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PubMed1998

Intracortical inhibition and facilitation in different representations of the human motor cortex.

Intracortical inhibition and facilitation in different representations of the human motor cortex. J. Neurophysiol. 80: 2870-2881, 1998. Intracortical inhibition (ICI) and intracortical facilitation (ICF) of the human motor cortex can be studied with paired transcranial magnetic stimulation (TMS). Plastic changes and some neurological disorders in humans are associated with changes in ICI and ICF. Although well characterized in the hand representation, it is not known if ICI and ICF vary across different body part representations. Therefore we studied ICI and ICF in different motor representations of the human motor cortex. The target muscles were rectus abdominus (RA), biceps brachii (BB), abductor pollicis brevis (APB), quadriceps femoris (QF), and abductor hallucis (AH). For each muscle, we measured the rest and active motor thresholds (MTs), the motor-evoked potential (MEP) stimulus-response curve (MEP recruitment), ICI, and ICF. The effects of different interstimulus intervals (ISIs) were studied with a conditioning stimulus (CS) intensity of 80% active MT. The effects of different CS intensities were studied at ISI of 2 ms for ICI and ISI of 15 ms for ICF. MT was lowest for APB, followed by BB, AH, and QF, and was highest for RA. Except for BB, MEP recruitment was generally steeper for muscles with lower MT. ICI and ICF were present in all the motor representations tested. The stimulus intensity necessary to elicit ICI was consistently lower than that required to elicit ICF, suggesting that they are mediated by separate mechanisms. Despite wide differences in MT and MEP recruitment, the absolute CS intensities (expressed as percentage of the stimulator's output) required to elicit ICI and ICF appear unrelated to MT and MEP recruitment in the different muscles tested. These findings suggest that the intracortical mechanisms for inhibition and facilitation in different motor representations are not related to the strength of corticospinal projections.

Action Potentials↗

PubMed2004

Superlinear population encoding of dynamic hand trajectory in primary motor cortex.

Neural activity in primary motor cortex (MI) is known to correlate with hand position and velocity. Previous descriptions of this tuning have (1) been linear in position or velocity, (2) depended only instantaneously on these signals, and/or (3) not incorporated the effects of interneuronal dependencies on firing rate. We show here that many MI cells encode a superlinear function of the full time-varying hand trajectory. Approximately 20% of MI cells carry information in the hand trajectory beyond just the position, velocity, and acceleration at a single time lag. Moreover, approximately one-third of MI cells encode the trajectory in a significantly superlinear manner; as one consequence, even small position changes can dramatically modulate the gain of the velocity tuning of MI cells, in agreement with recent psychophysical evidence. We introduce a compact nonlinear "preferred trajectory" model that predicts the complex structure of the spatiotemporal tuning functions described in previous work. Finally, observing the activity of neighboring cells in the MI network significantly increases the predictability of the firing rate of a single MI cell; however, we find interneuronal dependencies in MI to be much more locked to external kinematic parameters than those described recently in the hippocampus. Nevertheless, this neighbor activity is approximately as informative as the hand velocity, supporting the view that neural encoding in MI is best understood at a population level.

Action Potentials↗

PubMed2005

Basic fibroblast growth factor stimulates functional recovery after neonatal lesions of motor cortex in rats.

Rats were given bilateral lesions of the motor cortex on the tenth day of life, and then received a daily subcutaneously injection of either basic fibroblast growth factor (FGF-2) or vehicle for 7 consecutive days. In adulthood, they were trained and assessed on a skilled forelimb reaching task. Although all lesion groups were impaired at skilled reaching, the postnatal day 10-lesioned group that received FGF-2 was less impaired than the lesion group that received the vehicle. Furthermore, the lesioned rats that received FGF-2 showed a filling of the lesion cavity with tissue, whereas the lesioned vehicle-treated rats still had a prominent lesion cavity. The functionality of the tissue filling the cavity, tissue surrounding it, and tissue from the motor cortex (in control rats) was assessed using intracortical microstimulation, and showed that stimulation of some sites from the filled cavity could evoke movement. The rats were perfused and processed for Golgi-Cox staining. Medium spiny neurons from the striatum were drawn and analyzed, and the results suggest that postnatal day 10 lesions of the motor cortex induced an increase in the length and complexity of these cells compared with those of non-lesioned rats. Our results suggest that FGF-2 may play an important role in recovery from early brain damage.

