Movements of the Forearm and Shoulder Performed against the Gravitation Force: Target Positioning under Kinesthetic Control

In humans, we tested targeted movements of the forearm and shoulder performed in the vertical direction (in a parallel manner with respect to the sagittal plane). Movements were realized, first, with the possibility for visual control of the coincidence of the angle of the limb link axis vs the vertical and the target angle value (using an optic system and video recording), and, second, in the absence of the above control. Movements including flexion (i.e., movement against the gravitation force) – extension of the limb link with an individually selected convenient velocity were initiated and terminated according to the presentation of permissive sound signals; simultaneously, EMGs were recorded from a few muscles flexing and extending the elbow and shoulder joints. We analyzed systematic errors of target positioning of the forearm and shoulder in movements realized exclusively under kinesthetic control. In the case of isolated flexion of the forearm for a 90 deg target angle, such errors in all members of the examined group (n = 11) were positive. These errors were, on average, 8.1 ± 0.7 deg without loading and reached 11.2 ± 0.9 deg with introduction of a 10 to 30 N additional loading on the forearm. Isolated movements of the shoulder for a 70 deg target angle (performed without loading, with full extension of the forearm and its voluntary fixation) were accompanied by positive errors of 18.3 ± 1.1 deg, on average. Both the movements and positioning were performed due to changes in the levels of activity of the flexor muscles, with minimum involvement of the antagonists. The nonlinear properties of the receptor apparatus responsible for the formation of a kinesthetic estimate of the joint angle (first of all, of muscle spindles) are a fundamental reason for positive errors of target positioning of the limb links realized under kinesthetic control in the absence of the visual one.     

Effect of muscle fatigue on target positioning of the human forearm under conditions of restriction of visual control

We studied movements of the forearm within the limits of 0 deg (full extension)-flexion to a 100 deg angle in the elbow joint-a reverse movement with episodes of target positioning at an intermediate target level (50 deg). The standard trajectory and trajectory of the performed movement were visualized by movements of cursors on the screen of a monitor in joint angle vs time coordinates. Systematic errors of blindfold (kinesthetic) positioning (after removal of the visual feedback informing the subject on the characteristics of the performed movement) observed under control conditions and after realization of a fatigue-inducing series of flexions/extensions of the forearm with a high loading were compared. It was found that the development of fatigue evoked no fundamental changes in the pattern of systematic errors of kinesthetic positioning. Both considerable prevailing of positive systematic errors within the examined group, their high interindividual variability, and (in most cases) patterns of signs of errors after reaching the target level by movements of opposite directions typical of the given subject were preserved. Mean intragroup values after the development of fatigue demonstrated some trend toward a decrease, but these changes did not reach the significance level. Possible mechanisms of the influence of muscle fatigue on the process of target positioning of a limb link realized exclusively under proprioceptive control are discussed. Neirofiziologiya/Neurophysiology, Vol. 38, Nos. 5/6, pp. 432–439, September–December, 2006.     

Positioning of the Human Forearm in Tracking Movements and Their Reproduction under Conditions of Limited Visual Control

In 14 healthy persons, we studied movements of the forearm with its positioning on a target level. A double trapezium was used as the command trajectory (flexion in the elbow joint from the state of full extension, 0°, with positioning on the level of 50 or 60° and further flexion to the 100° angle, and a similar reverse movement). We compared (i) tracking movements, when the subject tried to adequately reproduce the movement of the target along the command trajectory visualized on the monitor screen and obtained visual information about the performed movement (shifts of the second light point in time/joint angle coordinates), and (ii) reproduction of these movements under conditions of limitation of the visual feedback (when there was no information about the performed movement). Parameters of the tracking movements and of their reproductions (delays of initiation of the movement phases as compared with the command signal, durations of these phases, and angle velocities of the forearm movement), as well as the quality of positioning after oppositely directed movements, were compared. Positioning on the target level performed under proprioceptive control (when visual control was limited) was accompanied by systematic errors, whose sign in most test series performed by most subjects coincided with the direction of the preceding movement phase. The pattern of signs of systematic positioning errors after movements of opposite directions was quite individual (typical of a given subject) and demonstrated no dependence on the value of the extensor loading. Averaged intragroup systematic errors of positioning after movement phase 1 (flexion to the target level) and phase 3 (extension to the same level) under conditions of a minimum extensor loading (0.5-1.0 N · m) were 2.57° and 2.52°, respectively. When the loading was substantial (3.6-6.0 N · m), the respective errors were 3.85° and 3.48°. The nonlinear properties of muscle stretch receptors in the elbow flexors and extensors (responsible for the significant dependence of the parameters of afferent signals produced in these receptors on the movement prehistory) are considered the primary reason for systematic errors when positioning is performed exclusively under proprioceptive control. The influence of alpha-gamma co-activation in active muscles on the characteristics of the above signals is discussed.     

