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PubMed1986

The supplementary motor area modulates perturbation-evoked discharges of neurones in the precentral motor cortex.

The hypothesis was tested that the supplementary motor cortex (SMA) may influence the responsiveness of area 4 neurones to kinesthetic stimuli. In the awake monkey, responses to arm displacements were recorded with and without conditioning intracortical stimulation of the SMA. In 14 of 26 tested area 4 neurones, there was an increase of the response latency and/or a decrease of the response magnitude when the peripheral stimulus was conditioned by SMA stimulation. Field potentials evoked by the displacements were reduced in 3 out of 7 recordings. These findings suggest that the SMA exerts subtle inhibitory effects on the motor cortex or its inputs.

Animals↗

PubMed1990

Formation of new synapses in the cat motor cortex following lesions of the deep cerebellar nuclei.

The effects of unilateral lesions of the deep cerebellar nuclei on the corticocortical (CC) projection from the somatosensory to the motor cortex were studied in adult cats, utilizing electrophysiological and electron microscopical methods. Axon terminals in the motor cortex belonging to CC afferents were labeled by degeneration induced by lesions of the somatosensory cortex; neurons in the motor cortex were labeled by the Golgi/EM method. In each cat, data from the motor cortex (MCx) contralateral (experimental) and ipsilateral (control) to the cerebellar lesion were compared. Cerebellar lesions produced marked motor deficits, which receded gradually and disappeared after 30 to 40 days. Subsequent lesions of the somatosensory cortex (area 2) contralateral to the cerebellar lesions resulted in the reappearance of the cerebellar symptoms. The number of CC synapses per unit area in experimental MCx was significantly higher than in control MCx. The increase in the number of CC synapses was apparent throughout layers II-V of the MCx, but was most prominent in layers II/III. The increase in the number of CC synapses in experimental MCx was due mainly to an increase of axon terminals synapsing with dendritic spines belonging to pyramidal neurons. In comparison, the numbers and spatial distribution of CC synapses with aspinous, nonpyramidal neurons from both experimental and control MCx were similar. Field potentials in the experimental MCx, evoked by stimulation of area 2, were altered following cerebellar lesions. In experimental MCx, the polarity of the early component of the field potentials reversed at cortical depths corresponding to layers II-III, whereas this reversal was not observed in control MCx. These findings suggest that lesions of the cerebellar nuclei induced sprouting of axon terminals in the MCx to establish a new function. The results provide the first anatomical evidence for the generation of new synapses in the adult CNS which is not induced by elimination of existing synapses.

Animals↗

PubMed1999

Motor cortex modulation of exteroceptive information at bulbar and thalamic lemniscal relays in the cat.

We have studied the control of the primary motor cortex on the activity of lemniscal neurons in semi-chronic implanted cats. In each experiment, two to three foci in the primary motor cortex were identified by intracortical microstimulation at low threshold (up to 20 microA) for their capacity to evoke movements of contralateral single limb joints. Neurons belonging to the dorsal column nuclei (main cuneate nucleus and gracile nucleus), or to the ventral posterolateral nucleus, were sampled for their response to stimulation of the peripheral cutaneous fields, as well as the antidromic response to stimulation of the contralateral medial lemniscus and ipsilateral somatosensory cortex, respectively. These neurons were then tested for stimulation of the cortical foci using a current intensity equal to the threshold needed to evoke motor effects, although we reduced the duration of the stimulating trains; thus, we avoided evoking movements which could elicit afferent volleys along the somatosensory paths. It was found that the primary motor cortex was able to modulate the transmission of exteroceptive signals at the level of both dorsal column nuclei and ventral posterolateral nucleus with analogous modalities. In particular: (i) a high percentage of responses, with a prevalence of excitatory effects, was observed when the receptive field of the neurons topographically corresponded to, or was very close to, the joint controlled by a given cortical focus; (ii) in these cases, higher percentages of excitations were observed in tests which concerned the distal segments of limbs than the proximal segments; (iii) the percentage of responses became lower as the neuronal receptive field was located further from the cortical motor target, the pattern being more frequently inhibitory in nature. From a functional point of view, the motor cortex control appears to be organized in a very precise manner. Its excitatory nature might subserve integrative mechanisms by which exteroceptive information arising in a given limb segment would be enhanced by a motor command inducing movements of the same body part. Moreover, a better definition of the afferent input could be obtained by a simultaneous depression of neurons, which send towards the cortex signals from adjacent or more distant cutaneous regions. It can be hypothesized that such an organization of the cortical control could improve the discriminative somatosensory aspects during the execution of explorative movements, besides supplying a sharper cutaneous feedback to the motor cortex.

