cord stimulation (EES), and specific postural training (SPT). The factors were used either alone (SPT group) or in combination (DOI+SPT, EES+SPT, and DOI+EES+SPT groups) or not used (control group). It was found that in none of these groups did normal postural corrective movements in response to lateral tilts of the supporting platform reappear within the month of treatment. In control group, reduced irregular electromyographic (EMG) responses, either correctly or incorrectly phased in relation to tilts, were observed. By contrast, in DOI+SPT and EES+SPT groups, a gradual threefold increase in the proportion of correctly phased EMG responses (compared with control) was observed. The increase was smaller in DOI+EES+SPT and SPT groups. Dissimilarly to these long-term effects, short-term effects of DOI and EES were weak or absent. In addition, gradual development of oscillatory EMG activity in the responses to tilts, characteristic for the control group, was retarded in DOI+SPT, EES+SPT, DOI+EES+SPT, and SPT groups. Thus regular application of the three tested factors and their combinations caused progressive, long-lasting plastic changes in the isolated spinal networks, resulting in the facilitation of spinal postural reflexes and in the retardation of the development of oscillatory EMG activity. The facilitated reflexes, however, were insufficient for normal postural functions.

Organization of functional synaptic connections between medullary reticulospinal neurons and lumbar descending commissural interneurons in the neonatal mouse

The medullary reticular formation (MRF) of the neonatal mouse is organized so that the medial and lateral MRF activate hindlimb and trunk motoneurons (MNs) with differential predominance. The goal of the present study was to investigate whether this activation is polysynaptic and mediated by commissural interneurons with descending axons (dCINs) in the lumbar spinal cord. To this end, we tested the polysynapticity of inputs from the MRF to MNs and tested for the presence of selective inputs from medial and lateral MRF to 574 individual dCINs in the L2 segment of the neonatal mouse. Reticulospinal-mediated postsynaptic Ca(2+) responses in MNs were reduced in the presence of mephenesin and after a midline lesion, suggesting the involvement of dCINs in mediating the responses. Consistent with this, stimulation of reticulospinal neurons in the medial or lateral MRF activated 51% and 57% of ipsilateral dCINs examined (255 and 352 dCINs, respectively) and 52% and 46% of contralateral dCINs examined (166 and 133 dCINs, respectively). The proportion of dCINs that responded specifically to stimulation of medial or lateral MRF was similar to the proportions of dCINs that responded to both MRF regions or to neither. The three responsive dCIN populations had largely overlapping spatial distributions. We demonstrate the existence of dCIN subpopulations sufficient to mediate responses in lumbar motoneurons from reticulospinal pathways originating from the medial and lateral MRF. Differential control of trunk and hindlimb muscles by the medullary reticulospinal system may therefore be mediated in part by identifiable dCIN populations.

Encoding of force increases and decreases by tibial campaniform sensilla in the stick insect, Carausius morosus

Detection of force increases and decreases is important in motor control. Experiments were performed to characterize the structure and responses of tibial campaniform sensilla, receptors that encode forces through cuticular strains, in the middle leg of the stick insect (Carausius morosus). The sensilla consist of distinct subgroups. Group 6A sensilla are located 0.3 mm distal to the femoro-tibial joint and have oval shaped cuticular caps. Group 6B receptors are 1 mm distal to the joint and have round caps. All sensilla show directional, phasico-tonic responses to forces applied to the tibia in the plane of joint movement. Group 6B sensilla respond to force increases in the direction of joint extension while Group 6A receptors discharge when those forces decrease. Forces applied in the direction of joint flexion produce the reverse pattern of sensory discharge. All receptors accurately encode the rate of change of force increments and decrements. Contractions of tibial muscles also produce selective, directional sensory discharges. The subgroups differ in their reflex effects: Group 6B receptors excite and Group 6A sensilla inhibit tibial extensor and trochanteral depressor motoneurons. The tibial campaniform sensilla can, therefore, encode force increases or decreases and aid in adapting motor outputs to changes in load.

Activity of red nucleus neurons in the cat during postural corrections

The dorsal-side-up body posture in standing quadrupeds is maintained by the postural system, which includes spinal and supraspinal mechanisms driven by somatosensory inputs from the limbs. A number of descending tracts can transmit supraspinal commands for postural corrections. The first aim of this study was to understand whether the rubrospinal tract participates in their transmission. We recorded activity of red nucleus neurons (RNNs) in the cat maintaining balance on the periodically tilting platform. Most neurons were identified as rubrospinal ones. It was found that many RNNs were profoundly modulated by tilts, suggesting that they transmit postural commands. The second aim of this study was to examine the contribution of sensory inputs from individual limbs to posture-related RNN modulation. Each RNN was recorded during standing on all four limbs, as well as when two or three limbs were lifted from the platform and could not signal platform displacements. By comparing RNN responses in different tests, we found that the amplitude and phase of responses in the majority of RNNs were determined primarily by sensory input from the corresponding (fore or hind) contralateral limb, whereas inputs from other limbs made a much smaller contribution to RNN modulation. These findings suggest that the rubrospinal system is primarily involved in the intralimb postural coordination, i.e., in the feedback control of the corresponding limb and, to a lesser extent, in the interlimb coordination. This study provides a new insight into the formation of supraspinal motor commands for postural corrections.