Reproduction of tracking movements and target positioning of the forearm in humans in the absence of visual control

In 17 healthy volunteers, we studied movements of the forearm, which included episodes of positioning on the target level. The trajectory of the non-ballistic (relatively slow) movement looked like a double trapezium (flexion of the elbow joint from the state of full extension, 0 deg, positioning on the 50 deg level, further flexion to the limit angle of 100 deg, and a similar reverse sequence). The command trajectory and the trajectory of the realized movement were visualized with movements of cursors on a monitor in time/joint angle coordinates. We compared parameters of the tracking movements (in the presence of visual feedback) and their blindfold reproduction (with the complete absence of visual control). It was found that blindfold reproduction movements differ from sample tracking movements and their reproduction with partial limitation of visual control [16] in higher peak velocities and shorter durations, i.e., a trend toward conversion of such movements into ballistic ones was observed. Under conditions of elimination of visual control, movements that led to positioning were mostly hypermetric, i.e., positioning was usually accompanied by positive systematic errors (whose sign coincided with the direction of the preceding movement phase). The mean intragroup value of the systematic error of the first positioning (after flexion to the target level) was +6.73 ± 1.15 deg, while the respective mean for the second positioning (after extension to the same level) was +4.00 ± 1.31 deg. The nonlinear properties of stretch receptors of muscles whose activity provides the formation of a proprioceptive estimate of the joint angle are considered the crucial reason for systematic errors of blindfold positioning.Neirofiziologiya/Neurophysiology, Vol. 36, Nos. 5/6, pp. 393–404, September–December, 2004.This revised version was published online in April 2005 with a corrected cover date and copyright year.     

Peculiarities of Activation of the Upper Limb Muscles in Realization of Two-Joint Movements in Humans

In tests on humans, we recorded EMG activity from the muscles flexing and extending the forearm and shoulder in the course of realization of sequential single-joint and simultaneous two-joint movements of the upper limb. As was shown, the shoulder muscles m. biceps brachii and m. triceps brachii are involved in flexion/extension of both elbow and shoulder joints. Central commands sent to the above muscles in the course of a two-joint movement could be considered a superposition of the central commands coming to the same muscles in realization of the corresponding sequential single-joint movements with the same changes in the angles of the elbow and shoulder joints. External loadings applied in the direction of extension of the elbow and shoulder joints induced, in general, similar changes in coordination of the activity of muscles moving the forearm and shoulder under conditions of both single-joint and two-joint movements. These facts allow us to suppose that coordination of the muscle activity in two-joint movements depends to a greater extent on the forces influencing limb links than on the mode of realization of the movements (two sequential single-joint movements vs a two-joint movement corresponding to the above motor events).     

Control of the Elbow Extensor Muscles in Slow Targeted Extensions of the Arm in Humans

Relations between the kinematic parameters of slow (non-ballistic) targeted extension movements in the elbow joint of humans and characteristics of the movement-related EMG activity in the two heads of the m. triceps brachii were analyzed. Test movements were performed under conditions of application of non-inertional external loadings directed toward flexion. It was shown that the movement-related EMG activity of the elbow extensors, similarly to what was observed in the flexors at flexion movements with the same parameters, demonstrates a complex structure and includes dynamic and stationary phases. In the former phase, in turn, initial and main components can be differentiated. The rising edge and decay of the main component of the dynamic extensor EMG phase could be approximated by exponential functions; this component was never split into a few subcomponents. Dependences between the amplitudes of m. triceps brachii EMG phases and the amplitude of the movement (or external loading) were, as a rule, nonlinear but monotonic. An increase in the test movement velocity led to an increase in the rate of rise of the rising edge of the dynamic EMG phase, while an increment in the amplitude was less significant. Under the used test conditions, the activity of the elbow extensors was usually accompanied by some coactivation of the antagonists (m. biceps brachii). It is concluded that motor commands coming to the elbow extensors at performance of the extension test movements differ from motor commands to the flexors at analogous flexion test movements by a simpler structure and more tonic pattern. Biomechanical specificities of fixation of the mentioned muscle groups to the arm bones (stability of the moment for application of the extensor force under conditions of changing the joint angle vs variable moment of the flexor force) are considered one of the main reasons for such specificity of the patterns of the extensor and flexor motor commands.     

