Human upright posture control in a virtual visual environment

The sagittal and frontal components of the stabilogram were monitored in 14 healthy subjects standing on a rigid or pliant support under three different conditions of visual control: with the eyes opened (EO), with the eyes closed (EC), or in a virtual visual environment (VVE). Under the VVE conditions, the subjects looked at a three-dimensional image of elements of a room (a 3-D artificial room) that was generated by a computer and locked to the fluctuations of the body center of gravity (CG) so that the visual connection between body sway and shifts of the visual environment typical of normal visual conditions was reproduced. Frequency filtration of the fluctuations of the foot’s center of pressure (FCP) was used to isolate the movements of the vertical projection of the CG and determine the difference between these two variables. The changes in the variables (CG and FCP-CG) were estimated using spectral analysis followed by the calculation of the root mean square (RMS) amplitudes of their spectral fluctuations. In subjects standing on a rigid support, the RMS amplitudes of the spectra of both variables were the highest under the VVE and EC conditions and the lowest under the EO conditions. In subjects standing on a pliant support, body sway was considerably enhanced, which was accompanied by a different pattern of visual influences. The RMS values were the highest under the EC conditions and were lower by a factor of 2–2.5 under the EO and VVE conditions. Thus, it has been demonstrated that the cerebral structures controlling posture ignore the afferent input from the eyes under VVE conditions, if the subject is standing on a rigid support and the CG fluctuations are relatively small; however, this afferentation is efficiently used for maintaining the posture on a pliable support, when the body sway is substantially enhanced.     

Reaction to the perturbation of the vertical posture under different conditions of standing and in the case of additional support

The reaction of equilibrium restoration in response to the perturbation of the vertical posture of a subject standing on a stable or unstable support was studied. Perturbation was induced by a sudden forward or backward shift of the support surface. In some of the experiments, the subject was holding onto a handrail suspended on a long soft belt with a fixed upper end. The results of the study showed that the reaction to support movement depended on the direction of the perturbation. The soleus muscle was activated first upon a backward movement, and the anterior tibial muscle was activated first upon a forward movement, with a latency of about 50 ms. Within 30–70 ms, bursts of activity were also detected in the respective antagonist muscles. Sudden movement of the platform caused bursts of impulses in the arm muscles even in the absence of contact with the handrail. These impulse bursts had a longer latency (80–130 ms) and probably played an auxiliary role in the process of the restoration of balance. In the case of standing on an unstable support, the latency of leg muscle activation increased. When a subject was holding onto a handrail, the intensity of impulse bursts decreased in the leg muscles and increased in the arm muscles, while the latent periods of the bursts in the arm muscles decreased. This effect proved to be still more pronounced in the case of balance maintenance on an unstable support. Thus, the change in the response to external perturbations during maintenance of the vertical posture on an unstable support demonstrates that an additional contact of the hand changes the adjustment of the posture control system.     

Effect of an additional manual motor task on the maintenance of equilibrium in the frontal and sagittal planes in standing subjects

The postural oscillations of a standing subject during an additional manual motor task consisting in holding a movable ball in the center of a flat box were studied. The movements of the center of pressure (CP) in the frontal and sagittal planes were studied when subjects were standing on a stable rigid support and on a movable unstable support. The effect of the additional motor task on the movement of the CP depended on the stability of the support. When the additional task was performed, the sagittal movements of the CP increased in the case a movable support and did not increase when the support was stable. The additional task decreased the frontal movements of CP in the case of a stable support, and it did not change the frontal movements of CP when the support was unstable. Thus, the performance of an additional motor task led to a reduction of the efficiency of the postural control system in maintaining equilibrium on an unstable support. This decrease may be due to a greater cortical influence on the posture control system in subjects standing on a movable support in comparison with this influence in the case of a stable support.     

