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ANTICIPATORY POSTURAL ADJUSTMENTS: FROM POSTURE TO MOVEMENT
Full text linkIn all the experiments we investigated the relationship between the voluntary movement and the postural control. We particularly focused our attention on the role played by the APAs, since the general hypothesis is that the postural feed forward control of the movement is strictly bound to the voluntary movement itself in a complex and flexible way.
In the first experiment we tested the hypothesis that APAs preceding an upper-limb target reaching movement could play a role also in controlling the movement accuracy. The aim of this study was seeking a direct proof of the relationship between the APAs amplitude and the endpoint of a target reaching movement.
The aim of the second study was to determine whether a short term immobilization (12 h) interferes in parallel with both the activation of the prime mover muscle, responsible for a given movement, and the postural muscles that are recruited to stabilize the limb.
In the third experiment was aimed at verifying whether the postural activation is affected by the phenomenon of the motor resonance as well as already described for the prime mover activation.
The results of the first experiment reinforce the hypothesis that a successful on-target pointing movement relies upon a specific tuning between APAs and prime mover activation, as that obtained at the end of the adaptation phase.
The most important result of the second experiment is that, although the prime mover activation remains unchanged after the immobilization, the trajectory described by the index finger is most likely changed between the two sessions due to the modification in the postural control that led to a less effective stabilization of the proximal joint, as was suggested by the mechanical model designed by Caronni and Cavallari (2009a).
In the last experiment we demonstrated that the resonant response in resting subjects replicates, under threshold, both the primary movement and postural activity. The precocious increase in excitability observed in BB may be the expression of the anticipatory activation observed during the execution of the movement. Given that MR reflects aspects that are intrinsic to motor programming also this result strongly support the idea that primary movement and the postural command are essential components of the same neural process
Presynaptic and postsynaptic effects of local cathodal DC polarization within the spinal cord in anaesthetized animal preparations
No full textThe main aim of the study was to compare presynaptic and postsynaptic actions of direct current polarization in the spinal cord, focusing on DC effects on primary afferents and motoneurons. In order to reduce the directly affected spinal cord region, weak polarizing direct current (0.1- 0.3 μA) was applied locally in deeply anaesthetized cats and rats; within the hindlimb motor nuclei in the caudal lumbar segments, or in the dorsal horn within the terminal projection area of low threshold skin afferents. Changes in excitability of primary afferents activated by intraspinal stimuli (20-50 μA), were estimated using increases or decreases in compound action potentials recorded from the dorsal roots or peripheral nerves as their measure. Changes in postsynaptic actions of the afferents were assessed from intracellularly recorded monosynaptic EPSPs in hindlimb motoneurons and monosynaptic extracellular field potentials (evoked by group Ia afferents in motor nuclei, or by low threshold cutaneous afferents in the dorsal horn). The excitability of motoneurons activated by intraspinal stimuli was assessed using intracellular records or motoneuronal discharges recorded from a ventral root or a muscle nerve. Cathodal polarization was found to affect motoneurons and afferents providing input to them to a different extent. Excitability of both was markedly increased during DC application but post-polarization facilitation was found to involve presynaptic afferents and some of their postsynaptic actions but only negligibly motoneurons themselves. Taken together the results indicate that long-lasting post-polarization facilitation of spinal activity induced by locally applied cathodal current primarily reflects facilitation of synaptic transmission
Latency of antIcIpatory postural adjustments depends on the intended, not on the actual movement velocIty
No full textLiterature reports that the Anticipatory Postural Adjustments (APAs) preceding voluntary movements are programmed according to several task parameters, including movement velocity (for a review: Bouisset & Do, Neurophysiol Clin 2008). However, the linkage between APAs and velocity has been highlighted within single subjects, who were asked to perform several movements at different velocities; therefore, till now, it has been impossible to discern whether the key factor determining the APA latency was the intended movement velocity or the actual one. Aim of this study was to distinguish between the above two factors.
We analyzed the APA chain that stabilizes the arm during a brisk flexion of the index finger (Caronni & Cavallari, Exp Brain Res 2009): such movement, driven by the prime mover Flexor Digitorum Superfcialis (FDS), is preceded by an APA chain in the upper limb, consisting of an excitatory burst in Triceps Brachii (TB) and in an almost contemporary inhibition in Biceps Brachii (BB) and Anterior Deltoid (AD). These APAs allow to counteract the elbow and shoulder flexion induced by the upward perturbation that the index finger movement causes on the metacarpophalangeal joint. Experiments were carried out in two groups of subjects: 1) 29 who composed our database from previous experiments and were asked to flex the finger “as fast as possible” (go-fast), but actually performed the movement with different speeds (from 238 to 1180°/s), and 2) ten new subjects who performed the go-fast movement at more than 500°/s and were then asked to go-slow at about 50% of their initial velocity, thus moving at 300 to 800°/s. Subjects sat on a chair with both arms along the body, elbows flexed at 90°, dominant hand prone, in axis with the forearm, and index-finger extended. Subjects produced a sequence of 30 finger flexions for each required velocity. Recorded variables were the index-finger movement and the rectified EMG from FDS, TB, BB and AD. For each sequence, the 30 traces were aligned on the FDS onset (taken as time zero) and averaged; then the latency of APAs in arm postural muscles was measured.