Peculiarities of Activation of Muscles of the Shoulder Belt in Voluntary Two-Joint Movements of the Upper Limb

We studied coordination of central motor commands (СMCs) coming to muscles of the shoulder and shoulder belt in the course of single-joint and two-joint movements including flexion and extension of the elbow and shoulder joints. Characteristics of rectified and averaged EMGs recorded from a few muscles of the upper limb were considered correlates of the CMC parameters. Special attention was paid to coordination of CMCs coming to two-joint muscles that are able to function as common flexors (m. biceps brachii, caput breve, BBcb) and common extensors (m. triceps brachii, caput longum, TBcl) of the elbow and shoulder joints. Upper limb movements used in the tests included planar shifts of the arm from one spatial point to another resulting from either simultaneous changes in the angles of the shoulder and elbow joints or isolated sequential (two-stage) changes in these joint angles. As was found, shoulder muscles providing movements of the elbow with changes in the angle of the elbow joint, i.e., BBcb and TBcl, were also intensely involved in the performance of single-joint movements in the shoulder joint. The CMCs coming to two-joint muscles in the course of two-joint movements appeared, in the first approximation, as sums of the commands received by these muscles in the course of corresponding single-joint movements in the elbow and shoulder joints. Therefore, if we interpret the isolated forearm movement performed due to a change in the angle of the elbow joint as the main motor event, while the shoulder movement is considered the accessory one, we can conclude that realization of a two-joint movement of the upper-limb distal part is based on superposition of CMCs related to basic movements (main and accessory). Neirofiziologiya/Neurophysiology, Vol. 41, No. 1, pp. 48–56, January–February, 2009.     

Facilitation and Inhibition of the <Emphasis Type="Italic">Soleus</Emphasis> H Reflex Related to Movements of the Upper Limb in Humans

In studies on healthy volunteers, we recorded an EMG discharge from the m. soleus corresponding to the H reflex evoked by transcutaneous stimulation of the n. tibialis comm. Changes in the magnitude of this reflex related to realization of brief voluntary movements of the ipsilateral upper limb were examined. The subjects were in a prone position. Fast flexion-extension of the forearm resulted first in 100- to 200-msec-long facilitation of the H reflex begun 30–40 msec before the appearance of EMG activity in the m. biceps brachii; this feature is indicative of the central nature of this effect related to the action of motor programs initiating the forearm movement. Facilitation of the H reflex was followed by its inhibition lasting several seconds. Within an interval corresponding to the maximum suppression of the H response, we tested the effect of additional conditioning stimulation of the n. peroneus comm. Occlusion of the inhibitory effects indicates that the same inhibitory neurons mediate the influences from both the peroneal input and the pathways transmitting inhibitory influences from the neuronal systems controlling upper limb muscles. Contractions of the ipsilateral m. biceps brachii evoked by direct electrical stimulation of the latter also resulted in inhibition of the soleus H reflex, which was rather similar in its time course to the above-mentioned inhibitory effects. There was no inhibition of the reflex after stimulations of the cutaneous receptors and n. medianus. These findings allow us to suppose that long-lasting inhibition of the H reflex induced by voluntary movements of the upper limb results from afferent influences from the receptors of contracting muscles. Such effects can be realized via the propriospinal pathways or long reflex arcs.     

Systematic positioning errors in human voluntary single-joint movements without visual guidance

Healthy subjects underwent analysis for positioning accuracy. The flexion-extension movements in the ankle and elbow joints performed without visual control were studied. The movements were produced by flexor muscles against small background loading (extensors were inactive). The subject was asked to memorize a certain target value of the joint angle reached under visual guidance in the phase of flexor contraction. After the flexion- (additional activation of the flexors) or extension-directed movement (relaxation of these muscles) the subject was asked to restore the target level from memory, without visual guidance. In the first case, when the target level was finally approached due to the flexor relaxation, a systematic overshoot of the target joint angle was observed. In the second case, when the target level was finally approached due to the additional activation of the flexors after their temporal relaxation, the positioning was much more accurate.Neirofiziologiya/Neurophysiology, Vol. 26, No. 2, pp. 91–98, March–April, 1994.     

