[Central Pattern Generators: Mechanisms of the Activity and Their Role in the Control of "Automatic" Movements].

Central pattern generators (CPGs) are a set of interconnected neurons capable of generating a basic pattern of motor output underlying "automatic" movements (breathing, locomotion, chewing, swallowing, and so on) in the absence of afferent signals from the executive motor apparatus. They can be divided into the constitutive CPGs active throughout the entire lifetime (respiratory CPGs) and conditional CPGs controlling episodic movements (locomotion, chewing, swallowing, and others). Since a motor output of CPGs is determined by their internal organization, the activities of the conditional CPGs are initiated by simple commands coming from higher centers. We describe the structural and functional organization of the locomotor CPGs in the marine mollusk Clione limacina, lamprey, frog embryo, and laboratory mammals (cat, mouse, and rat), CPGs controlling the respiratory and swallowing movements in mammals, and CPGs controlling discharges of the electric organ in the gymnotiform fish. It is shown that in all these cases, the generation of rhythmic motor output is based both on the endogenous (pacemaker) activity of specific groups of interneurons and on interneural interactions. These two interrelated mechanisms complement each other, ensuring the high reliability of CPG functionality. We discuss how the experience obtained in studying CPGs can be used to understand mechanisms of more complex functions of the brain, including its cognitive functions.

Effects of galvanic vestibular stimulation on postural limb reflexes and neurons of spinal postural network.

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.

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.

Activity of motor cortex neurons during backward locomotion.

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.

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-HT(2) 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.

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.

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.

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.

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.

Neural mechanisms of single corrective steps evoked in the standing rabbit.

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 toward 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.

Pattern generation.

Central pattern generators are neuronal ensembles capable of producing the basic spatiotemporal patterns underlying 'automatic' movements (e.g. locomotion, respiration, swallowing and defense reactions), in the absence of peripheral feedback. Different experimental approaches, from classical electrophysiological and pharmacological methods to molecular and genetic ones, have been used to understand the cellular and synaptic bases of central pattern generator organization and reconfiguration of generator operation in behaviorally relevant contexts. Recently, it has been shown that the high reliability and flexibility of central pattern generators is determined by their redundant organization. Everything that is crucial for generator operation is determined by a number of complementary mechanisms acting in concert; however, various mechanisms are weighted differently in determining different aspects of central pattern generator operation.

Effect of acute lateral hemisection of the spinal cord on spinal neurons of postural networks.

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.

Heterogeneity of the population of command neurons in the lamprey.

The effects of signals transmitted from the brain to the spinal locomotor networks by a population of command neurons are determined by specific functional projections of each individual neuron. To reveal these projections, we used a simple vertebrate model, the lamprey, in which responses of the spinal networks to spikes in single reticulospinal axons were detected by using the spike-triggered averaging of the motoneuronal activity. We found that individual neurons exert a uniform effect on the segmental motor output along the whole extent of their axons. Twenty different patterns of effect, that is, combinations of influences on the segmental motoneuron pools, were found. The widespread projections and heterogeneity of the population of command neurons present a basis for formation of different gross motor synergies.

Analysis of the central pattern generator for swimming in the mollusk Clione.

The pteropod mollusk Clione limacina swims by rhythmic movements of two wings. The central pattern generator (CPG) for swimming, located in the pedal ganglia, is formed by three groups of interneurons. The interneurons of the groups 7 and 8 are of crucial importance for rhythm generation. They are endogenous oscillators capable of generating rhythmic activity with a range of frequencies typical of swimming after extraction from the ganglia. This endogenous rhythmic activity is enhanced by serotonin. The interneurons 7 and 8 produce one prolonged action potential (about 100 ms in duration) per cycle. Prolonged action potentials contribute to determining the duration of the cycle phases. The interneurons of two groups inhibit one another determining their reciprocal activity. The putative transmitters of groups 7 and 8 interneurons are glutamate and acetylcholine, respectively. Transition from one phase to the other is facilitated by the plateau interneurons of group 12 that contribute to termination of one phase and to initiation of the next phase. Maintaining the rhythm generation and transition from one phase to the other is also promoted by postinhibitory rebound. The redundant organization of the swimming generator guarantees the high reliability of its operation. Generation of the swimming output persisted after the inhibitory input from interneurons 8 to 7 had been blocked by atropine. Activity of the swimming generator is controlled by a set of command neurons that activate, inhibit or modulate the operation of the swimming CPG in relation to a behaviorally relevant context.

Vestibular compensation in the lamprey.

Vestibular compensation, i.e. recovery after unilateral labyrinthectomy (UL) in the lamprey (a lower vertebrate, Cyclostome), as well as different means of promoting the compensation, is described. UL results in a loss of equilibrium and rotation during swimming which is compensated in about 60 days. The deficit can, however, be immediately abolished by applying asymmetrical tonic visual or vestibular input (electrical stimulation of the optic or vestibular nerve, illumination of one eye, removal of one eye). In addition, asymmetrical visual input promotes plastic changes underlying the vestibular compensation. The compensation is discussed in relation to the neuronal network responsible for postural control in the lamprey and identified in our previous studies.

