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Biomedical subjects

K Popov

Publications and source records attributed to K Popov.

At least 19 recordsLinked to original sources
PubMed2000

Evidence for a vestibulo-cardiac reflex in man.

Changes in posture demand rapid cardiovascular adjustments to maintain blood pressure and volume distribution. We demonstrated a vestibulo-cardiac reflex in supine individuals by measuring electrocardiogram and arterial blood pressure after small backwards drops of the head triggered at varying intervals after the R-spike. In normal volunteers heart rate was accelerated by drops occurring within 500-600 ms of a beat, but no rapid effect was noted in patients with a vestibular defect. The speed at which the vestibular signal of head drop accelerated heart rate implies a direct reflex. Impairment of the vestibulo-cardiac reflex would help to explain the vaso-vagal consequences of labyrinthitis.

Adult↗
PubMed1998

Postural responses to vibration of neck muscles in patients with uni- and bilateral vestibular loss.

Postural responses to vibration applied unilaterally to the dorsal neck muscles were recorded with a sway platform in 11 patients with bilateral vestibular loss (BLD), 13 patients with unilateral vestibular lesions (ULD) and 19 normal subjects. In the normals, the vibration induced a forward postural deviation. Vibration failed to induce postural sway in the BLD patients but induced a backwards head movement in 7 patients. In the ULD patients, vibration contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway directed to the side of the lesion and backwards. The findings demonstrate the importance of concurrent vestibular signals in determining the behavioural response to neck afferent input. We propose that in normal subjects the intact vestibular signal gives no confirmation that a head movement has occurred so it is assumed that the lower body has tilted forwards which provokes a compensatory sway. In the total absence of vestibular function the neck signal may represent a real head movement so the preferential response is a head tilt to restore upright posture. The vestibular imbalance in the ULD patients is roughly equivalent to the asymmetrical signals obtained in a normal subject during head rotation to the intact side. The stretch signal induced by ipsi-lesional vibration confirms possible head rotation, thus provoking a compensatory postural sway. Copyright 1998 Elsevier Science B.V. All rights reserved

Journal Article↗
PubMed1998

Proprioceptive information processing in weightlessness.

The "illusions" experiment carried out on five astronauts during the last two French-Russian flights (Antarès in 1992 and Altaïr in 1993) and in the Russian Post-Antarès mission (1993) was designed to investigate the adaptive changes in human proprioceptive functions occurring in weightlessness at both the sensorimotor and cognitive levels, focusing on two kinds of responses: (1) whole-body postural reflexes, and (2) whole-body movement perception. These kinesthetic and motor responses were induced using the tendon-vibration method, which is known to selectively activate the proprioceptive muscular sensory channel and to elicit either motor reactions or illusory movement sensations. Vibration (70 Hz) was therefore applied to ankle (soleus or tibialis) and neck (splenii) muscles. The subject's whole-body motor responses were analyzed from EMG and goniometric recordings. The perceived vibration-induced kinesthetic sensations were mimicked by the subjects with a joystick. The main results show that a parallel in-flight attenuation of the vibration-induced postural responses and kinesthetic illusions occurred, which seems to indicate that the proprioceptive system adapts to the microgravity context, where standing posture and conscious coding of anteroposterior body movements are no longer relevant. The same sensory messages are used at the same time in different sensory motor loops and in the coding of newly developed behavioral movements under microgravity. These results suggest that the human proprioceptive system has a high degree of adaptive functional plasticity, at least as far as the perceptual and motor aspects are concerned.

Adaptation, Physiological↗
PubMed1997

Is the erect posture in microgravity based on the control of trunk orientation or center of mass position?

In the present experiments carried out in microgravity two questions were addressed. First, when the subject was instructed to adopt a vertical erect posture in microgravity with his feet fixed to the floor of the space cabin, would he control anteroposterior position with respect to the ankle joint axis of the "vertical projection" of his center of mass (CM) or trunk axis orientation with respect to the "vertical" (perpendicular to the floor of the space cabin)? Secondly, is CM anteroposterior position regulated during upper trunk movements in microgravity, in the absence of equilibrium constraint? Two subjects were tested in a long-term space flight. Video camera recordings were performed and analyzed off line. The results show that during erect vertical posture in microgravity, the trunk axis with respect to the "vertical" is inclined some 7 degrees forward. The anteroposterior position of the CM "vertical" projection is not shifted forward, as might be expected in view of the trunk inclination, but remains close to the ankle joint axis. At the end of the upper trunk forward or backward bending movement, the final position of the vertical CM projection remains close to the ankle joint axis in microgravity. These results are interpreted as indicating that CM anteroposterior position continues to be accurately controlled in microgravity; the forward inclination of the trunk axis observed in microgravity is interpreted as being due to a misevaluation of the "vertical" axis on the basis of biased information from proprioceptive inputs.

Analysis of Variance↗
PubMed1997

Postural responses to vibration of neck muscles in patients with idiopathic torticollis.

