Direct current stimulation modulates the excitability of the sensory and motor fibres in the human posterior tibial nerve, with a long-lasting effect on the H-reflex

Several studies demonstrated that transcutaneous direct current stimulation (DCS) may modulate central nervous system excitability. However, much less is known about how DC affects peripheral nerve fibres. We investigated the action of DCS on motor and sensory fibres of the human posterior tibial nerve, with supplementary analysis in acute experiments on rats. In forty human subjects, electric pulses at the popliteal fossa were used to elicit either M-waves or H-reflexes in the Soleus, before (15 min), during (10 min) and after (30 min) DCS. Cathodal or anodal current (2 mA) was applied to the same nerve. Cathodal DCS significantly increased the H-reflex amplitude; the post-polarization effect lasted up to ~ 25 min after the termination of DCS. Anodal DCS instead significantly decreased the reflex amplitude for up to ~ 5 min after DCS end. DCS effects on M-wave showed the same polarity dependence but with considerably shorter after-effects, which never exceeded 5 min. DCS changed the excitability of both motor and sensory fibres. These effects and especially the long-lasting modulation of the H-reflex suggest a possible rehabilitative application of DCS that could be applied either to compensate an altered peripheral excitability or to modulate the afferent transmission to spinal and supraspinal structures. In animal experiments, DCS was applied, under anaesthesia, to either the exposed peroneus nerve or its Dorsal Root, and its effects closely resembled those found in human subjects. They validate therefore the use of the animal models for future investigations on the DCS mechanisms

Antagonists\u2019 alternation during voluntary oscillations of the extremities adapts to the mechanical context. Experimental evidences and a neural control model

Many common life gestures require the ability to combine voluntary movements of different limb segments into a variety of co-ordinated patterns. When two limb segments are moved together, non-mechanical constraints act to facilitate certain associations and to hinder, or even impede others. The effect of such constraints is especially manifest when coupling isorhythmic oscillations. For example, it is quite simple to couple flexion of the hand prone with plantarflexion of the foot, while it is apparently more difficult to couple hand extension with foot plantarflexion. A similar behaviour is also shown by many other associations of limb segments. When studying coupled oscillations of the ipsilateral hand and foot, Baldissera et al. (1991, 2000; Baldissera and Cavallari 2001) measured the inter-limb phase-relations, both between hand and foot movements and, respectively, between EMG onset in Extensor Carpi Radialis (ECR) and Tibialis Anterior (TA). Moreover, they described the phase-relation intrinsic to each of the two extremities, expressed as the frequency-dependent (phase) delay between the onset of EMG activity and the onset of the related movement. In the hand, such relation could be well fitted by a pendulum model, while this was not possible for the foot oscillations, since at low-frequency (<1.5 Hz) onset of TA EMG paradoxically phase-lagged the onset of dorsiflexion, suggesting that the initial part of the movement was sustained by the recoil of elastic structures that were stretched during plantarflexion. Since the recoil should return the moving segment to its equilibrium position, i.e. that position in which the limb rests when muscles are fully relaxed, it may be argued that TA contraction was only needed to move the foot away from the equilibrium. As a consequence, the phase lag should disappear if the EMG onset were referred to the foot passive equilibrium position rather than to the movement onset. This hypothesis was tested in 10 subjects who voluntarily oscillated their right foot at various frequencies (0.2 to 3Hz) over 3 angular ranges: a central-range (foot crossing the equilibrium symmetrically), a high-range (whole excursion above equilibrium) and a low-range (whole excursion below equilibrium). In the central-range, phase-relations were measured between the crossing of equilibrium position and the onset of the TA EMG during dorsiflexion or the onset of Soleus (Sol) EMG during plantarflexion. In both cases, the phase-curves started around zero, without showing any paradoxical lag of EMG on movement. Phase-curves with similar features were also obtained in the high- and low-ranges (no crossing of equilibrium) by correlating the onset of the EMG burst to the onset of the related movement. The patterns of muscle activation recorded in all these conditions may be conceived as the result of one single motor output, which is split and distributed to the antagonists as soon as the segment crosses its equilibrium position. However, the passive equilibrium position may vary following the application of external loads or changes in limb orientation. In the hand, for instance, the equilibrium is reached in one single position when the hand is prone (flexion-extension vertical) but it covers an angular range when the hand is semi-prone, i.e., midway between prone and supine (flexion-extension horizontal). The analysis performed on foot oscillations was thus extended to the hand to determine how antagonists\u2019 alternation is regulated in the presence of an equilibrium range. Activity distribution between ECR and Flexor Carpi Radialis (FCR) during rhythmic flexion-extension of the hand was analysed in three different mechanical conditions (5 subjects). In the first condition (hand prone, flexion-extension in a vertical parasagittal plane) the hand passive equilibrium position was ~50\ub0 in flexion. During hand oscillations FCR and ECR were alternatively recruited to move the hand symmetrically away from the equilibrium and de-recruited to allow conservative forces to restore the equilibrium. Switching between antagonists occurred at the centre of the oscillation (equilibrium crossing), just like it happened for the foot. In the second condition (hand semi-prone, flexion-extension in a horizontal transversal plane) the hand equilibrium was attained over an angle of about 26\ub0. When the hand was oscillated symmetrically around this equilibrium range, each muscle was recruited when the hand entered the equilibrium range, i.e. in advance of the oscillation centre. Both vertical and horizontal oscillations were also performed all externally to the equilibrium position or range: in these cases only one muscle was recruited over the entire cycle, the EMG burst starting at the onset of the related movement. In the third condition (same orientation as condition 2) the application of a frictional load expanded the equilibrium range to encompass the entire hand oscillation. Now concentric muscle contraction was needed throughout each phase of the movement and switching between antagonists occurred at the movement reversal, i.e. ~90\ub0 in advance of the oscillation centre. Altogether, these findings show that during both hand and foot oscillations contractile force starts developing when an intrinsic or external resistance has to be overcome in order to continue the movement. This behaviour indicates that the nervous system monitors the mechanical characteristics of the moving limb and consequently adapts to them the pattern of antagonists\u2019 activation. To account for such a control, a neural network is proposed that compares the afferent information about joint position with a position central command, thus detecting the position error caused by the forces that resist to movement. From the sign and amplitude of the error signal, the network determines the direction (agonist vs antagonist) and the amount of motor activation required. The control capability of the network was tested by connecting it to a realistic model of the limb, which accounted for the mechanical properties of the segment and, when present, of the load. When simulating each of the mechanical conditions in which the hand or the foot were oscillated, the proposed neural network reproduced the patterns of antagonists\u2019 alternation observed in the real experiments

