Biomedical subjects

R Mazzocchio

Publications and source records attributed to R Mazzocchio.

At least 37 records · Page 2Linked to original sources

PubMed1995

Distribution of Ia effects onto human hand muscle motoneurones as revealed using an H reflex technique.

1. The possibility of eliciting H reflexes in relaxed hand muscles using a collision between the orthodromic impulses generated by magnetic cortical stimulation and the antidromic motor volley due to a supramaximal (SM) peripheral nerve stimulus was investigated in seven subjects. 2. Magnetic stimuli, applied through a circular coil (outer diameter, 13 cm) centred at the vertex, evoking EMG responses of 3-5 mV amplitude in the relaxed abductor digit minimi (ADM) muscle, and SM test stimuli to the ulnar nerve at the wrist producing a direct maximal motor response (Mmax) in the ADM muscle, were given either alone or combined. 3. In all subjects, combined cortical and SM ulnar stimulation produced a response after the Mmax with the latency of an H reflex evoked by the ulnar stimulus. This response occurred only within interstimulus intervals (1-20 ms) compatible with collision in the motor axons. The response behaved like an H reflex being time-locked to the SM ulnar stimulus, facilitated by voluntary activation of ADM muscle, depressed by vibration (4 s, 100 Hz) of ADM tendon and by a submotor-threshold ulnar nerve stimulus applied 50 and 80 ms before the combined stimulation, respectively. 4. In some subjects, it was also possible to distinguish an earlier response preceding the H reflex by 3 ms. Evidence is given that this response is probably of cortical origin. 5. Varying the intensity of magnetic stimulation resulted in a non-linear relationship between the H reflex size and the size of the cortical response. When the latter was between 5-25% of Mmax, H reflexes were small (2.5-7.5% of Mmax); with cortical responses between 25-50% of Mmax, there was a steep increase in H reflex amplitude (10-30% of Mmax). We suggest that this behaviour is due to an uneven distribution of Ia effects within the motoneurone pool.

Adult↗

PubMed1995

Group Ia non-reciprocal inhibition from wrist extensor to flexor motoneurones in humans.

Interneurones mediating disynaptic inhibition from extensor to flexor carpi radialis muscles were characterized by pharmacological stimulation of Renshaw cells. It is, indeed, known that only Ia interneurones are blocked by recurrent inhibition. Renshaw cell potentiation, induced by intravenous administration of 2 g levo-acetylcarnitine, blocked Ia reciprocal inhibition from triceps to biceps muscles but not disynaptic inhibition from extensor to flexor carpi radialis muscles. It is concluded that the interneurones mediating this latter inhibition are not Ia interneurones. This kind of inhibition could be an example of a Ia non-reciprocal inhibitory pathway.

Adult↗

PubMed1994

Depression of Renshaw recurrent inhibition by activation of corticospinal fibres in human upper and lower limb.

1. This study tested whether the recurrent inhibition of soleus and wrist flexor motoneurones could be modified by transcranial magnetic stimulation in human subjects. 2. Magnetic stimulation was given through a circular coil centred at the vertex. The intensity of the magnetic stimulus was subthreshold for evoking a motor response in the active soleus and wrist flexor muscles. The recurrent inhibition brought about by a conditioning H1 reflex discharge was estimated by a test H' reflex. The modifications of the recurrent inhibition after cortical stimulation were distinguished from the motoneuronal changes by comparing H' to a reference H reflex. 3. In the soleus motoneurones, the reference H reflex was inhibited at a minimum conditioning-test interval of -2 ms (H reflex stimulus before magnetic stimulation). In contrast, the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. In the wrist flexor motoneurones, both H' and reference H reflexes were facilitated. However, at lower cortical stimulus intensities, only the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. 4. In both motoneurone pools, H' facilitation started 3-4 ms later than the earliest changes in the reference H reflex. Also, the threshold of H' facilitation was lower than that of reference H reflex. 5. It is concluded that facilitation of the H' reflex is produced by corticospinal inhibition of Renshaw cells via a short interneuronal chain in both the upper and lower limb.