No correlation between the APA latency and the actual movement speed was observed when all subjects had to go-fast (for all muscles: r2 0.50), while APAs delayed of about 20-25 ms were found in the ten new subjects when they had to go-slow (p < 0.001). Moreover, in the speed range between 300 and 800°/s, the APA latency depended only on movement instruction: subjects going fast showed APAs that were about 15-30 ms earlier than those of subjects going slow (p < 0.001).
These data suggest a stronger role of the intended movement velocity versus the actual one in setting the timing of postural muscles recruitment with respect to the prime mover. Such linkage between the APA programming and the intended velocity of voluntary movement also strengthens the idea that the postural and prime mover muscles are driven by a “shared” motor command, as proposed by Bruttini et al. (Exp Brain Res 2014)
Transcranial direct current stimulation of the supplementary motor area enhances intra-limb anticipatory postural adjustments
No full textIt has been recently demonstrated that alterations in anticipatory postural adjustments (APAs), which represent a fundamental component of any correct voluntary movement, contribute to the motor impairment after stroke. APAs circuitry is not fully understood, but several studies suggest that the supplementary motor area (SMA) is deeply involved.
To confirm its involvement in APA generation, and verify the possibility to interfere with the latter, we applied trascranial direct current stimulation (tDCs) over the SMA, looking for effects on the postural control of the upper-limb during a simple index-finger flexion. Indeed, as shown in our lab (Caronni and Cavallari, Exp Brain Res 194:59,2009), finger flexion is preceded by inhibitory APAs carved in Biceps and Anterior Deltoid EMG, and by an excitatory APA appearing on Triceps EMG.
Subjects (n=12) performed 3 sessions of 30 brisk flexions of the index-finger, while sitting on a chair with the right upper-arm along the body, elbow at 90° and hand prone. Before each session, subjects underwent tDCs (1mA, 20min) with CATHODAL, ANODAL or SHAM configuration.
During ANODAL stimulation the average APAs in Biceps and Triceps were greater than during SHAM (BB: +26.5%, TB: +66%) and CATHODAL (BB: +20.5%, TB: +63.4%) stimulation (for both muscles, ANOVA p<0.02 Tukey p<0.05). Instead, the APA recorded on Anterior Deltoid was unchanged in all sessions.
These results confirm the SMA involvement in APA generation and suggest that tDCs, which has been shown to be a valuable tool for treating different neurological conditions, may be as well employed for postural rehabilitation
Anticipatory postural adjustments associated to reaching movements are likely programmed according to the expectancy of visual information
No full textDuring goal-directed movements, the head, eyes and limb are dynamically coordinated so as to look at and reach a given target. This study examined whether changes in eye-head-arm coordination affects Anticipatory Postural Adjustments (APAs) in lower limb muscles when performing an arm reaching movement, toward a target of known position.
Standing right-handed subjects (n=10) flexed their shoulder and reached, with the index-fingertip, a target placed in front of them, at shoulders height. Four conditions were studied: 1) steadily looking at the target while reaching (Visually-Guided Reaching); 2) starting with the head flexed watching the floor, then Look and Reach the target; 3) as in 2, but Without Reaching and 4) keeping the head flexed until having reached the target (Blind Reaching). Subjects memorized the target position by performing several reaching movements, until they felt confident. Thereafter, they performed in random order 7 sequences of 15 trials (separated by 5 min of rest): one in Visually-Guided Reaching, one in Without Reaching, one in Blind Reaching, and four in Look and Reach. Recorded variables were: arm and head kinematics, vertical electro-oculogram, EMG from right Anterior Deltoid (prime mover), pectoralis major and biceps brachii, EMG from right and left tibialis anterior and hamstring, forces exerted to the ground.
For what concerns Look and Reach movements, two coordination strategies were found: trials in which both the eyes and head movements preceded the arm prime mover activity (Look-First, 35.0±6.5% of total), and trials in which the opposite occurred (Arm-First, 35.5±6.6%). In the remaining trials, which were discarded, the prime mover recruitment followed the head extension but preceded the eye movements, or vice-versa. Data analysis revealed that APAs in leg muscles and forces to the ground were more anticipated in Look-First than in Arm-First, as it was in Visually-Guided than in Blind Reaching (p0.6). Moreover, in Without Reaching trials the APAs associated to head extension always lagged the APAs observed in both Look-First and Arm-First conditions. Thus, the head extension alone could not be responsible for these latter effects.