Animals↗

PubMed2001

Abnormal postexcitatory and interhemispheric motor cortex inhibition in writer's cramp.

Focal transcranial magnetic stimulation (TMS) of the motor cortex was used to study excitatory and inhibitory stimulation effects in 25 patients with writer's cramp and 25 healthy volunteers. We investigated excitatory and inhibitory corticospinally mediated motor effects in muscles contralateral to the stimulation side as well as interhemispheric inhibition of tonic motor activity in muscles ipsilateral to stimulation. Motor evoked potentials (MEPs) were recorded from both first dorsal interosseus muscles. Motor thresholds at rest and amplitudes and latencies of MEPs obtained during maximal contraction were always bilaterally normal. The duration of postexcitatory inhibition was significantly shortened (168+/-55 vs. 198+/-39 ms in normal subjects, P=0.001) and the duration of interhemispheric inhibition prolonged (30.3+/-6.6 vs. 26+/-3.9 ms in normal subjects, P < 0.001). Both observations would be compatible with a decreased inhibition of corticospinal and transcallosal outputs of the motor cortex. The results were not influenced by fatigue effects. Abnormal motor cortex inhibition seems to be a generalized phenomenon in writer's cramp since it was detected in both hemispheres and during a simple isometric motor task which did not evoke dystonic symptoms.

Adult↗

PubMed2000

Facilitatory I wave interaction in proximal arm and lower limb muscle representations of the human motor cortex.

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits direct and indirect (I) waves in the corticospinal tract. Facilitatory I wave interaction has been demonstrated with a suprathreshold first stimulus (S1) followed by a subthreshold to threshold second stimulus (S2). Intracortical inhibition (ICI) and intracortical facilitation (ICF) can be studied by another paired TMS paradigm with a subthreshold conditioning stimulus (CS) followed by a suprathreshold test stimulus. Facilitatory I wave interaction in motor representations other than the hand area and its relationship to ICI and ICF has not been studied. We studied I wave interaction, ICI and ICF in an intrinsic hand muscle (abductor pollicis brevis, APB), in a proximal arm muscle (biceps brachii, BB) and in a lower limb muscle (tibialis anterior, TA) in 11 normal subjects. I wave facilitation was studied by paired TMS at 24 interstimulus intervals (ISIs) from 0.5 to 5.1 ms. For APB and TA, facilitation occurred in three distinct peaks at ISIs of 0.9-1.7, 2. 5-3.5, and 4.1-5.1 ms. For BB, facilitation was significant for the first two peaks. The latencies of the peaks were similar among different muscles, but the magnitude of facilitation was much greater for APB and TA compared with BB. For all three muscles, changing the S2 to transcranial electrical stimulation (TES) resulted in much less facilitation of the first peak. For APB, there was significant I wave facilitation with S2 at 72% motor threshold (MT). The same stimulus used as the CS did not elicit ICF at ISI of 15 ms, suggesting that the threshold for eliciting I wave facilitation is lower than that for ICF. For BB and TA, there was no I wave facilitation with S2 at 90% of APB MT, and the same stimulus used as CS led to ICI. Thus in BB and TA the threshold for eliciting ICI is lower than that for I wave facilitation. We conclude that the circuits that mediate I wave interactions are present in the proximal arm and lower limb representations of the motor cortex. I wave facilitation occurs predominately in the cortex and may be primarily related to the monosynaptic corticomotoneuronal (CM) system. The reduced I wave facilitation for BB compared with APB and TA may be related to less extensive CM projection and involvement of other polysynaptic descending pathways. I wave facilitation, ICI, and ICF appears to be mediated by different neuronal circuits.