Adult↗

PubMed1994

Effect of tonic voluntary activity on the excitability of human motor cortex.

1. The threshold for obtaining EMG responses after transcranial magnetic stimulation of the brain is reduced by voluntary contraction of the target muscle. The present experiments tested whether some of this effect is due to increased cortical, as opposed to spinal, excitability during the contraction. 2. Magnetic stimulation was delivered with a figure-of-eight coil oriented with the junction region along the interaural line and also (in 4 of 7 subjects) with a circular coil centred at the vertex. The intensity of the conditioning stimulus was subthreshold for evoking a motor response in the relaxed wrist flexor muscles of the forearm. The presence of a small descending corticospinal volley in both the relaxed and active conditions was detected by measuring the facilitation of test H reflexes elicited in the flexor muscles of the forearm. 3. In all subjects, magnetic stimulation with either coil facilitated the H reflex at conditioning-test intervals of -1 to -3 ms (median nerve stimulus before magnetic). This was followed by a long-lasting facilitation. In three of the seven subjects stimulation with the figure-of-eight coil elicited an additional, earlier peak of facilitation at a conditioning-test interval of -3 to -5 ms. 4. In all subjects, the threshold for obtaining facilitation of the H reflex using a conditioning-test interval of -1 to -3 ms was reduced, and the amount of facilitation was larger, if subjects performed a weak tonic voluntary contraction. In contrast, with a conditioning-test interval of -3 to -5 ms voluntary contraction had no effect on the threshold. 5. It is suggested that H reflex facilitation at the conditioning-test interval of -1 to -3 ms was produced by indirect activation of corticospinal neurones by the magnetic stimulus, whereas at -3 to -5 ms, the facilitation was produced by direct activation of corticospinal axons. It is concluded that tonic voluntary contraction of a target muscle decreases the threshold for indirect activation of corticospinal neurones but not for direct stimulation of their axons.

Conditioning, Psychological↗

PubMed1993

Distribution of recurrent inhibition in the human upper limb.

The distribution of homonymous and heteronymous recurrent inhibition among the motor nuclei innervating the main muscles of the human upper limb has been investigated in 25 healthy subjects. Homonymous recurrent inhibition was studied with a specially designed electrophysiological method with paired H-reflexes, previously described by Bussel & Pierrot-Deseilligny (1977), combined with a pharmacological study using a cholinergic agonist, the L-acetylcarnitine (Rossi & Mazzocchio 1991). These methods were used to investigate the Flexor Carpi Radialis (FCR), Extensor Carpi Radialis (ECR), Opponens Pollicis (OP) and Abductor Digiti Minimi (ADM) motor nuclei. In the Deltoid, Triceps, Flexor Carpi Ulnaris (FCU) and Extensor Carpi Ulnaris (ECU) motor nuclei in which it was impossible to evoke clearly distinguishable H-reflexes, homonymous recurrent inhibition was studied with the PSTH technique: homonymous recurrent inhibition was found in Deltoid, Triceps, FCR, FCU, ECR, ECU motor nuclei but not in OP and ADM motor nuclei. Heteronymous recurrent inhibition was studied with the PSTH technique in the Deltoid, Biceps, Triceps, FCR, ECR, FCU, ECU, Flexor Digitorum Communis (FDC), Extensor Digitorum Communis (EDC) motor nuclei as well as those innervating the intrinsic muscles of the hand. The following results were obtained: (1) motor neurones innervating muscles acting at digits do not receive any heteronymous recurrent inhibition; (2) motor neurones innervating muscles acting at the wrist give recurrent inhibition to motor nuclei of proximal muscles but do not receive any recurrent projections from the latter; and (3) motor neurones innervating proximal muscles (acting at shoulder or elbow) are interconnected by recurrent inhibition and receive heteronymous recurrent projections from some wrist muscles but not from intrinsic hand muscles.