Analysis of Variance↗

PubMedDate not supplied

[Magnetic stimulation of the motor cortex in healthy persons].

In 49 healthy volunteers the motor cortex was stimulated transcranially by the intensive short magnetic field and the responses of the m. abductor digiti minimi (m. ADM) in 20 volunteers were recorded and of the m. tibialis anterior (m. TA) in 29 volunteers in relaxation and in mild contraction. In all volunteers by the electric stimulation of the ulnar nerve in the region of the wrist and the peroneal nerve in the region of the fibular capitulum there have been measured the F wave latency, distal latency of M responses and the conduction time of the peripheral motor neuron has been calculated. The difference between the latency of m. ADM and n. TA responses and the conduction time of the peripheral motor nerve represented the central motor conduction time from the motor cortex to the motor neurons of the anterior horns of the spinal cord (C8 and L4 segments). The normative values of the absolute latencies of the muscle responses and the central motor conduction time in the relaxed and contracted muscle are calculated as well as the normatives of the normal differences in latencies and the central motor conduction time between the left and right side of the same subject.

Fingers↗

PubMed2004

Anesthetized Long Evans rats show similar protein expression and long-term potentiation as Fischer 344 rats but reduced short-term potentiation in motor cortex.

A number of studies describe strain-related differences in the motor behavior of rats. Inbred albino F344 rats are found to be impaired in procedural spatial learning, skilled reaching, and over ground locomotion in relation to pigmented out bred Long Evans (LE) rats. These deficits could be related to the functional differences in the motor cortex of the two strains, and the objective of the present study was to examine this hypothesis. Synaptic transmission was examined in the two rat strains, using long-term potentiation (LTP) and short-term potentiation (STP), two electrophysiological measures of neural function and learning. Field potentials were evoked in the motor cortex of anesthetized Long Evans and Fischer 344 (F344) rats in response to contralateral white matter stimulation. The main findings indicated that (1) baseline-evoked responses in the two strains was similar, indicating similar basal levels of synaptic strength, (2) LTP was induced in both strains of rats, suggesting similar synaptic efficacy in the two strains of rats, and (3) STP was enhanced in the Fischer 344 rats, suggesting differences in synaptic function. Protein expression also revealed that the two strains did not differ with respect to structural or synaptic protein expression. Thus, the two strains exhibit motor skill differences despite a great degree of physiological similarity in motor cortex. The results are discussed in relation to the greater utility of using the Long Evans rat for examining the neural basis of plasticity and models of disease, especially if motor tasks are evaluated.

Analysis of Variance↗

PubMedDate not supplied

Reorganization of the human motor cortex by sensory signals: a selective review.

1. The normal human motor cortex can be made to reorganize by repeated stimulation of proprioceptive inputs, with or without concurrent stimulation of the motor cortex by transcranial magnetic nerve stimulation. Appropriate stimulation induces a focal increase in the excitability of corticospinal projections to specific muscles and, possibly, an increase in the area of the cortex projecting to those muscles. 2. We have shown that repeated stimulation on several successive days causes this 'plastic' reorganization to persist for at least several days. We have also used this approach to determine whether increases in the excitability of the motor cortex can be induced in stroke patients (in whom cortical excitability is usually depressed) and whether this is accompanied by functional changes. 3. The results of these studies were mixed but, in patients in whom plastic changes were induced, there were improvements and sometimes marked improvements in both motor function and some electrophysiological parameters. The reasons for the inconsistent results are not clear, but do not appear to relate to the site, size or nature of the lesion.

