Effects of direct current (DC) on nerve fibers have primarily been investigated during or just after DC application. However, locally applied cathodal DC was recently demonstrated to increase the ...excitability of intraspinal preterminal axonal branches for >1 h. The aim of this study was therefore to investigate whether DC evokes a similarly long-lasting increase in the excitability of myelinated axons within the dorsal columns. The excitability of dorsal column fibers stimulated epidurally was monitored by recording compound action potentials in peripheral nerves in acute experiments in deeply anesthetized rats. The results show that
) cathodal polarization (0.8-1.0 µA) results in a severalfold increase in the number of epidurally activated fibers and
) the increase in the excitability appears within seconds,
) lasts for >1 h, and
) is activity independent, as it does not require fiber stimulation during the polarization. These features demonstrate an unexplored form of plasticity of myelinated fibers and indicate the conditions under which it develops. They also suggest that therapeutic effects of epidural stimulation may be significantly enhanced if it is combined with DC polarization. In particular, by using DC to increase the number of fibers activated by low-intensity epidural stimuli, the low clinical tolerance to higher stimulus intensities might be overcome. The activity independence of long-lasting DC effects would also allow the use of only brief periods of DC polarization preceding epidural stimulation to increase the effect.
The study indicates a new form of plasticity of myelinated fibers. The differences in time course of DC-evoked increases in the excitability of myelinated nerve fibers in the dorsal columns and in preterminal axonal branches suggest that distinct mechanisms are involved in them. The results show that combining epidural stimulation and transspinal DC polarization may dramatically improve their outcome and result in more effective pain control and the return of impaired motor functions.
Electrically interfacing the skin for monitoring personal health condition is the basis of skin‐contact electrophysiology. In the clinical practice the use of stiff and bulky pregelled or dry ...electrodes, in contrast to the soft body tissues, imposes severe restrictions to user comfort and mobility while limiting clinical applications. Here, in this work dry, unperceivable temporary tattoo electrodes are presented. Customized single or multielectrode arrays are readily fabricated by inkjet printing of conducting polymer onto commercial decal transfer paper, which allows for easy transfer on the user's skin. Conformal adhesion to the skin is provided thanks to their ultralow thickness (<1 µm). Tattoo electrode–skin contact impedance is characterized on short‐ (1 h) and long‐term (48 h) and compared with standard pregelled and dry electrodes. The viability in electrophysiology is validated by surface electromyography and electrocardiography recordings on various locations on limbs and face. A novel concept of tattoo as perforable skin‐contact electrode, through which hairs can grow, is demonstrated, thus permitting to envision very long‐term recordings on areas with high hair density. The proposed materials and patterning strategy make this technology amenable for large‐scale production of low‐cost sensing devices.
Temporary tattoo electrodes are fabricated by inkjet printing and tested as unperceivable skin‐contact electrodes in various electrophysiology applications. Ultralow thickness (<1 µm) endows tattoos with conformable adhesion to skin and capability to be perforated by growing hairs. Custom multielectrodes arrays can be designed in view of specific applications in diagnostics, human machine interfacing, and personal health monitoring.
•tDCS was used for interfering with the central pathways governing intra-limb APAs.•SMA was stimulated in anodal, cathodal and sham configuration during finger tapping.•Anodal tDCS increased APAs in ...biceps and triceps, muscle that stabilize the elbow.•tDCS had no effect on prime mover recruitment and voluntary movement kinematics.•The postural and the voluntary commands seem already separated before entering SMA
Recent works provide evidences that anticipatory postural adjustments (APAs) are programmed with the prime mover recruitment as a shared posturo-focal command. However the ability of the CNS to adjust APAs to changes in the postural context implies that the postural and voluntary components should take different pathways before reaching the representation of single muscles in the primary motor cortex. Here we test if such bifurcation takes place at the level of the supplementary motor area (SMA).