Adult↗

PubMed1991

Cortical projection of putative group Ib afferent fibres from the human forearm.

The possibility was investigated that impulses in group Ib afferents from fore-arm flexors have access to cerebral cortex in man. A long-lasting increase in the threshold (Th) of group Ia afferent fibres from flexor carpi radialis muscle (Fcr) was obtained after prolonged (100 Hz for 20 min) tendon vibration at the wrist. It was assumed that under this condition a weak electrical stimulation of the median nerve at the elbow, insufficient to reactivate Ia fibres because of the rise in their threshold (as verified by the method of homonymous Ia facilitation of Fcr H reflex), engaged Ib fibres only. Peripheral volleys at Erb's point and cerebral cortical potentials to median nerve stimulation at the elbow were evoked before and after prolonged vibration of Fcr. During 10-30 min after the end of vibration, in which homonymous facilitation of Fcr H reflex was abolished, both the ongoing peripheral volley and the cortical responses were markedly reduced with respect to their control values. Recovery to pre-vibration control amplitudes coincided with recovery of Fcr H reflex homonymous facilitation. In order to verify if activity in afferents other than group Ib fibres might contribute to the cortical response after vibration, specially designed experiments were also performed. It is concluded that the cortical wave recorded after 20 min vibration represents the arrival of Ib impulses from flexor carpi radialis to the human cerebral cortex.

Adult↗

PubMed1991

Presence of homonymous recurrent inhibition in motoneurones supplying different lower limb muscles in humans.

The pattern of recurrent inhibition to motoneurones (MNs) innervating different lower limb muscles was investigated in ten healthy subjects. Three complementary experimental designs, all based on the electrophysiological method introduced by Bussel and Pierrot-Deseilligny in 1977, were used in Quadriceps (Qu), Pretibial (Pt), Soleus (Sol) and Abductor hallucis (Abh) motornuclei: 1) measurement of the relationship between the amplitude of conditioning H (H1) and test (H') reflexes; 2) measurement of the effect of the intravenous administration of L-Acetylcarnitine (L-Ac) on the amplitude of the test H' reflex after a constant H1 conditioning reflex; 3) measurement of the modifications of the test H' reflex in relation to a reference H (Ref H) reflex during a weak tonic voluntary contraction of the homonymous muscle. A complete agreement among results obtained with the different experimental paradigms was observed. Similarly to the Sol, both Pt and Qu MNs were found to be recurrently inhibited: a) the test H' reflex exhibited a progressive and consistent depression with increasing amplitude of the H1 conditioning reflex beyond a specific value; b) an additional decrement of the test H' reflex was obtained after intravenous administration of L-Ac; c) a decrease in the size of the test H' reflex, with respect to its value at rest, was observed during a weak tonic voluntary contraction, in spite of the enhanced MN excitability (as shown by the increase in the Ref H). By contrast, no evidence of recurrent inhibition to the Abh MNs was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcarnitine↗

PubMed1991

Changes in alpha motoneuron excitability of the soleus muscle in relation to vestibular stimulation assessed by angular acceleration in man.

Variations in spinal motoneuron excitability, tested by the monosynaptic H reflex of the soleus muscle, were studied in man in relation to angular acceleration of the body in a damped rotating chair. Clockwise and anticlockwise rotation produced similar changes on the same spinal motoneurons, consisting of a first clear-cut facilitatory phase starting at 0.4 degrees of rotation (corresponding to 156 ms) with a peak between 10 and 30 degrees, followed by a second excitatory phase between 50 and 100 degrees; then, the amplitude of the H reflex progressively recovered to its control value. Both facilitatory phases showed a significant decrease by reducing the angular velocity and acceleration. Control experiments ruled out that both the startle reaction of the subject and variations in the somatosensory input during rotation could be responsible for generating the facilitatory effects on the H reflex. The mean value of the voluntary reaction to turning sensation was 1.1 degrees, corresponding to about 335 ms. It is concluded that the changes described in motoneuron excitability could represent a vestibulospinal reflex response originating from the horizontal semicircular canals.