Dependence of joint stiffness on the conditions of visual control in upright undisturbed stance in humans

We recorded the sagittal and frontal components of the stabilogram of healthy humans in upright undisturbed stance under five conditions of visual control: (i) open eyes (OE); (ii) closed eyes (CE); (iii) visual inversion (VI); (iv) central vision (CV), and (v) diffused light (DL). Through a low-pass filter of trajectories of the center of pressure of feet (CPF), the vertical projection of the center of gravity (CG) and, consequently, the difference CPF-CG were estimated. The former represents the controlled variable, while the latter is proportional to the horizontal acceleration and assumed to express the resultant joint stiffness (mostly in the ankle joints). The stiffness was characterized through a method based on spectral analysis of the CPF-CG variable and subsequent calculations of the median frequency (MF) and the root mean square (RMS) of the spectra. The median frequencies of the spectra of the CPF-CG variable changed slightly under various visual conditions. At standing on a rigid support, they varied from 0.97 to 0.99 Hz and from 0.93 to 0.97 Hz for the CPF-CG, calculated from the sagittal and frontal components of the stabilogram, respectively. Under conditions of a pliable support, the corresponding frequencies varied within the limits of 0.79–0.83 Hz and 0.74–0.78 Hz. In contrast to the median frequencies, the RMSs demonstrated greater variability depending on different visual conditions. At standing on a rigid support, paired comparisons showed significant differences between the RMSs of the spectra of the CPF-CG variable of the sagittal direction under CE and OE conditions (0.14 ± 0.030 and 0.09 ± 0.020 mm, respectively) and under DL and OE conditions (0.130 ± ± 0.025 and 0.090 ± 0.020 mm, respectively). The RMS of the CPF-CG variable calculated for the frontal stabilogram differed significantly from each other for the VI and OE conditions (0.115 ± 0.020 and 0.075 ± ± 0.015 mm, respectively). In case of standing on a pliable support, a greater variability of visual influences on the CPF-CG variable was found. The RMS for its sagittal motion was the greatest under CE conditions (0.19 ± 0.03 mm); it was significantly greater than the respective values under OE, CV, and DL conditions (0.097 ± ± 0.020, 0.110 ± 0.020, and 0.140 ± 0.030 mm, respectively). The means of RMSs of the spectra of the frontal CPF-CG was also the greatest under CE conditions (0.20 ± 0.03 mm) and the smallest under OE conditions (0.095 ± 0.020 mm). In addition, the value of the RMS fluctuations under CE conditions (0.150 ± 0.025 mm) differed significantly from the respective values under OE conditions (0.095 ± 0.020 mm) and CV conditions (0.110 ± 0.020 mm). Thus, our findings support the statement that the influence of visual conditions on the maintenance of vertical stance is mediated (at least partially) by the mechanisms controlling the ankle joint stiffness. This regulation is mostly manifested in changes of a single parameter, the amplitude of fluctuations of the CPF-CG variable. We also found that the joint stiffness can be modulated by both nonspecific visual influences (which, in particular, reflect the perception of illumination) and specific visual influences, related to information on the position of the body and on its movements with respect to external objects. Neirofiziologiya/Neurophysiology, Vol. 38, No. 2, pp. 157–166, March–April, 2006.     

The Sense of Effort and two Models of Single-Joint Motor Control

Two sets of experiments were carried out. In the first set, human subjects were asked to make the same effort with the elbow flexors at different joint angles under isometric conditions. In some experiments, the subjects were standing with the arm in a vertical (parasagittal) plane; in others, they were seated with the arm in a horizontal (transverse) plane. When muscular torque at a given effort level (ordinate) was plotted as a function of elbow joint angle (abscissa), the resulting isoeffort torque-angle profiles tended to be flat or negatively sloping over a range from 45° to 135°, and they were often nonmonotonic. Increases in effort up to near-maximal levels caused the isoeffort torque-angle profiles to shift upward with little alteration in shape. In the second set of experiments, seated subjects with the arm horizontal resisted baseline torques produced by a motor that acted to extend the elbow joint. Unexpected increases and decreases in torque were superimposed on the baseline torque. The subjects either were instructed to intervene and return the elbow to the initial (90°) position, or were told, “Do not intervene voluntarily; let the motor move your arm.” Effort was reported both under baseline conditions and after the changes in torque. It was found that changes in effort were a function of the changes in torque opposed by the elbow flexors, and were similar whether the subject had repositioned the arm or allowed it to be moved by the motor. In the latter case, the arm came to rest after displacements that were a function of the size and direction of the torque change. For individual subjects, the largest angular displacements ranged from ° 10° to °20° for changes in torque of ° 10 N.m. There was no evidence for any angular dependence of the effort judgments at a given torque over this angular range. Depending on whether effort is primarily an efferent perception proportional to voluntary motor activity or also has a significant afferent (involuntary) component, different models of motor control are supported by these data.     

