Biomedical subjects

R Forget

Publications and source records attributed to R Forget.

28 records · Page 2Linked to original sources

PubMed1991

Evidence for a contribution of the motor cortex to the long-latency stretch reflex of the human thumb.

1. In normal subjects, transcranial magnetic stimulation of the hand region of the motor cortex evokes motor responses only in contralateral hand muscles at a latency of about 19-24 ms. In contrast, stimulation of the motor cortex of three mirror movement subjects evoked, nearly simultaneously, motor responses in hand muscles on both sides of the body at latencies similar to those of normal subjects. In these subjects no other neuroanatomical pathways appear to be abnormally directed across the mid-line. Thus, their mirror movements are probably due to a projection of the corticospinal tract to homologous motoneurone pools on each side of the body. 2. We reasoned that if the motor cortex contributes to the generation of long-latency stretch reflex responses then in these mirror movement subjects stretching a muscle on one side of the body should produce long-latency reflex responses in the ipsilateral and the homologous contralateral muscle. 3. To test this idea experiments were done on normal human subjects and on the subjects with mirror movements. The electromyographic (EMG) activity of the flexor pollicis longus muscle (FPL) on each side of the body was recorded. Stretch of the distal phalanx of the thumb of one hand produced a series of distinct reflex EMG responses in the ipsilateral FPL. The earliest response, when present, began at 25 ms (S.D. = 3.5 ms) and was followed by responses at 40 (S.D. = 3.9 ms) and 56 ms (S.D. = 4.3 ms). There was no difference, either in timing or intensity, between the ipsilateral FPL EMG responses of normal subjects and those of the mirror movement subjects. 4. No response of any kind was observed in the contralateral (unstretched) FPL of normal subjects. In contrast, we observed in all three mirror movement subjects EMG responses in the contralateral (unstretched) FPL beginning at 45-50 ms. The latency of this response is considerably shorter than the fastest voluntary kinaesthetic reaction time, which was on average 130 ms (S.D. = 11 ms). The contralateral long-latency EMG response observed in the mirror movement subjects was on average 30% (range 5-60%) of that on the ipsilateral side. No short-latency response (25 ms) was ever observed in the contralateral FPL of these subjects. 5. These observations are quite consistent with the idea that the long-latency stretch reflex responses of hand and finger muscles are produced, at least in part, by the motor cortex.

Electromyography↗

PubMed1990

Anticipatory postural adjustment in the absence of normal peripheral feedback.

The response to voluntary unloading (by the subject's contralateral arm) and imposed unloading (by the experimenter) of a 1 kg weight supported at the wrist were studied in normal volunteers (controls) and in a deafferented subject (patient). The patient had no touch, pressure or kinesthetic sensations in either of the arms, but the motor nerve fibers were unaffected. The reflex activity generated by imposed unloading in the controls was never observed in the patient. The displacement's amplitude of the unloaded forearm was 3 x smaller with voluntary than with imposed unloading. In both the controls and the patient, the displacement was of similar amplitude and preceded by an anticipatory postural adjustment. It is concluded that this postural adjustment is of central origin since it can be generated in the absence of peripheral feedback.

Adult↗

PubMed1989

Facilitation of quadriceps motoneurones by group I afferents from pretibial flexors in man. 1. Possible interneuronal pathway.