Acceleration↗

PubMed1990

Clonus in man: a rhythmic oscillation maintained by a reflex mechanism.

The mechanisms underlying clonus were studied in 7 patients with spasticity of the lower limbs arising from upper motor neuron lesions. Clonus sensitivity to resetting was studied by evoking a soleus H reflex at different time intervals between two successive clonic beats. The relationship between the interval separating the H reflex from the preceding beat and the inter-beat interval in which the H reflex was delivered was used to calculate the resetting index. A high resetting index, close to 1.0, was found. Calf compression, applied to the upper third of the leg to block group I afferent fibres from gastrocnemius-soleus muscle, completely abolished ankle clonus within 10 min of the onset of compression. Clonus disappearance was preceded by a progressive reduction in duration and frequency. Finally, evidence is presented that clonus duration is contingent upon joint angle and therefore presumably on the amount of stretch placed on the triceps surae muscle group. Our findings demonstrate that clonus is largely dependent on reflex and mechanical factors and do not confirm the recent hypothesis that clonus relies on a central spinal generator.

Adolescent↗

PubMed1990

Enhancement of recurrent inhibition by intravenous administration of L-acetylcarnitine in spastic patients.

The recurrent inhibition of the soleus alpha-motoneurons at rest, evaluated by a specially designed method of paired H reflexes, was estimated in 10 patients with spastic paraparesis. In three of these patients, Renshaw cell activity produced inhibition of the corresponding alpha-motor neurons comparable to that obtained in normal subjects, while the inhibition was reduced in five and absent in two. The effects of intravenously administered L-acetylcarnitine on the activity of Renshaw cells were studied in these patients. In all patients except the two with no evidence of recurrent inhibition, L-acetylcarnitine was found to significantly increase the amount of recurrent inhibition. Its effect became evident at about 30 minutes, reached a maximum around 50 minutes and vanished about 70 minutes from the onset of administration. A significant correlation was found between Renshaw cell activity and the ability of the drug to increase it. L-acetylcarnitine appeared to act specifically by enhancing recurrent inhibition, since no variation in the excitability of the monosynaptic reflex arc was observed.

Acetylcarnitine↗

PubMed1989

Further evidence for Renshaw inhibition in man. A combined electrophysiological and pharmacological approach.

Experiments involving the acute administration of a central cholinergic substance (L-acetylcarnitine) were performed on 6 healthy subjects to obtain additional evidence that the electrophysiological method developed by Bussel and Pierrot-Deseilligny in 1977 does actually assess recurrent inhibition in man. In all the subjects, the drug further decreased the amplitude of a test H reflex (H') following a conditioning H reflex (H1). The amount of this supplementary inhibition was found to be related to the size of H1 reflex. Experimental evidence is also presented that H' reflex supplementary depression is not contaminated by homonymous Ib effects. It is concluded that the method effectively tests the excitability of Renshaw cells in man.

Acetylcarnitine↗

PubMed1989

Recurrent inhibition in human spinal spasticity.

The study was performed on a group of 17 patients with spastic paraparesis: 12 with hereditary spastic paraparesis, 3 with cord compression and 2 with complete spinal transection. 10 healthy volunteers acted as controls. Recurrent inhibition of the soleus alpha-motoneurones was estimated at rest and during voluntary contraction of triceps surae. At rest, there was evidence for a substantial decrease in the excitability of Renshaw cells in 9 out of the 12 patients with hereditary spastic paraparesis; this was also observed in the 2 patients with complete spinal transection, while the 3 patients with cord compression exhibited a normal Renshaw cell activity. In 3 out of 4 patients with hereditary spastic paraparesis, the changes in Renshaw cell excitability expected to occur during voluntary contraction were not found, whereas in one patient with hereditary spastic paraparesis and one with spastic paraparesis due to cord compression recurrent inhibition was normally influenced by the motor command. Our results indicate that recurrent inhibition is likely to be differently affected according to the type and/or localization of the lesion. It is also suggested that the central nervous system might control the excitability of Renshaw cells at rest and during voluntary contraction via partly separate pathways. The role of recurrent inhibition in spasticity is discussed.