Adult↗

PubMed1997

Treatment of deafferentation pain by chronic stimulation of the motor cortex: report of a series of 20 cases.

Twenty patients with deafferentation pain were treated by chronic stimulation of the motor cortex. The central fissure was localized using stereotactic MRI and the motor cortex was mapped using intra-operative somatosensory evoked potentials. Seven patients with trigeminal neuropathic pain experienced definite pain relief varying between 40 and 100%. Ten patients had central pain secondary to central nervous system lesions. A satisfactory long-lasting pain control (pain relief > 40%) was obtained in five of them (50% of cases). One patient with pain from peripheral nerve injury obtained more than 80% pain relief. Two patients had pain from spinal cord lesions. One did not respond but the other obtained an excellent long-term result. The location of the effective stimulation plots was in agreement with the somatotopic maps of the primary motor cortex. One patient developed a small extradural haematoma which resolved spontaneously. None of the patients developed seizure activity. This study confirms the potential value of motor cortex stimulation in the treatment of certain forms of intractable pain, especially in cases with trigeminal neuropathic pain.

Adult↗

PubMed2002

Glutamate induces different neuronal conditioned responses than ACPD when used as a locally ionophoresed unconditioned stimulus in the cat motor cortex.

Single unit recordings were made from the motor cortex of conscious cats with glass micropipettes that allowed ionophoretic application of 0.5 M glutamate in 2 M NaCl or 0.5 M ACPD (1S,3R-1-amino-cyclopentane-1,3-dicarboxylic acid, a mGluR agonist) in 2 M NaCl. Activity in response to a 70 dB click (1 ms rectangular pulse to loudspeaker) was studied before, during, and immediately after applying each agent locally as a paired US (90 nA current 570 ms after click for 300 ms in combination with glabella tap). A 70 dB hiss sound was presented 4.4 sec after the click as a discriminative stimulus (DS). CS and DS were presented 10 times initially (adaptation); then CS, US plus tap, and DS (approximately 10 times as conditioning); and then CS and DS (2-10 times to test post-conditioning). Glutamate potentiated the mean, early, 8-16 ms response to the click after conditioning (t=18.2, p<0.0001), but not the baseline activity which decreased from a mean of 17 spk/sec to 7 spk/sec (t=3.71, p<0.001). Baseline activity increased to 31 spk/sec when glutamate was applied during conditioning (t=3.30, p<0.005). ACPD reduced the intermediate, 64-72 ms response to the click after conditioning (t=8.18, p<0.0001), and potentiated the late 104-112 ms response (t=15.4, p<0.0001). Baseline activity was slightly increased after conditioning with ACPD. Saline did not potentiate the response to click. The results indicate that glutamate agonists that differ in their receptor affinities can induce different CRs when used as locally applied USs to condition neuronal responses to a click CS in the motor cortex of cats.

Acoustic Stimulation↗

PubMed2005

On the relations between single cell activity in the motor cortex and the direction and magnitude of three-dimensional dynamic isometric force.

The role of the motor cortex in the control of both the direction and magnitude of dynamic force, when both are allowed to vary in 3D, is not known. We recorded the activity of 504 cells in the motor cortex of two monkeys during a behavioral task in which the subjects used a manipulandum to vary both the direction and magnitude of isometric force in 3D space. The majority (86%) of cells active in the task related to the direction, a tiny number (2.5%) to the magnitude, and a moderate number (11.5%) to both the direction and magnitude of dynamic force output. Finally, we compared neural activity in the same population of neurons during dynamic and static force output and found that the relations to direction and magnitude were very similar in both epochs. Our results indicate that during dynamic force production, cells in the motor cortex are primarily concerned with specifying the direction of force. The magnitude signal is not prominent in motor cortex neurons, and in general, magnitude and direction of force are specified together. Furthermore, the data suggest that the control of static and dynamic motor systems is based, to a great extent, on a common control process.