A vector-sum process produces curved aiming paths under rotated visual-motor mappings

Under a 90° rotation of motor space relative to visual space, human two-dimensional aiming movements frequently take the form of smooth arcs such as spirals and semi-circles. A time-independent differential equation explains this tendency in terms of a rotation-induced vector field made up, at each point in the two-dimensional space, of two input vectors. One vector represents a visual error signal and the other represents a motor error signal. A trajectory's instantaneous direction of movement at each point can be described as the resultant of the two vectors. This mathematical formulation incorporates plausible visual-motor mechanisms and, when expressed in polar coordinates, leads to a new method for analyzing the spatial properties of movements (i.e., movement paths). Plots of the angle between the resultant and the target vector () against distance from the target (r, in the polar representation) summarize the arc-shaped movement paths as a simple relation that can be analyzed statistically with respect to properties such as monotonicity. The polar representation is a plausible representation of visually-guided movements, with the visual error vector functioning as an objective function relative to which behavior is optimized. We extend the model and ther, movement path analysis to non-90° rotations, and we find that the model predicts an observed qualitative shift in behavior for rotations greater than 90°. It also predicts qualitatively different path shapes observed under visual-motor reflections.This work was performed while the first author was under the support of Grant IST-8511589 from the National Science Foundation and Grant NCC2-307 from the National Aeronautics and Space Administration     

Influence of adaptation to horizontal body position on pointing errors and visual estimation of a target position in humans

The central program of a targeted movement includes a component intended for to compensate for the weight of the arm; this is why the accuracy of pointing to a memorized position of the visual target in darkness depends on orientation of the moving limb in relation to the vertical axis. Transition from the vertical to the horizontal body position is accompanied by a shift of the final hand position along the body axis towards the head. We studied how pointing errors and visual localization of the target are modified due to adaptation to the horizontal body position; targeted movements to a real target were repeatedly performed during the adaptation period. Three types of experiments were performed: a basic experiment, and two different experiments with adaptation realized under somewhat dissimilar conditions. In the course of the first adaptation experiment, subjects received no visual information on the hand’s position in space, and targeted movements of the arm to a luminous target could be corrected using proprioceptive information only. With such a paradigm, the accuracy of pointing to memorized visual targets showed no adaptation-related changes. In the second adaptation experiment, subjects were allowed to continuously view a marker (a light-emitting diode taped to the fingertip). After such adaptation practice, the accuracy of pointing movements to memorized targets increased: both constant and variational errors, as well as both components of constant error (i.e.,X andY errors) significantly dropped. Testing the accuracy of visual localization of the targets by visual/verbal adjustment, performed after this adaptation experiment, showed that the pattern of errors did not change compared with that in the basic experiment. Therefore, we can conclude that sensorimotor adaptation to the horizontal position develops much more successfully when the subject obtains visual information about the working point position; such adaptation is not related to modifications in the system of visual localization of the target.     

Reciprocal aiming precision and central adaptations as a function of mechanical constraints

The present study investigated the influence of mechanical constraints (load and movement velocity) on the movement accuracy during a reciprocal aiming precision task. Seven participants had to point rhythmically and alternatively (with flexion–extension of the right elbow) a cursor at two targets as accurately as possible. Two loads (applied to the limb effectors; 500 and 2500 g), two movement frequencies (1.25 and 1.75 Hz) as well as two target sizes (1 and 5 cm) were manipulated. Surface EMG activity of both biceps brachii and triceps brachii was recorded. Attentional demands, reflecting the central cost associated with the performance of aiming movements was assessed using a dual-task paradigm (via a probe reaction time task – RT). While the results demonstrated a significant degradation of pointing accuracy with mechanical loading (mean absolute error – AE for 500 g load: 0.32 mm ± 0.64; mean AE for 2500 g load: 0.51 ± 0.74 mm), no significant effect of movement frequency was found. For the two mechanical constraints, the mental effort to meet the task demands remained the same (mean RT_−500g: 370 ± 123 ms; mean RT_−2500g: 395 ± 119 ms). Electromyographic activity of both biceps brachii and triceps brachii muscles evidenced neural adaptations to changes in mechanical constraints. Put together, the present findings suggest that the cause of the observed loss of movement accuracy may probably result from more peripheral alterations such as an impairment of the afferent information processing.     