Adult↗

PubMed1989

Motoneurone recurrent inhibition is enhanced by L-acetylcarnitine in humans.

Renshaw cells, which mediate the recurrent inhibition of spinal a-motoneurones, are activated by acetylcholine, both through motoneurone collaterals and the reticulo-spinal system. Since it is known that L-acetylcarnitine (L-AC) has central cholinergic effects, we tested in ten normal subjects and three spastic patients the ability of L-AC to induce changes in excitability of the Renshaw cells. These were activated by a conditioning monosynaptic reflex of the soleus muscle, evoked by electrical stimulation of the tibial nerve, and the resulting recurrent inhibition of the motoneurones was assessed by a subsequent monosynaptic reflex (H'). Recurrent inhibition was tested prior to, during and after an intravenous administration of a solution containing 2000 mg of L-AC. L-AC administration proved to be able to induce in all subjects a decrease in the H'-reflex. This effect ensued approximately 30 min after onset of L-AC administration, reached the peak after 40 min and vanished in about one hour. The extent of the decrease in H' varied among subjects, being on the average 22% of the control values. A relationship was found between duration of L-AC administration and time for reaching the maximal effect. These results show that L-AC is able to decrease a-motoneurone excitability by increasing Renshaw cell activity, both in normal and spastic subjects.

Acetylcarnitine↗

PubMed1988

Changes in Ia reciprocal inhibition from the peroneal nerve to the soleus alpha-motoneurons with different static body positions in man.

Experiments were conducted in man to evaluate the changes in Ia inhibitory interneurons activated from the anterior tibial muscle and projecting to the soleus alpha-motoneurons in relation to different static body positions. Subjects were fixed to a tilting chair and the effects of body rotation were evaluated at 80 degrees (normal sitting position) and 40 degrees of backward inclination (head supine, nose-up). A test H-reflex was used to assess changes in excitability of the soleus alpha-motoneurons after a conditioning stimulus applied to the deep peroneal nerve. In 5 out of 6 subjects, we observed a significant increase in the reciprocal inhibition after backward inclination of the body (40 degrees) with respect to the control position (80 degrees). Such increase was attributed to facilitation of the Ia inhibitory interneurons projecting to the soleus motoneurons. We consider the possibility that the observed increment in reciprocal inhibition after backward inclination be sustained by variations of tonic vestibular activity.

Adult↗

PubMed1988

Cutaneous control of group I pathways from ankle flexors to extensors in man.

Reciprocal inhibition from the anterior tibial muscle onto antagonist motoneurones of the soleus muscle was studied in normal man under control conditions and after low intensity stimulation of cutaneous afferent fibres from the sole and dorsal region of the ipsilateral and contralateral foot. Ipsilateral cutaneous stimulation increased the reciprocal inhibition to the soleus motoneurones, without qualitative differences between the effect from the sole and that from the dorsal region of the foot. Stimulation of cutaneous afferent fibres from the contralateral foot produced the reverse effect, i.e., depression of the Ia reciprocal inhibition from the tibialis anterior to the soleus motoneurones. No effects could be observed when cutaneous areas other than those of the foot were stimulated. The effects of cutaneous stimulation on the reciprocal inhibition became evident only when this inhibition approached its maximum and, thus, they most strongly influenced its recovery phase. Since cutaneous stimulation does not modify the test reflex when given alone, it is likely that there must be convergence on common premotoneuronal interneurones. Indirect evaluation of central delay suggests that the cutaneous afferent fibres from the foot have oligosynaptic spinal connections with interneurones belonging to the group I pathways to the antagonists. Our findings furnish additional evidence that short-latency inhibition of soleus motoneurones after a single conditioning stimulation of group I afferents from the tibialis anterior muscle constitutes a true example of disynaptic Ia reciprocal inhibition in man.