Action Potentials↗

PubMed1996

Intraoperative myogenic motor evoked potentials induced by direct electrical stimulation of the exposed motor cortex under isoflurane and sevoflurane.

We monitored myogenic motor evoked potentials (MEPS) during intracranial surgery in 21 patients anesthetized with nitrous oxide in oxygen, fentanyl, and 0.75-1.5 minimum alveolar anesthetic concentration (MAC) isoflurane (n = 11) or sevoflurane (n = 10). The exposed motor cortex was stimulated with a single or train-of-five rectangular pulses at a high frequency (500 Hz), while the compound muscle action potentials (CMAPS) were recorded from the abductor pollicis brevis muscle. Neuromuscular block was monitored by recording the CMAPs from the abductor pollicis brevis muscle in response to electrical stimulation of the median nerve at the wrist (M-response). Stimulation of the motor cortex with a single pulse elicited MEPs in none of the patients, while stimulation with a train-of-five rectangular pulses at high frequency elicited MEPs in all patients. The relationship between MEP amplitude and the level of neuromuscular block induced by vecuronium infusion was evaluated in seven patients. For comparison of the individual measurements, the MEP amplitude at a M-response amplitude of 100% was calculated by means of the individual regression curve as 100% of MEP amplitude. There was a linear correlation between percent MEP amplitude and percent M-response amplitude (r = 0.81; P < 0.01). Intraoperative monitoring of MEP could be performed at a M-response amplitude above 90 % of the baseline value in 10 patients and at a M-response amplitude of 20%-50% of the baseline value in 11 patients. During monitoring of the 21 patients, MEPs did not change in 18 patients and disappeared in two patients. In the remaining patient, MEP amplitudes were attenuated to approximately 10% of the baseline value and recovered after cessation of surgical manipulation. In the two patients in whom MEPs disappeared, motor paresis developed postoperatively. We conclude that 1) intraoperative myogenic MEP monitoring is feasible during isoflurane or sevoflurane anesthesia if stimulation is performed with a short train of rectangular pulses, and 2) that electromyographic monitoring of neuromuscular block is useful to assess intraoperative MEP changes under partial neuromuscular block.

Action Potentials↗

PubMedDate not supplied

Long-term potentiation of the late NMDA-dependent components of neuron responses in the cat motor cortex to stimulation of the direct cortical input from field 5 of the parietal cortex.

Bicuculline-filled microelectrodes were used to record responses to weak stimulation of the parietal cortex in field 5 of the motor cortex of anesthetized cats, and revealed late excitatory responses of neurons similar to those seen in the motor cortex of conscious cats in response to conditioned stimulation of the parietal cortex triggering a conditioned reflex consisting of placing the paw on a support. Tetanic stimulation of the parietal cortex (10-20 sec, 100 Hz) in the same conditions evoked long-term potentiation of late responses, resulting in the formation and enhancement of responses, along with decreases in the latent period of responses.

Afferent Pathways↗

PubMed1985

Trained slow tracking. II. Bidirectional discharge patterns of cerebellar nuclear, motor cortex, and spindle afferent neurons.

