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Results for “Motor Cortex”

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At least 505 records · Page 28Linked to original sources

PubMed1989

Modified impulse diminishes discomfort of transcranial electrical stimulation of the motor cortex.

Transcranial electrical stimulation of the motor cortex has been developed in 1980 for electrophysiological assessment of the descending pathways. However, the widespread use of this procedure is limited by its painfulness due to simultaneous excitation of the sensory receptors and muscles of the scalp, especially if higher stimulus strengths are necessary in patients with motor deficits to evoke electromyographic responses. The present invention concerns a device which allows modification of the impulse delivered by a commercially available motor stimulator. By interruption on the exponentially decreasing impulse, "cutting" it after a freely selectable time by a low resistant short circuit on the patient's side, discomfort due to excitation of the sensory receptors and muscles of the scalp can be diminished with identical electromyographic responses regarded amplitudes and latencies of the potentials, thus making electrical stimulation less painful. Considering the applied charges we found a marked reduction of charge per phase using the modified impulse as compared to the original one with the difference increasing linearily depended on the preselected voltage.

Electric Stimulation↗

PubMed2006

Theta burst stimulation does not reliably depress all regions of the human motor cortex.

OBJECTIVE: The excitability of the human motor cortex projecting to hand muscles can be reduced by theta burst transcranial magnetic repetitive stimulation (TBS). This study compared the magnitude and variability of changes evoked by TBS for a distal and proximal arm muscle. METHODS: Eight subjects participated in three studies. In each study, electromyographic responses (MEPs) to single-pulse transcranial magnetic stimulation assessed cortical excitability before and after 40s of TBS. In the first two studies, TBS (intensity, 80% active motor threshold) was delivered to the optimal locations for biceps or first dorsal interosseous (FDI). In the final study, weaker intensity TBS was delivered over the biceps representation. RESULTS: TBS targeting biceps produced highly variable results among subjects. For the group, MEPs were not significantly depressed. Repeat studies in individual subjects highlighted the variability of responses. For FDI, MEPs were significantly depressed 5min after TBS and remained depressed for >30min (p<0.05). No significant changes in biceps MEPs occurred with weaker TBS. CONCLUSIONS: The magnitude and reliability of TBS depends on the region of the cortex targeted. SIGNIFICANCE: Results obtained for the hand should not be considered indicative of changes that will occur in other regions of the motor cortex or the brain.

Adult↗

PubMed2000

Motor cortex disinhibition in acute stroke.

OBJECTIVES: To test whether a disinhibition occurs in the human motor cortex after stroke. METHODS: Patients with a mild to moderate hemiparesis after an acute unilateral ischemic stroke were compared with age-matched healthy controls. We used paired transcranial magnetic stimuli (TMS) to investigate intracortical inhibition and facilitation. Single TMS were applied to obtain a cortical silent period. RESULTS: Intracortical inhibition was significantly reduced in the affected hemisphere at interstimulus intervals of 2, 3 and 4 ms. The cortical silent period was significantly prolonged when compared to the unaffected hemisphere of the patients and to the control group. Motor cortex disinhibition observed in stroke patients was associated either with minimal impairment at the onset of symptoms or with rapidly improving motor functions. CONCLUSIONS: Motor cortex disinhibition occurs in humans after stroke. We suggest that this disinhibition is indicative of compensatory mechanisms, which are involved in recovery-related reorganization.

Acute Disease↗

PubMed1986

Do thalamic regions which project to rostral primate motor cortex receive spinothalamic input?

The purpose of this study was to examine whether spinal pathways terminate in regions of the primate thalamus which project directly to the motor cortex. In order to examine this issue we employed a 'double labeling' procedure which utilized transport of both anterograde and retrograde tracers in the same macaque. One tracer substance was injected into the cervical spinal cord and the other was injected into the rostral zone of primary motor cortex. We observed no overlap of spinothalamic terminations and thalamic regions which project to the motor cortex. Thus, our results provide no anatomical support for a direct thalamic relay of peripheral afferent input to the motor cortex of primates.