Electrophysiology↗

PubMed1991

Chronic motor cortex stimulation for the treatment of central pain.

Twelve patients with deafferentation pain secondary to central nervous system lesions were subjected to chronic motor cortex stimulation. The motor cortex was mapped as carefully as possible and the electrode was placed in the region where muscle twitch of painful area can be observed with the lowest threshold. 5 of the 12 patients reported complete absence of previous pain with intermittent stimulation at 1 year following the initiation of this therapy. Improvements in hemiparesis was also observed in most of these patients. The pain of these patients was typically barbiturate-sensitive and morphine-resistant. Another 3 patients had some degree of residual pain but considerable reduction of pain was still obtained by stimulation. Thus, 8 of the 12 patients (67%) had continued effect of this therapy after 1 year. In 3 patients, revisions of the electrode placement were needed because stimulation became incapable of inducing muscle twitch even with higher stimulation intensity. The effect of stimulation on pain and capability of producing muscle twitch disappeared simultaneously in these cases and the effect reappeared after the revisions, indicating that appropriate stimulation of the motor cortex is definitely necessary for obtaining satisfactory pain control in these patients. None of the patients subjected to this therapy developed neither observable nor electroencephalographic seizure activity.

Adult↗

PubMed2005

Motor cortex excitability after thalamic infarction.

OBJECTIVE: We examined 8 patients with hemihypesthesia due to an ischemic thalamic lesion to explore the effects of a central sensory dysfunction on motor cortex excitability. METHODS: Motor excitability was assessed using transcranial magnetic stimulation techniques and electrical peripheral nerve stimulation. Motor function was evaluated by the Nine-Hole-Peg Test and measurement of hand grip strength. The affected side was compared with the non-lesioned side and with an age-matched control group. RESULTS: Patients had a loss of inhibition and an increase of facilitation in the motor cortex of the affected side. The silent period was prolonged and motor function was impaired on the affected side. CONCLUSIONS: A thalamic lesion can modulate motor cortical excitability. SIGNIFICANCE: This study suggests that, under normal conditions, somatosensory afferents influence inhibitory and excitatory properties in the motor cortex.

Adult↗

PubMed2006

Large-scale organization of preferred directions in the motor cortex. II. Analysis of local distributions.

The spatial arrangement of preferred directions (PDs) in the primary motor cortex has revealed evidence for columnar organization and short-range order. We investigated the large-scale properties of this arrangement. We recorded neural activity at sites on a grid covering a large region of the arm area of the motor cortex while monkeys performed a 3D reaching task. Sites were projected to the cortical surface along anatomically defined cortical columns and a PD was extracted from each site with directionally tuned activity. We analyzed the resulting 2D surface map of PDs. Consistent with previous studies, we found that any particular reaching direction was re-represented at many points across the recorded area. In particular, we determined that the median radius of a cortical region required to represent the full complement of reaching directions is at most 1 mm. We also found that for the majority of regions of this size, the distribution of PDs within them exhibits an enrichment for the representation of forward and backward reaching directions (see companion paper). Finally, we found that the error of a population vector estimate of reaching direction constructed from neural activity within these regions is small on average, but varies significantly across different sections of the motor cortex, with the highest levels of error sustained near the fundus of the central sulcus and lowest levels achieved near the crown. We interpret these findings in the context of two well-known features of motor cortex, that is, its highly distributed anatomical organization and its behaviorally dependent plasticity.

Algorithms↗

PubMed1975

Responses of neurones in motor cortex and in area 3A to controlled stretches of forelimb muscles in cebus monkeys.