TDCS was applied over the SMA in 14 subjects, who produced a brisk index-finger flexion. This activity is preceded by inhibitory APAs, carved in the tonic activity of Biceps Brachii and Anterior Deltoid, and by an excitatory APA in Triceps Brachii. Subjects performed a series of 30 flexions before, during and after 20min of tDCS in CATHODAL, ANODAL or SHAM configuration.
The inhibitory APA in Biceps and the excitatory APA in Triceps were both greater in ANODAL than in SHAM and CATHODAL configurations, while no difference was found among the latter two (ANODAL vs. SHAM: biceps +26.5%, triceps +66%; ANODAL vs. CATHODAL: biceps +20.5%, triceps: +63.4%; for both muscles, ANOVA p<0.02, Tukey p<0.05). Instead, the APA in anterior deltoid was unchanged in all configurations. No changes were observed in prime mover recruitment and index-finger kinematics.
Results show that the SMA is involved in modulating APAs amplitude. Moreover, the differential effect of tDCS observed on postural and voluntary commands suggests that these two components of the motor program are already separated before entering SMA.
Key points
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Transcranial constant current polarization of the human brain is on the increase in neurological practice because it improves several motor and cognitive functions of the human nervous ...system and because it is non‐invasive and technically simple.
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Here we show that transcranial brain polarization in anaesthetized animals not only affects cortical neurons, as is often assumed, but also facilitates activation of neurons in all investigated subcortical motor systems.
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In addition, the subcortical facilitation greatly outlasts (by at least hours) the period of transcranial polarization. These findings provide new evidence of plasticity at subcortical levels, the mechanisms for which remain to be investigated.
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In clinical practice, the subcortical effects of transcranial polarization may thus make an essential contribution to the beneficial effects of the treatment of motor impairments.
The main aim of the study was to examine the effects of transcranial polarization on neurons in two descending motor systems, rubro‐ and reticulospinal. Anodal DC current was applied through an electrode in contact with the skull over the contralateral sensori‐motor cortex, against an electrode placed between the skull and the ipsilateral temporal muscles in deeply anaesthetized cats. Its effects were estimated from changes in descending volleys evoked by electrical stimuli applied in the red nucleus (RN), medial longitudinal fascicle (MLF; to reticulospinal fibres) and the pyramidal tract (PT; to corticospinal or corticoreticular fibres). The descending volleys were recorded from the surface of the spinal cord at a cervical level. Rubrospinal neurones were activated either directly or indirectly, via interpositorubral fibres. Reticulospinal neurons were likewise activated directly and indirectly, via other reticulospinal or corticospinal fibres. Transcranial polarization facilitated transsynaptic activation of both rubrospinal and reticulospinal neurons, shortening the latency of the indirect descending volleys and/or increasing them, Direct activation of descending axons was much less affected. The facilitation of all subcortical neurons examined was potentiated by repeated applications of transcranial direct current stimulation (tDCS) and outlasted the polarization by at least 1–2 h, replicating tDCS effects on indirect activation of cortical neurons. The results indicate that the beneficial effects of tDCS on motor performance in humans may be due to more efficient activation of not only cortical but also subcortical neuronal systems. Combined actions of tDCS on cortical and subcortical neurones might thus further improve recovery of motor functions during rehabilitation after central injuries. 249/250
Anticipatory Postural Adjustments (APAs) are commonly described as unconscious muscular activities aimed to counterbalance the perturbation caused by the primary movement, so as to ensure the ...whole-body balance, as well as contributing to initiate the displacement of the body center of mass when starting gait or whole-body reaching movements. These activities usually create one or more fixation chains which spread over several muscles of different limbs, and may be thus called
. However, it has been reported that APAs also precede voluntary movements involving tiny masses, like a flexion/extension of the wrist or even a brisk flexion of the index-finger. In particular, such movements are preceded by an
APA chain, that involves muscles acting on the proximal joints. Considering the small mass of the moving segments, it is unlikely that the ensuing perturbation could threaten the whole-body balance, so that it is interesting to enquire the physiological role of
APAs and their organization and control compared to
APAs. This review is focused on
APAs and highlights a strict correspondence in their behavior and temporal/spatial organization with respect to
APAs. Hence it is suggested that both are manifestations of the same phenomenon. Particular emphasis is given to
APAs preceding index-finger flexion, because their relatively simple biomechanics and the fact that muscular actions were limited to a single arm allowed peculiar investigations, leading to important conclusions. Indeed, such paradigm provided evidence that by granting a proper fixation of those body segments proximal to the moving one APAs are involved in refining movement precision, and also that APAs and prime mover activation are driven by a shared motor command.