Combined recruitment of two \ufb01xation chains during cyclic movements of one arm

Voluntary adduction-abduction movements of one arm in the horizontal plane discharge a reaction torque which would rotate the trunk in the direction opposite to arm acceleration. Rotation is impeded by muscular fixation chains that exert forces counterbalancing the reaction torque. We examined how two different fixation chains cooperate in stabilising the trunk during the above movements. Standing subjects (n=6), with shoulders ante-flexed, performed cyclic adductions-abductions of the right arm (1.5. Hz) while grasping a fixed handle with the left hand. In this set-up, reaction torque is contrasted by: (1) a leg fixation chain, exerting on the ground a torque around the vertical axis (Tz), recorded by a force platform; and (2) a left arm fixation chain, exerting on the handle a force in the medial-lateral direction (Fh), recorded by a load cell. Subjects performed 20 trials (15 cycles each). It was found that Tz and Fh underwent sinusoidal changes at the same frequency as arm movements and contributed in counteracting the reaction torque. The intensity of the handle grip, monitored by EMG activity in the left Flexor Digitorum Superficialis, was changed from trial to trial and kept constant during each trial. As grip strength increased, Fh amplitude increased linearly while amplitude of Tz linearly decreased. In conclusion, voluntarily strengthening the handle grip progressively deviates the postural actions from the legs to the left arm

Changes in \u201cintra-limb\u201d anticipatory postural adjustments after a short-term immobilization of both wrist and fingers

Neuromuscular effects of limb immobilization are widely reported in the literature, however, most papers describe changes in the motor pathways deserving the prime movers 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 postural adjustments that occur 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 12h immobilization affects the APAs development. Subjects (n=7) were sitting on a chair with the right arm along the trunk, the elbow at 90\ub0 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 and from the postural muscles (BB, TB and AD). At the end of the session, the EMG electrodes were left in place and the fingers and wrist joints immobilized by a cast which was removed 12 hours later. We then repeated the whole protocol. Short-term immobilization effectively reduced the excitatory APA in TB and increased the inhibitory APA in BB and AD. The movement amplitude and duration, as well as the magnitude of the prime mover activation were unchanged. We conclude that the overall motor impairment following immobilization of a joint may be partly due to APAs modifications in muscles acting on other non-immobilized joints of the same limb