Adult↗

PubMed1988

Influence of different static head-body positions on spinal lumbar interneurons in man: the role of the vestibular system.

The present experiments were made in man with the aim of studying the possible influences of different head-body tilts on the activity of the interneurons Ia, Ib and the Renshaw cells functionally coupled to the soleus alpha-motoneurons. Subjects were seated on a chair, rotable with respect to the vertical axis, and were studied at 80 degrees and 40 degrees to the horizontal. The excitability of the soleus alpha-motoneurons slightly decreased when the body was placed at 40 degrees of backward inclination whereas the Renshaw cell activity showed a reinforcement of inhibition on the same motoneurons. The reciprocal inhibition from the anterior tibial to the soleus muscle increased at 40 degrees of backward inclination with respect to the control values at 80 degrees. Finally, short-latency homonymous facilitation and inhibition showed no significant change in relation to body position. The results indicate that different head-body positions are able to modify the bias of spinal interneurons in man. We discuss the hypothetical role of the vestibular system in producing the effects seen.

Female↗

PubMed1988

The H reflex recovery curve reinvestigated: low-intensity conditioning stimulation and nerve compression disclose differential effects of presumed group Ia fibres in man.

The recovery curve of the soleus H reflex, evoked by stimulation of the tibial nerve in the popliteal fossa, was studied by applying an electrical conditioning stimulus to the inferior soleus nerve. Under these conditions a long-latency facilitatory phase could be superimposed on a long-lasting inhibition, the excitability cycle being therefore similar to that obtained by means of a paired shock to the tibial nerve. In order to identify the afferent fibres responsible for the effects observed, various conditioning stimulus strengths and nerve compression were used. A low-intensity stimulus induced only a facilitatory phase, while the inhibition promptly ensued on increasing stimulus strength, which remained however subliminal for activation of group II fibres. During calf compression exerted by a sphygmomanometer cuff placed between the conditioning and test stimuli, the facilitatory effects disappeared within 10 to 15 min, and the inhibitory ones disappeared within 25 to 30 min from the onset of compression. Tonic voluntary contraction enhanced both the inhibition and the facilitation. In a subject with complete spinal section, both inhibitory and facilitatory phases could be demonstrated on low-intensity stimulation. The present data, and previous results of ours, allow the following conclusions. (1) Facilitation and inhibition are produced by fibres likely belonging to group Ia spindle afferents. (2) Both effects are of spinal origin. (3) The spinal circuits mediating the effects may be modulated by descending commands. (4) The facilitation is sustained by tonic supraspinal influences while the inhibition is independent of it. Arguments are proposed against the hypotheses that the inhibition be due to transmitter depletion or to presynaptic mechanisms.

Adult↗

PubMed1987

Evidence for Renshaw cell-motoneuron decoupling during tonic vestibular stimulation in man.

The influence of static head-body tilts in the sagittal plane on the activity of Renshaw cells coupled to the soleus extensor alpha-motoneurons was studied in eight human subjects. Head-body rotation was carried out using a tilting seat and its effect was evaluated at 80 degrees (normal sitting position) and at 40 degrees of backward inclination (nose-up). Renshaw cell activity was assessed through a specially designed method of paired H-reflexes first described by Bussel and Pierrot-Deseilligny. alpha-Motoneuron excitability was also independently studied by mapping a reference H-reflex amplitude as a function of static head-body displacements. In almost all subjects Renshaw cell activity was increased at 40 degrees backward inclination with respect to control values at 80 degrees. These changes were attributed to the tonic labyrinthine reflexes capable of decoupling Renshaw cell activity from their motoneurons when the body was tilted backward from the upright position. We discuss the hypothetical functional modalities of the recurrent inhibitory circuit during postural adjustments elicited by labyrinthine input.

Electric Stimulation↗