1. The experiments were designed to investigate the effects of longitudinal muscle displacements on neurones of the motor cortex of anaesthetized Cebus monkeys and thus test the hypothesis that signals from muscle spindles may modify motor cortical output. The effects of sinusoidal stretching of the extensor digitorum communis (EDC) at frequencies varying from 6 to 300 Hz and of step and rhomboidal stretches were studied in neurones of the motor cortex. For comparison, neurones of the primary receiving area for low-threshold muscle afferents, cortical area 3a, were also included in this study. Neurones of the motor cortex were subdivided into corticospinal (PT) neurones and non-corticospinal (non-PT) neurones. 2. Threshold stretch amplitudes were clearly higher for neurones of area 4 (PT and non-PT) than for 3a neurones. However, a conspicuous fall in threshold stretch amplitude was observed for all three neurone populations when the frequency of sinusoidal stretching was increased (highest frequency: 300 Hz). A small number of non-PT and PT neurones responded to vibration amplitudes of less than 100 mum and some of these low-threshold cells of area 4 also responded to rhomboidal stretches of 8 mm/sec ramp velocity and 80 mum plateau amplitude. Increasing the stretch amplitude to twice threshold nearly doubled the output magnitude in all three cell types. Neurones of area 3a and non-PT neurones of area 4 had similar latencies, and these were significantly shorter than the latencies of PT neurones tested with trains of high frequency vibration. Dynamic response patterns were observed in all three cell types, but most frequently in 3a neurones. 3. It is concluded that, in Cebus monkeys, signals from both primary and secondary muscle spindle endings from forelimb muscles reach the motor cortex. Under the present experimental conditions, the input from the primaries to the motor cortex was effective only if these spindle receptors were driven maximally by vibratory stimuli. The particularly low probability of stretch-evoked discharges of cortico-spinal neurones in the anaesthetized preparation may be explained by a low gain in transmission from input to output cells of the motor cortex.

Animals↗

PubMed1978

Postnatal development of visually evoked activity within motor cortex of cat.

1. The development of visually evoked activity within motor cortex was studied in chloralose-anesthetized kittens of 3, 4, and 5 mo of age and adult cats. 2. The slow-wave response from motor cortex of the adult cat is a triphasic negative-positive-negative wave. In 3-mo-old kittens the response is only a long-duration negative wave, while at 4 mo the positive and late negative waves emerge, and at 5 mo of age the adult form of the response is present. 3. Single neurons in the adult respond to the light during the positive component of the slow-wave response. When only the negative wave is present in kittens of 3 and 4 mo of age, neurons do not respond to the photic stimulus. At 5 mo of age the correlation between unit activity and the positive component of the response is identical to the adult. 4. The laminar analysis of the positive component of the response suggests that it originates relatively superficially in cortex and migrates down toward the deeper layers. Its correlation with unit activity indicates that the positive component represents excitatory postsynaptic potentials. 5. These results are consistent with an excitatory drive on neurons within motor cortex undergoing development change until the 5th mo of life.

Aging↗

PubMed1987

Cortico-cortical connections of the motor cortex in the brushtailed possum (Trichosurus vulpecula).

Cortico-cortical connections of motor cortex in the marsupial brushtailed possum were traced by making injections of horseradish peroxidase (HRP) into two parts of motor cortex: the rostral agranular part which does not overlap somatosensory cortex, and the caudal part which does. Following injections in motor cortex, labelled neurons were observed on the same side of the brain within somatosensory areas 1 and 2 and in parietal cortex just caudal to S1, with most neurons in cortical Layers 2-4. Commissural connections were found in half of the experiments, with many labelled neurons in cortical Layer 5. We have compared the pattern of cortico-cortical connections in the possum with those seen in some other mammals, which appear generally similar.

Anatomy, Comparative↗

PubMed1990

Morphological and physiological identification of neurons in the cat motor cortex which receive direct input from the somatic sensory cortex.

The population of neurons in the cat motor cortex which receives monosynaptic input from a specific functional region of the somatic sensory cortex was identified with the techniques of intracellular recording and staining with HRP. Both pyramidal and nonpyramidal cells located in the superficial layers of the pericruciate cortex responded to stimulation of the sensory cortex with short latency, excitatory postsynaptic potentials. More than half of the labeled cells were classified as pyramidal cells and the remainder as sparsely spinous or aspinous nonpyramidal cells. The characteristics of the EPSP's of the 2 groups of cells, ie. latency, time from beginning to peak and amplitude were found to vary only slightly. The results suggest that input from the sensory cortex impinges upon neurons which may in turn have an excitatory or inhibitory effect on cortico-fugal neurons in the motor cortex.