Afferent Pathways↗

PubMed1992

The contribution of magnetic stimulation of the motor cortex to the diagnosis of cervical spondylotic myelopathy. Correlation of central motor conduction to distal and proximal upper limb muscles with clinical and MRI findings.

Magnetic stimulation of the motor cortex and cervical spine was performed on 24 patients with cervical spondylotic myelopathy documented by MRI. Compound motor action potentials (CMAPs) were recorded from the biceps and thenar muscles to study the central motor pathways of two different myotomes, C5-C6 and C8-D1. Central motor conduction was abnormal in all 24 patients for thenar muscles and in 5 patients for biceps brachii. In patients with a single compression level, central motor conduction abnormalities were confined to the myotomes caudal to the site of compression documented by MRI, in both proximal and distal upper limb muscles in the patients with upper spondylotic compression, and in distal muscles only in the patients with lower compression. In the patients with multilevel compression, central motor conduction time was abnormal for thenar muscles and always normal for the biceps muscle, but its mean value was significantly greater than in the control subjects, suggesting a slight involvement of central motor pathways for proximal upper limb muscles and major damage of the lower cervical segments. Owing to their high degree of sensitivity, central motor conduction studies may be of considerable value in the functional assessment of central motor pathways in cervical spondylotic myelopathy.

Action Potentials↗

PubMed2000

The effect of brain tumors on BOLD functional MR imaging activation in the adjacent motor cortex: implications for image-guided neurosurgery.

BACKGROUND AND PURPOSE: Functional MR (fMR) imaging data coregistered to a neurosurgical navigation system have been proposed as guides for the resection of brain tumor in or adjacent to eloquent cortices. The purpose of this study was to compare data obtained from the side of the brain affected by tumor with the contralateral side and to determine if there are physiological limitations of fMR imaging in accurately determining the location of the primary motor cortex. METHODS: Ten patients with tumors in or directly adjacent to the motor cortex were studied with fMR imaging (finger-tapping paradigm). fMR imaging data were analyzed using multiple R values. These data were coregistered to a real-time intraoperative neurosurgical navigation system. RESULTS: Significant variability of motor cortex activation patterns was noted among individual patients. The activation volumes on the side of the tumor were significantly smaller compared with the contralateral side for all tumors not previously resected (0.66+/-0.47). This was most pronounced in glioblastomas (0.27+/-0.21). We propose that these differences were caused by a loss of autoregulation in the tumor vasculature of glioblastomas and venous effects. CONCLUSION: Notwithstanding the differences noted, the motor cortex was identified successfully in all patients. This was confirmed by intraoperative physiological identification of the motor cortex and a lack of postoperative neurologic deficit.

Brain Neoplasms↗

PubMedDate not supplied

Motor cortex plasticity--from physiology to clinical neurology.

Neurophysiologic, neuroanatomic and neuroimaging studies conducted over the past two decades reveal that the cerebral cortex is functionally and structurally dynamic. The functional topography of the motor cortex can be modified by a variety of experimental manipulations, including peripheral or central injury, electrical stimulation, pharmocologic treatment or behavioral experience. Recent evidence demonstrates that functional alterations in motor cortex organization are accompanied by changes in dendritic and synaptic structure, as well as alterations in the regulation of cortical neurotransmitter systems. This article describes the state of the science regarding the main mechanisms implicated in the motor cortex plasticity, the main tools used for its investigation and the consequence of the recent discoveries on the therapeutic and rehabilitation procedures for the brain-injured persons.

Animals↗

PubMed1995

The origin of the soleus late response evoked by magnetic stimulation of human motor cortex.