In vivo longitudinal MRI and behavioral studies in experimental spinal cord injury

Comprehensive in vivo longitudinal studies that include multi-modal magnetic resonance imaging (MRI) and a battery of behavioral assays to assess functional outcome were performed at multiple time points up to 56 days post-traumatic spinal cord injury (SCI) in rodents. The MRI studies included high-resolution structural imaging for lesion volumetry, and diffusion tensor imaging (DTI) for probing the white matter integrity. The behavioral assays included open-field locomotion, grid walking, inclined plane, computerized activity box performance, and von Frey filament tests. Additionally, end-point histology was assessed for correlation with both the MRI and behavioral data. The temporal patterns of the lesions were documented on structural MRI. DTI studies showed significant changes in white matter that is proximal to the injury epicenter and persisted to day 56. White matter in regions up to 1 cm away from the injury epicenter that appeared normal on conventional MRI also exhibited changes that were indicative of tissue damage, suggesting that DTI is a more sensitive measure of the evolving injury. Correlations between DTI and histology after SCI could not be firmly established, suggesting that injury causes complex pathological changes in multiple tissue components that affect the DTI measures. Histological evidence confirmed a significant decrease in myelin and oligodendrocyte presence 56 days post-SCI. Multiple assays to evaluate aspects of functional recovery correlated with histology and DTI measures, suggesting that damage to specific white matter tracts can be assessed and tracked longitudinally after SCI.

Bilateral postsynaptic actions of pyramidal tract and reticulospinal neurons on feline erector spinae motoneurons

Trunk muscles are important for postural adjustments associated with voluntary movements but little has been done to analyze mechanisms of supraspinal control of these muscles at a cellular level. The present study therefore aimed to investigate the input from pyramidal tract (PT) neurons to motoneurons of the musculus longissimus lumborum of the erector spinae and to analyze to what extent it is relayed by reticulospinal (RS) neurons. Intracellular records from motoneurons were used to evaluate effects of electrical stimulation of medullary pyramids and of axons of RS neurons descending in the medial longitudinal fasciculus (MLF). The results revealed that similar synaptic actions were evoked from the ipsilateral and contralateral PTs, including disynaptic and trisynaptic EPSPs and trisynaptic IPSPs. Stimulation of the MLF-evoked monosynaptic and disynaptic EPSPs and disynaptic or trisynaptic IPSPs in the same motoneurons. All short-latency PSPs of PT origin were abolished by transection of the MLF, while they remained after transection of PT fibers at a spinal level. Hence, RS neurons might serve as the main relay neurons of the most direct PT actions on musculus (m.) longissimus. However, longer-latency IPSPs remaining after MLF or PT spinal lesions and after ipsilateral or contralateral hemisection of spinal cord indicate that PT actions are also mediated by ipsilaterally and/or contralaterally located spinal interneurons. The bilateral effects of PT stimulation thereby provide an explanation why trunk movements after unilateral injuries of PT neurons (e.g., stroke) are impaired to a lesser degree than movements of the extremities.

Facilitation of postural limb reflexes with epidural stimulation in spinal rabbits

It is known that after spinalization animals lose their ability to maintain lateral stability when standing or walking. A likely reason for this is a reduction of the postural limb reflexes (PLRs) driven by stretch and load receptors of the limbs. The aim of this study was to clarify whether spinal networks contribute to the generation of PLRs. For this purpose, first, PLRs were recorded in decerebrated rabbits before and after spinalization at T12. Second, the effects of epidural electrical stimulation (EES) at L7 on the limb reflexes were studied after spinalization. To evoke PLRs, the vertebrate column of the rabbit was fixed, whereas the hindlimbs were positioned on the platform. Periodic lateral tilts of the platform caused antiphase flexion-extension limbs movements, similar to those observed in intact animals keeping balance on the tilting platform. Before spinalization, these movements evoked PLRs: augmentation of extensor EMGs and increase of contact force during limb flexion, suggesting their stabilizing postural effects. Spinalization resulted in almost complete disappearance of PLRs. After EES, however, the PLRs reappeared and persisted for up to several minutes, although their values were reduced. The post-EES effects could be magnified by intrathecal application of quipazine (5-HT agonist) at L4-L6. Results of this study suggest that the spinal cord contains the neuronal networks underlying PLRs; they can contribute to the maintenance of lateral stability in intact subjects. In acute spinal animals, these networks can be activated by EES, suggesting that they are normally activated by a tonic supraspinal drive.