Animals↗

PubMed1993

Reversal of paw preference after ablation of the preferred forelimb primary motor cortex representation of the rat depends on the size of the forelimb representation.

Animals were tested for forelimb preference in a grasp or in a force task before microstimulation mapping of the primary motor cortex contralateral to the preferred forelimb. The size and location of the preferred forelimb primary motor cortex representation was determined and ablated. Seven days later, forelimb preference was again evaluated. Reversal of the initial preference after cortical ablation depended on the size of the preferred forelimb primary motor cortex representation. The mean size of the forelimb representation of the animals that immediately reversed forelimb preference after the cortical ablation was significantly smaller than the mean size of the representation of the animals that did not reverse forelimb preference in three consecutive tests seven days after the lesion. In another experiment, the size, location, and threshold currents of the forelimb representations were evaluated bilaterally before the forelimb preference test. The mean size of the preferred representations did not differ from the mean size of the non-preferred representations. However, when the primary motor cortex representation of the preferred forelimb was ablated, reversal of the initial preference depended, as in the previous experiment, on the size of the forelimb representation. In conclusion, reversal of forelimb preference after ablation of the preferred forelimb primary motor cortex representation depends on the size of the forelimb representation. Moreover, forelimb preference in a behavioral task is not associated with a larger forelimb representation in the contralateral primary motor cortex.

Animals↗

PubMed1980

Peripheral input pathways to the monkey motor cortex.

We have shown (Asanuma et al., 1979c) that the monkey motor cortex receives peripheral somesthetic inputs directly from the thalamus. In the present experiments, we studied the pathways which mediated these inputs by stimulating superficial radial (SR) and deep radial (DR) nerves and recording evoked potentials from the motor and sensory cortices and the following results were obtained: 1. The focus for SR and DR evoked potentials in the sensory cortex was located in a circumscribed small area whereas in the motor cortex, the evoked potentials were distributed in a wide area along the central sulcus including the distal forelimb area. 2. Ablation of the sensory cortex reduced the size, but neither abolished nor changed the latency of the evoked potentials in the motor cortex. 3. Section of dorsal column nearly abolished the evoked potentials in the motor cortex, but only halved their size in the sensory cortex. 4. Section of ventrolateral cervical column including the spinothalamic tract halved the size of evoked potentials in the sensory cortex, but did not change the size in the motor cortex. 5. It is concluded that direct peripheral inputs to the motor cortex are mediated primarily through the dorsal column system whereas the peripheral inputs to the sensory cortex are mediated through both dorsal column and spinothalamic tract.

Animals↗

PubMed1990

Effects of motor cortex and single muscle stimulation on neurons of the lateral vestibular nucleus in the rat.

