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Single steps in different directions are often used for postural corrections. However, our knowledge about the neural mechanisms underlying their generation is scarce. This study was aimed to characterize the corrective steps generated in response to disturbances of the basic body configuration caused by forward, backward or outward displacement of the hindlimb, as well as to reveal location in the CNS of the corrective step generating mechanisms. Video recording of the motor response to translation of the supporting surface under the hindlimb along with contact forces and activity of back and limb muscles was performed in freely standing intact and in fixed postmammillary rabbits. In intact rabbits, displacement of the hindlimb in any direction caused a lateral trunk movement towards the contralateral hindlimb, and then a corrective step in the direction opposite to the initial displacement. The time difference between onsets of these two events varied considerably. The EMG pattern in the supporting hindlimb was similar for all directions of corrective steps. It caused the increase in the limb stiffness. EMG pattern in the stepping limb differed in steps with different directions. In postmammillary rabbits the corrective stepping movements, as well as EMG patterns in both stepping and standing hindlimbs were similar to those observed in intact rabbits. This study demonstrates that the corrective trunk and limb movements are generated by separate mechanisms activated by sensory signals from the deviated limb. The neuronal networks generating postural corrective steps reside in the brainstem, cerebellum, and spinal cord.

KEY POINTS Spinal reflexes are substantial components of the motor control system in all vertebrates and centrally driven reflex modifications are essential to many behaviours, but little is known about the neuronal mechanisms underlying these modifications. To study this issue, we took advantage of an in vitro brainstem-spinal cord preparation of the lamprey (a lower vertebrate), in which spinal reflex responses to spinal cord bending (caused by signals from spinal stretch receptor neurons) can be evoked during different types of fictive behaviour. Our results demonstrate that reflexes observed during fast forward swimming are reversed during escape behaviours, with the reflex reversal presumably caused by supraspinal commands transmitted by a population of reticulospinal neurons. NMDA receptors are involved in the formation of these commands, which are addressed primarily to the ipsilateral spinal networks. In the present study the neuronal mechanisms underlying reflex reversal have been characterized for the first time. ABSTRACT Spinal reflexes can be modified during different motor behaviours. However, our knowledge about the neuronal mechanisms underlying these modifications in vertebrates is scarce. In the lamprey, a lower vertebrate, body bending causes activation of intraspinal stretch receptor neurons (SRNs) resulting in spinal reflexes: activation of motoneurons (MNs) with bending towards either the contralateral or ipsilateral side (a convex or concave response, respectively). The present study had two main aims: (i) to investigate how these spinal reflexes are modified during different motor behaviours, and (ii) to reveal reticulospinal neurons (RSNs) transmitting commands for the reflex modification. For this purpose in in vitro brainstem-spinal cord preparation, RSNs and reflex responses to bending were recorded during different fictive behaviours evoked by supraspinal commands. We found that during fast forward swimming MNs exhibited convex responses. By contrast, during escape behaviours, MNs exhibited concave responses. We found RSNs that were activated during both stimulation causing reflex reversal without initiation of any specific behaviour, and stimulation causing reflex reversal during escape behaviour. We suggest that these RSNs transmit commands for the reflex modification. Application of the NMDA antagonist (AP-5) to the brainstem significantly decreased the reversed reflex, suggesting involvement of NMDA receptors in the formation of these commands. Longitudinal split of the spinal cord did not abolish the reflex reversal caused by supraspinal commands, suggesting an important role for ipsilateral networks in determining this type of motor response. This is the first study to reveal the neuronal mechanisms underlying supraspinal control of reflex reversal.

Central pattern generators consist of sets of interconnected neurons able to generate a basic motor output pattern underlying automatic movements (respiration, locomotion, chewing, swallowing, etc.) without any afferent signals from the executive motor apparatus. They are divided into constitutive pattern generators, which are active throughout life (the respiratory generator), and conditional pattern generators, which control episodic movements (locomotion, chewing, swallowing, etc.). As the motor output of a pattern generator is defined by its internal organization, the activity of conditional pattern generators is initiated by a simple command arriving from the higher centers. The structural-functional organization of the locomotor pattern generators in the marine mollusk Clione, the lamprey, the frog embryo, and laboratory mammals (cats, mice, and rats) are described, along with pattern generators controlling respiratory and swallowing movements in mammals and pattern generators for discharges of the electric organs of gymnotiform fish. Generation of the rhythmic motor output is shown in all cases to be based on the endogenous (pacemaker) activity of specific groups of interneurons and interneural interactions. These two interacting mechanisms supplement each other, ensuring the reliable operation of pattern generators. The question of whether experience gained from studies of central pattern generators can be used for understanding the mechanisms of more complex brain functions, including cognitive functions, is discussed.

The pelagic marine mollusk Clione limacina (class Gastropoda, subclass Opisthobranchaea, order Pteropoda), 3–5 cm in length, swims by rhythmically moving (1–2-Hz) two winglike appendages. Each swim cycle consists of two phases—the dorsal (D) and ventral (V) wing flexions. The nervous system of Clione consists of five pairs of ganglia. The wing movements are controlled by the pedal ganglia giving rise to the wing nerves. The neuronal circuit of the swim central pattern generator (CPG) is located in the pedal ganglia, which is able to generate the basic pattern of rhythmic activity after isolation from the organism (fictive swimming). Approximately 120 pedal neurons exhibit rhythmic activity during fictive swimming. According to their morphology, rhythmic neurons are divided into motoneurons (MNs), with axons exiting via the wing nerves to wing muscles, and interneurons (INs), with axons projecting to the contralateral ganglion.

The lamprey has two principal behavioral states—a quiescent state, when it is attached to the substrate with its sucker mouth; and an active state, when it locomotes. It is capable of several forms of locomotion, but it actively stabilizes its body orientation in space only during the main form, fast forward swimming. During fast forward swimming, orientation of the lamprey in the sagittal (pitch) and transversal (roll) planes is stabilized in relation to the gravity vector by means of the postural control systems driven by vestibular input. Any deviations from the stabilized orientation are reflected in vestibular signals, which cause corrective motor responses. In the pitch and yaw planes, the corrections occur due to the body bending in the corresponding plane. In the roll plane, the corrections occur due to a change in the direction of locomotor body undulations, from lateral to oblique.