Posture and hand load alter muscular response to sudden elbow perturbations

Joint stiffness and stability are reliant on coordinated muscle activity which may differ depending on initial posture and loading during sudden perturbations. This study investigated the effects of arm posture and hand load on muscle activity during perturbations of the arm. Fifteen male participants experienced perturbations to the wrist causing elbow extension using a combination of three body postures (standing, supine, sitting) and three hand load conditions (no, solid, and fluid loads), with known and unknown timing. Surface EMG was collected from eight muscles of the right upper extremity. The response to sudden loading was examined using muscle activities pre (baseline) and post (reflex) perturbation. During the baseline period, known perturbation timing resulted in greater muscular activity than for unknown timing, while the opposite was found for the reflex period. During the reflex period with fluid load, biceps brachii and brachioradialis demonstrated increased activity of 2.4% and 4.0% of maximum respectively, from supine to standing. During the reflex period, the fluid load resulted in forearm co-contraction 23% and 47% greater than the solid and no load conditions. Body orientation and hand loading influenced muscular response to elbow perturbations. Muscle co-contraction at the elbow during known timing suggests a contribution to elbow joint stability that may reduce injury risk caused by sudden elbow loading.     

The role of visual information in maintaining postural stability after the maximum exercise for the upper and lower limb muscles

The postural stability on a seesaw generating anterior–posterior instability with the eyes open (EO) and then the eyes closed (EC) in young healthy subjects (n = 28) before and 6 min after the maximum bicycle exercise (Wingate test) performed using lower limbs (“leg exercise”) or upper limbs (“hand exercise”) was investigated. It was found that “hand exercise” caused the same increase in average velocity (V, mm/s) and in the average range of sway of the centre of pressure (Qy, mm) as “leg exercise.” However, the duration of V recovery (EC: 2 min 30 s and 50 s; EO: 60 s and 40 s after “leg exercise” and “hand exercise,” respectively) and Qy (EC: 1 min 10 s and 30 s after “leg exercise” and “hand exercise,” respectively; EO: no changes from baseline) was shorter after “hand exercise.” In the presence of visual information, the increment in V decreased more than 2 times after “leg exercise” (+100.5% and + 40.5%, p < 0.01 for EC and EO, respectively) and after “hand exercise” (+73.0% and +30.3%, p < 0.01 for EC and EO, respectively). Moreover, Qy after both exercises remained at the initial level under EO conditions but significantly increased under EC conditions (+42.8%, p < 0.01 after “leg exercise” and +40.3%, p < 0.01 after “hand exercise”). Thus, the maximum exercise for the muscles of the upper limbs causes the same reduction in postural stability as analogous exercise for the muscles of the lower limbs, but the recovery period after “hand” exercise was shorter. The presence of visual information allows the baseline maintenance of postural stability and significantly reduces the strain of postural regulation while standing on a movable support after the maximum “leg exercise” and “hand exercise.”     

Simulating mechanical consequences of voluntary movement upon whole-body equilibrium: the arm-raising paradigm revisited

Voluntary arm-raising movement performed during the upright human stance position imposes a perturbation to an already unstable bipedal posture characterised by a high body centre of mass (CoM). Inertial forces due to arm acceleration and displacement of the CoM of the arm which alters the CoM position of the whole body represent the two sources of disequilibrium. A current model of postural control explains equilibrium maintenance through the action of anticipatory postural adjustments (APAs) that would offset any destabilising effect of the voluntary movement. The purpose of this paper was to quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, knee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation described mechanical consequences of the arm-raising movement for different initial conditions. The variables tested were arm inertia, the presence or not of gravity field, the initial standing position and arm movement direction. Simulations showed that the mechanical effect of arm-raising movement was mainly local, that is to say at the level of trunk and lower limbs and produced a slight forward displacement of the CoM (1.5 mm). Backward arm-raising movement had the same effect on the CoM displacement as the forward arm-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compared to arm movement performed without load. Postural disturbance was minimised for an initial standing posture with the CoM vertical projection corresponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural perturbation due to arm-raising movement increased compared to the four-joints model. On the basis of these results the classical assumption that APAs stabilise the CoM is challenged.     