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits a primary motor evoked potential (MEP) in the soleus muscle at a latency of approximately 30 msec, which may be followed by a late potential with a variable latency of 80-120 msec (soleus late response, SLR). While the MEP is thought to arise from stimulation of the corticospinal tract, the origin of the SLR is uncertain. In the present study we have investigated the properties of the SLR in order to elucidate its origin. An SLR was evoked at a latency of 100-120 msec in 5 out of 10 normal subjects in the relaxed state and at a latency of approximately 100 msec in all subjects when tibialis anterior (TA) was slightly facilitated. The SLR was largest with 5-10% TA contraction, decreased in size with increasing levels of TA contraction and was negligible in all subjects when the foot was immobilised. The latency of the SLR fell by 23 msec when the foot was passively dorsiflexed 20 degrees. A similar response to the SLR, at a latency of 77 msec, was present in all subjects following electrical stimulation of the peroneal nerve. Our findings suggest that the SLR is a soleus stretch reflex resulting from dorsiflexion of the foot due to preferential activation of TA following cortical stimulation.

Adolescent↗

PubMed2001

Reorganization of human motor cortex after hand replantation.

In 10 patients, reorganizational changes of the motor cortex contralateral to a replanted hand (MCreplant) were studied one to 14 years after complete traumatic amputation and consecutive successful replantation of the hand. The organizational state of MCreplant was assessed for the deafferentated and peripherally deefferentated hand-associated motor cortex and the adjacent motor representation of the proximal arm. For this, response maps were established for the first dorsal interosseus and biceps brachii muscle using focal transcranial magnetic stimulation (TMS) on a skull surface grid. Characteristics of the maps were center of gravity (COG), number of effective stimulation sites, amplitude sum, and amplitudes and response threshold at the optimal stimulation point. The COG is defined by the spatial distribution of response amplitudes on the map and lies over the cortex region with the most excitable corticospinal neurones supplying the recorded muscle. The COG of the biceps map in MCreplant was shifted laterally by 9.8 +/- 3.6 mm (range 5.0-15.7 mm). The extension of the biceps map in MCreplant was increased and the responses were enlarged and had lowered thresholds. For the muscles of the replanted hand, the pattern of reorganization was different: Response amplitudes were enlarged but thresholds, COG, and area of the cortical response map were normal. The different reorganizational phenomena observed for the motor cortical areas supplying the replanted hand and the biceps brachii of the same arm may be influenced by a different extent of deafferentation and by their different role in hand motor control.

Adolescent↗

PubMed2004

Abnormalities of inhibitory neuronal mechanisms in the motor cortex of patients with schizophrenia.

BACKGROUND: Focal transcranial magnetic stimulation (TMS) of the motor cortex was used to study two cortically activated inhibitory neuronal mechanisms that suppress ongoing tonic voluntary electromyographic activity in contralateral (postexcitatory inhibition [PI]) and ipsilateral (transcallosal inhibition [TI]) hand muscles. The PI follows the corticospinally mediated excitatory motor response (MEP) and is influenced by dopaminergic neurotransmission. TI reflects transcallosally mediated inhibition of the contralateral motor cortex, leading to motor inhibition in muscles ipsilateral to stimulation. PI and TI were studied to explore whether dopaminergic neurotransmission or interhemispheric transfers are altered in schizophrenia. METHODS: TMS was performed in 16 patients with this disease and in 16 healthy control subjects. Surface electromyographic activity was recorded bilaterally from the first dorsal interosseous muscle during a sustained strong isometric contraction. RESULTS: When compared with the findings in healthy subjects, patients with schizophrenia had a significantly longer PI and TI. The changes of the PI support the notion of an overactivity of the central dopaminergic system in schizophrenia. CONCLUSION: The prolonged TI suggests an abnormal activation of interhemispheric connections between the motor cortices and may be related to previously reported pathology of the corpus callosum in schizophrenic patients.

Adolescent↗

PubMed1998

Polarization of the human motor cortex through the scalp.