In quadrupeds, the most critical aspect of postural control during locomotion is lateral stability. However, neural mechanisms underlying lateral stability are poorly understood. Here, we studied lateral stability in decerebrate cats walking on a treadmill with their hindlimbs. Two destabilizing factors were used: a brief lateral push of the cat and a sustained lateral tilt of the treadmill. It was found that the push caused considerable trunk bending and twisting, as well as changes in the stepping pattern, but did not lead to falling. Due to postural reactions, locomotion with normal body configuration was restored in a few steps. It was also found that the decerebrate cat could keep balance during locomotion on the laterally tilted treadmill. This postural adaptation was based on the transformation of the symmetrical locomotor pattern into an asymmetrical one, with different functional lengths of the right and left limbs. Then, we analyzed limb and trunk neural mechanisms contributing to postural control during locomotion. It was found that one of the limb mechanisms operates in the transfer phase and secures a standard (relative to the trunk) position for limb landing. Two other limb mechanisms operate in the stance phase; they counteract distortions of the locomotor pattern by regulating the limb stiffness. The trunk configuration mechanism controls the body shape on the basis of sensory information coming from trunk afferents. We suggest that postural reactions generated by these four mechanisms are integrated, thus forming a response of the whole system to perturbation of balance during locomotion.

Forward locomotion has been extensively studied in different vertebrate animals, and the principal role of spinal mechanisms in the generation of this form of locomotion has been demonstrated. Vertebrate animals, however, are capable of other forms of locomotion, such as backward walking and swimming, sideward walking, and crawling. Do the spinal mechanisms play a principal role in the generation of these forms of locomotion? We addressed this question in lampreys, which are capable of five different forms of locomotion - fast forward swimming, slow forward swimming, backward swimming, forward crawling, and backward crawling. To induce locomotion in lampreys spinalised at the second gill level, we used either electrical stimulation of the spinal cord at different rostrocaudal levels, or tactile stimulation of specific cutaneous receptive fields from which a given form of locomotion could be evoked in intact lampreys. We found that any of the five forms of locomotion could be evoked in the spinal lamprey by electrical stimulation of the spinal cord, and some of them by tactile stimulation. These results suggest that spinal mechanisms in the lamprey, in the absence of phasic supraspinal commands, are capable of generating the basic pattern for all five forms of locomotion observed in intact lampreys. In spinal lampreys, the direction of swimming did not depend on the site of spinal cord stimulation, but on the stimulation strength. The direction of crawling strongly depended on the body configuration. The spinal structures presumably activated by spinal cord stimulation and causing different forms of locomotion are discussed.

Postural limb reflexes (PLRs) represent a substantial component of the postural system responsible for stabilization of dorsal-side-up trunk orientation in quadrupeds. Spinalization causes spinal shock, that is a dramatic reduction of extensor tone and spinal reflexes, including PLRs. The goal of our study was to determine changes in activity of spinal interneurons, in particular those mediating PLRs, that is caused by spinalization. For this purpose, in decerebrate rabbits, activity of individual interneurons from L5 was recorded during stimulation causing PLRs under two conditions: (1) when neurons received supraspinal influences and (2) when these influences were temporarily abolished by a cold block of spike propagation in spinal pathways at T12 (“reversible spinalization”; RS). The effect of RS, that is a dramatic reduction of PLRs, was similar to the effect of surgical spinalization. In the examined population of interneurons (n= 199), activity of 84% of them correlated with PLRs, suggesting that they contribute to PLR generation. RS affected differently individual neurons: the mean frequency decreased in 67% of neurons, increased in 15%, and did not change in 18%. Neurons with different RS effects were differently distributed across the spinal cord: 80% of inactivated neurons were located in the intermediate area and ventral horn, whereas 50% of nonaffected neurons were located in the dorsal horn. We found a group of neurons that were coactivated with extensors during PLRs before RS and exhibited a dramatic (>80%) decrease in their activity during RS. We suggest that these neurons are responsible for reduction of extensor tone and postural reflexes during spinal shock.

In lampreys, stretch receptor neurons (SRNs) are located at the margins of the spinal cord and activated by longitudinal stretch in that area caused by body bending. The aim of this study was a comprehensive analysis of motor responses to bending of the lamprey body in different planes and at different rostrocaudal levels. For this purpose, in vitro preparation of the spinal cord isolated together with notochord was used, and responses to bending were recorded from SRNs, as well as from motoneurons innervating the dorsal (dMNs) and ventral (vMNs) parts of a myotome. It was found that SRNs were activated on the convex (stretched) side of the preparation during bending both in the yaw and in the pitch plane. By contrast, responses of motoneurons depended on the site and plane of bending. In the yaw plane, concave responses to bending of rostral segments and convex responses to bending of mid-body segments prevailed. In the pitch plane, convex responses in dMNs and concave responses in vMNs to bending in mid-body segments prevailed. These spinal reflexes could contribute to feedback regulation of locomotor body undulations and to the control of body configuration during locomotion. After a longitudinal split of the spinal cord, only convex responses in motoneurons were present, suggesting an important role of contralateral networks in determining the type of motor response. Stimulation of the brainstem changed the type of motor response to bending, suggesting that these spinal reflexes can be modified by supraspinal signals in accordance with different motor behaviors. J. Comp. Neurol. 521:3847–3862, 2013. © 2013 Wiley Periodicals, Inc.

In quadrupeds, acute lateral hemisection of the spinal cord (LHS) severely impairs postural functions, which recover over time. Postural limb reflexes (PLRs) represent a substantial component of postural corrections in intact animals. The aim of the present study was to characterize the effects of acute LHS on two populations of spinal neurons (F and E) mediating PLRs. For this purpose, in decerebrate rabbits, responses of individual neurons from L5 to stimulation causing PLRs were recorded before and during reversible LHS (caused by temporal cold block of signal transmission in lateral spinal pathways at L1), as well as after acute surgical LHS at L1. Results obtained after Sur-LHS were compared to control data obtained in our previous study. We found that acute LHS caused disappearance of PLRs on the affected side. It also changed a proportion of different types of neurons on that side. A significant decrease and increase in the proportion of F- and non-modulated neurons, respectively, was found. LHS caused a significant decrease in most parameters of activity in F-neurons located in the ventral horn on the lesioned side and in E-neurons of the dorsal horn on both sides. These changes were caused by a significant decrease in the efficacy of posture-related sensory input from the ipsilateral limb to F-neurons, and from the contralateral limb to both F- and E-neurons. These distortions in operation of postural networks underlie the impairment of postural control after acute LHS, and represent a starting point for the subsequent recovery of postural functions.