The decerebrate cat generates the essential features of the force constraint strategy

Cats actively respond to horizontal perturbations of the supporting surface according to the force constraint strategy. In this strategy, the force responses fall into two groups oriented in either rostral and medial directions or caudal and lateral directions, rather than in strict opposition to the direction of perturbation. When the distance between forelimbs and hindlimbs is decreased, the responses are less constrained and directed more in line with the perturbation. We have recently shown that electromyographic responses from limb muscles of the decerebrate cat resemble those obtained in the intact animal. Our objectives here were to determine whether the decerebrate cat preparation would also exhibit the force constraint strategy and whether that strategy would exhibit the characteristic dependence on limb position on the strategy. Horizontal support surface perturbations were delivered and three-dimensional exerted forces were recorded from all four limbs. Clustered force responses were generated by all four limbs and were found to be statistically indistinguishable between animals decerebrated using two different levels of transection. The directionality of the force responses was preserved throughout successive time epochs during the perturbations. In addition, the clustering of force responses increased with distance between forelimbs and hindlimbs. These results indicate that the force constraint strategy used by terrestrial animals to maintain stability can be generated without the assistance of the cerebral cortices and without prior training. This suggests an important role for the lower brain stem and spinal cord in generating an appropriate strategy to maintain stability.

Normative values for three clinical measures of motor performance used in the neurological assessment of sports concussion

Postural control and motor coordination are essential components of normal athletic activity. Tasks involving balance and coordination are used to determine neurological function in sports-related concussion. Determining normative values for these tasks is therefore essential to provide sports medicine professionals with a frame of reference with which to interpret clinical measures obtained from players suspected of sustaining a concussion. One hundred and seventytwo healthy subjects (16-37 yrs) performed three timed tests: Tandem Gait (TG); Finger-to-Nose (FTN); Single-Leg-Stance (SLS) on firm and foam surfaces. Unadjusted geometric means (+/-SD) for each measure were averaged across three trials. Time to complete TG was 11.2+/-1.2s. FTN for the dominant and non-dominant arm were 2.9+/-1.1s and 3.0+/-1.2s, respectively. SLS values for dominant and non-dominant leg were 20.4+/-3.0s (firm), 3.4+/-1.6s (foam), and 21.0+/-2.9s (firm), 3.3+/-1.6s (foam), respectively. For TG, there was an order effect (P<.001) but no age, sex or BMI effects. FTN demonstrated a dominant arm preference (P<.001), sex (P=.006), BMI (P=.043) and order effects (P<.001). SLS demonstrated an order effect on the firm surface (P=.009) and an order (P<.001) and BMI (P=.001) effect on foam. Intra-rater reliability, as measured by ICC (3,3), demonstrated that TG and FTN had excellent reliability compared to SLS. FTN and TG should continue to be used in test batteries to determine neurological function in sports-related concussion.

Activity of pyramidal tract neurons in the cat during standing and walking on an inclined plane

To keep balance when standing or walking on a surface inclined in the roll plane, the cat modifies its body configuration so that the functional length of its right and left limbs becomes different. The aim of the present study was to assess the motor cortex participation in the generation of this left/right asymmetry. We recorded the activity of fore- and hindlimb-related pyramidal tract neurons (PTNs) during standing and walking on a treadmill. A difference in PTN activity at two tilted positions of the treadmill (+/- 15 deg) was considered a positional response to surface inclination. During standing, 47% of PTNs exhibited a positional response, increasing their activity with either the contra-tilt (20%) or the ipsi-tilt (27%). During walking, PTNs were modulated in the rhythm of stepping, and tilts of the supporting surface evoked positional responses in the form of changes to the magnitude of modulation in 58% of PTNs. The contra-tilt increased activity in 28% of PTNs, and ipsi-tilt increased activity in 30% of PTNs. We suggest that PTNs with positional responses contribute to the modifications of limb configuration that are necessary for adaptation to the inclined surface. By comparing the responses to tilts in individual PTNs during standing and walking, four groups of PTNs were revealed: responding in both tasks (30%); responding only during standing (16%); responding only during walking (30%); responding in none of the tasks (24%). This diversity suggests that common and separate cortical mechanisms are used for postural adaptation to tilts during standing and walking.

