Biomedical subjects

Grigori N Orlovsky

Publications and source records attributed to Grigori N Orlovsky.

9 recordsLinked to original sources

PubMed2006

Interlimb postural coordination in the standing cat.

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.

Animals↗

PubMed2006

Comparison of activity of individual pyramidal tract neurons during balancing, locomotion, and scratching.

Neuronal mechanisms of the spinal cord, brainstem, and cerebellum play a key role in the control of complex automatic motor behaviors-postural corrections, stepping, and scratching, whereas the role of the motor cortex is less clear. To assess this role, we recorded fore and hind limb-related pyramidal tract neurons (PTNs) in the cat during postural corrections and during locomotion; hind limb PTNs were also tested during scratching. The activity of nearly all PTNs was modulated in the rhythm of each of these motor patterns. The discharge frequency, averaged over the PTN population, was similar in different motor tasks, whereas the degree of frequency modulation was larger during locomotion. In individual PTNs, a correlation between analogous discharge characteristics (frequency or its modulation) in different tasks was very low, suggesting that input signals to PTNs in these tasks have a substantially different origin. In about a half of PTNs, their activity in different tasks was timed to the analogous (flexor/extensor) parts of the cycle, suggesting that these PTNs perform similar functions in these tasks (e.g., control of the value of muscle activity). In another half of PTNs, their activity was timed to opposite parts of the cycle in different tasks. These PTNs seem to perform different motor functions in different tasks, or their targets are active in different parts of the cycle in these tasks, or their effects are not directly related to the control of motor output (e.g., they modulate transmission of afferent signals).

Animals↗

PubMed2006

Pattern of motor coordination underlying backward swimming in the lamprey.

The main form of locomotion in the lamprey (a lower vertebrate, cyclostome) is forward swimming (FS) based on periodical waves of lateral body flexion propagating from head to tail. The lamprey is also capable of backward swimming (BS). Here we describe the kinematical and electromyographic (EMG) pattern of BS, as well as the effects on this pattern exerted by different lesions of the spinal cord. The BS was evoked by tactile stimulation of a large area in the anterior part of the body. Swimming was attributed to the waves of lateral body undulations propagating from tail to head. The EMG bursts on the two sides alternated, and the EMG in more caudal segments led in phase the EMG in more rostral segments. Main kinematical characteristics of BS strongly differed from those of FS: the amplitude of undulations was much larger and their frequency lower. Also, the maintenance of the dorsal-side-up body orientation ascribed to vestibular postural reflexes (typical for FS) was not observed during BS. A complete transection of the spinal cord did not abolish the generation of forward-propagating waves rostral to the lesion. After a lateral hemisection of the spinal cord, the BS pattern persisted on both sides rostral to the lesion; caudal to the lesion, it was present on the intact side and reduced or abolished on the lesioned side. The role of the spinal cord in generation of different forms of undulatory locomotion (FS and BS) is discussed.

Animals↗

PubMed2006

Neural bases of postural control.

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.

Animals↗

PubMed2004

Vestibular compensation in lampreys: restoration of symmetry in reticulospinal commands.

Removal of a vestibular organ (unilateral labyrinthectomy, UL) in the lamprey results in a loss of equilibrium, so that the animal rolls (rotates around its longitudinal axis) when swimming. Owing to vestibular compensation, UL animals gradually restore postural equilibrium and, in a few weeks, swim without rolling. Important elements of the postural network in the lamprey are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections to the spinal cord. As shown previously, a loss of equilibrium after UL is associated with disappearance of vestibular responses in the contralateral group of RS neurons. Are these responses restored in animals after compensation? To answer this question, we recorded vestibular responses in RS neurons (elicited by rotation of the compensated animal in the roll plane) by means of chronically implanted electrodes. We found that the responses re-appeared in the compensated animals. This result supports the hypothesis that the loss of equilibrium after UL was caused by asymmetry in supraspinal motor commands, and the recovery of postural control in compensated animals was due to a restoration of symmetry.

Action Potentials↗

PubMed2003

Activity of different classes of neurons of the motor cortex during postural corrections.

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.

Action Potentials↗

PubMed2003

The pattern of motor coordination underlying the roll in the lamprey.

The 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 degrees 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.

Animals↗

PubMed2002

Encoding and decoding of reticulospinal commands.

In the lamprey, the reticulospinal (RS) system is the main descending system transmitting commands to the spinal cord. We investigated these commands and their effect on the spinal mechanisms. The RS commands were studied by recording responses of RS neurons to sensory stimuli eliciting different motor behaviors. Initiation of locomotion was associated with symmetrical bilateral massive activation of RS neurons, whereas turns in different planes were associated with asymmetrical activation of corresponding neuronal groups. The sub-populations of RS neurons causing different motor behaviors partly overlap. We suggest that commands for initiation of locomotion and regulation of its vigour, encoded as the value of bilateral RS activity, are decoded in the spinal cord by integrating all RS signals arriving at the segmental locomotor networks. Commands for turns in different planes, encoded as an asymmetry in the activities of specific groups of RS neurons, are decoded by comparing the activities of those groups. This hypothesis was supported by the experiments on a neuro-mechanical model, where the difference between the activities in the left and right RS pathways was used to control a motor rotating the animal in the roll plane. Transformation of the descending commands into the motor responses was investigated by recording the effects of individual RS neurons on the motor output. Twenty patterns of influences have been found. This great diversity of the patterns allows the RS system to evoke body flexion in any plane. Since most neurons have asymmetrical projections we suggest that, for rectilinear swimming, RS neurons with opposite asymmetrical effects are co-activated.

Animals↗

PubMed2002

Comparative neurobiology of postural control.

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.

Anatomy, Comparative↗