Postural limb reflexes (PLRs) represent a substantial component of postural corrections. Spinalization results in loss of postural functions, including disappearance of PLRs. The aim of the present study was to characterize the effects of acute spinalization on two populations of spinal neurons (F and E) mediating PLRs, which we characterized previously. For this purpose, in decerebrate rabbits spinalized at T12, responses of interneurons from L5 to stimulation causing PLRs before spinalization, were recorded. The results were compared to control data obtained in our previous study. We found that spinalization affected the distribution of F- and E-neurons across the spinal grey matter, caused a significant decrease in their activity, as well as disturbances in processing of posture-related sensory inputs. A two-fold decrease in the proportion of F-neurons in the intermediate grey matter was observed. Location of populations of F- and E-neurons exhibiting significant decrease in their activity was determined. A dramatic decrease of the efficacy of sensory input from the ipsilateral limb to F-neurons and from the contralateral limb to E-neurons was found. These changes in operation of postural networks underlie the loss of postural control after spinalization and represent a starting point for the development of spasticity.

During locomotion, neurons in motor cortex exhibit profound step-related frequency modulation. The source of this modulation is unclear. The aim of this study was to reveal the contribution of different limb controllers (locomotor mechanisms of individual limbs) to the periodic modulation of motor cortex neurons during locomotion. Experiments were conducted in chronically instrumented cats. The activity of single neurons was recorded during regular quadrupedal locomotion (control), as well as when only one pair of limbs (fore, hind, right, or left) was walking while another pair was standing. Comparison of the modulation patterns in these neurons (their discharge profile with respect to the step cycle) during control and different bipedal locomotor tasks revealed several groups of neurons that receive distinct combinations of inputs from different limb controllers. In the majority (73%) of neurons from the forelimb area of motor cortex, modulation during control was determined exclusively by forelimb controllers (right, left, or both), while in the minority (27%), hindlimb controllers also contributed. By contrast, only in 30% of neurons from the hindlimb area was modulation determined exclusively by hindlimb controllers (right or both), while in 70% of them, the controllers of forelimbs also contributed. We suggest that such organization of inputs allows the motor cortex to contribute to the right–left limbs' coordination within each of the girdles during locomotion, and that it also allows hindlimb neurons to participate in coordination of the movements of the hindlimbs with those of the forelimbs.

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.

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.

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 T12 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 (≤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.

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).

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.

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.

In a previous study we demonstrated that the activity of pyramidal tract neurons (PTNs) of the motor cortex is modulated in relation to postural corrections evoked by periodical tilts of the animal. The modulation included an increase in activity in one phase of the tilt cycle and a decrease in the other phase. It is known that the motor cortex contains a large population of inhibitory GABAergic neurons. How do these neurons participate in periodic modulation of PTNs? The goal of this study was to investigate the role of GABAA inhibitory neurons of the motor cortex in the modulation of postural-related PTN activity. Using extracellular electrodes with attached micropipettes, we recorded the activity of PTNs in cats maintaining balance on a tilting platform both before and after iontophoretic application of the GABAA receptor antagonists gabazine or bicuculline. The tilt-related activity of 93% of PTNs was affected by GABAA receptor antagonists. In 88% of cells, peak activity increased by 75 ± 50% (mean ± SD). In contrast, the trough activity changed by a much smaller value and almost as many neurons showed a decrease as showed an increase. In 73% of the neurons, the phase position of the peak activity did not change or changed by no more than 0.1 of a cycle. We conclude that the GABAergic system of the motor cortex reduces the posture-related responses of PTNs but has little role in determining their response timing.

The body posture during standing and walking is maintained due to the activity of a closed-loop control system. In the review, we consider different aspects of postural control: its functional organization, the distribution of postural functions in different parts of the central nervous system, and the activity of neuronal networks controlling posture.

The dorsal-side-up body posture in standing quadrupeds is maintained by coordinated activity of four limbs. We studied this coordination in the cat standing on the platform periodically tilted in the frontal plane. By suspending different body parts, we unloaded one, two, or three limbs. The activity of selected extensor muscles and the contact forces under the limbs were recorded. With all four limbs on the platform, extensors of the fore- and hindlimbs increased their activity in parallel during ipsilateral downward tilt. With two forelimbs on the platform, this muscular pattern persisted in the forelimbs and in the suspended hindlimbs. With two hindlimbs on the platform, the muscular pattern persisted only in the hindlimbs, but not in the suspended forelimbs. These results suggest that coordination between the two girdles is based primarily on the influences of the forelimbs upon the hindlimbs. However, these influences do not necessarily determine the responses to tilt in the hindlimbs. This was demonstrated by antiphase tilting of the fore- and hindquarters. Under these conditions, the extensors of the fore- and hindlimbs appeared uncoupled and modulated in antiphase, suggesting an independent control of posture in the fore- and hindquarters. With only one limb supporting the shoulder or hip girdle, a muscular pattern with normal phasing was observed in both limbs of that girdle. This finding suggests that reflex mechanisms of an individual limb generate only a part of postural corrections; another part is produced on the basis of crossed influences.