The neuronal responses to stimulation of motor cortical sites and of forelimb single muscles were studied in the lateral vestibular nucleus of anaesthetized rats. Of the 228 neurons tested for response to stimulation of contralateral motor cortex, 63% responded to cortical sites controlling extensor muscles and 30% to those controlling flexors. The corresponding figures for responders to ipsilateral stimulation were 34 and 21%. Vestibulospinal units responded to cortical sites controlling extensor and flexor muscles whereas the remaining lateral vestibular nucleus neurons, very reactive to cortical sites controlling extensor muscles, responded little to contralateral and not at all to ipsilateral cortical sites controlling flexor muscles. The effects evoked by contralateral cortical sites controlling extensors varied, those induced by cortical sites controlling flexors were inhibitory in 77% of cases. The responses to ipsilateral motor cortex stimulation differed not so much by cortical sites controlling extensor or flexor muscles as by whether the neuron was in the dorsal or ventral zone of the lateral vestibular nucleus: mixed in the former, all inhibitory in the latter. Of the lateral vestibular nucleus units tested for response to stimulation of ipsilateral or contralateral forelimb distal muscles, only 11% responded. All the vestibulospinal units responsive to muscle stimulation lay in the dorsal zone of the nucleus. The remainder, dorsal or ventral, were not responsive to contralateral muscles. Single lateral vestibular nucleus cells influenced both by ipsilateral muscle and by contralateral motor cortex made up 24% of the pool, vestibulospinal and non-vestibulospinal. They fell into three groups: responsive to one or both structures but responding more strongly to combined stimulation; responsive to each of the two structures but showing a response to combined stimulation not significantly different from that evoked by the cortex alone; responsive only to combined stimulation. The lateral vestibular nucleus units included in these three groups accounted for 29% of those tested for response to extensor muscles and cortical sites controlling extensors and 15% of those tested for response to flexor muscles and cortical sites controlling flexors. Twenty-five per cent of the vestibulospinal neurons responded both to contralateral muscles and to ipsilateral motor cortex stimulation but none of the non-vestibulospinal neurons responded to both. All the responders to both were in the dorsal zone of the lateral vestibular nucleus and responded to extensor stimuli, always in the same way. These results indicate that motor cortex output exerts a major influence on lateral vestibular nucleus discharges, while the muscle afferents have a modulatory influence on the lateral vestibular nucleus responses to cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1996

Evaluation of the motor cortex by magnetic stimulation in patients with Alzheimer disease.

Motor evoked potentials (MEPs) from abductor pollicis brevis (APB) and tibialis anterior (TA) muscles elicited by transcranial magnetic stimulation of the motor cortex were studied in 15 patients with Alzheimer disease (AD). An abnormally higher MEP threshold in APB, frequently associated with absence of the MEP in relaxed TA muscles, was found in 40% of patients, almost all of them in the more severe stage of the disease. The MEP amplitude and averaged MEP/MAP ratio were reduced respectively by 20% and 26% in the APB muscle, and by 46.7% and 53.3% in the TA muscle. The less frequent prolongation of the central conduction time (CCT) (20%) might reflect preservation of the impulse propagation along the surviving pyramidal fibers. In 63.6% of the patients the central silent period (cSP) duration in the APB muscle was shortened; the mean value was significantly different between patients and controls. The results of this study suggest that loss and/or dysfunction of motor cortex neurones, including pyramidal cells and inhibitory interneurones may occur in AD patients before clinical signs become apparent.

Aged↗

PubMed2004

Increased facilitation of the primary motor cortex following 1 Hz repetitive transcranial magnetic stimulation of the contralateral cerebellum in normal humans.

Connections between the cerebellum and the contralateral motor cortex are dense and important, but their physiological significance is difficult to measure in humans. We have studied a group of 10 healthy subjects to test whether a modulation of the excitability of the left cerebellum can affect the excitability of the contralateral motor cortex. We used repetitive transcranial magnetic stimulation (rTMS) at 1 Hz frequency to transiently depress the excitability of the left cerebellar cortex and paired-pulse TMS testing of intracortical inhibition (ICI) and intracortical facilitation (ICF) to probe the excitability of cortico-cortical connections in the right motor cortex. The cortical silent period was also measured before and after cerebellar rTMS. Motor evoked potentials (MEPs) were significantly larger after than before conditioning rTMS trains (p < 0.01). Moreover, left cerebellar rTMS increased the ICF of the right motor cortex as measured with paired-pulses separated by an interstimulus interval (ISI) of 15 ms. The effect lasted for up to 30 min afterward and was specific for the contralateral (right) motor cortex. The cortical silent period was unaffected by cerebellar rTMS. The implication is that rTMS of the cerebellar cortex can shape the flowing of inhibition from Purkinje cells toward deep nuclei, thereby increasing the excitability of interconnected brain areas.

Adult↗