<Emphasis Type="Italic">Myxobolus imparfinis</Emphasis> n. sp. (Myxozoa: Myxosporea), a new gill parasite of <Emphasis Type="Italic">Imparfinis mirini</Emphasis> Haseman (Siluriformes: Heptapteridae) in Brazil

A new species of myxozoan, Myxobolus imparfinis n. sp. is described based on material from the gills of Imparfinis mirini (Haseman) (Heptapteridae). Mature myxospores are round, measuring 7.1–8.4 (7.9 ± 0.3) μm in length, 4.5–6.2 (5.5 ± 0.5) μm in width and 3.1–4.2 (3.7 ± 0.3) μm in thickness. The polar capsules are of unequal size, the larger polar capsule measuring 3.4–4.5 (3.9 ± 0.3) μm in length and 1.4–2.0 (1.7 ± 0.1) μm in width and the smaller capsule measuring 3.1–3.8 (3.4 ± 0.2) μm in length and 1.2–1.8 (1.5 ± 0.2) μm in width. The polar filament presents 6–7 coils. Spores had a prevalence of infection of 75% (6/8). In histological analyses we detected the development site of spores in primary filaments, in afferent branchial artery, thus classifying the type of infection to the filamental type and vascular subtype. The phylogenetic analyses of a dataset including species Myxobolus Bütschli, 1882 and Henneguya Thélohan, 1892 from South America recovered M. imparfinis n. sp. as a sister species of Myxobolus flavus Carriero, Adriano, Silva, Ceccarelli & Maia, 2013. To our knowledge, this is the first record of a myxozoan species parasitising I. mirini.     

Maintenance of upright stance in humans in normal and virtual visual environments

We recorded in 16 healthy subjects the sagittal and frontal components of the stabilogram when standing on a rigid motionless or movable (oscillating) platform under four conditions of visual control: (i) open eyes, OE; (ii) closed eyes, CE; (iii) central vision, CV, and (iv) virtual visual environment, VVE. Under the latter condition, subjects observed the 3D image of a room, which was generated by the computer; the image was adhered to head movements in such a manner that a peculiar connection for normal visual conditions between movements of the head and displacements of the visible visual environment was reproduced. Through a low-pass filtration of a trajectory of the center of pressure of feet (CPF), two elementary variables were received, horizontal motions of the center of gravity (CG) and the difference between the CPF and the CG (CPF-CG). Changes in these variables (CG and CPF-CG) were estimated using spectral analysis and subsequent calculation of the median frequency (MF) and root mean square value (RMS) of the spectra. The MFs of the spectra of the investigated variables were approximately identical under conditions of standing on oscillating and motionless supports and showed no clear dependence on various visual conditions. Unlike MFs, the RMSs of the spectra of body sways appeared more dependent on changes of conditions of standing and the mode of visual control (differing from each other in the higher sensitivity to modifications of conditions of standing and visual control). With standing on the motionless support, the RMSs of the spectra of both variables were the greatest under VVE and CE conditions and the smallest under OE condition. The body oscillations were considerably amplified under conditions of standing on a movable support, and a different pattern of visual influences on the RMS of both investigated variables was revealed. The RMSs had the greatest value under CE condition and were much smaller (50–40%) under other visual conditions, including the VVE condition. Therefore, our findings show that, under VVE conditions, visual sensory afferentation is ignored by cerebral structures controlling postural adjustments if standing occurs on a motionless support but is effectively used at the maintenance of upright stance on an oscillating support. Neirofiziologiya/Neurophysiology, Vol. 39, No. 6, pp. 476–485, November–December, 2007.     

<Emphasis Type="Italic">Epichloë occultans</Emphasis> enhances micropropagation efficiency in <Emphasis Type="Italic">Lolium multiflorum</Emphasis>

Italian ryegrass (Lolium multiflorum) is an annual grass considered as one of the most important temperate forage grasses in the world. However, it is recalcitrant to plant tissue culture techniques hindering its genetic manipulation. Epichloë occultans is an endophytic fungus associated with L. multiflorum. This symbiosis causes improvements in physiological and ecological traits of the host plants. The objective of this work was to study the effect of E. occultans on L. multiflorum micropropagation. We compared the response of endophyte-infected (E+) and endophyte-free (E?) seeds in different micropropagation stages. The E+ seeds were more successful than E? seeds in in vitro germination (83?±?5 vs. 63?±?6%), callus induction (78?±?5 vs. 57?±?6%), callus proliferation (average diameter of 21.5?±?1.3 mm in two subcultures vs 17.3?±?0.8 mm in three subcultures) and plant regeneration from callus (83?±?7 vs. 30?±?8%). These results indicate that E. occultans enhances significantly L. multiflorum micropropagation. The use of endophyte-infected (E+) seeds can be a solution to make this grass more amenable to different biotechnological tools, such as the genetic transformation.     