A deviation from the dorsal-side-up body posture in quadrupeds activates the mechanisms for postural corrections. Operation of these mechanisms was studied in the rabbit maintaining balance on a platform periodically tilted in the frontal plane. First, we characterized the kinematics and electromyographic (EMG) patterns of postural responses to tilts. It was found that a reaction to tilt includes an extension of the limbs on the side moving down and flexion on the opposite side. These limb movements are primarily due to a modulation of the activity of extensor muscles. Second, it was found that rabbits can effectively maintain the dorsal-side-up body posture when complex postural stimuli are applied, i.e., asynchronous tilts of the platforms supporting the anterior and posterior parts of the body. These data suggest that the nervous mechanisms controlling positions of these parts of the body can operate independently of each other. Third, we found that normally the somatosensory input plays a predominant role for the generation of postural responses. However, when the postural response appears insufficient to maintain balance, the vestibular input contributes considerably to activation of postural mechanisms. We also found that an asymmetry in the tonic vestibular input, caused by galvanic stimulation of the labyrinths, can affect the stabilized body orientation while the magnitude of postural responses to tilts remains unchanged. Fourth, we found that the mechanisms for postural corrections respond only to tilts that exceed a certain (threshold) value.

The dorsal side-up body orientation in quadrupeds is maintained by a postural system that is driven by sensory feedback signals. The spinal cord, brainstem, and cerebellum play essential roles in postural control, whereas the role of the forebrain is unclear. In the present study we investigated whether the motor cortex is involved in maintenance of the dorsal side-up body orientation. We recorded activity of neurons in the motor cortex in awake rabbits while animals maintained balance on a platform periodically tilting in the frontal plane. The tilts evoked postural corrections, i.e., extension of the limbs on the side moving down and flexion on the opposite side. Because of these limb movements, rabbits maintained body orientation close to the dorsal side up. Four classes of efferent neurons were studied: descending corticofugal neurons of layer V (CF5s), those of layer VI (CF6s), corticocortical neurons with ipsilateral projection (CCIs), and those with contralateral projection (CCCs). One class of inhibitory interneurons [suspected inhibitory neurons (SINs)] was also investigated. CF5 neurons and SINs were strongly active during postural corrections. In most of these neurons, a clear-cut modulation of discharge in the rhythm of tilting was observed. This finding suggests that the motor cortex is involved in postural control. In contrast to CF5 neurons, other classes of efferent neurons (CCI, CCC, CF6) were much less active during postural corrections. This suggests that corticocortical interactions, both within a hemisphere (mediated by CCIs) and between hemispheres (mediated by CCCs), as well as corticothalamic interactions via CF6 neurons are not essential for motor coordination during postural corrections.

SUMMARYThe lamprey swims by caudally directed lateral undulations of its body. During swimming the animal is oriented with its dorsal side up, and any deviation from this posture (roll tilt) elicits a corrective motor response aimed at restoring the normal orientation. Video recording was used to study the kinematic pattern of the response to a 90° roll tilt imposed in the intact lamprey. The corrective responses were associated with specific modifications of the swimming movements. The plane of locomotor undulations deviated from the normal, i.e. frontal plane in one direction at the beginning of the rotation and in the opposite direction at its end. A similar motor pattern was also observed in the anterior part of the body of lampreys in which the spinal cord had been transected in the mid-body area, when performing postural corrections. It could also be observed during roll turns performed by lampreys after a rostral hemisection of the spinal cord. We argue that these modifications of the locomotor pattern generate the moments of force necessary for initiation and termination of the corrective roll turn. Possible neuronal mechanisms causing the corrective movements are discussed.

In recent years, studies of nervous mechanisms for the control of body posture have been performed on animal models of different complexity - cat, rabbit, lamprey and the mollusc Clione. These studies have greatly expanded our knowledge of how the control system operates, how the system can change the stabilized body orientation and how the postural functions are distributed within different parts of the CNS. For simpler animal models, the postural network has been analyzed in considerable detail and main cell types and their interactions have been identified.

Different species maintain a particular body orientation in space due to activity of the closed-loop postural control system. In this review we discuss the role of neurons of descending pathways in operation of this system as revealed in animal models of differing complexity: lower vertebrate (lamprey) and higher vertebrates (rabbit and cat). In the lamprey and quadruped mammals, the role of spinal and supraspinal mechanisms in the control of posture is different. In the lamprey, the system contains one closed-loop mechanism consisting of supraspino-spinal networks. Reticulospinal (RS) neurons play a key role in generation of postural corrections. Due to vestibular input, any deviation from the stabilized body orientation leads to activation of a specific population of RS neurons. Each of the neurons activates a specific motor synergy. Collectively, these neurons evoke the motor output necessary for the postural correction. In contrast to lampreys, postural corrections in quadrupeds are primarily based not on the vestibular input but on the somatosensory input from limb mechanoreceptors. The system contains two closed-loop mechanisms – spinal and spino-supraspinal networks, which supplement each other. Spinal networks receive somatosensory input from the limb signaling postural perturbations, and generate spinal postural limb reflexes. These reflexes are relatively weak, but in intact animals they are enhanced due to both tonic supraspinal drive and phasic supraspinal commands. Recent studies of these supraspinal influences are considered in this review. A hypothesis suggesting common principles of operation of the postural systems stabilizing body orientation in a particular plane in the lamprey and quadrupeds, that is interaction of antagonistic postural reflexes, is discussed.

The marine mollusk Clione limacina, when swimming, normally stabilizes the vertical body orientation by means of the gravitational tail reflexes. Horizontal swimming or swimming along inclined ascending trajectories is observed rarely. Here we report that GABA injection into intact Clione resulted in a change of the stabilized orientation and swimming with a tilt of approximately 45 degrees to the left. The analysis of modifications in the postural network underlying this effect was done with in vitro experiments. The CNS was isolated together with the statocysts. Spike discharges in the axons of two groups of motoneurons responsible for the left and right tail flexion, as well as in the axons of CPB3 interneurons mediating signals from the statocyst receptors to the motoneurons, were recorded extracellularly when the preparation was rotated in space. Normally the tail motoneurons of the left and right groups were activated with the contralateral tilt of the preparation. Under the effect of GABA, the gravitational responses in the right group of motoneurons and in the corresponding interneurons were dramatically reduced while the responses in the left group remained unchanged. The most likely site of the inhibitory GABA action is the interneurons mediating signals from the statocysts to the right group of tail motoneurons. The GABA-induced asymmetry of the left and right gravitational tail reflexes, observed in the in vitro experiments, is consistent with a change of the stabilized orientation caused by GABA in the intact Clione.

