1,721,323 research outputs found
TMS investigations into the task-dependent functional interplay between human posterior parietal and motor cortex
No full textTranscranial magnetic stimulation (TMS) can be used in two different ways to investigate the contributionof cortical areas involved in grasp/reach movements in humans. It can produce “virtual lesions” thatinterfere with activity in particular cortical areas at specific times during a task, or it can be used in a twincoil design to test the excitability of cortical projections to M1 at different times during a task. The formermethod has described how cortical structures such as the ventral premotor cortex (PMv), dorsal premotorcortex (PMd) and the anterior intraparietal sulcus (aIPS) are important for specific aspects of reaching,grasping and lifting objects. In the latter method, a conditioning stimulus (CS) is first used to activateputative pathways to the motor cortex from, for example, posterior parietal cortex (PPC) or PMd, whilea second, test stimulus (TS), delivered over the primary motor cortex a few ms later probes any changesin excitability that are produced by the input. Thus changes in the effectiveness of the conditioning pulsegive an indication of how the excitability of the connection changes over time and during a specifictask. Here we review studies describing the time course of operation of parallel intracortical circuits andcortico-cortical connections between the PMd, PMv, PPC and M1, thus demonstrating that functionalinterplay between these areas and the primary motor cortices is not fixed, but can change in a highlytask-, condition- and time-dependent manne
Galvanic vestibular stimulation evokes short-latency EMG responses in human masseter muscles
No full textA short latency vestibulomasseteric reflex evoked by electrical stimulation over the mastoid in healthy humans
No full textWe describe EMG responses recorded in active masseter muscles following unilateral and bilateral electrical vestibular stimulation (EVS, current pulses of 5 mA intensity, 2 ms duration, 3 Hz frequency). Averaged responses in unrectified masseter EMG induced by unilateral EVS were examined in 16 healthy subjects; effects induced by bilateral (transmastoid) stimulation were studied in 10 subjects. Results showed that unilateral as well as bilateral EVS induces bilaterally a clear biphasic response (onset latency ranging from 7.2 to 8.8 ms), that is of equal amplitude and latency contra- and ipsilateral to the stimulation site. In all subjects, unilateral cathodal stimulation induced a positive-negative response termed p11/n15 according to its mean peak latency; the anodal stimulation induced a response of opposite polarity (n11/p15) in 11/16 subjects. Cathodal responses were significantly larger than anodal responses. Bilateral stimulation induced a p11/n15 response significantly larger than that induced by the unilateral cathodal stimulation. Recordings from single motor units showed that responses to cathodal stimulation corresponded to a brief (2-4 ms) silent period in motor unit discharge rate. The magnitude of EVS-induced masseter response was linearly related to current intensity and scaled with the mean level of EMG activity. The size of the p11/n15 response was asymmetrically modulated when subjects were tilted on both sides; in contrast head rotation did not exert any influence. Control experiments excluded a possible role of cutaneous receptors in generating the masseter response. We conclude that transmastoid electrical stimulation evokes vestibulomasseteric reflexes in healthy humans at latencies consistent with a di-trisynaptic pathway
Depression of Renshaw recurrent inhibition by activation of corticospinal fibres in human upper and lower limb.
Full text link1. 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
Distribution of Ia effects onto human hand muscle motoneurones as revealed using an H reflex technique.
Full text link1. 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
Relaxation from a voluntary contraction is preceded by increased excitability of motor cortical inhibitory circuits.
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Transcranial magnetic stimulation investigations of reaching and grasping movements
No full textTranscranial magnetic stimulation (TMS) has emerged as a suitable technique to investigate
the network of cortical areas involved in human grasp/reach movements. Applied over the
primary motor cortex (M1), TMS reveals the pattern of activation of different muscles
during complex reaching-to-grasp tasks. Repetitive TMS (rTMS) used to induce “virtual
lesions” of other cortical areas has allowed investigation of other cortical structures such as
the ventral premotor cortex (PMv), dorsal premotor cortex (PMd) and the anterior intraparietal sulcus (aIPS). Each of these appears to contribute to specific aspects of reaching,
grasping and lifting objects. Finally, twin-coil TMS studies can illustrate the time course of
operation of parallel intracortical circuits that mediate functional connectivity between the
PMd, PMv, the posterior parietal cortex and the primary motor cortices.Transcranial magnetic stimulation (TMS) has emerged as a suitable technique to investigate the network of cortical areas involved in human grasp/reach movements. Applied over the primary motor cortex (M1), TMS reveals the pattern of activation of different muscles during complex reaching-to-grasp tasks. Repetitive TMS (rTMS) used to induce “virtual lesions” of other cortical areas has allowed investigation of other cortical structures such as the ventral premotor cortex (PMv), dorsal premotor cortex (PMd) and the anterior intraparietal sulcus (aIPS). Each of these appears to contribute to specific aspects of reaching, grasping and lifting objects. Finally, twin-coil TMS studies can illustrate the time course of operation of parallel intracortical circuits that mediate functional connectivity between the PMd, PMv, the posterior parietal cortex and the primary motor cortices.: Introduction: The ease with which we can make reach-to-grasp movements conceals a good deal of the underlying co..
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