Impairment of postural control in rabbits with extensive spinal lesions

Our previous studies on rabbits demonstrated that the ventral spinal pathways are of primary importance for postural control in the hindquarters. After ventral hemisection, postural control did not recover, whereas after dorsal or lateral hemisection it did. The aim of this study was to examine postural capacity of rabbits after more extensive lesion (3/4 section of the spinal cord at T(12) level), that is, with only one ventral quadrant spared (VQ animals). They were tested before (control) and after lesion on the platform periodically tilted in the frontal plane. In control animals, tilts of the platform regularly elicited coordinated electromyographic (EMG) responses in the hindlimbs, which resulted in generation of postural corrections and in maintenance of balance. In VQ rabbits, the EMG responses appeared only in a part of tilt cycles, and they could be either correctly or incorrectly phased in relation to tilts. Because of a reduced value and incorrect phasing of EMG responses on both sides, this muscle activity did not cause postural corrective movements in the majority of rabbits, and the body swayed together with the platform. In these rabbits, the ability to perform postural corrections did not recover during the whole period of observation (< or =30 days). Low probability of correct EMG responses to tilts in most rabbits as well as an appearance of incorrect responses to tilts suggest that the spinal reflex chains, necessary for postural control, have not been specifically selected by a reduced supraspinal drive transmitted via a single ventral quadrant.

Electromyographic responses from the hindlimb muscles of the decerebrate cat to horizontal support surface perturbations

The sensory and neural mechanisms underlying postural control have received much attention in recent decades but remain poorly understood. Our objectives were 1) to establish the decerebrate cat as an appropriate model for further research into the sensory mechanisms of postural control and 2) to observe what elements of the postural response can be generated by the brain stem and spinal cord. Ten animals were decerebrated using a modified premammillary technique, which consists of a premammillary decerebration that is modified with a vertical transection near the subthalamic nucleus to eliminate spontaneous locomotion. Horizontal support surface perturbations were applied to all four limbs and electromyographic recordings were collected from 14 muscles of the right hindlimb. Muscle activation was quantified with tuning curves, which compared increases and decreases in muscle activity to background and graphed the difference against perturbation direction. Parallels were drawn between these tuning curves, which were further quantified with a principal direction and breadth (range of directions of muscle activation), and data collected by other researchers from the intact animal. We found a strong similarity in the direction and breadth of the tuning curves generated in the decerebrate and intact cat. These results support our hypothesis that directionally specific tuning of muscles in response to support surface perturbations does not require the cortex, further indicating a strong role for the brain stem and spinal cord circuits in mediating directionally appropriate muscle activation patterns.

Quantification of deficits in lateral paw positioning after spinal cord injury in dogs

Background

Previous analysis of the behavioural effects of spinal cord injury has focussed on coordination in the sagittal plane of movement between joints, limb girdle pairs or thoracic and pelvic limb pairs. In this study we extend the functional analysis of the consequences of clinical thoracolumbar spinal cord injury in dogs to quantify the well-recognised deficits in lateral stability during locomotion. Dogs have a high centre of mass thereby facilitating recognition of lateral instability.

Results

We confirm that errors in lateral positioning of the pelvic limb paws can be quantified and that there is a highly significant difference in variability of foot placement between normal and spinal cord injured dogs. In this study there was no detectable difference in lateral paw positioning variability between complete and incomplete injuries, but it appears that intergirdle limb coordination and appropriate lateral paw placement recover independently from one another.

Conclusion

Analysis of lateral paw position in the dog provides an additional tier of analysis of outcome after spinal cord injury that will be of great value in interpreting the effects of putative therapeutic interventions.

Maintenance of lateral stability during standing and walking in the cat

During free behaviors animals often experience lateral forces, such as collisions with obstacles or interactions with other animals. We studied postural reactions to lateral pulses of force (pushes) in the cat during standing and walking. During standing, a push applied to the hip region caused a lateral deviation of the caudal trunk, followed by a return to the initial position. The corrective hindlimb electromyographic (EMG) pattern included an initial wave of excitation in most extensors of the hindlimb contralateral to push and inhibition of those in the ipsilateral limb. In cats walking on a treadmill with only hindlimbs, application of force also caused lateral deviation of the caudal trunk, with subsequent return to the initial position. The type of corrective movement depended on the pulse timing relative to the step cycle. If the force was applied at the end of the stance phase of one of the limbs or during its swing phase, a lateral component appeared in the swing trajectory of this limb. The corrective step was directed either inward (when the corrective limb was ipsilateral to force application) or outward (when it was contralateral). The EMG pattern in the corrective limb was characterized by considerable modification of the hip abductor and adductor activity in the perturbed step. Thus the basic mechanisms for balance control in these two forms of behavior are different. They perform a redistribution of muscle activity between symmetrical limbs (in standing) and a reconfiguration of the base of support during a corrective lateral step (in walking).