Kinematic and EMG characteristics of simple shoulder movements with proprioception and visual feedback.

The objective of this study was to determine if simple, shoulder movements use the dual control hypothesis strategy, previously demonstrated with elbow movements, and to see if this strategy also applies in the absence of visual feedback. Twenty subjects were seated with their right arm abducted to 90 degrees and externally rotated in the scapular plane. Subjects internally rotated to a target position using a custom shoulder wheel at three different speeds with and without visual feedback. Kinematics were collected with a motion analysis system and electromyographic (EMG) recordings of the pectoralis major (PECT), infraspinatus (INFRA), anterior and posterior (ADELT, PDELT) deltoid muscles were used to evaluate muscle activity patterns during movements. Kinematics changed as movement speed increased with less accuracy (p<0.01). Greater EMG activity was observed in the PECT, PDELT, and INFRA with shorter durations for the ADELT, PDELT and INFRA. Movements with only kinesthetic feedback were less accurate (p<0.01) and performed faster (p<0.01) than movements with visual feedback. EMG activity suggests no major difference in CNS control strategies in movements with and without visual feedback. Greater resolution with visual feedback enables the implementation of a dual control strategy, allowing greater movement velocity while maintaining accuracy.     

Different programming modes of human saccadic eye movements as a function of stimulus eccentricity: Indications of a functional subdivision of the visual field

1. Voluntary saccadic eye movements were made toward flashes of light on the horizontal meridian, whose duration and distance from the point of fixation were varied; eye movements were measured using d.c.-electrooculography.—2. Targets within 10°–15° eccentricity are usually reached by one saccadic eye movement. When the eyes turn toward targets of more than 10°–15° eccentricity, the first saccadic eye movement falls short of the target by an angle usually not exceeding 10°. The presence of the image of the target off the fovea (visual error signal) subsequent to such an undershoot elicits, after a short interval, corrective saccades (usually one) which place the image of the target on the fovea. In the absence of a visual error signal, the probability of occurrence of corrective saccades is low, but it increases with greater target eccentricities. These observations suggest that there are different, eccentricity-dependent modes of programming saccadic eye movements.—3. Saccadic eye movements appear to be programmed in retinal coordinates. This conclusion is based on the observations that, irrespective of the initial position of the eyes in the orbit, a) there are different programming modes for eye movements to targets within and beyond 10°–15° from the fixation point, and b_ the maximum velocity of saccadic eye movements is always reached at 25° to 30° target eccentricity. —4. Distributions of latency and intersaccadic interval (ISI) are frequently multimodal, with a separation between modes of 30 to 40 msec. These observations suggest that saccadic eye movements are produced by mechanisms which, at a frequency of 30 Hz, process visual information. —5. Corrective saccades may occur after extremely short intervals (30 to 60 msec) regardless of whether or not a visual error signal is present; the eyes may not even come to a complete stop during these very short intersaccadic intervals. It is suggested that these corrective saccades are triggered by errors in the programming of the initial saccadic eye movements, and not by a visual error signal. —6. The exitence of different, eccentricity-dependent programming modes of saccadic eye movements, is further supported by anatomical, physiological, psychophysical, and neuropathological observations that suggest a dissociation of visual functions dependent on retinal eccentricity. Saccadic eye movements to targets more eccentric than 10°–15° appear to be executed by a mechanism involving the superior colliculus (perhaps independent of the visual cortex), whereas saccadic eye movements to less eccentric targets appear to depend on a mechanism involving the geniculo-cortical pathway (perhaps in collaboration with the superior colliculus).     