Somatic organic content of the ctenophores <Emphasis Type="Italic">Mnemiopsis leidyi</Emphasis> (Ctenophora: Lobata) and <Emphasis Type="Italic">Beroe ovata</Emphasis> (Ctenophora: Beroida) in early ontogenetic stages

The organic matter content in the eggs and early larvae of the ctenophores Mnemiopsis leidyi and Beroe ovata from the Black Sea was determined using the adapted microtechnique of dichromate oxidation. The content of organic matter in the eggs of M. leidyi (0.25 μg/indiv.) was 5 times less than in B. ovata (1.28 ± 0.29 μg/indiv.). The somatic content of organic matter was 0.25 ± 0.09 μg/indiv. (25.1 ± 8.3 μg/mg, wet wt) for 2-day-old larvae of M. leidyi (0.2–0.3 mm in body length) and 1.37 ± 0.19 μg/indiv. (67.1 ± 5.7 μg/mg wet wt) for larvae of B. ovata (0.4 mm in body length). The specific organic content of larvae of both species steadily decreased with an increase in ctenophore body size and weight, approaching 3–4 μg/mg of wet weight for 2 mm specimens of M. leidyi and 3–5 μg/mg of wet weight for 6 mm B. ovata. The specific organic content of early larvae was 20–30 times higher than that of adult ctenophores. The results of this investigation could be useful in the evaluation of the energy budget for somatic growth and generative production in these species. Calculations indicate that with specific wet weight growth rates of 0.43/day for M. leidyi larvae and 0.29/day for B. ovata larvae, their true organic increases are respectively 30 and 38% less, i.e., no more than 0.31/day for the former and 0.18/day for the latter species.     

Influence of a movable support under one leg on human vertical posture during standing with asymmetrical load on legs

Changes in the vertical posture maintenance were studied when the legs were placed on supports of different degrees of mobility and part of the body weight was voluntarily transferred to one leg. The aim of these experiments was to explore how the mobility of support under the feet affects the balance and how this effect depends on the load distribution between the legs during standing. When both legs were on rigid immovable supports, the vertical posture was maintained by control of the center of pressure (CP) on both legs. When the subject transferred the weight to one foot, the posture was maintained mainly due to the control of CP of the loaded leg. When the legs were on supports of different degrees of mobility, the balance was maintained by the leg on the immovable support. This result was observed both when the subject stood with symmetrical load on the legs and when the load was transferred to one leg. Even when the leg was unloaded but placed on the immovable support, its CP moved more compared to the CP of the loaded leg on a movable support. The results obtained show that the support mobility is a factor that determines the mechanisms of posture maintenance, and this factor is more significant than load distribution between the legs. Thus, the upright posture is maintained with the physical properties of support under the feet taken into account.     

Dietary characteristics of co-occurring polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>) in the Canadian Arctic,Darnley Bay

Reduction in sea ice due to climate change is expected to have a negative impact on habitat availability for Arctic marine fishes and induce range expansion of species from southern environments. Such an effect will likely be observed in the abundance of polar cod, Boreogadus saida (Lepechin, 1774), as well as interspecific interactions of this intermediate-level trophic taxon, particularly in more southerly fringing seas in the Arctic. Polar cod and capelin, Mallotus villosus (Müller, 1776), are pelagic, planktivorous forage fishes, which occupy similar dietary niches and are the primary prey of marine predators. Co-occurring polar cod and capelin were collected at seven stations in Darnley Bay, NT, during August 2013. Standard length (SL), used as a proxy for age, suggested that polar cod (mean ± 1 SD: 71.1 ± 10.3 mm) were predominantly age 1+ and capelin (96.2 ± 13.4 mm) were mostly age 2+. Stomach content analyses indicated that both species feed extensively on calanoid copepods (Calanus hyperboreus, C. glacialis, Metridia longa) and amphipods (Themisto libellula). There was high dietary overlap between capelin and polar cod, evidenced by Schoener’s index (0.80). Additionally the quantity of dietary items, biomass and energetic content consumed differed among size classes in both capelin (SL, 70.5–132.0 mm) and polar cod (SL, 42.1–114.4 mm). This study illustrates that the diets of these sympatric forage fishes in an Arctic ecosystem are very similar, indicating a high potential for interspecific competition as the sub-Arctic capelin expands its range into Arctic regions with climate change.     