A phenomenological model of the mechanism of stabilization of the dorsal-side-up orientation in the lamprey is suggested. Mathematical modeling is based on the experimental results on investigation of postural control in lampreys using combined in vivo and robotics approaches. Dynamics of the model agrees qualitatively with the experiment. It is shown by computer simulations that postural correction commands from one or several reticulospinal neurons provide information which may be sufficient for stabilization of body orientation in the lamprey.

The reticulospinal (RS) system is the main descending system transmitting commands from the brain to the spinal cord in the lamprey. It is responsible for initiation of locomotion, steering, and equilibrium control. In the present study, we characterize the commands that are sent by the brain to the spinal cord in intact animals via the reticulospinal pathways during locomotion. We have developed a method for recording the activity of larger RS axons in the spinal cord in freely behaving lampreys by means of chronically implanted macroelectrodes. In this paper, the mass activity in the right and left RS pathways is described and the correlations of this activity with different aspects of locomotion are discussed. In quiescent animals, the RS neurons had a low level of activity. A mild activation of RS neurons occurred in response to different sensory stimuli. Unilateral eye illumination evoked activation of the ipsilateral RS neurons. Unilateral illumination of the tail dermal photoreceptors evoked bilateral activation of RS neurons. Water vibration also evoked bilateral activation of RS neurons. Roll tilt evoked activation of the contralateral RS neurons. With longer or more intense sensory stimulation of any modality and laterality, a sharp, massive bilateral activation of the RS system occurred, and the animal started to swim. This high activity of RS neurons and swimming could last for many seconds after termination of the stimulus. There was a positive correlation between the level of activity of RS system and the intensity of locomotion. An asymmetry in the mass activity on the left and right sides occurred during lateral turns with a 30% prevalence (on average) for the ipsilateral side. Rhythmic modulation of the activity in RS pathways, related to the locomotor cycle, often was observed, with its peak coinciding with the electromyographic (EMG) burst in the ipsilateral rostral myotomes. The pattern of vestibular response of RS neurons observed in the quiescent state, that is, activation with contralateral roll tilt, was preserved during locomotion. In addition, an inhibition of their activity with ipsilateral tilt was clearly seen. In the cases when the activity of individual neurons could be traced during swimming, it was found that rhythmic modulation of their firing rate was superimposed on their tonic firing or on their vestibular responses. In conclusion, different aspects of locomotor activity—initiation and termination, vigor of locomotion, steering and equilibrium control—are well reflected in the mass activity of the larger RS neurons.

The marine mollusk Clione limacina, when swimming, can stabilize different body orientations in the gravitational field. Here we describe one of the modes of operation of the postural network in Clione-maintenance of the vertical, head-up orientation. Experiments were performed on the CNS-statocyst preparation. Spike discharges in the axons of different types of neurons were recorded extracellularly when the preparation was rotated in space through 360 degrees in different planes. We characterized the spatial zones of activity of the tail and wing motor neurons as well as of the CPB3 interneurons mediating the effects of statocyst receptor cells on the tail motor neurons. It was found that the activity of the tail motor neurons increased with deviation of the preparation from the normal, rostral-side-up orientation. Their zones of activity were very wide ( approximately 180 degrees ). According to the zone position, three distinct groups of tail motor neuron (T1-T3) could be distinguished. The T1 group had a center of the zone near the ventral-side-up orientation, whereas the zones of T2 and T3 had their centers near the left-side-up and the right-side-up positions, respectively. By comparing the zone of activity with the direction of tail bending elicited by each of the groups, one can conclude that gravitational reflexes mediated by the T1, T2, and T3 groups will evoke turning of the animal toward the head-up orientation. Two identified wing motor neurons, 1A and 2A, causing the wing beating, were involved in gravitational reactions. They were activated with the downward inclination of the ipsilateral side. Opposite reactions were observed in the motor neurons responsible for the wing retraction. A presumed motor effect of these reactions is an increase of oscillations in the wing that is directed downward and turning of Clione toward the head-up orientation. Among the CPB3 interneurons, at least four groups could be distinguished. In three of them (IN1, IN2, and IN3), the zones of activity were similar to those of the three groups (T1, T2, and T3) of the tail motor neurons. The group IN4 had the center of its zone in the dorsal-side-up position; a corresponding group was not found among the tail motor neurons. In lesion experiments, it was found that gravitational input mediated by a single CPB3 interneuron produced activation of its target tail motor neurons in their normal zones, but the strength of response was reduced considerably. This finding suggests that several interneurons with similar spatial zones converge on individual tail motor neurons. In conclusion, because of a novel method, activity of the neuronal network responsible for the postural control in Clione was characterized in the terms of gravitational responses in different neuron groups comprising the network.

Wannier, T., T. G. Deliagina, G. N. Orlovsky, and S. Grillner. Differential effects of the reticulospinal system on locomotion in lamprey. J. Neurophysiol. 80: 103–112, 1998. Specific effects of stimulating different parts of the reticulospinal (RS) system on the spinal locomotor pattern are described in lamprey. In the in vitro brain stem and spinal cord preparation, microstimulation in different areas of the reticular formation was performed by ejecting a small amount of d-glutamate from a micropipette. These areas were distributed over the four reticular nuclei of the brain stem: the mesencephalic reticular nucleus (MRN) and the anterior, middle and posterior rhombencephalic reticular nuclei (ARRN, MRRN, and PRRN, respectively). To prevent synaptic spread of excitation within the brain stem, the synaptic transmission was blocked by using a low Ca2+, high Mn2+physiological saline in the brain stem pool. “Fictive” locomotion was evoked by applying N-methyl-d-aspartate (NMDA) to the spinal cord. Rhythmical discharges of motoneurons were recorded bilaterally in the midbody area, from the ventral roots that had been subdivided in dorsal and ventral branches, supplying the dorsal and ventral part of the myotome, respectively. Two major effects of brain stem stimulation were elicited: a change in the frequency of the locomotory rhythm and an induction of asymmetry (left/right, dorsal/ventral) in the segmental motor output. Approximately 50% of the stimulated sites evoked a change in locomotor frequency. In the PRRN almost all effective sites evoked an increase in frequency (10–50%). In the other nuclei, increase and decrease (10–30%) were observed equally frequently. Most of the stimulated sites (50–80%) in any reticular nucleus evoked asymmetry in the segmental motor output. Distortion of the segmental output symmetry was classified into eight categories by comparing the intensity of locomotor bursts in the dorsal and ventral branches of the two ventral roots, ipsilateral and contralateral to the stimulated side. These categories differed in the direction of the body flexion, which would be evoked during normal swimming: ipsilateral (I), contralateral (C), dorsal (D), ventral (V), ipsilateral and dorsal (ID), ipsilateral and ventral (IV), contralateral and dorsal (CD), and contralateral and ventral (CV). The different categories were not equally represented in each nucleus and across the nuclei. The most pronounced categories for each nucleus were as follow. In MRN: I (33%); ARRN: C (44%); MRRN: rostral part, I (36%) and caudal part, CV (42%); and PRRN: rostral part, I (40%) and caudal part, IV (35%). Other categories were also present but less common in each nucleus. To examine if the effects of brain stem stimulation were uniform along the spinal cord, recordings were performed from distal parts of the cord. Stimulation of a given point in the brain stem produced similar pattern of effects in 59% of cases and different patterns in 41% of cases. The main conclusion of the present study is that the proportion of RS neurons with different influences on the spinal locomotor network differs significantly among different parts of the reticular formation of the lamprey. The specificity of RS influences may represent a basis for modifications of the segmental locomotor output necessary for the control of equilibrium and steering during locomotion.