Differential origin of reticulospinal drive to motoneurons innervating trunk and hindlimb muscles in the mouse revealed by optical recording

To better understand how the brainstem reticular formation controls and coordinates trunk and hindlimb muscle activity, we used optical recording to characterize the functional connections between medullary reticulospinal neurons and lumbar motoneurons of the L2 segment in the neonatal mouse. In an isolated brainstem-spinal cord preparation, synaptically induced calcium transients were visualized in individual MNs of the ipsilateral and contralateral medial and lateral motor columns (MMC, LMC) following focal electrical stimulation of the medullary reticular formation (MRF). Stimulation of the MRF elicited differential responses in MMC and LMC, according to a specific spatial organization. Stimulation of the medial MRF elicited responses predominantly in the LMC whereas stimulation of the lateral MRF elicited responses predominantly in the MMC. This reciprocal response pattern was observed on both the ipsilateral and contralateral sides of the spinal cord. To ascertain whether the regions stimulated contained reticulospinal neurons, we retrogradely labelled MRF neurons with axons coursing in different spinal funiculi, and compared the distributions of the labelled neurons to the stimulation sites. We found a large number of retrogradely labelled neurons within regions of the gigantocellularis reticular nucleus (including its pars ventralis and alpha) where most stimulation sites were located. The existence of a mediolateral organization within the MRF, whereby distinct populations of reticulospinal neurons predominantly influence medial or lateral motoneurons, provides an anatomical substrate for the differential control of trunk and hindlimb muscles. Such an organization introduces flexibility in the initiation and coordination of activity in the two sets of muscles that would satisfy many of the functional requirements that arise during postural and non-postural motor control in mammals.

Effect of intrathecal administration of serotoninergic and noradrenergic drugs on postural performance in rabbits with spinal cord lesions

Our previous studies have shown that extensive spinal lesions at T12 in the rabbit [ventral hemisection (VHS) or 3/4-section that spares one ventral quadrant (VQ)] severely damaged the postural system. When tested on the platform periodically tilted in the frontal plane, VHS and VQ animals typically were not able to perform postural corrective movements by their hindlimbs, although EMG responses (correctly or incorrectly phased) could be observed. We attempted to restore postural control in VHS and VQ rabbits by applying serotoninergic and noradrenergic drugs to the spinal cord below the lesion through the intrathecal cannula. It was found that serotonin and quipazine (5-HT1,2,3 agonist) did not re-establish postural corrective movements. However, when applied during a 10-day period after lesion, these drugs produced a twofold increase of the proportion of correct EMG responses to tilts. It was also found that methoxamine (alpha1 noradrenergic agonist), as well as the mixture of methoxamine and quipazine, did not re-establish postural corrective movements and did not increase the proportion of correct EMG responses. Serotonin (at later stages) and methoxamine induced periodical bursting in EMGs, suggesting activation of spinal rhythm-generating networks. Appearance of bursting seems to perturb normal operation of postural mechanisms, as suggested by methoxamine-induced abolishment of postural effects of quipazine. When applied in an intact animal, none of the tested drugs affected the value of postural corrections or evoked periodical bursting. We conclude that activation of the serotoninergic system (but not the noradrenergic one) causes selective enhancement of spinal postural reflexes during the earlier postlesion period.

Metamorphosis-induced changes in the coupling of spinal thoraco-lumbar motor outputs during swimming in Xenopus laevis

Anuran metamorphosis includes a complete remodeling of the animal's biomechanical apparatus, requiring a corresponding functional reorganization of underlying central neural circuitry. This involves changes that must occur in the coordination between the motor outputs of different spinal segments to harmonize locomotor and postural functions as the limbs grow and the tail regresses. In premetamorphic Xenopus laevis tadpoles, axial motor output drives rostrocaudally propagating segmental myotomal contractions that generate propulsive body undulations. During metamorphosis, the anterior axial musculature of the tadpole progressively evolves into dorsal muscles in the postmetamorphic froglet in which some of these back muscles lose their implicit locomotor function to serve exclusively in postural control in the adult. To understand how locomotor and postural systems interact during locomotion in juvenile Xenopus, we have investigated the coordination between postural back and hindlimb muscle activity during free forward swimming. Axial/dorsal muscles, which contract in bilateral alternation during undulatory swimming in premetamorphic tadpoles, change their left-right coordination to become activated in phase with bilaterally synchronous hindlimb extensions in locomoting juveniles. Based on in vitro electrophysiological experiments as well as specific spinal lesions in vivo, a spinal cord region was delimited in which propriospinal interactions are directly responsible for the coordination between leg and back muscle contractions. Our findings therefore indicate that dynamic postural adjustments during adult Xenopus locomotion are mediated by local intraspinal pathways through which the lumbar generator for hindlimb propulsive kicking provides caudorostral commands to thoracic spinal circuitry controlling the dorsal trunk musculature.