Vibration of the dorsal muscles of the neck, simulating lengthening, in standing man causes a visible forwards tilt of the body shown on posturography as a tonic sagittal sway deviation. According to the theory that posture is organized with respect to a 'body schema' this deviation is a result of an interpretation of the concurrent neck afferent and vestibular signals. Considering the hypothesis that neck afferent signals may be misinterpreted in patients with spasmodic torticollis (ST) causing abnormal postural responses, we recorded body sway induced by unilateral dorsal neck muscle vibration in 22 idiopathic ST patients (19 treated with botulinum toxin) during upright stance with eyes closed. Comparison groups were 19 normal subjects and 11 patients with bilateral loss of vestibular function (labyrinthine defective, LD) in whom neck afference should be intact. Both treated and untreated ST and LD patients had absent or diminished sway deviations. When sway deviation did occur, it was sagitally oriented as with normal subjects and unrelated to ST head turns. In most ST and LD patients, neck vibration induced neck extension, an effect which is observed in normal subjects only if the torso is retrained. The results suggest that neck proprioceptive input retains local postural functions in ST, however, it is relatively ignored in the context of the whole body postural control and spatial orientation. The mild disorders of vestibular function reported in torticollis patients may be due to an inability to calibrate vestibular signals by reference to corroborative signals from neck proprioception.

Achilles Tendon↗
PubMed1996

Postural responses to vibration of neck muscles in patients with unilateral vestibular lesions.

Postural responses to vibration applied unilaterally to dorsal neck muscles were recorded with a sway platform in nine patients with unilateral vestibular lesions and 19 normal subjects. In normals, the vibration induced a forward postural deviation. In patients, vibration of the neck contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway of lower amplitude than normal and predominantly in the direction of the lesion or backwards. It is suggested that the proprioceptive error signal introduced by the neck vibration combined with an asymmetrical vestibular input due to a unilateral vestibular lesion provoked an erroneous representation of head position in patients resulting in a redirection of their body sway.

Humans↗
PubMed1993

Sensorimotor and perceptual function of muscle proprioception in microgravity.

Adaptive properties of the human proprioceptive systems were studied during the French-Soviet orbital flight (Aragatz mission, December 1988). The present space experiment investigated the hypothesis that the modifications of both biomechanical and physiological conditions occurring under microgravity involve considerable reorganization of body perception and postural control. The proprioceptive information originating in muscles is known to contribute, together with visual, vestibular, and sole cutaneous information to postural regulation. Moreover, by specifically activating the proprioceptive channel, muscle vibration is able to elicit both illusory movement sensations and postural responses. This experimental tool was used in microgravity in order to test various aspects of muscle sensory function. Ankle flexor and extensor vibration was applied under different experimental conditions. Quantitative analysis of motor responses was carried out on leg muscle EMG, goniometric, and kinesigraphic recordings. Joystick recordings and astronauts' comments were used to describe the kinaesthetic sensations. The main results were as follows: 1) Under microgravity, the sensitivity of muscle receptors remains unchanged. 2) During the flight, the tonic vibration reflexes (TVR) increased significantly in flexor muscles, which exhibited a sustained tonic activity. 3) The whole-body postural responses normally induced by ankle flexor muscle vibration were suppressed, whereas they remained unchanged or were only reduced when vibrations were applied to the ankle extensor muscles. In all cases, the postural response velocity decreased. 4) A disfacilitation of the vibration-induced postural illusions was observed to occur during long-term exposure to microgravity. These illusions became atypical however. For example: body lift illusion could be induced by tibialis anterior muscle vibration, whereas it was never induced in the controls. The characteristics of the illusory body movements described under normal gravity can be restored by artificially increasing the axial foot support forces during the flight. In conclusion, these data suggest that a functional reorganization of the proprioceptive information processing occurs in microgravity, affecting both perceptual and motor aspects of behavior. It is possible that these proprioceptive adaptations may be partly attributable to the new whole-body propulsive foot functions imposed by exposure to weightlessness and to the adaptation of motor behavior to the third dimension of space.

Adaptation, Physiological↗
PubMed1993

Axial synergies under microgravity conditions.

Fast forward and backward upper trunk movements were analyzed and compared under microgravity and under preflight and postflight conditions. The kinematic analysis showed that the upper trunk movements were accompanied by hip and knee movements in the opposite direction both under microgravity and under normal gravitational conditions. This suggests that the center of mass position with respect to the feet is still regulated under microgravity when the feet are attached to the floor. The EMG analysis during backward movements shows that under preflight conditions a set of muscles (ErSp, BF, Sol) in the back of the body are activated early on. Under microgravity, the early Sol activation was replaced by an early TA activation, which was still present at the first postflight recording and was then replaced by the early Sol activation observed under preflight conditions. This finding shows that the EMG pattern underlying the axial synergies is flexible and that adaptive changes take place both under microgravity and after return to Earth.

Adaptation, Physiological↗
PubMed1993

Gaze control in microgravity. 1. Saccades, pursuit, eye-head coordination.