Anticipatory postural adjustments (APAs) are the coordinated muscular activities that precede the voluntary movements to counteract the associated postural perturbations. Many studies about gait ...initiation call APAs those activities that precede the heel-off of the leading foot, thus taking heel-off as the onset of voluntary movement. In particular, leg muscles drive the center of pressure (CoP) both laterally, to shift the body weight over the trailing foot and backward, to create a disequilibrium torque pushing forward the center of mass (CoM). However, since subjects want to propel their body rather than lift their foot, the onset of gait should be the CoM displacement, which starts with the backward CoP shift. If so, the leg muscles driving such a shift are the prime movers. Moreover, since the disequilibrium torque is mechanically equivalent to a forward force acting at the pelvis level, APAs should be required to link the body segments to the pelvis: distributing such concentrated force throughout the body would make all segments move homogeneously. In the aim of testing this hypothesis, we analyzed gait initiation in 15 right-footed healthy subjects, searching for activities in trunk muscles that precede the onset of the backward CoP shift. Subjects stood on a force plate for about 10 s and then started walking at their natural speed. A minimum of 10 trials were collected. A force plate measured the CoP position while wireless probes recorded the electromyographic activities. Recordings ascertained that at gait onset APAs develop in trunk muscles. On the right side, Rectus Abdominis and Obliquus Abdominis were activated in 11 and 13 subjects, respectively, starting on average 33 and 54 ms before the CoP shift; Erector Spinae (ES) at L2 and T3 levels was instead inhibited (9 and 7 subjects, 104 and 120 ms). On the contralateral side, the same muscles showed excitatory APAs (abdominals in 11 and 12 subjects, 27 and 82 ms; ES in 10 and 7 subjects, 75 and 32 ms). The results of this study provide a novel framework for distinguishing postural from voluntary actions, which may be relevant for the diagnosis and rehabilitation of gait disorders.
Voluntary movements induce postural perturbations, which are counteracted by anticipatory postural adjustments (APAs) that preserve body equilibrium. Little is known about the neural structures ...generating APAs, but several studies suggested a role of sensory–motor areas, basal ganglia, supplementary motor area and thalamus. However, the role of the cerebellum still remains an open question. The aim of this present paper is to shed further light on the role of cerebellum in APAs organization. Thus, APAs that stabilize the arm when the index finger is briskly flexed were recorded in 13 ataxic subjects (seven sporadic cases, four dominant ataxia type III and two autosomal recessive), presenting a slowly progressive cerebellar syndrome with four-limb dysmetria, and compared with those obtained in 13 healthy subjects. The pattern of postural activity was similar in the two groups excitation in triceps and inhibition in biceps and anterior deltoid (AD), but apparent modifications in timing were observed in all ataxic subjects in which, on average, triceps brachii excitation lagged the onset of the prime mover flexor digitorum superficialis by about 27 ms and biceps and AD inhibition were almost synchronous to it. Instead, in normal subjects, triceps onset was synchronous to the prime mover and biceps and AD anticipated it by about 40 ms. The observed disruption of the intra-limb APA organization confirms that the cerebellum is involved in APA control and, considering cerebellar subjects as a model of dysmetria, also supports the view that a proper APA chain may play a crucial role in refining movement metria.