The neuronal pathway of the facilitation of quadriceps (Q) motoneurones (MNs) evoked by stimulation of the common peroneal nerve (CPN) has been reinvestigated using both the post-stimulus time histogram (PSTH) method for measurement of the firing probability of individual units and the H reflex technique. It has been found that Ia (and to an unknown extent Ib) afferents from pretibial flexors - but not from peroneal muscles - are responsible for this excitation. The central latency of the CPN-induced excitation of Q MNs was estimated to be 3-3.7 ms longer than that of their monosynaptic Ia excitation. To further investigate the neuronal pathway of the CPN-induced excitation the spatial facilitation technique was used, the effects on the Q H reflex of two conditioning stimuli (applied to the CPN and the femoral nerve - FN) being compared when applied separately and together. When the two conditioning volleys were timed to reach the spinal cord simultaneously the facilitation of the H reflex on combined stimulation was larger than the algebraic sum of the effects by separate stimuli in 40% of the cases. It is argued that this additional facilitation reflects summation at a premotoneuronal level and it is concluded that non-monosynaptic Ia excitation of Q MNs from Q and pretibial flexors is, at least partly, mediated through a common pathway. In those individual units in which stimulation of the FN and/or the CPN evoked a non-monosynaptic Ia excitation, this excitation was reduced on combined stimulation of the two nerves. It is argued that this reflects inhibition of the interneurones mediating the excitation, i.e. consists in a disfacilitation of the MNs. It is suggested that the non-monosynaptic (homonymous and heteronymous) Ia excitation of Q MNs in man (and the inhibition of this excitation) is mediated through a system of neurones similar to the system recently described in the cat by Edgley and Jankowska (1987).

Adult↗

PubMed1989

Facilitation of quadriceps motoneurones by group I afferents from pretibial flexors in man. 2. Changes occurring during voluntary contraction.

The facilitation of the quadriceps (Q) H reflex evoked by stimulation of group I afferents from pretibial flexors exhibits biphasic changes during Q voluntary contraction. At short conditioning-test intervals the facilitation is increased, whereas it is decreased at longer intervals and/or at high conditioning stimulus intensities and/or when the contraction strength is increased. The spatial facilitation at a premotoneuronal level observed at rest on combined stimulation - common peroneal nerve (CPN) and femoral nerve (FN) - regularly disappeared during contraction. It is argued that the increase in facilitation at the onset of the weakest Q contraction reflects a descending facilitation of the interneurones mediating the CPN-induced excitation to Q MNs. An occlusion of descending and peripheral excitation in these interneurones can easily account for the depression of the facilitation when the conditioning stimulus intensity and/or the strength of the contraction is increased. However, occlusion cannot explain all the results and inhibition of the neurones mediating excitation of MNs, i.e. disfacilitation of the MNs, probably contributes to the decrease in facilitation observed during contraction.

Adult↗

PubMed1987

Rapid elbow flexion in the absence of proprioceptive and cutaneous feedback.

Rapid goal-directed movements of elbow flexion were studied in normal human subjects and in patients deprived of proprioceptive and cutaneous feedback. All normal subjects showed a burst of electromyographic (EMG) activity in the extensor muscle (antagonist) that served to arrest the limb precisely in the target zone. The magnitude of this burst co-varied with the magnitude of the initial accelerating burst in the flexor muscle (agonist). In patients, there was a small decelerating burst poorly correlated with the agonist activity. All patients had difficulty to control the amplitude of their movements due to improper adjustment of the size and time of onset of the decelerating burst. It is concluded that the central nervous system can generate a sequence of commands to accelerate and decelerate a limb in the absence of peripheral feedback. However, information from the moving limb is required to adjust the magnitude and time of onset of deceleration.

Adult↗

PubMed1986

Electromyographic studies of congenital mirror movements.

We studied two patients with congenital mirror movements. In one, the movements were associated with impaired sensation and a cervicodorsal meningocele. In the other, no abnormality of the nervous system was found. In an EMG study, the normal temporal characteristics, response latency, duration, and amount of EMG on the normal and mirror sides suggest that similar motor commands are responsible for both voluntary and mirror movements.

Adolescent↗

PubMed1985

Initial agonist burst duration changes with movement amplitude in a deafferented patient.

Changes in the duration of the initial agonist burst were studied in a deafferented human. The patient had been functionally deafferented for five years, having no touch, vibration, pressure or kinesthetic sensation nor any tendon reflexes in the four limbs. Pain and temperature sensation were intact and motor fibres were unaffected. The subject made visually guided step-tracking movements using flexion/extension movements about the elbow. Initial agonist burst duration increased with movement amplitude. Burst duration was approximately 65 ms in small movements (6-12 deg) increasing to 136 ms in intermediate (36 deg) and 200 ms in large (54 and 60 deg) movements. Similar changes in initial burst duration with movement amplitude were seen when the subject made non-visually guided movements. It is concluded that the duration of the initial agonist burst is centrally determined.

