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Biomedical subjects

G W Thickbroom

Publications and source records attributed to G W Thickbroom.

At least 19 recordsLinked to original sources
PubMed2001

Long-term changes in motor cortical organisation after recovery from subcortical stroke.

The present study has investigated the long-term changes in the organisation of the corticomotor projection to the hand in a group of subjects who had sustained a subcortical hemispheric stroke up to 15 years previously and had subsequently recovered normal or near-normal motor function. Transcranial magnetic cortical stimulation (TMCS) was employed to map the topography of the primary corticomotor projection to the hand and to obtain measures of cortical motor threshold, long-latency intracortical inhibition and corticospinal conduction. Changes in motor threshold and in motor-evoked potential (MEP) amplitude and latency in keeping with persisting impairment of conduction in the corticospinal pathway were still present in the majority of subjects, whereas the duration of the post-MEP silent period, reflecting the strength of long-latency intracortical inhibition, was usually normal. Topographic shifts in the corticomotor representation relative to the unaffected side were found in the majority of subjects. In some the shifts were in the mediolateral axis suggesting reorganisation within the primary motor cortex, while in the others anteroposterior shifts were present in keeping with recruitment of premotor or postcentral cortex. The present findings indicate that changes in the physiological properties of the corticomotor projection to the hand are frequently present in subjects who have recovered motor function after a subcortical stroke and may persist indefinitely. We postulate that these changes are the result of reorganisation at cortical level and that cortical reorganisation is one of the processes which contribute to motor recovery after a subcortical lesion and which may compensate for persisting impairment of conduction in the corticospinal pathway.

Adult↗
PubMed2001

Patterns of muscle involvement in inclusion body myositis: clinical and magnetic resonance imaging study.

The differential patterns of muscle involvement in the upper and lower limbs in sporadic inclusion body myositis (sIBM) were examined in 18 patients using both quantitative and manual muscle testing as well as magnetic resonance imaging (MRI) in 9 patients. Weakness of the quadriceps femoris and the forearm flexors was present in most patients, but there was considerable variability in the patterns and severity of muscle involvement. MRI disclosed preferential patterns of muscle involvement within functional groups such as the quadriceps femoris, in which there was severe involvement of the vasti with relative sparing of the rectus femoris, and the triceps surae, in which selective involvement of the medial gastrocnemius was common. Involvement of flexor digitorum profundus on MRI was found in only one third of patients. The results emphasize the variability in the clinical phenotype and differential susceptibility of muscles to the disease process in sIBM.

Adult↗
PubMed2001

Differences in sensory and motor cortical organization following brain injury early in life.

There have been a number of physiological studies of motor recovery in hemiplegic cerebral palsy which have identified the presence of novel ipsilateral projections from the undamaged hemisphere to the affected hand. However, little is known regarding the afferent projection to sensory cortex and its relationship to the reorganized cortical motor output. We used transcranial magnetic stimulation (TMS) to investigate the corticomotor projection to the affected and unaffected hands in a group of subjects with hemiplegic cerebral palsy, and also performed functional magnetic resonance imaging (fMRI) studies of the patterns of activation in cortical motor and sensory areas following active and passive movement of the hands. Both TMS and fMRI demonstrated a normal contralateral motor and sensory projection between the unaffected hand and the cerebral hemisphere. However, in the case of the affected hand, the TMS results indicated either a purely ipsilateral projection or a bilateral projection in which the ipsilateral pathway had the lower motor threshold, whereas passive movement resulted in fMRI activation in the contralateral hemisphere. These results demonstrate that there is a significant fast-conducting corticomotor projection to the affected hand from the ipsilateral hemisphere in this group of subjects, but that the predominant afferent projection from the hand is still directed to the affected contralateral hemisphere, resulting in an interhemispheric dissociation between afferent kinesthetic inputs and efferent corticomotor output. The findings indicate that there can be differences in the organization of sensory and motor pathways in cerebral palsy, and suggest that some of the residual motor dysfunction experienced by these subjects could be due to an impairment of sensorimotor integration at cortical level as a result of reorganization in the motor system.

Adolescent↗
PubMed2000

The role of the supplementary motor area in externally timed movement: the influence of predictability of movement timing.

A significant role in the planning and preparation for voluntary movement has been ascribed to secondary motor areas located on the medial wall of the cerebral hemispheres, and in particular to the supplementary motor area (SMA). Within the SMA, rostral and caudal subdivisions have been described, and differential roles have been attributed to these regions in relation to movement planning, preparation and execution. We have used functional magnetic resonance imaging (fMRI) to investigate the role of the SMA in the timing of movement execution, by recording the fMRI signal from mesial pre-motor areas and primary sensorimotor cortex (SM1) during the execution of a simple motor task externally cued at predictable (regular) and unpredictable (irregular) time intervals. The mean rate of movement was matched in both experiments. There was a greater activation of caudal than rostral SMA with both predictably and unpredictably cued movements, and a doubling of the signal when the timing of the motor response was unpredictable. In contrast, there was no difference in the activation of primary sensorimotor cortex with the two tasks. The data demonstrate that the caudal SMA has an important role in the execution of externally cued movements. The results also suggest a greater role for this region in the performance of unpredictably timed compared with predictably timed movements, however a model is proposed (based on electrophysiological data) which shows how the difference in functional signal in these two situations can be explained on the basis of a difference in the time course of neuronal activation in the SMA, rather than in the overall degree of activation.