The main function of postural nervous mechanisms in different species, from mollusc to man, is to counteract the force of gravity and stabilize body orientation in space. Here we investigate the basic principles of postural control in a simple animal model, the marine mollusc Clione limacina. When swimming, C. limacina maintains its vertical orientation because of the activity of the postural neuronal network. Driven by gravity-sensing organs (statocysts), the network causes postural corrections by producing tail flexions. To understand how this function occurs, we studied network activity by using a new method. We used an in vitro preparation that consisted of the central nervous system isolated with the statocysts. Output signals from the network (electrical activity of tail motor neurons) controlled an electrical motor which rotated the preparation in space. We analysed the activity of individual neurons involved in postural stabilization under opened or closed feedback loop. When we closed this artificial feedback loop, the network stabilized the vertical orientation of the preparation. This stabilization is based on the tendency of the network to minimize the difference between the activities of the two antagonistic groups of neurons, which are driven by orientation-dependent sensory inputs.

The lamprey normally swims with the dorsal side up. Illumination of one eye shifts the set-point of the vestibular roll control system, however, so that the animal swims with a roll tilt towards the source of light (the dorsal light response). A tilted orientation is often maintained for up to 1 min after the stimulation. In present study, the basis for this behaviour was investigated at the neuronal level. The middle rhombencephalic reticular nucleus (MRRN) is considered a main nucleus for the control of roll orientation in lampreys. Practically all MRRN neurons receive vestibular and visual input and project to the spinal cord. Earlier extracellular experiments had shown that optic nerve stimulation potentiates the response to vestibular stimulation in the ipsilateral MRRN. This most likely represents a neural correlate of the dorsal light response. Experiments were carried out in vitro on the isolated brainstem of the silver lamprey (Ichthyomyzon unicuspis). MRRN cells were recorded intracellularly, and the overall activity of descending systems was monitored with bilateral extracellular electrodes. The responses to 10 Hz optic nerve stimulation and 1 Hz vestibular nerve stimulation, and the influence of optic nerve stimulation on the vestibular responses, were investigated. In most preparations, optic nerve stimulation excited practically all ipsilateral MRRN cells. After stimulation, the cell was typically depolarized and showed an increased level of synaptic noise for up to 80 s. In contralateral MRRN neurons, optic nerve stimulation usually evoked hyperpolarization or no response. Vestibular nerve stimulation evoked compound excitatory postsynaptic potentials (EPSPs) or spikes in approximately 90% of the cells, both ipsilaterally and contralaterally. A smaller subpopulation of MRRN cells (approximately 10%) received vestibular inhibition. In 26 of 48 recorded MRRN cells, the response to vestibular stimulation was potentiated after ipsilateral optic nerve stimulation. The potentiation was seen in cells receiving either excitatory or inhibitory vestibular input as an increase in EPSP amplitude/spiking (85%) and a decrease in inhibitory postsynaptic potential amplitude (15%) respectively. In most cases the vestibular responses did not return to control levels during the testing period (10-30 min), and thus the visual stimulation most likely induced long-lasting changes in the functional connectivity of the role control network, in addition to the short-lasting afteractivity. In four of the 11 cells recorded contralateral to the stimulated optic nerve, a depression of the vestibular response could be seen. In potentiated cells, single vestibular pulses often evoked longer episodes of large synaptic noise and sometimes spiking. In the latter case, the action potentials appeared with highly variable latency after each stimulation pulse. This indicates that an important mechanism underlying the potentiation may be a long-lasting increase in excitability in a pool of unidentified interneurons located either upstream of the MRRN cells, relaying vestibular and visual inputs, or downstream, providing positive feedback.

In lampreys, stretch receptor neurons (SRNs) are located at the margins of the spinal cord and activated by longitudinal stretch in that area caused by body bending. The aim of this study was a comprehensive analysis of motor responses to bending of the lamprey body in different planes and at different rostrocaudal levels. For this purpose, in vitro preparation of the spinal cord isolated together with notochord was used, and responses to bending were recorded from SRNs, as well as from motoneurons innervating the dorsal (dMNs) and ventral (vMNs) parts of a myotome. It was found that SRNs were activated on the convex (stretched) side of the preparation during bending both in the yaw and in the pitch plane. By contrast, responses of motoneurons depended on the site and plane of bending. In the yaw plane, concave responses to bending of rostral segments and convex responses to bending of mid-body segments prevailed. In the pitch plane, convex responses in dMNs and concave responses in vMNs to bending in mid-body segments prevailed. These spinal reflexes could contribute to feedback regulation of locomotor body undulations and to the control of body configuration during locomotion. After a longitudinal split of the spinal cord, only convex responses in motoneurons were present, suggesting an important role of contralateral networks in determining the type of motor response. Stimulation of the brainstem changed the type of motor response to bending, suggesting that these spinal reflexes can be modified by supraspinal signals in accordance with different motor behaviors.