Relations between the errors of targeted movements and inexactitude of visual perception of space

Errors of targeted movements of the arm to the places of presentation of light targets (in darkness) were studied in healthy subjects kept in a vertical position or laying on their backs. An error along theY axis (corresponding to the longitudinal body axis) changed its sign depending on the body orientation with respect to the gravitation vector. In the vertical position, the arm shifted to the feet at the movement’s termination, while in the laying position it shifted to the head. AnX error showed no dependence on the position of the body in space. The errors reached their maxima in the absence of visual control, but became two-three times smaller when the tested subject could observe the position of an indicator (light diodes) fixed on the end of the index finger (or of a pointer rod). When the spatial positions of targets were reconstructed according to verbal “indications”, the amplitudes ofX andY errors appeared similar to those at real movements (indication under visual control). In this case, the sign ofY errors also depended on the body orientation, but their direction was opposite. We suppose that systematicY errors at the targeted arm movements are determined not only by an antigravitation component of the motor program, but also by shifting of a sensory visual estimations of the spatial target position.     

Effects of Local and Remote Muscle Pain on Stretch ReflexActivities in the Elbow Joint Flexors and Extensors of Unanesthetized Cats

In experiments on unanesthetized cats, we compared the effects of experimentally induced pain in the m. biceps brachii or in the neck muscles on EMG activity of the flexors and extensors of the elbow joint (mm. biceps et triceps brachii, respectively) evoked by a passive extension-flexion of the above joint. Muscle pain was induced by injections of 0.5 ml of a hypertonic (7%) NaCl solution into the above-mentioned muscles. In the case of pain in the biceps, i.e., in the muscle directly involved in realization of the reflex, we observed an increase in the amplitude and significant shortening of the latency of EMG responses of this muscle. The amplitude of a short-latency (supposedly monosynaptic) component of the biceps reflex (М1 response) increased by 65%, while an increment of the latter (supposedly polysynaptic) М2 component was 117%. When pain was induced in anatomically remote neck muscles, the stretch reflex in the biceps was considerably suppressed. The maximum amplitudes of the М1 and М2 components decreased by 25 and 30%, respectively, but the latencies of these components decreased significantly, similarly to what was observed in the case of induction of experimental pain in the biceps. Under both conditions of experimental pain, changes in the parameters of EMG responses of the forearm extensor (m. triceps brachii) demonstrated similarity with those of the biceps responses. The maximum effect of pain induction was observed within the first 5 min after injections of the hypertonic solution; full recovery of the stretch reflex parameters was observed on the 20th to 30th min. We conclude that the effects of pain induction on the reflex under study are not generalized. They depend on the site of such induction with respect to the muscle where the stretch reflex is elicited. Unidirectional effects of both types of pain on the antagonist muscles allow us to suppose that modulation of the reflex reactions upon pain induction is mediated by influences from the supraspinal CNS structures. Induction of pain in the biceps increased the amplitude of EMG manifestations of the stretch reflex, while such induction in the neck muscles decreased such responses; nonetheless, in both cases the latency of the reflexes decreased. This fact allows us to believe that the sensitivity of muscle spindles increased under both conditions of the pain influence.     

Displacement and Return Movement of Chloroplasts in the Marine Dinophyte Pyrocystis noctiluca. Experiments with Optical Tweezers

Infrared laser traps (optical tweezers) were used to study laser-induced organelle movements in the marine alga Pyrocystis noctiluca (Dinophyta). These cells are highly suitable for optical micromanipulation due to their large size and extensive vacuole. Experiments were done with plastids held by optical tweezers and moved from the nuclear area into the vacuole. The subsequent retraction movement was analysed for speed. The displaced organelles remained connected to their original position by a thin cytoplasmic strand, often less than 1 μm in diameter. When the organelles were released they rapidly returned at an initial rate of 81.7 ± 7.8 μm . s^?1 (overall displacement 50 μm, measured distance 20 μm, 25 °C ± 1 °C, number of cells 22), slowing down with progressive retraction of the connecting strand. The return movement was reduced to 4.2 ± 0.2 μ .s^?1 (n = 10) when the organelles were displaced and held for 1 min. Displacement to a longer distance increased the rate of return movement. A change from a high to a low environmental temperature significantly reduced movement from 94.5 ± 9.0 . s^?1 (30 °C ± 1 °C, n = 22) to 34.5 ± 2.7 μm .s^?1 (5°C ± 1 °C, n = 22). Nocodazole and N-ethylmaleimide (NEM), inhibitors of microtubules and acto-myosin, respectively, did not affect the retraction of the connecting strand, but at high concentrations of NEM it became increasingly difficult to move organelles away from the nuclear area. We suggest that the return movement of organelles within laser-induced artificial strands mainly depends on the viscoelastic properties of the tonoplast. The quantification of these properties by optical tweezers allows determination of reactions of plant cells to temperature changes.     

