Little is known about anticipatory postural adjustments (APAs) developing when body segments of t... more Little is known about anticipatory postural adjustments (APAs) developing when body segments of tiny mass are moved. Thus, APAs in the human upper-limb were investigated during a gentle and small index finger tap (35 mm stroke in 50 ms). This task was fulfilled by ten subjects either with prone or supine hand. EMG was recorded from Flexor Digitorum Superficialis (FDS), the prime mover, and from several upper-limb muscles under slight tonic contraction. Regardless of hand posture, EMG was inhibited in Flexor Carpi Radialis and facilitated in Extensor Carpi Radialis well before the FDS burst. With the prone hand, the prime mover activity was preceded by Biceps inhibition and Triceps facilitation; this effect reverted in sign with the supine hand. A postural reversal was also observed in Anterior Deltoid and Trapezius which were both inhibited with the prone hand. The effect in Trapezius was present only with the unsupported forearm. It is thus demonstrated that a gentle small finger tap produces well-defined anticipatory natural synergies behaving as the most “classical” APAs: (1) they are distributed to several upper-limb muscles creating a postural chain aiming to prevent the effects of the interaction torques generated by the voluntary movement; (2) they change in amplitude according to the level of postural stability and (3) they revert in sign when movement direction is reverted. These results are also corroborated by data obtained from a simple mechanical model simulating finger tapping in a fictive upper-limb. A possible role of APAs in controlling movements’ accuracy is also discussed.
We previously observed that index finger flexion, with the prone hand, is preceded by an inhibito... more We previously observed that index finger flexion, with the prone hand, is preceded by an inhibitory postural adjustment (iAPA) in the Biceps Brachii (BB) electromyogram (EMG). The aim of this work is to locate the iAPA origin by testing the changes in spinal and cortico-spinal excitability during its development. Subjects were asked to flex their index finger, at will after an acoustic signal, while surface EMGs from Flexor Digitorum Superficialis (prime mover) and from BB (postural muscle) were recorded. In each experiment, tendon (T) reflexes in the resting BB and motor-evoked potentials elicited by magnetic cortical stimulation (CMEPs) in the resting, or in the slightly contracted BB, were collected both during the iAPA development and before the go signal. When BB was fully relaxed, T-reflexes falling in the iAPA temporal window were un-modulated, while CMEPs were significantly inhibited; this inhibition occurred, on average, 37 ms before the iAPA onset indicating that iAPA sets up and develops at supraspinal level. Changes in CMEPs’ excitability clearly anticipated the iAPA onset (about 50 ms) also when cortico-spinal excitability was probed in the active BB, thus indicating that this effect is likely due to a de-recruitment of spinal motoneurones. Thus, (1) iAPA is associated with voluntary movement even when postural muscles are at rest, (2) during iAPA spinal motoneurones are more likely disfacilitated than inhibited, and (3) the iAPA timing is regulated similarly to prime mover activation. All together these results suggest that iAPAs is fully sustained by inhibitory circuits located in the supra-spinal centres.
It has been observed that mechanical stimulation of the skin of the index fingertip causes a weak... more It has been observed that mechanical stimulation of the skin of the index fingertip causes a weak short-latency inhibition followed by a strong long-lasting facilitation of the flexor carpi radialis (FCR) H-reflex. Based on threshold and latency, these cutaneous reflexes are thought to be routed to motoneurons by parallel pathways. As recent studies have shown predominant inhibitory potentials in slow motoneurons and predominant excitatory potentials in faster ones, the question arises as to whether or not the two cutaneous pathways converge onto the same motoneuron. The poststimulus time histogram technique was used to investigate the changes in firing frequency of low-threshold FCR motor units (MUs), induced by passive mechanical or focal electrical stimuli to the index skin. After gently tapping the finger pulp a small sharp inhibition appeared in 20 MUs. On average, inhibition started 10.2 ± 1.6 ms from the homonymous Ia monosynaptic effect, and its central delay was estimated to be 1.2 ± 1.6 ms. The subsequent facilitation, more consistent, had a mean latency of 13.5 ± 1.7 ms. Inhibition and excitation were statistically significant (P < 0.05). A similar biphasic effect was observed in seven other FCR-MUs, also after focal electrical stimulation of the same skin area. Comparison with the time course of the H-reflex, representing the whole population of MUs, showed striking similarities in time course and latency to the present MU effect. It is thus suggested that cutaneous spinal pathways may have a homogeneous distribution within the FCR motoneuron pool, and that the skewed distribution of cutaneous afferents onto motoneurons should be not taken as a rule.