Adult↗

PubMedDate not supplied

The modulation of somatosensory evoked potentials during the foreperiod of a forewarned reaction time task.

During the foreperiod of a forewarned reaction time (RT) task reflexes in the executing limb increase to a lesser extent than those in the contralateral limb. This is possibly due to input modulation. The present study investigates the possibility of cutaneous sensory modulation during motor preparation by studying the amplitudes of somatosensory evoked potentials (SEPs). Eighteen subjects performed a forewarned RT task with the same fingers as the ones which were electrically stimulated. SEPs evoked during the 4 sec preparatory period were compared to those evoked during movement execution and during the resting period after the motor response respectively. During response execution most SEP components showed smaller amplitudes, i.e., they were gated, which agrees with other studies. In the first part of the foreperiod no SEP modulation was observed. Towards the end of the foreperiod, 500 msec before the response stimulus (RS), the amplitude of the contralateral parietal N70-P100 was significantly decreased, while the P45-N70 showed a similar tendency. However, at the same time the P100-N140 was increased in amplitude. The decrease of the intermediate latency components towards the end of the foreperiod is discussed in terms of gating, while the increase in the long latency component is discussed with respect to a decrease in RT on trials where the fingers were stimulated just before the RS, pointing to the role of attentional mechanisms.

Adult↗

PubMedDate not supplied

Multidirectional analysis of maximal voluntary contractions of the thumb.

A multidirectional dynamometer was used to investigate the maximal static force (MSF) exerted at the proximal phalanx of the dominant and nondominant thumbs of 12 normal women subjects. These MSFs were measured in two sessions within a two-week interval. Eight directions covering 360 degrees, at increments of 45 degrees within the transverse plane of the longitudinal axis of the thumb, were examined. A three-way analysis of variance (ANOVA) with repeated measures (factors: direction, session, handedness) was performed. No significant interaction was found between the main factors. The ANOVA showed that MSFs vary according to the direction of effort. No significant difference, however, was observed between the dominant and nondominant thumbs or between the MSFs obtained in the two sessions. Post-hoc analysis revealed that MSFs exerted in directions containing an abduction component were significantly lower than MSFs exerted in the other directions. It is concluded that directional strength of the thumb is maximized for functional grasping activities and reflects the anatomic distribution of muscles acting at the thumb.

Adult↗

PubMedDate not supplied

Interaction between attention demanding motor and cognitive tasks and static postural stability.

BACKGROUND: Due to the often-reported decrease in postural stability in the elderly, it is important to understand factors that may contribute to reduced postural stability. It is possible that attention-demanding focal tasks performed concurrent with postural regulation influence postural stability. OBJECTIVE: This study utilized dual-task methodology to determine if motor or cognitive focal tasks interact with center of pressure (COP) excursion during static bipedal stance in healthy young and healthy elderly subjects (n = 18). METHODS: The cognitive task involved silently solving an orally-presented multi-step arithmetic problem over a 30-second period. The motor task was a 30-second bilateral static finger-thumb pinch task performed at 10% of maximal voluntary contraction with a pair of pinch-force transducers. Each focal task was performed separately, and in a condition in which both tasks were performed simultaneously. COP excursion was compared in quiet standing (no focal task) and during performance of the focal tasks with full vision and with vision occluded. RESULTS: Performance on the focal tasks was unaffected by increased postural demands during stance as compared to a seated baseline condition. This was the case for both age groups, and for the full vision and occluded vision conditions. Medio-lateral COP excursion was reduced over the quiet standing pretest condition when attentional focus was on the cognitive task, suggesting that COP was influenced centrally during cognition. In contrast, COP excursion increased over the quiet standing pretest condition when performing the motor focal task, suggesting a reduced ability to suppress sway when the motor system was concurrently occupied with a voluntary task that shared the same input-output resources. CONCLUSION: The ability to share attentional resources among focal and postural tasks was similar in healthy young and elderly subjects.

Adult↗