Postural performance in decerebrated rabbit

It is known that animals decerebrated at the premammillary level are capable of standing and walking without losing balance, in contrast to postmammillary ones which do not exhibit such behavior. The main goals of the present study were, first, to characterize the postural performance in premammillary rabbits, and, second, to activate the postural system in postmammillary ones by brainstem stimulation. For evaluation of postural capacity of decerebrated rabbits, motor and EMG responses to lateral tilts of the supporting platform and to lateral pushes were recorded before and after decerebration. In addition, the righting behavior (i.e., standing up from the lying position) was video recorded. We found that, in premammillary rabbits, responses to lateral tilts and pushes were similar to those observed in intact ones, but the magnitude of responses was reduced. During righting, premammillary rabbits assumed the normal position slower than intact ones. To activate the postural system in postmammillary rabbits, we stimulated electrically two brainstem structures, the mesencephalic locomotor region (MLR) and the ventral tegmental field (VTF). The MLR stimulation (prior to elicitation of locomotion) and the VTF stimulation caused an increase of the tone of hindlimb extensors, and enhanced their responses to lateral tilts and to pushes. These results indicate that the basic mechanisms for maintenance of body posture and equilibrium during standing are present in decerebrated animals. They are active in the premammillary rabbits but need to be activated in the postmammillary ones.

Spinal and supraspinal postural networks

Different species maintain a particular body orientation in space (upright in humans, dorsal-side-up in quadrupeds, fish and lamprey) due to the activity of a closed-loop postural control system. We will discuss operation of spinal and supraspinal postural networks studied in a lower vertebrate (lamprey) and in two mammals (rabbit and cat). In the lamprey, the postural control system is driven by vestibular input. The key role in the postural network belongs to the reticulospinal (RS) neurons. Due to vestibular input, deviation from the stabilized body orientation in any (roll, pitch, yaw) plane leads to generation of RS commands, which are sent to the spinal cord and cause postural correction. For each of the planes, there are two groups of RS neurons responding to rotation in the opposite directions; they cause a turn opposite to the initial one. The command transmitted by an individual RS neuron causes the motor response, which contributes to the correction of posture. In each plane, the postural system stabilizes the orientation at which the antagonistic vestibular reflexes compensate for each other. Thus, in lamprey the supraspinal networks play a crucial role in stabilization of body orientation, and the function of the spinal networks is transformation of supraspinal commands into the motor pattern of postural corrections. In terrestrial quadrupeds, the postural system stabilizing the trunk orientation in the transversal plane was analyzed. It consists of two relatively independent sub-systems stabilizing orientation of the anterior and posterior parts of the trunk. They are driven by somatosensory input from limb mechanoreceptors. Each sub-system consists of two closed-loop mechanisms - spinal and spino-supraspinal. Operation of the supraspinal networks was studied by recording the posture-related activity of corticospinal neurons. The postural capacity of spinal networks was evaluated in animals with lesions to the spinal cord. Relative contribution of spinal and supraspinal mechanisms to the stabilization of trunk orientation is discussed.

Neurotech for neuroscience: unifying concepts, organizing principles, and emerging tools

The ability to tackle analysis of the brain at multiple levels simultaneously is emerging from rapid methodological developments. The classical research strategies of "measure," "model," and "make" are being applied to the exploration of nervous system function. These include novel conceptual and theoretical approaches, creative use of mathematical modeling, and attempts to build brain-like devices and systems, as well as other developments including instrumentation and statistical modeling (not covered here). Increasingly, these efforts require teams of scientists from a variety of traditional scientific disciplines to work together. The potential of such efforts for understanding directed motor movement, emergence of cognitive function from neuronal activity, and development of neuromimetic computers are described by a team that includes individuals experienced in behavior and neuroscience, mathematics, and engineering. Funding agencies, including the National Science Foundation, explore the potential of these changing frontiers of research for developing research policies and long-term planning.

Neuromechanics of muscle synergies for posture and movement

Recent research suggests that the nervous system controls muscles by activating flexible combinations of muscle synergies to produce a wide repertoire of movements. Muscle synergies are like building blocks, defining characteristic patterns of activation across multiple muscles that may be unique to each individual, but perform similar functions. The identification of muscle synergies has strong implications for the organization and structure of the nervous system, providing a mechanism by which task-level motor intentions are translated into detailed, low-level muscle activation patterns. Understanding the complex interplay between neural circuits and biomechanics that give rise to muscle synergies will be crucial to advancing our understanding of neural control mechanisms for movement.