Little is known about anticipatory postural adjustments (APAs) developing when body segments of t... more Little is known about anticipatory postural adjustments (APAs) developing when body segments of tiny mass are moved. Thus, APAs in the human upper-limb were investigated during a gentle and small index finger tap (35 mm stroke in 50 ms). This task was fulfilled by ten subjects either with prone or supine hand. EMG was recorded from Flexor Digitorum Superficialis (FDS), the prime mover, and from several upper-limb muscles under slight tonic contraction. Regardless of hand posture, EMG was inhibited in Flexor Carpi Radialis and facilitated in Extensor Carpi Radialis well before the FDS burst. With the prone hand, the prime mover activity was preceded by Biceps inhibition and Triceps facilitation; this effect reverted in sign with the supine hand. A postural reversal was also observed in Anterior Deltoid and Trapezius which were both inhibited with the prone hand. The effect in Trapezius was present only with the unsupported forearm. It is thus demonstrated that a gentle small finger tap produces well-defined anticipatory natural synergies behaving as the most “classical” APAs: (1) they are distributed to several upper-limb muscles creating a postural chain aiming to prevent the effects of the interaction torques generated by the voluntary movement; (2) they change in amplitude according to the level of postural stability and (3) they revert in sign when movement direction is reverted. These results are also corroborated by data obtained from a simple mechanical model simulating finger tapping in a fictive upper-limb. A possible role of APAs in controlling movements’ accuracy is also discussed.
We previously observed that index finger flexion, with the prone hand, is preceded by an inhibito... more We previously observed that index finger flexion, with the prone hand, is preceded by an inhibitory postural adjustment (iAPA) in the Biceps Brachii (BB) electromyogram (EMG). The aim of this work is to locate the iAPA origin by testing the changes in spinal and cortico-spinal excitability during its development. Subjects were asked to flex their index finger, at will after an acoustic signal, while surface EMGs from Flexor Digitorum Superficialis (prime mover) and from BB (postural muscle) were recorded. In each experiment, tendon (T) reflexes in the resting BB and motor-evoked potentials elicited by magnetic cortical stimulation (CMEPs) in the resting, or in the slightly contracted BB, were collected both during the iAPA development and before the go signal. When BB was fully relaxed, T-reflexes falling in the iAPA temporal window were un-modulated, while CMEPs were significantly inhibited; this inhibition occurred, on average, 37 ms before the iAPA onset indicating that iAPA sets up and develops at supraspinal level. Changes in CMEPs’ excitability clearly anticipated the iAPA onset (about 50 ms) also when cortico-spinal excitability was probed in the active BB, thus indicating that this effect is likely due to a de-recruitment of spinal motoneurones. Thus, (1) iAPA is associated with voluntary movement even when postural muscles are at rest, (2) during iAPA spinal motoneurones are more likely disfacilitated than inhibited, and (3) the iAPA timing is regulated similarly to prime mover activation. All together these results suggest that iAPAs is fully sustained by inhibitory circuits located in the supra-spinal centres.
It has been observed that mechanical stimulation of the skin of the index fingertip causes a weak... more It has been observed that mechanical stimulation of the skin of the index fingertip causes a weak short-latency inhibition followed by a strong long-lasting facilitation of the flexor carpi radialis (FCR) H-reflex. Based on threshold and latency, these cutaneous reflexes are thought to be routed to motoneurons by parallel pathways. As recent studies have shown predominant inhibitory potentials in slow motoneurons and predominant excitatory potentials in faster ones, the question arises as to whether or not the two cutaneous pathways converge onto the same motoneuron. The poststimulus time histogram technique was used to investigate the changes in firing frequency of low-threshold FCR motor units (MUs), induced by passive mechanical or focal electrical stimuli to the index skin. After gently tapping the finger pulp a small sharp inhibition appeared in 20 MUs. On average, inhibition started 10.2 ± 1.6 ms from the homonymous Ia monosynaptic effect, and its central delay was estimated to be 1.2 ± 1.6 ms. The subsequent facilitation, more consistent, had a mean latency of 13.5 ± 1.7 ms. Inhibition and excitation were statistically significant (P < 0.05). A similar biphasic effect was observed in seven other FCR-MUs, also after focal electrical stimulation of the same skin area. Comparison with the time course of the H-reflex, representing the whole population of MUs, showed striking similarities in time course and latency to the present MU effect. It is thus suggested that cutaneous spinal pathways may have a homogeneous distribution within the FCR motoneuron pool, and that the skewed distribution of cutaneous afferents onto motoneurons should be not taken as a rule.
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Papers by Antonio Caronni