Single-unit discharge was recorded in the dentate and interposed cerebellar nuclei, motor cortex, and C7 and C8 dorsal root ganglia during trained, slow hold-ramp-hold tracking, rapid alternating movement, torque-pulse perturbation, and action tremor of the monkey's wrist. Fifty-seven dentate and 45 interposed neurons were found in two monkeys that discharged in relation to slow tracking movement. Nearly all neurons had a distinct bidirectional pattern of discharge consisting of an abrupt increase (or decrease) in firing frequency at or before the onset of movement that was variably maintained throughout the ramp and was independent of movement direction. None of the neurons showed a clear relationship to direction, position, velocity, or load during the performance of this task. Nevertheless, many of these neurons discharged in relation to rapid alternation and (for interpositus) torque pulses in patterns that were directionally reciprocal. Some interpositus neurons showed a modulation related to tremor superimposed on the bidirectional discharge related to slow ramps. Twenty-nine neurons in motor cortex of one monkey discharged during slow hold-ramp-hold tracking in two patterns. Class I neurons (14 of 29) showed gradually changing, directionally reciprocal modulations of firing frequency for movements in opposite directions. These neurons were often related to torque load and/or to wrist position but not to velocity. The discharge pattern was similar to the pattern of activity of forearm muscles. Class II neurons (15 of 29) showed an abrupt change in firing frequency that was bidirectional. They were often related to torque load and/or to velocity but not to position. Motor cortex neurons discharged in relation to rapid alternating movements, torque pulses, and tremor in similar patterns that did not distinguish the two classes. Five units in dorsal root ganglia were identified as muscle spindle afferents. During ramps, their pattern of discharge was bidirectional and resembled the bidirectional discharge patterns of neurons in motor cortex (class II) and cerebellum. For some cells the bidirectional pattern varied slightly in relation to the direction and velocity of movement and the amount of torque load, but it was not related to the large changes in wrist position (muscle length). Modulation in relation to tremor was superimposed on the bidirectional pattern related to ramps. The comparison of spindle afferent discharge with the concurrent electromyogram (EMG) of the parent muscle suggested that spindles were driven by gamma-fusimotor activity dissociated from that of homonymous alpha-skeletomotor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1999

Real-time control of a robot arm using simultaneously recorded neurons in the motor cortex.

To determine whether simultaneously recorded motor cortex neurons can be used for real-time device control, rats were trained to position a robot arm to obtain water by pressing a lever. Mathematical transformations, including neural networks, converted multineuron signals into 'neuronal population functions' that accurately predicted lever trajectory. Next, these functions were electronically converted into real-time signals for robot arm control. After switching to this 'neurorobotic' mode, 4 of 6 animals (those with > 25 task-related neurons) routinely used these brain-derived signals to position the robot arm and obtain water. With continued training in neurorobotic mode, the animals' lever movement diminished or stopped. These results suggest a possible means for movement restoration in paralysis patients.

Animals↗

PubMed2000

Loss of the innate cortical engram for action patterns used in skilled reaching and the development of behavioral compensation following motor cortex lesions in the rat.

Damage to the motor cortex of the rat (Rattus norvegicus) impairs skilled movements used in reaching for food with the contralateral forepaw. Nevertheless, there is substantial recovery in success over a two-week postsurgical period. The profile of behavioral recovery is believed to reflect the eventual normalization of behavior, but this idea has not been explicitly examined. The present experiments examined postsurgical reaching success and reaching movements as a function of (1) lesion type, (2) lesion size, (3) lesion location, (4) depletion of forebrain noradrenaline, and (4) presurgical and postsurgical experience. The results show that at least two separate processes contribute to recovery in postsurgical performance. The early postsurgical period was characterized by extreme difficulties in making reaching movements. The experiments suggest that this initial impairment was due to the loss of the innate cortical engram that supports the action patterns used for skilled movements. Subsequent recovery in reaching success was not due to the reacquisition of normal movements, but was due rather to the use of compensatory movements. The results are discussed in relation to the idea that true recovery from motor cortex injury will require that damaged neurons and their connections be rescued or replaced.

Animals↗

PubMed1994

Non-NMDA receptors in motor neuron disease (MND): a quantitative autoradiographic study in spinal cord and motor cortex using [3H]CNQX and [3H]kainate.