Direct currents (DC) applied directly to central nervous system structures produce substantial and long-lasting effects in animal experiments. We tested the functional effects of very weak scalp DC (< 0.5 mA, 7 s) on the human motor cortex by assessing the changes in motor potentials evoked by transcranial magnetic brain stimulation. We performed four different experiments in 15 healthy volunteers. Our findings led to the conclusion that such weak (< 0.5 mA) anodal scalp DC, alternated with a cathodal DC, significantly depresses the excitability of the human motor cortex, providing evidence that a small electric field crosses the skull and influences the brain. A possible mechanism of action of scalp DC is the hyperpolarization of the superficial excitatory interneurones in the human motor cortex.

Adult↗

PubMed2000

Coherence between low-frequency activation of the motor cortex and tremor in patients with essential tremor.

BACKGROUND: In healthy people, rhythmic activation of the motor cortex in the 15-30 Hz frequency range accompanies and contributes to voluntarily-generated postural contractions of contralateral muscle. In patients with Parkinson's disease, an abnormal low-frequency activation of the motor areas of the cortex occurs and has been directly linked to the characteristic 3-6 Hz rest tremor of this disease. We therefore investigated whether the motor cortex is involved in the transmission of the rhythmic motor drive responsible for generating essential tremor. METHODS: Non-invasive recordings of activity from the hand area of the motor cortex were made from six patients with essential tremor by magnetoencephalography. The recordings were made simultaneously with the electromyogram recorded from contralateral finger muscles during periods of postural tremor. A statistical spectral analysis was done to determine at which frequencies the two signals were correlated. FINDINGS: Spectral analysis of the electromyogram signals showed a significant low-frequency component at the frequency of the tremor bursts. However, there was no coherence between magnetoencephalogram and electromyogram recordings at the tremor frequency, indicating that no correlation existed between the tremor signal and low-frequency activity recorded from the primary motor cortex in individuals with essential tremor. Coherence at frequencies higher than the tremor frequency was similar to that in healthy individuals performing voluntary postural contractions. INTERPRETATION: The absence of significant coherence between the magnetoencephalogram and electromyogram at tremor frequencies suggests that in essential tremor the tremor is imposed on the active muscle through descending pathways other than those originating in the primary motor cortex. These findings challenge the model widely used to explain the efficacy of neurosurgical treatment of essential tremor, are in contrast to those of previous studies of parkinsonian rest tremor, and highlight an important difference in the pathophysiology of essential and parkinsonian tremor.

Adolescent↗

PubMedDate not supplied

Functional interneuronal connections in the motor cortex of alert and anesthetized cats.

Neuronal interconnections in the motor cortex of alert and anesthetized cats were investigated. It is shown that the number of connections between neurons did not decrease within microareas of the cortex during deepening of anesthesia, but their reorganization occurred in the form of leveling of the input and output properties of all cells of the microareas. The number of interneuronal connections of various microareas with deepening of anesthesia considerably decreased and those neurons which were relatively inert in the cortex of alert cats were considerably activated during restructuring of the functional interconnections between microareas. The results of the investigations permitted the assumption of differentiated participation of neurons of the cat motor cortex generating spikes of various amplitude in information processing at the level of microareas when alert and under anesthesia.

Anesthesia↗

PubMed1999

An alternative interpretation of population vector rotation in macaque motor cortex.

Neural recordings from the primary motor cortex of monkeys performing movements at an angle to a cue stimulus have yielded two main results: (A) the population vector rotates from the direction of the cue to the direction of movement, and (B) cells with intermediate preferred directions are recruited during the middle of this rotation. These results have been interpreted as the neural correlates of a process of 'mental rotation'. Here we propose that results A and B are also consistent with an alternate hypothesis of 'response substitution', given four well known features of cortical neurophysiology.

Animals↗

PubMed1995

Reorganisation in human motor cortex.