In lamprey, distinct groups of reticulospinal neurones utilize different neurotransmitters such as glutamate or serotonin. The present study demonstrates that a group of reticulospinal neurones inhibit their target neurones by an action on glycine receptors. Simultaneous intracellular recordings from a reticulospinal neurone and spinal target neurone shows that the former may evoke an IPSP in the latter. These IPSPs are elicited at a constant latency and amplitude, and follow high frequency stimulation (100 Hz). Furthermore, the IPSPs are maintained when the excitatory amino acid synaptic transmission is blocked, suggesting that the effects are not elicited via a powerful disynaptic pathway. These data taken together establish the monosynaptic nature of the pathway. IPSPs elicited from single reticulospinal neurones or from electrical stimulation of the reticular formation are suppressed by administration of strychnine, suggesting that glycine is the neurotransmitter of these inhibitory reticulospinal neurones.

1. The pteropod mollusk Clione limacina swims by rhythmical oscillations of two wings, and its spatial orientation during locomotion is determined by tail movements. The majority of neurons responsible for generation of the wing and tail movements are located in the pedal ganglia. On the other hand, the majority of sensory inputs that affect wing and tail movements project to the cerebral ganglia. The goal of the present study was to identify and characterize cerebropedal neurons involved in the control of the swimming central generator or motor neurons of wing and tail muscles. Cerebropedal neurons affecting locomotion-controlling mechanisms are located in the rostromedial (CPA neurons), caudomedial (CPB neurons), and central (CPC neurons) zones of the cerebral ganglia. According to their morphology and effects on pedal mechanisms, 10 groups of the cerebropedal neurons can be distinguished. 2. CPA1 neurons project through the ipsilateral cerebropedal connective to both pedal ganglia. Activation of a CPA1 by current injection resulted in speeding up of the locomotor rhythm and intensification of the firing of the locomotor motor neurons. 3. CPA2 neurons send numerous thin fibers into the ipsi- and contralateral pedal and pleural ganglia through the cerebropedal and cerebropleural connectives. They strongly inhibit the wing muscle motor neurons and, to a lesser extent, slow down the locomotor rhythm. 4. CPB1 neurons project through the contralateral cerebropedal connective to both pedal ganglia. They activate the locomotor generator. 5. CPB2 neurons also project, through the contralateral cerebropedal connective, to both pedal ganglia. They affect wing muscle motor neurons. 6. CPB3 neurons have diverse morphology: they project to the pedal ganglia either through the ipsilateral cerebropedal connective, or through the contralateral one, or through both of them. They affect putative motor neurons of the tail muscles. 7. CPC1, CPC2, and CPC3 neurons project through the ipsilateral cerebropedal connective to both pedal ganglia. They activate the locomotor generator. 8. CPC4 and CPC5 neurons project through the contralateral cerebropedal connective to the contralateral pedal ganglia. They activate the locomotor generator. 9. Serotonergic neurons were mapped in the CNS of Clione by immunohistochemical methods. Location and size of cells in two groups of serotonin-immunoreactive neurons in the cerebral ganglia appeared to be similar to those of CPA1 and CPB1 neurons. This finding suggests a possible mechanism for serotonin's ability to exert a strong excitatory action on the locomotor generator of Clione. 10. The role of different groups of cerebropedal neurons is discussed in relation to different forms of Clione's behavior in which locomotor activity is involved.

The networks of the brainstem and spinal cord that co-ordinate locomotion and body orientation in lamprey are described. The cycle-to-cycle pattern generation of these networks is produced by interacting glutamatergic and glycinergic neurones, with NMDA receptor-channels playing an important role at lower rates of locomotion. The fine tuning of the networks produced by 5-HT, dopamine and GABA systems involves a modulation of Ca2+-dependent K+ channels, high- and low-threshold voltage-activated Ca2+ channels and presynaptic inhibitory mechanisms. Mathematical modelling has been used to explore the capacity of these biological networks. The vestibular control of the body orientation during swimming is exerted via reticulospinal neurones located in different reticular nuclei. These neurones become activated maximally at different angles of tilt.

Quadrupeds maintain the dorsal side up body orientation due to the activity of the postural control system driven by limb mechanoreceptors. Binaural galvanic vestibular stimulation (GVS) causes a lateral body sway toward the anode. Previously, we have shown that this new position is actively stabilized, suggesting that GVS changes a set point in the reflex mechanisms controlling body posture. The aim of the present study was to reveal the underlying neuronal mechanisms. Experiments were performed on decerebrate rabbits. The vertebral column was rigidly fixed, whereas hindlimbs were positioned on a platform. Periodic lateral tilts of the platform caused postural limb reflexes (PLRs): activation of extensors in the loaded and flexing limb and a decrease in extensor activity in the opposite (unloaded and extending) limb. Putative spinal interneurons were recorded in segments L4–L5 during PLRs, with and without GVS. We have found that GVS enhanced PLRs on the cathode side and reduced them on the anode side. This asymmetry in PLRs can account for changes in the stabilized body orientation observed in normal rabbits subjected to continuous GVS. Responses to platform tilts (frequency modulation) were observed in 106 spinal neurons, suggesting that they can contribute to PLR generation. Two neuron groups were active in opposite phases of the tilt cycle of the ipsi-limb: F-neurons in the flexion phase, and E-neurons in the extension phase. Neurons were driven mainly by afferent input from the ipsi-limb. If one supposes that F- and E-neurons contribute, respectively, to excitation and inhibition of extensor motoneurons, one can expect that the pattern of response to GVS in F-neurons will be similar to that in extensor muscles, whereas E-neurons will have an opposite pattern. We have found that ∼40% of all modulated neurons meet this condition, suggesting that they contribute to the generation of PLRs and to the GVS-caused changes in PLRs.

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.