During the long-duration spaceflight Aragatz on board the Mir station, an experiment exploring the different oculomotor subsystems involved in gaze control during orientation to a fixed target or when tracking a moving target was executed by two cosmonauts. Gaze orientation: with head fixed, the "main sequence" relationships of primary horizontal saccades were modified, peak velocity was higher and saccade duration was shorter in flight than on earth, latency was decreased and saccade accuracy was better in flight. With head free, gaze orientation toward the target was achieved by coordinated eye and head movements, their timing was maintained in the horizontal plane; when gaze was stabilized on the target, there was a trend of a larger eye than head contribution not seen in preflight tests. Pursuit: Horizontal pursuit at 0.25 and 0.5 Hz frequency remained smooth with a 0.98 gain and minor phase lag, on earth and in flight. In the vertical plane, the eye did not track the target with a pure smooth pursuit eye movement, but the saccadic system contributed to gaze control. Upward tracking was mainly achieved with a succession of saccades, whereas downward tracking was due to combined smooth pursuit and catch-up saccades. This asymmetry was maintained during flight in head fixed and head free situations. On earth, head peak velocity was maximal upward, and in flight it was maximal downward.

Electrooculography↗
PubMed1993

Gaze control in microgravity. 2. Sequences of saccades toward memorized visual targets.

The reproduction, in complete darkness, of sequences of 5 horizontal saccades towards previously presented visual targets has been investigated in human subjects on the ground (control subjects) and one cosmonaut in microgravity. The incidence of corrective saccades during the execution of the memory-guided saccades in darkness has been examined. It was quite large for the control subjects (more than half of all saccades), and increased during the flight, while the corrective visually guided saccades incidence decreased. Direction errors occurred in about the third of all sequences on the ground, and this parameter also increased in microgravity. Memory-guided sequences were mostly hypermetric. Whereas the absolute error continuously increased with the target rank, it was not the case with the amplitude ratio, which presented a peak at the third rank, that is, at the middle of the sequence. The accuracy of the reproduction of the sequences did depend on the sequence pattern as much as on the subject. Some learning was observed in repeated reproduction of the same pattern. Although the average error did not change in microgravity, the linear regression coefficient between the visually guided and memory-guided saccades decreased.

Adult↗
PubMed1992

[Strategy and synergy: two levels of equilibrium control during movement. Effects of the microgravity].

Equilibrium is ensured during forward or backward upper trunk movements by displacing the hip and knee simultaneously in opposite directions. When fast movements are performed, a muscle synergy characterized by the early activation of a set of trunk, thigh and leg muscles precedes the onset of the kinematic changes. The question which is addressed concerns the possibility that two levels of equilibrium control might exist during upper trunk movements: the strategy level, which is relatively invariant, is characterized by the displacement in opposite directions of the upper and low segments, and the muscle synergy level at which the strategy is implemented, which may be adaptable to the environmental constraints. When upper trunk movements are performed under microgravity with the subject's feet fixed to the floor of the space cabin, the displacement of upper and lower body segments in opposite directions still occurs, although this is no longer necessary to maintain the equilibrium. This kinematic strategy seems to be aimed at regulating the centre of inertia position with respect to the feet. The muscle synergies associated with these kinematic changes are modified, however, under microgravity. After returning to the ground, the previous synergies do not reappear immediately, but only after a few days. This suggests that a short period of learning is needed to change the synergy. These data are compatible with the hypothesis that two levels of equilibrium control actually exist during upper trunk movements, the strategy level, which is kinematic and invariant, and the synergy level, which is adaptable to the environmental constraints through a short learning process.

Electromyography↗
PubMed1990

[The dissociation of a mutant strain of Streptomyces levoris 41-08, a producer of ichthiomycin].

The population of Streptomyces levoris 41-08, producer of ichtiomicin, is established to be unhomogeneous and consisting of four types of morphologically different varieties: light grey, white, oligospore and asporogenous. The morphological varieties differ in their biochemical activity. The most promising for the ichtiomicine biosynthesis is the grey variant, some of whose representatives manifest up to 30% higher activity than the controls. The statistical processing of data shows that the oligospore and the asporogene variants with their limits of activity to 121.8 and 119.3% respectively are promising too, while the existence of white representatives in the population is not recommended since they are low-active minus variants.

Anti-Bacterial Agents↗
PubMed1989

[Alpha-amylase polymorphism. 1. A comparative study of alpha-amylase Hp and Gm].

Individual phenotypes, phenotypical and genetic frequencies of the alpha-amylase enzyme have been established by means of populational genetic researches. The most common phenotype is AmylA Amyl2A (85.15%) followed by AmylA Amyl2A 2B (6.27%), AmylAIB Amyl2A (5.37%), Amyl IA Amyl2A 2B (2.15%), AmylA Amyl2B (0.53%), AmylC Amyl2B (0.35%), AmylC Amyl2A 2B (0.18%). The difference between the observed and theoretically expected phenotypes of Amy, Hp and M Gm(1) is insignificant. The examined contingent from the Bulgarian population is found to be in genetic balance. Statistical analysis of the reuö results does not prove a genetic link between Amy, Hp and Gm (1).

Humans↗