The distribution and density of non-NMDA receptors in spinal cord and motor cortex was compared in 10 cases of motor neuron disease (MND) and 8 neurologically normal controls by quantitative autoradiography using [3H]CNQX and [3H]kainate. In the motor cortex of MND cases, an increased density of [3H]kainate binding sites was observed which was most marked in the deep layers. No significant differences were observed in [3H]CNQX binding in the motor cortex between MND and control cases. In the spinal cord significantly increased densities of both [3H]CNQX and [3]kainate binding sites were found in the substantia gelatinosa and the intermediate grey matter in the MND group. The changes in [3H]kainate binding were observed only in the amyotrophic lateral sclerosis (ALS) subgroup of MND, while the changes in [3H]CNQX binding in the spinal cord were more marked in ALS compared to progressive muscular atrophy (PMA) cases. These findings provide evidence in support of a disturbance of glutamatergic neurotransmission in MND and suggest that there may be an increased excitatory drive to motor neurons via non-NMDA receptors. It is unclear at present whether the changes observed represent a compensatory response to loss of motor neurons in MND or a pathophysiological phenomenon contributing to motor neuron degeneration. Modulation of non-NMDA receptor activity may represent a possible target for therapeutic intervention in this disease.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMedDate not supplied

Specific patterns of intrinsic connections between representation zones in the rat motor cortex.

The organization of intrinsic connections in rat motor cortex was studied by combining microstimulation and tract-tracing techniques. Maps of forelimb and vibrissal movements were constructed from the distribution of cortical sites from which movements were evoked in response to intracortical microstimulation. Then, a single injection of a fluorescent dextran was placed into either a vibrissal or a wrist representation zone, or into a region bordering these zones, resulting in anterograde labeling of long intrinsic, horizontal axons. Following injection into the vibrissal area, axons were largely restricted to the whisker representation zone and to the border region with the forelimb representation. Injections into a wrist zone labeled projections largely restricted to the forelimb area and to the border with the vibrissal area. Injections into a border region labeled dense projections throughout most of the forelimb and vibrissal areas. These findings indicate that intrinsic axon collaterals in the motor cortex form specific and extensive connections among representation zones related to movements of the same body part. These connections may be involved in the coordination of activity in different representation zones for the execution of complex movement patterns. The projection of axon collaterals into border regions may be the anatomical substrate for the rapid reorganization of motor cortical maps that occurs following various experimental manipulations.

Animals↗

PubMed2000

Modulation of single motor unit discharges using magnetic stimulation of the motor cortex in incomplete spinal cord injury.

OBJECTIVES: Motor evoked potentials (MEPs) and inhibition of voluntary contraction to transcranial magnetic stimulation (TMS) of the motor cortex have longer latencies than normal in patients with incomplete spinal cord injury (iSCI) when assessed using surface EMG. This study now examines the modulation of single motor unit discharges to TMS with the aim of improving resolution of the excitatory and inhibitory responses seen previously in surface EMG recordings. METHODS: A group of five patients with iSCI (motor level C4-C7) was compared with a group of five healthy control subjects. Single motor unit discharges were recorded with concentric needle electrodes from the first dorsal interosseus muscle during weak voluntary contraction (2%-5% maximum). TMS was applied with a 9 cm circular stimulating coil centred over the vertex. Modulation of single motor unit discharges was assessed using peristimulus time histograms (PSTHs). RESULTS: Mean (SEM) threshold (expressed as percentage of maximum stimulator output (%MSO)) for the excitatory peak (excitation) or inhibitory trough (inhibition) in the PSTHs was higher (p<0.05) in the patients (excitation = 47.1 (5.9) %MSO; inhibition = 44.3 (3.2) %MSO) than in controls (excitation=31.6 (1.2) %MSO; inhibition = 27.4 (1.0) %MSO). Mean latencies of excitation and inhibition were longer (p<0.05) in the patients (excitation=35 (1.8) ms; inhibition = 47.1 (1.8) ms) than in the controls (excitation = 21.1 (1.6) ms; inhibition = 27 (0.4) ms). Furthermore, the latency difference (inhibition-excitation) was longer (p<0.05) in the patients (10.4 (2.1) ms) than in the controls (6.2 (0.6) ms). CONCLUSION: Increased thresholds and latencies of excitation and inhibition may reflect degraded corticospinal transmission in the spinal cord. However, the relatively greater increase in the latency of inhibition compared with excitation in the patients with iSCI may reflect a weak or absent early component of cortical inhibition. Such a change in cortical inhibition may relate to the restoration of useful motor function after iSCI.