It is a common experience to exhibit a greater dexterity when performing a pointing movement with the preferred limb (PREF) vs. the non-preferred (NON-PREF) one. Here we provide evidence that the ...higher precision in pointing movements of the PREF vs. NON-PREF hand is associated with an earlier occurrence of the anticipatory postural adjustments (APAs). In this aim, we compared the APAs which stabilize the left or the right arm when performing a pen-pointing movement (prime mover flexor carpi radialis (FCR)). Moreover, we analyzed the elbow and wrist kinematics as well as the precision of the pointing movement. The mean kinematics of wrist movement and its latency, with respect to prime mover recruitment, were similar in the two sides, while APAs in triceps brachii (TB), biceps brachii (BB) and anterior deltoid (AD) were more anticipated when movements were performed with the PREF than with the NON-PREF hand (60-70 vs. 20-30 ms). APAs amplitudes were comparable in the muscles of the two sides. Earlier APAs in the preferred limb were associated with a better fixation of the elbow, which showed a lower excursion, and with a less scattered pointing error (PREF: 10.1 ± 0.8 mm; NON-PREF: 16.3 ± 1.7). Present results suggest that, by securing the more proximal joints dynamics, an appropriate timing of the intra-limb APAs is necessary for refining the voluntary movement precision, which is known to be scarce on the NON-PREF side.
Evidence shows that the postural and focal components within the voluntary motor command are functionally unique. In 2015, we reported that the supplementary motor area (SMA) processes Anticipatory ...Postural Adjustments (APAs) separately from the command to focal muscles, so we are still searching for a hierarchically higher area able to process both components. Among these, the parietal operculum (PO) seemed to be a good candidate, as it is a hub integrating both sensory and motor streams. However, in 2019, we reported that transcranial Direct Current Stimulation (tDCS), applied with an active electrode on the PO contralateral to the moving segment vs. a larger reference electrode on the opposite forehead, did not affect intra-limb APAs associated to brisk flexions of the index-finger. Nevertheless, literature reports that two active electrodes of opposite polarities, one on each PO (dual-hemisphere, dh-tDCS), elicit stronger effects than the "active vs. reference" arrangement. Thus, in the present study, the same intra-limb APAs were recorded before, during and after dh-tDCS on PO. Twenty right-handed subjects were tested, 10 for each polarity: anode on the left vs. cathode on the right, and vice versa. Again, dh-tDCS was ineffective on APA amplitude and timing, as well as on prime mover recruitment and index-finger kinematics. These results confirm the conclusion that PO does not take part in intra-limb APA control. Therefore, our search for an area in which the motor command to prime mover and postural muscles are still processed together will have to address other structures.
It is a common experience, immediately after the removal of a cast or a splint, to feel motor awkwardness, which is usually attributed to muscular and joint immobilization. However, the same feeling ...may also be perceived after a brief period of immobilization. We provide evidence that this last effect stems from changes in the cortical organization of the focal movement as well as in the associated anticipatory postural adjustments. Indeed, these two aspects of the motor act are strongly correlated, although scaled in different manners. In fact, they are both shaped in the primary motor cortex, they both undergo similar amplitude and latency modulation and, as we will show, they are both impaired by the immobilization of the lone prime mover. Neuromuscular effects of limb immobilization are well known; however, most papers focus on changes occurring in the pathways projecting to the prime mover, which acts on the immobilized joint. Conversely, this study investigates the effect of immobilization on anticipatory postural adjustments. Indeed, we show that 12 h of wrist and fingers immobilization effectively modify anticipatory postural adjustments of the elbow and the shoulder, that is, those joints not immobilized within the fixation chain. Accordingly, the motor impairment observed after short-term immobilization most likely stems from the unbalance between anticipatory postural adjustments and the focal movement.
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