Non-undulatory locomotion in the lamprey.

The lamprey (a lower vertebrate, cyclostome), in addition to ordinary swimming, is also capable of crawling. Here we describe crawling forward in a narrow U-shaped tunnel. A rapid movement along the tunnel was evoked by stimulating the tail. The muscle activity responsible for propulsion was confined to the area around the body bend. Muscles on the inner (concave) side were activated when approaching the turn, and inactivated on the top of the arc. Muscles on the outer (convex) side were co-active with their antagonists, but also active in the area of straightening of the body bend. This pattern of muscle activity propagated along the body. The role of central and reflex mechanisms in the generation of locomotor movements is discussed.

Role of dermal photoreceptors and lateral eyes in initiation and orientation of locomotion in lamprey.

The response to illumination, and the functional roles of skin photoreceptors and lateral eyes, were examined in the adult river lamprey (Lampetra fluviatilis L.). Illumination of one side of the lamprey evoked a turning movement away from the light source followed by locomotion. The lateral eyes were responsible for directing the movements away from the source of light. A selective illumination of one lateral eye consistently evoked a negative phototactic reaction, whereas a selective illumination of tail skin photoreceptors evoked locomotion, without any preferential orientation relative to the source of light. Experiments were performed by video recording the locomotor responses to localized illumination, and analyzed frame by frame. The horizontal turning movement during negative phototaxis consisted of an asymmetric laterally directed mechanical wave, of higher amplitude and lower velocity than the normal locomotory waves, which was propagated from the rostral to the caudal end of the body.

Activity of Ia inhibitory interneurons during fictitious scratch reflex in the cat.

(1) The fictitious scratch reflex was observed in decerebrate cats immobilized with Flaxedil. The activity of interneurons in the inhibitory pathways from Ia afferents to motoneurons of antagonistic muscles was recorded during scratching. The selected interneurons were supplied by Ia afferents from m. vastus, posterior biceps-semitendinosus and sartorius. (2) Almost all recorded interneurons showed periodic modulation of activity. Their maximal activity in the scratch cycle usually coincided with the maximal activity of motoneurons of those muscles from which the interneurons receive Ia afferents. Excitation of Ia afferents by passive stretch of the muscle or by electrical stimulation of the muscle nerve resulted in the increase of the interneuron activity, without changing its timing in the scratch cycle.

Role of glutamate receptor subtypes in the lamprey respiratory network.

The respiratory role of glutamate receptors was investigated in the isolated lamprey brain preparation by analyzing the changes in respiratory activity induced by bath application of specific antagonists of ionotropic and metabotropic glutamate receptors. The results show that these antagonists differentially affect the pattern of breathing and provide the first evidence that both ionotropic and metabotropic glutamate receptors are involved in neurotransmission within the lamprey respiratory network.

Activity of motoneurons during fictitious scratch reflex in the cat.

(1) Intracellular recording of motoneurons of different hindlimb muscles: tibialis anterior (TA), gastrocnemius and soleus (GS), vastus crureus (Vast), posterior biceps and semitendinosus (PBSt), was carried out during the fictitious scratch reflex in decerebrate cats. (2) During the postural stage of the reflex, a depolarizaiton (3.8 mV on average) was observed in TA motoneurons accompanied by tonic discharge. No change of the membrane potential (MP) and no discharge were observed during this stage in GS, Vast and PBSt motoneurons. (3) In the rhythmical stage of the reflex, the MP of TA motoneurons changed only slightly during the 'long' (L) phase of the scratch cycle and remained at approximately the same level as during the postural stage. In this phase, motoneurons discharged at frequencies of 20-100 pps. In the 'short' (S) phase of the scratch cycle a strong repolarization occurred, the MP reached the same level as observed during resting conditons (MP0), and the discharge discontinued. (4) GS motoneurons were gradually depolarized during the second half of the L-phase. The depolarization reached its maximum (5.5 mV on average in relation to the MP0) on average in relation to the MP0) in the S-phase, and several action potentials were generated with intervals of 5-10 msec. Then, at the beginning of the L-phase, the motoneurons were repolarized and the MP reached the level of the MP0. The behavior of Vast motoneurons was essentially similar to that of GS motoneurons. (5) The PBSt motoneurons usually had two peaks of depolarization per cycle--in the S-phase and at the beginning of the L-phase. The maximal depolarization was 3.5 mV (on average). The motoneurons generated action potentials at one or both peaks of depolarization. (6) The possible organization of the central influences upon motoneurons of different muscles during scratching is discussed.