Adult↗

PubMed1975

Functional significance of projection from the cerebellar nuclei to the motor cortex in the cat.

The functional organization of projections from the cerebellar subcortical nuclei to the motor cortex through the nucleus ventralis lateralis (VL) of the thalamus was studied using tungsten microelectrodes for stimulation and recordings in acute and chronic cats. The following results were obtained. (1) Microstimulation of a small area of the ventral thalamus produced contraction of a single limb muscle as well as movements of whiskers. (2) The stimulus parameters for producing low threshold contraction of limb muscles were different from those for face muscles. The decrease of the frequency gradualy increased the threshold values for face muscles whereas the decrease abruptly increased the threshold for limb muscles. The optimum duration of the train for the lowest thresholds was longer for face muscles. (3) Stimulation of cerebellar nuclei (interpositus and lateralis) produced contraction of limb muscles. The stimulus parameters for the minimum threshold were similar to those for producing contraction of limb muscles from the ventral thalamus. (4) The peripheral receptive fields of neurons located around the low threshold sites in the thalamus were diffuse, i.e. they were driven insecurely by twisting the joints or pressure to the deep structures, but could not be driven by touch or light pressure on a circumscribed area of the body. (5) Chronic ablation of the motor cortex did not abolish the muscle contractions produced by thalamic stimulation, excluding the possibility that the effects were produced by stimulation of the branches of the pyramidal tract fibers reaching the ventral thalamus. (6) Chronic section of the brachium conjunctivum abolished or changed the characteristics of the contractions produced by thalamic stimulation indicating that the previous effects were produced by stimulation of cerebellar efferent fibers reaching the thalamus. (7) From these results it was concluded that the efferent impulses originating from the cerebellar nuclei can produce contraction of a particular muscle through activation of the red nucleus. These impulses are, at the same time, transmitted to a small group of neurons in the VL and then forwarded to the neurons in the motor cortex. (8) The functional significance of the VL projection system has been discussed in relation to the efferent zones within the motor cortex.

Animals↗

PubMed2005

Modulating parameters of excitability during and after transcranial direct current stimulation of the human motor cortex.

Weak transcranial direct current stimulation (tDCS) of the human motor cortex results in excitability shifts which occur during and after stimulation. These excitability shifts are polarity-specific with anodal tDCS enhancing excitability, and cathodal reducing it. To explore the origin of this excitability modulation in more detail, we measured the input-output curve and motor thresholds as global parameters of cortico-spinal excitability, and determined intracortical inhibition and facilitation, as well as facilitatory indirect wave (I-wave) interactions. Measurements were performed during short-term tDCS, which elicits no after-effects, and during other tDCS protocols which do elicit short- and long-lasting after-effects. Resting and active motor thresholds remained stable during and after tDCS. The slope of the input-output curve was increased by anodal tDCS and decreased by cathodal tDCS. Anodal tDCS of the primary motor cortex reduced intracortical inhibition and enhanced facilitation after tDCS but not during tDCS. Cathodal tDCS reduced facilitation during, and additionally increased inhibition after its administration. During tDCS, I-wave facilitation was not influenced but, for the after-effects, anodal tDCS increased I-wave facilitation, while cathodal tDCS had only minor effects. These results suggest that the effect of tDCS on cortico-spinal excitability during a short period of stimulation (which does not induce after-effects) primarily depends on subthreshold resting membrane potential changes, which are able to modulate the input-output curve, but not motor thresholds. In contrast, the after-effects of tDCS are due to shifts in intracortical inhibition and facilitation, and at least partly also to facilitatory I-wave interaction, which is controlled by synaptic activity.

Adult↗