Mechanism of signal production in the vibratory communication of the wandering spider Cupiennius getazi (Arachnida,Araneae)

The communication with substrate vibrations produced by vibrations of the body or its appendages is widespread among arthropods, especially among spiders. Its biomechanics, however, is poorly understood. Males of the wandering spider Cupiennius getazi produce such substrate vibrations during courtship by means of dorsoventral movements of their opisthosoma without hitting their dwelling plant.Simultaneous recordings of the plant vibrations (accelerometry), of the opisthosoma movements (laser Doppler vibrometry) and of the electromyograms of the opisthosomal depressor muscle, revealed that the main frequency of the vibratory signal of about 80 Hz originates from the activity of the opisthosomal depressor muscle. The transfer functions of the spider's body show resonances which could amplify the main frequency before it is transmitted into the plant.A low frequency component of the opisthosomal movement (duration c. 0.3 s, displacement c. 6 mm (peak-peak) 30° deflection angle, frequency 10–20 Hz) can be distinguished from a main frequency component (duration c. 0.1 s, displacement c. 0.5 mm 2.5° deflection angle, frequency c. 80 Hz). The main frequency component is superimposed on an upward movement of the low frequency component.     

The investigation of locomotor activity control system in humans under the air-stepping conditions during passive and voluntary leg movements

The degree of activation of the central stepping program during passive leg movement was studied in healthy subjects under unloading conditions; the excitability of spinal motoneurons was studied during passive and voluntary stepping movements. Passive stepping movements with characteristics maximally close to those during voluntary stepping were accomplished by the experimenter. The bursts of muscular activity during voluntary and imposed stepping movements were compared. In addition, the influence on the leg movement of artificially created loading onto the foot was studied. The excitability of spinal motoneurons was estimated by the amplitude of modulation of the m. soleus H reflex. Changes in the H reflex (Hoffmann’s reflex) after fixation of the knee and hip joints were also studied. In most subjects, passive movements were accompanied by bursts of electromyographic (EMG) activity in the hip muscles (sometimes in shank muscles); the timing of the EMG burst during the step cycle coincided with the burst’s timing during voluntary stepping. In many cases, the bursts in EMG activity exceeded the activity of homonymous muscles during voluntary stepping. Simulation of foot loading influenced significantly the distal part of the moving extremity during both voluntary and passive movements, which was expressed in the appearance of movements in the ankle joint and an increase in the phasic EMG activity of the shank muscles. The excitability of motoneurons during passive movements was higher than during voluntary movements. Changes and modulation of the H reflex throughout the step cycle were similar without restriction of joint mobility and without hip joint mobility. Fixation of the knee joint was of great importance. It is supposed that imposed movements activate the same mechanisms of rhythm generation as supraspinal commands during voluntary movements. During passive movements, presynaptic inhibition depends mostly on the afferent influences from the moving leg rather than on the central commands. Under the conditions of “air-stepping,” the afferent influences from the foot pressure receptors are likely to interact actively with the central program of stepping and to determine the final activity pattern irrespective of the movement type (voluntary or passive).     

Fine‐scale movements and habitat use of juvenile southern flounder Paralichthys lethostigma in an estuarine seascape

Habitat use of juvenile southern flounder Paralichthys lethostigma was examined within a shallow estuarine seascape during June and July 2011 using acoustic telemetry. Fine‐scale movement and habitat use of P. lethostigma was investigated with an acoustic positioning system placed in a seascape that varied in habitat type, physicochemical conditions and bathymetry. The use of different habitat types was examined with Euclidean distance‐based analyses, and generalized additive models were used to determine the relative importance of habitat type relative to physicochemical conditions and bathymetry. Tracks of P. lethostigma ranged in distance between 1477 and 8582 m and speed was 4·2 ± 1·1 m min^?1 (mean ± s.e .) for all P. lethostigma combined. Depth, slope and habitat type had the most influence on P. lethostigma occurrence and deep sandy areas with shallow slopes were used most frequently. In addition, depth use by P. lethostigma was influenced by tidal cycles, indicating habitat use varies temporally and is dynamic. Finally, temperatures <30·5° C were used more than warmer waters within the study area. The results successfully identify movements by juvenile P. lethostigma, and indicate that definitions of essential habitats need to account for dynamics in habitat use.