Influences of sensory input from the limbs on feline corticospinal neurons during postural responses

The dorsal-side-up body posture of standing quadrupeds is maintained by coordinated activity of all limbs. Somatosensory input from the limbs evokes postural responses when the supporting surface is perturbed. The aim of this study was to reveal the contribution of sensory inputs from individual limbs to the posture-related modulation of pyramidal tract neurons (PTNs) arising in the primary motor cortex. We recorded the activity of PTNs from the limb representation of motor cortex in the cat maintaining balance on a platform periodically tilted in the frontal plane. Each PTN was recorded during standing on four limbs, and when two or three limbs were lifted from the platform and thus did not signal its displacement to motor cortex. By comparing PTN responses to tilts in different tests we found that the amplitude and the phase of the response in the majority of them were determined primarily by the sensory input from the corresponding contralateral limb. In a portion of PTNs, this input originated from afferents of the peripheral receptive field. Sensory input from the ipsilateral limb, as well as input from limbs of the other girdle made a much smaller contribution to the PTN modulation. These results show that, during postural activity, a key role of PTNs is the feedback control of the corresponding contralateral limb and, to a lesser extent, the coordination of posture within a girdle and between the two girdles.

Epidural spinal cord stimulation plus quipazine administration enable stepping in complete spinal adult rats

We hypothesized that epidural spinal cord stimulation (ES) and quipazine (a serotonergic agonist) modulates the excitability of flexor and extensor related intraspinal neural networks in qualitatively unique, but complementary, ways to facilitate locomotion in spinal cord-injured rats. To test this hypothesis, we stimulated (40 Hz) the S(1) spinal segment before and after quipazine administration (0.3 mg/kg, ip) in bipedally step-trained and nontrained, adult, complete spinal (mid-thoracic) rats. The stepping pattern of these rats was compared with control rats. At the stimulation levels used, stepping was elicited only when the hindlimbs were placed on a moving treadmill. In nontrained rats, the stepping induced by ES and quipazine administration was non-weight bearing, and the cycle period was shorter than in controls. In contrast, the stepping induced by ES and quipazine in step-trained rats was highly coordinated with clear plantar foot placement and partial weight bearing. The effect of ES and quipazine on EMG burst amplitude and duration was greater in flexor than extensor motor pools. Using fast Fourier transformation analysis of EMG bursts during ES, we observed one dominant peak at 40 Hz in the medial gastrocnemius (ankle extensor), whereas there was less of dominant spectral peak in the tibialis anterior (ankle flexor). We suggest that these frequency distributions reflect amplitude modulation of predominantly monosynaptic potentials in the extensor and predominantly polysynaptic pathways in the flexor muscle. Quipazine potentiated the amplitude of these responses. The data suggest that there are fundamental differences in the circuitry that generates flexion and extension during locomotion.

Vestibular signal processing in a subject with somatosensory deafferentation: the case of sitting posture

Background

The vestibular system of the inner ear provides information about head translation/rotation in space and about the orientation of the head with respect to the gravitoinertial vector. It also largely contributes to the control of posture through vestibulospinal pathways. Testing an individual severely deprived of somatosensory information below the nose, we investigated if equilibrium can be maintained while seated on the sole basis of this information.

Results

Although she was unstable, the deafferented subject (DS) was able to remain seated with the eyes closed in the absence of feet, arm and back supports. However, with the head unconsciously rotated towards the left or right shoulder, the DS's instability markedly increased. Small electrical stimulations of the vestibular apparatus produced large body tilts in the DS contrary to control subjects who did not show clear postural responses to the stimulations.

Conclusion

The results of the present experiment show that in the lack of vision and somatosensory information, vestibular signal processing allows the maintenance of an active sitting posture (i.e. without back or side rests). When head orientation changes with respect to the trunk, in the absence of vision, the lack of cervical information prevents the transformation of the head-centered vestibular information into a trunk-centered frame of reference of body motion. For the normal subjects, this latter frame of reference enables proper postural adjustments through vestibular signal processing, irrespectively of the orientation of the head with respect to the trunk.

Neural bases of goal-directed locomotion in vertebrates--an overview

The different neural control systems involved in goal-directed vertebrate locomotion are reviewed. They include not only the central pattern generator networks in the spinal cord that generate the basic locomotor synergy and the brainstem command systems for locomotion but also the control systems for steering and control of body orientation (posture) and finally the neural structures responsible for determining which motor programs should be turned on in a given instant. The role of the basal ganglia is considered in this context. The review summarizes the available information from a general vertebrate perspective, but specific examples are often derived from the lamprey, which provides the most detailed information when considering cellular and network perspectives.