Transcranial magnetic stimulation over the motor cortex was used to construct a map of the effective sites on the scalp from which short-latency electromyogram responses could be evoked in muscles proximal to either an amputation stump (two subjects) or an ischemically anesthetized forearm (two subjects). At rest, the maps were larger and the responses bigger when stimulating contralateral to the amputated arm or after anesthesia than they were in the intact arm or before anesthesia. However, this difference disappeared when the maps were constructed during a small tonic voluntary contraction of the target muscle. We conclude that reorganisation of the corticospinal projection to a muscle at rest may no longer be present during activity. If so, this calls into question the possible functional benefits of such reorganisation in the control of movement after peripheral damage.

Amputation Stumps↗

PubMed2000

Plasticity and primary motor cortex.

One fundamental function of primary motor cortex (MI) is to control voluntary movements. Recent evidence suggests that this role emerges from distributed networks rather than discrete representations and that in adult mammals these networks are capable of modification. Neuronal recordings and activation patterns revealed with neuroimaging methods have shown considerable plasticity of MI representations and cell properties following pathological or traumatic changes and in relation to everyday experience, including motor-skill learning and cognitive motor actions. The intrinsic horizontal neuronal connections in MI are a strong candidate substrate for map reorganization: They interconnect large regions of MI, they show activity-dependent plasticity, and they modify in association with skill learning. These findings suggest that MI cortex is not simply a static motor control structure. It also contains a dynamic substrate that participates in motor learning and possibly in cognitive events as well.

Animals↗

PubMed2001

Constraints on somatotopic organization in the primary motor cortex.

Since the 1870s, the primary motor cortex (M1) has been known to have a somatotopic organization, with different regions of cortex participating in control of face, arm, and leg movements. Through the middle of the 20th century, it seemed possible that the principle of somatotopic organization extended to the detailed representation of different body parts within each of the three major representations. The arm region of M1, for example, was thought to contain a well-ordered, point-to-point representation of the movements or muscles of the thumb, index, middle, ring, and little fingers, the wrist, elbow, and shoulder, as conveyed by the iconic homunculus and simiusculus. In the last quarter of the 20th century, however, experimental evidence has accumulated indicating that within-limb somatotopy in M1 is not spatially discrete nor sequentially ordered. Rather, beneath gradual somatotopic gradients of representation, the representations of different smaller body parts or muscles each are distributed widely within the face, arm, or leg representation, such that the representations of any two smaller parts overlap extensively. Appreciation of this underlying organization will be essential to further understanding of the contribution to control of movement made by M1. Because no single experiment disproves a well-ordered within-limb somatotopic organization in M1, here I review the accumulated evidence, using a framework of six major features that constrain the somatotopic organization of M1: convergence of output, divergence of output, horizontal interconnections, distributed activation, effects of lesions, and ability to reorganize. Review of the classic experiments that led to development of the homunculus and simiusculus shows that these data too were consistent with distributed within-limb somatotopy. I conclude with speculations on what the constrained somatotopy of M1 might tell us about its contribution to control of movement.

Brain Mapping↗

PubMed1995

Contribution of NMDA and nonNMDA glutamate receptors to synchronized excitation and cortical output in the primary motor cortex of the rat.

Application of a GABA (gamma-aminobutyric acid) type A receptor antagonist through a microdialysis probe into the forelimb primary motor cortex (MI) of ketamine anesthetized rats induced the appearance of paroxysmal field potentials recorded in the supragranular layers of the MI and concomitant electromyographic (EMG) activity in the contralateral forelimb. Application of a nonNMDA (N-methyl-D-aspartate) glutamate receptor antagonist in conjunction with the GABA type A receptor antagonist completely blocked the paroxysmal field potentials and the EMG activity of the contralateral forelimb, while a NMDA receptor antagonist had no effect. The results indicate that the spread of activity within the primary motor cortex and the motor cortex output are mediated by nonNMDA receptors.

6-Cyano-7-nitroquinoxaline-2,3-dione↗