Modulation of the soleus muscle H reflex caused by ballistic voluntary arm movements in humans

In healthy humans, we studied the influence of conditioning voluntary arm movements on the H reflex induced by transcutaneous stimulation of the tibial nerve and recorded from the soleus muscle. We examined the effects of flexion and extension of the forearm, as well as of finger clenching performed with the maximum rate. Conditioning arm movements were self-induced or realized upon presentation of a visual signal (light flash). We found that the pattern of changes in the H reflex is determined by the position of the subject’s body in the course of tests. The ipsilateral arm flexion in the elbow joint in the standing position resulted in depression of the H reflex lasting about 100 msec from the beginning of the movement, while the effect observed in the lying position (on the couch with the feet hanging free in the air) looked like a facilitation of the reflex lasting about 100 to 200 msec. The direction and dynamics of modifications of the H reflex under conditions of the use of different conditioning movements (forearm flexions/extensions and finger clenching of the ipsilateral arm, as well as contralateral forearm flexions in the elbow joint) were rather similar. We also showed that the observed facilitation of the H reflex began earlier than the voluntary arm movement (40 to 50 msec prior to the beginning). We hypothesize that these conditioning influences result from the action of central motor commands and represent the factor related to anticipatory postural rearrangements. Such rearrangements are directed toward the maintenance of equilibrium of the body in the course of a future movement. These commands depend significantly on the spatial position of the subject’s body. Neirofiziologiya/Neurophysiology, Vol. 40, No. 2, pp. 147–154, March–April, 2008.     

Age and growth assessment of western North Atlantic spiny butterfly ray <Emphasis Type="Italic">Gymnura altavela</Emphasis> (L. 1758) using computed tomography of vertebral centra

Life history strategies of batoid fishes have evolved within dynamic marine ecosystems. Adaptations in reproductive and developmental biology are paramount to the survival of species, and therefore knowledge of growth rates to maturity is fundamental for identifying constraints on the conservation of populations. The butterfly rays (Myliobatiformes: Gymnuridae) are highly derived batoids with generally low reproductive potentials for which age and growth information remains unknown. In this study we applied high-resolution X-ray computed tomography (HRXCT) to vertebral centra from a stingray for the first time to estimate age, and used a multimodel approach to investigate growth of spiny butterfly ray, Gymnura altavela. Estimated ages of the oldest male and female were 11 and 18 yrs. at disk widths (WD) 1355 mm and 2150 mm, respectively. Disk width-at-age data were analyzed using three growth models (von Bertalanffy, logistic, Gompertz), and the most parsimonious and empirically supported model was the logistic function with sex treated as a fixed effect on asymptotic disk width (WD _∞ ) and k parameters. Model parameter estimates were (males) WD _∞  = 1285.46 ± 67.27 mm, k = 0.60 ± 0.10, and (females) WD _∞  = 2173.51 ± 129.78 mm, k = 0.27 ± 0.04. Results indicated sexually dimorphic growth patterns, with males growing faster and reaching asymptotic size at earlier ages than females. These age and growth results are the first reported for the genus, and suggest that G. altavela grows at a similar rate as some teleosts and batoids, and relatively fast among chondrichthyans.     

Influence of Achilles tendon vibration on the vertical posture during standing with asymmetrical leg loading

The shift of the common center of pressure (CCP) and the center of pressure (CP) of one leg was studied during the Achilles tendon vibration of one or both legs while the subject was standing with symmetrical load on the legs or with the load transferred to one leg. The CP shift of the standing subject during unilateral Achilles tendon vibration depended on both the side of application of vibration and on the distribution of the leg load. During standing with a asymmetrical load on the legs, the shift of the CCP was larger than when the vibration was applied to the loaded leg. The CP shift of one leg was greater if both vibration and the load were applied to it. Vibration of the unloaded leg caused a CP shift in the loaded contralateral leg. In this case, vibration of the left unloaded leg did not cause any noticeable CP shift of the left leg, while vibration of the unloaded right leg caused a CP shift of the right leg. Under the similar conditions of loading and vibration, the displacement of the CP of the right leg was larger than the displacement of the CP of the left leg. It may be suggested that postural asymmetry and unilateral vibration of the leg muscles change the internal representation of the position of the body axis in relation to the vertical, which affects the displacement of the CP of one leg in response to afferent stimulation of the leg muscles.     