1. In the intact pond snail Planorbis corneus, tactile or electrical stimulation of the skin evoked a biphasic general defense reaction. A weak stimulation evoked only the first phase of the reaction, represented as a fast pulling of the shell towards the head. With stronger stimulation, this phase was followed by the second phase that was comprised of three components: detachment from the substrate, slow retraction of the body into the shell, and letting out of air from the lung through the pneumostome. 2. About 70 motor neurons (MNs) of the columellar muscle have been revealed in different ganglia by means of their cobalt back-filling through the cut columellar nerve. A complicated pattern of electrical coupling was found for different groups of MNs. Excitation of individual MNs, evoked by current injection, resulted in contraction of the columellar muscle (CNS-columellar muscle preparation). The strongest contraction was evoked by the cerebral MNs; fast small contraction by the parietal MNs; and slow, long-latency contraction, by the pedal MNs. 3. In the same preparation, electrical stimulation of the cutaneous (lip) nerve evoked biphasic contraction of the columellar muscle (a first phase lasting approximately 3 s, and a second phase of up to 1 min). The temporal pattern of this response was similar to that of the defense reaction in the intact animal. A weak stimulation evoked only the first phases of the reaction, while a stronger stimulation evoked both phases. The amplitude of both the first and the second phase was graded with the strength of stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

The postural system maintains a specific body orientation and equilibrium during standing and during locomotion in the presence of many destabilizing factors (external and internal). Numerous studies in humans have revealed essential features of the functional organization of this system. Recent studies on different animal models have significantly supplemented human studies. They have greatly expanded our knowledge of how the control system operates, how the postural functions are distributed within different parts of CNS, and how these parts interact with each other to produce postural corrections and adjustments. This review outlines recent advances in the studies of postural control in quadrupeds, with special attention given the neuronal postural mechanisms.

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.

Forward walking (FW) and backward walking (BW) are two important forms of locomotion in quadrupeds. Participation of the motor cortex in the control of FW has been intensively studied, whereas cortical activity during BW has never been investigated. The aim of this study was to analyze locomotion-related activity of the motor cortex during BW and compare it with that during FW. For this purpose, we recorded activity of individual neurons in the cat during BW and FW. We found that the discharge frequency in almost all neurons was modulated in the rhythm of stepping during both FW and BW. However, the modulation patterns during BW and FW were different in 80% of neurons. To determine the source of modulating influences (forelimb controllers vs. hindlimb controllers), the neurons were recorded not only during quadrupedal locomotion but also during bipedal locomotion (with either forelimbs or hindlimbs walking), and their modulation patterns were compared. We found that during BW (like during FW), modulation in some neurons was determined by inputs from limb controllers of only one girdle, whereas the other neurons received inputs from both girdles. The combinations of inputs could depend on the direction of locomotion. Most often (in 51% of forelimb-related neurons and in 34% of the hindlimb-related neurons), the neurons received inputs only from their own girdle when this girdle was leading and from both girdles when this girdle was trailing. This reconfiguration of inputs suggests flexibility of the functional roles of individual cortical neurons during different forms of locomotion.

1. A contact of the pteropod mollusk Clione limacina with its prey (small pteropod mollusk Limacina helicina) evokes a complex pattern of hunting and feeding behavior: protraction of tentacles to seize the prey, activation of buccal apparatus to swallow the prey, activation of locomotor system (speeding up of wing beating), reversal of reaction to tactile stimulation of the head, loss of normal (vertical) orientation in space, and swimming in circles. After injection of physostigmine (PhS), the acetylcholinesterase inhibitor, into the hemocoel of intact Clione, all these manifestations of the hunting and feeding behavior could be evoked by tactile stimulation of the head, or they arose spontaneously. 2. In the preparation of the isolated CNS, the effect of PhS on the neural networks controlling different aspects of the hunting and feeding behavior was studied by recording from neurons monitoring activity of different networks (< or = 4 neurons simultaneously). Tactile stimulation of the head was mimicked by a short-term electrical stimulation of the corresponding nerve. Before PhS application, the nerve stimulation evoked elements of the defense reaction, i.e., long-lasting inhibition of all main motor control systems: the locomotor network in the pedal ganglia, the tentacle control network in the cerebral ganglia, and the network controlling radula and hook movements in the buccal ganglia. However, after PhS application, the same stimulus evoked a long-lasting bout of excitation in all the three networks accompanied by activation of the heart-exciting neuron as well as by a modification of the activity of statocyst receptor cells controlling Clione's spatial orientation (the "fictive hunting bout"). Similar hunting bouts could arise spontaneously. 3. Injection of acetylcholine (ACh) into the hemocoel of intact Clione was less effective than injection of PhS. After ACh injection, reversal of reaction to head stimulation was observed in < or = 20% of the experiments (the percentage of positive results was higher if ACh was injected into the head just over the CNS). Bath application of ACh to the isolated CNS did not produce the hunting bouts. However, a short-term local application of ACh to the cerebral ganglia in the isolated CNS resulted in activation of the main motor systems controlling locomotion, protraction of tentacles, and movements of buccal mass. 4. During spontaneous PhS-induced bouts, excitation of different networks involved in hunting behavior was sometimes not quite synchronous. Different networks could be excited in variable order over a period of up to several seconds.(ABSTRACT TRUNCATED AT 400 WORDS)

The pteropod mollusc Clione limacina is a predator, feeding on another pteropod mollusc, Limacina helicina. Essential elements of Clione's hunting and feeding behaviour can be evoked pharmacologically. The sensory input driven by the presence of Limacina is mimicked by physostigmine (PhS), the acetylcholinesterase inhibitor. Injection of PhS into Clione brought it to the state of hunting excitation accompanied by a reversal of the reaction to head stimulation. As a result, head stimulation, instead of a defence reaction, evoked active hunting behaviour. The 'fictive hunting behaviour', that is activation of main motor systems participating in hunting behaviour, could be evoked in a preparation of the isolated central nervous system (CNS) by PhS application. To some extent, activation of the motor systems was realized through serotonergic mechanisms. GABAergic mechanisms also play an important role in the organization of Clione's feeding behaviour. GABA application to the preparation of the isolated CNS resulted in activation of the feeding rhythm generator and excitation of the motoneurons supplying the protractor tentacle muscles. The results show that in gastropod molluscs complex behaviour involving different functional systems may be described in terms of the activity of single identified neurons.

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.