Active spatial perception in the vibrissa scanning sensorimotor system

Haptic perception is an active process that provides an awareness of objects that are encountered as an organism scans its environment. In contrast to the sensation of touch produced by contact with an object, the perception of object location arises from the interpretation of tactile signals in the context of the changing configuration of the body. A discrete sensory representation and a low number of degrees of freedom in the motor plant make the ethologically prominent rat vibrissa system an ideal model for the study of the neuronal computations that underlie this perception. We found that rats with only a single vibrissa can combine touch and movement to distinguish the location of objects that vary in angle along the sweep of vibrissa motion. The patterns of this motion and of the corresponding behavioral responses show that rats can scan potential locations and decide which location contains a stimulus within 150 ms. This interval is consistent with just one to two whisk cycles and provides constraints on the underlying perceptual computation. Our data argue against strategies that do not require the integration of sensory and motor modalities. The ability to judge angular position with a single vibrissa thus connects previously described, motion-sensitive neurophysiological signals to perception in the behaving animal.

Rats explore the world with their whiskers (vibrissae). Although the sensations of touch that an animal experiences while exploring an object either in front of its head or to its side can be similar, the two sensations tell the animal different things about its nearby environment. The translation from passive touch to knowledge of an object's location requires that the nervous system keep track of the location of the animal's body as it moves. We studied this process by restricting a rat's whisking information to that provided by a single actively moving vibrissa. We found that even with such limited information, rats can search for, locate, and differentiate objects near their heads with astonishing speed. Their behavior during this search reflects the computations performed by their nervous systems to locate objects based on touch, and this behavior demonstrates that rats keeps track of their vibrissa motion with a resolution of less than 0.1 s. Understanding how these computations are performed will bring us closer to understanding how the brain integrates the sense of touch with its sense of self.

Rats can localize objects with their specialized vibrissa system by the integration of feed-forward sensory events and motor feedback. This discovery provides a behavioral model for understanding how sensorimotor loops derive the perception of space from the sensation of touch.

Sensory-motor transformation by individual command neurons

Animals and humans maintain a definite body orientation in space during locomotion. Here we analyze the system for the control of body orientation in the lamprey (a lower vertebrate). In the swimming lamprey, commands for changing the body orientation are based on vestibular information; they are transmitted to the spinal cord by reticulospinal (RS) neurons. The aim of this study was to characterize the sensory-motor transformation performed by individual RS neurons. The brainstem-spinal cord preparation with vestibular organs was used. For each RS neuron, we recorded (1) its vestibular responses to turns in different planes and (2) responses in different motoneuron pools of the spinal cord to stimulation of the same RS neuron; the latter data allowed us to estimate the direction of torque (caused by the RS neuron) that will rotate the animal's body during swimming. For each of the three main planes (roll, pitch, and yaw), two groups of RS neurons were found; they were activated by rotation in opposite directions and caused the torques counteracting the rotation that activated the neuron. In each plane, the system will stabilize the orientation at which the two groups are equally active; any deviation from this orientation will evoke a corrective motor response. Thus, individual RS neurons transform sensory information about the body orientation into the motor commands that cause corrections of orientation. The closed-loop mechanisms formed by individual neurons of a group operate in parallel to generate the resulting motor responses.

Responses of reticulospinal neurons in the lamprey to lateral turns

When swimming, the lamprey maintains a definite orientation of its body in the vertical planes, in relation to the gravity vector, as the result of postural vestibular reflexes. Do the vestibular-driven mechanisms also play a role in the control of the direction of swimming in the horizontal (yaw) plane, in which the gravity cannot be used as a reference direction? In the present study, we addressed this question by recording responses to lateral turns in reticulospinal (RS) neurons mediating vestibulospinal reflexes. In intact lampreys, the activity of axons of RS neurons was recorded in the spinal cord by implanted electrodes. Vestibular stimulation was performed by periodical turns of the animal in the yaw plane (60 degrees peak to peak). It was found that the majority of responding RS neurons were activated by the contralateral turn. By removing one labyrinth, we found that yaw responses in RS neurons were driven mainly by input from the contralateral labyrinth. We suggest that these neurons, when activated by the contralateral turn, will elicit the ipsilateral turn and thus will compensate for perturbations of the rectilinear swimming caused by external factors. It is also known that unilateral eye illumination elicits a contralateral turn in the yaw plane (negative phototaxis). We found that a portion of RS neurons were activated by the contralateral eye illumination. By eliciting an ipsilateral turn, these neurons could mediate the negative phototaxis.