Neonatal intermittent hypoxia and hypertension

Obstructive apnea during sleep is accompanied by intermittent hypoxia (IH) leading to hypertension and other cardiovascular disturbances. A comparative evaluation of long-term effects of the neonatal IH on the cardiovascular functions was performed in normotensive Sprague-Dawley and spontaneously hypertensive rats (SHR). The newborn rats were placed for 30 days to conditions of IH (8% and 21% O₂, alternating every 90 s for 12 h/day). Control groups of rats were constantly kept in normoxia. By 6 months, in the spontaneously hypertensive rats exposed to IH at the period of wakefulness there was a statistically significant increase (as compared with control) of the systolic (185.8 ± 1.7 and 169.9 ± 1.4 mm Hg, correspondingly, p < 0.010 and the diastolic pressure (96.2 ± 4.9 and 86.0 ± 2.6 mm Hg, correspondingly, p < 0.01). During sleep, the systolic and diastolic pressure in these rats was higher than in control animals by 10 mm Hg (p < 0.01) and 12 mm Hg (p < 0.01), its decrease during sleep being absent. In SHR submitted to IH there was an increase in the power ratio of the heart rate variability from 0.9 ± 0.15 to 1.5 ± 0.17, which indicates a shift of the sympathico-parasympathetic balance in this group towards predominance of the sympathetic component. In the Sprague-Dawley rats exposed to neonatal hypoxia, the above-indicated changes were not prominent. These peculiarities of the hypertensive rats allow establishing connection of the genetic factor with the sympathetic mechanism providing long-term consequences of the neonatal IH for the cardiovascular control in the SHR.     

Influence of fingertip contact on illusory arm movements.

Recent evidence suggests that reaching movements are more accurate when end point contact occurs, suggesting that fingertip contact contributes to a final estimation of arm position. In the present study we tested two hypotheses: 1). that fingertip contact influences illusions of arm movement produced by muscle vibration and 2). that this influence depends on the a priori context of the stability of the contact surface. Subjects sat with their elbows on a table and eyes closed. They demonstrated the perceived orientation of the left (cue) arm by mirroring the location with the right (report) arm. We manipulated deep proprioceptive cues by vibrating the left biceps brachia, causing illusions of elbow extension, and tested whether these illusions were altered when the fingertip remained in contact with a stable external surface. The context at this point represents a prior assumption that the external contact surface is stable. Midway through the experiment, the context was changed by challenging the prior assumption that the contact surface was stable by demonstrating that it could move. Unbeknownst to the subject, the external contact surface remained stable during data collection throughout the experiment. As expected, without tactile cues, biceps vibration caused illusory elbow extension. Conditions with fingertip contact and biceps vibration in the stable context demonstrated that contact largely eliminated the overestimation of cue arm elbow angle. However, in the context of a possibly unstable (movable) contact surface, the reports of elbow extension returned. Thus a priori notions about the stability context of an external contact surface influence how this tactile cue is integrated with proprioceptive sensory modalities to generate an estimate of arm location in space. These findings support the notion that tactile cues are used to calibrate proprioception against external spatial frameworks.     

Mechanisms of reference posture correction in the system of upright posture control

It was earlier shown that ultraslow tilts of the support under quiet standing conditions evoke an unusual response reflecting the operation of compensatory mechanisms: postural sway is a superposition of postural oscillations typical of quiet standing and greater, slower inclinations of the body caused by the tilt. This may be explained by the presence of two hierarchical levels of upright posture control: real-time control compensates for small deviations of the body from the reference posture prescribed by presetting control. Mathematical simulation methods have been used to study the mechanisms of reference posture control. The results are compared with available experimental data. It is demonstrated that the reference posture can be corrected according to the gravitational vertical with the use of a kinesthetic reference alone. It is hypothesized that, when correcting the reference posture, the nervous system “assumes” the support to be immobile. The afferent input from sole pressure receptors is an important factor in reference posture correction. The advantages of the putative two-level control over control based on an explicit internal model are discussed.