In conclusion, according to the chosen strategy, Look and Reach movements are programmed by taking into account the availability of online visual information about target position. In fact, when Looking First, the CNS takes into account that such information will be already available when approaching the target, like in Visually-Guided Reaching; instead, when moving the Arm First, the CNS seems not to rely on such information, like in Blind Reaching. Neither the ocular movements nor the head extension seem to be responsible for the change in APA programming, given the similarity of the Look-First to Arm-First and the Visually-Guided to Blind Reaching time-lags. Thus, considering also the time for cortical processing, it is likely that the expectancy of visual information influences the programming of arm movement
Effects of DC stimulation on the excitability of posterior tibial nerve
No full textSeveral studies demonstrated that transcutaneous direct current stimulation (DCs) may non-invasively modulate the excitability of several CNS structures. Its effect is typically facilitatory when using anodal polarity and inhibitory when using the cathodal one. In most studies, DCs has been applied on cortical or spinal structures, while little is known about its effects on peripheral nerves fibres. This research aims at highlighting such effects.
In twenty subjects, square pulses electrical stimulation of the posterior tibial nerve (1 ms duration, 1 shock every 9 seconds) at the popliteal fossa was used to elicit Soleus H-reflexes before (15 min), during (10 min) and after (30 min) DCs. Cathodal or anodal current (1 to 3 mA) was applied just proximally to the same nerve.
Cathodal DCs induced a significant increase of the H-reflex amplitude (about +35%) with respect to the control value (before). The excitatory effect lasted about 25 min after DCs end. Anodal DCs induced instead a significant decrease (about -25%) of the reflex amplitude, which lasted just about 5 min after DCs end.
In conclusion, DCs is able to elicit acute and short-term polarity-dependent effects on peripheral nerve excitability. This neuromodulation, which is likely due to a local change in nerve excitability, is however opposite in sign to the effects usually found on cortical or spinal structures
Availability of visual information modulates timing of anticipatory postural adjustments during arm reaching movements
No full textThis study examined whether changes in eye-head-arm coordination affect Anticipatory Postural Adjustments (APAs) in lower limbs, when performing arm reaching movements toward a target of known position.
Ten standing subjects reached a target placed in front of them with their index-fingertip. Four conditions were studied: 1) steadily looking at the target while reaching (Visually Guided, VG); 2) watching the floor with the head flexed, then Look and Reach the target (L&R); 3) Looking as in 2, but without Reaching (LnoR); 4) keeping the head flexed while reaching (Blind Reaching, BR). Firstly, subjects memorized the target position. Then they performed 15 reaching trials in VG, LnoR, BR, and 60 in L&R, in random order. Recorded variables were: arm, head and eyes movements, EMG from Anterior Deltoid (AD, prime mover) and legs postural muscles, forces exerted to the ground.
Data from L&R trials were classified in two groups: those in which both the head and eyes movements preceded AD-EMG (look first) and those in which the AD preceded head and eyes (arm first). APAs in leg muscles and forces to the ground were more anticipated in look first than in arm first, as it was in VG than in BR (p<0.0005), i.e. when a visual information was searched for or already present before starting the arm movement. Data from LnoR trials excluded that the head extension by itself could have caused the APAs advance, confirming that this should be ascribed to the availability of visual information
Immobilization of the hand affects arm and shoulder postural control
No full textObjectives: Neuromuscular consequences of limb immobilization are widely reported in literature, however, most papers describe changes in the motor pathways deserving muscles of the immobilized joint. Conversely, the present study investigates the effect of a short-term immobilization on the activation of both the prime mover and the associated postural muscles. It has been recently observed that when rapidly flexing the index finger, the forearm equilibrium is preserved thanks to the postural adjustments occurring in arm and shoulder muscles prior to the movement onset (APAs). These postural adjustments are excitatory in Triceps Brachii (TB) and inhibitory in Biceps Brachii (BB) and Anterior Deltoid (AD). In this study we tested if and how a 12 hours immobilization affects the APAs development in these muscles.
Methods: Subjects (n=10) were sitting on a chair with the right arm along the trunk, the elbow flexed at 90° and the prone hand in axis with the forearm. Starting with the index finger extended, subjects performed a rapid flexion (about 5-7 cm), repeated every 4s for 120 times. The metacarpo-phalangeal and elbow joints angles were recorded, as well as the EMG activity from the prime mover Flexor Digitorum Superficialis as well as from BB, TB and AD. At the end of the session, the EMG electrodes were left in place and fingers and wrist joints immobilized by a cast which was removed 12 hours later. The whole experimental protocol was then repeated.
Results: Short-term immobilization significantly reduced the excitatory APA in TB (-36.0%) and increased the inhibitory APAs in BB and AD (+19.4% and +30.3% respectively). The movement amplitude and duration, as well as the magnitude of the prime mover activation were instead unchanged.
Conclusions: Following immobilization of a joint, even of brief duration, the overall motor impairment may be partly due to APAs modifications in muscles acting on other non-immobilized joints of the same limb
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