Adult↗
PubMed2000

Changes in corticomotor excitability after fatiguing muscle contractions.

To investigate whether the type and duration of activity influences corticomotor excitability following fatiguing exercise, we compared motor evoked potential (MEP) responses of the biceps brachii to transcranial magnetic stimulation (TMS) during recovery from two different exercise regimens. Responses were recorded in both the resting state and during a weak contraction. Ten subjects performed a 60-s maximal voluntary contraction (MVC) and, on a subsequent occasion, a sustained 20% MVC to the point of exhaustion. Resting MEP amplitude declined following maximal and submaximal protocols, reaching 34% and 31% of pre-exercise means, respectively (P < 0.001 for both). In contrast, mean facilitated MEP amplitude showed a smaller and more transient decrement following the sustained submaximal effort (64%; P < 0.05), but not the 60-s MVC. Abolition of the postexercise depression in resting MEP amplitude by a weak tonic contraction indicates that decreases in excitability at the spinal level contribute to the reduced corticomotor excitability observed after fatiguing exercise.

Action Potentials↗
PubMed2000

Functional reorganisation of the corticomotor projection to the hand in skilled racquet players.

While it is known that relatively rapid changes in functional representation may occur in the human sensorimotor cortex in short-term motor-learning studies, there have been few studies of changes in organisation of the corticomotor system associated with the long-term acquisition of motor skills. In the present study, we have used transcranial magnetic stimulation (TMS) to investigate the corticomotor projection to the hand in a group of elite racquet players, who have developed and maintained a high level of skill over a period of many years, and have compared the findings with those in a group of social players and a group of non-playing control subjects. Increased motor-evoked-potential (MEP) amplitudes and shifts in the cortical motor maps for the playing hand were found in all of the elite players and cortical motor thresholds were reduced in some players, whereas in the social players all parameters were within the normal range. The findings in the elite players are interpreted as being indications of a process of functional reorganisation with the motor cortex or corticomotor pathway that are associated with the acquisition and retention of complex motor skills.

Adult↗
PubMed1999

Differences in functional magnetic resonance imaging of sensorimotor cortex during static and dynamic finger flexion.

Functional magnetic resonance imaging (fMRI) studies of the human motor system have commonly used movement paradigms which contain a dynamic component; however, the relationship between the fMRI signal for motor tasks with and without a dynamic component is not known. We have investigated the relationship between the fMRI signal during a static finger flexion task and during dynamic finger flexion at 1-3 Hz, each at two levels of force (5% and 10% of maximum voluntary contraction). A small fMRI response could be recorded from only a few subjects during the static tasks. In contrast, a substantial fMRI response occurred during dynamic tasks in all subjects at both levels of force. The fMRI response was not significantly correlated with force or movement rate during the dynamic tasks. It is concluded that the factors responsible for generating an fMRI response are fundamentally different during steady contractions compared to those involving a dynamic component, and that the fMRI signal may be more sensitive to changes in the pattern of neural activation rather than the ongoing firing rate or extent of activation.

Adult↗
PubMed1999

Physiological studies of the corticomotor projection to the hand after subcortical stroke.

OBJECTIVE: The mechanisms which lead to recovery of motor function after a stroke are poorly understood. Functional reorganization of cortical motor centres is thought to be one of the factors which may contribute to recovery. We have investigated the extent of reorganization which occurs at the level of the primary motor cortex after a lesion of the corticospinal pathway. METHODS: Transcranial magnetic stimulation was used to map the topography of the primary corticomotor projection to the abductor pollicis brevis muscle and study changes in cortical motor thresholds and corticospinal conduction in a group of 20 subjects with subcortical infarcts of varying duration (1 week to 15 years) and varying degrees of motor deficit. RESULTS: There was a broad correlation between motor evoked potential (MEP) amplitude and motor thresholds on the one hand and the severity of motor deficit and site and extent of the lesion on the other. Shifts in the cortical motor maps were found in both early and late cases, irrespective of the site of the lesion, but were more frequent in the longer standing cases. Shifts were usually along the mediolateral axis but anteroposterior shifts were found in some late cases. CONCLUSION: Our findings indicate that there is functional reorganization of the corticomotor projection in subjects who regain a degree of motor control following a subcortical lesion sparing the motor cortex.

Adolescent↗
PubMed1999

A model of the effect of MEP amplitude variation on the accuracy of TMS mapping.

We have modelled the effect of motor evoked potential (MEP) amplitude variation on transcranial magnetic stimulation (TMS) maps. The range of variability in TMS map parameters was estimated by randomly altering the MEP amplitude associated with each stimulus site and re-calculating the map parameters. TMS map position and area were remarkably stable, with variations of the order of 1 mm for map position and less than 5% for map area. The results indicate that reliable and accurate mapping studies can be carried out in the presence of an intrinsic variability in MEP amplitude measurements.

Adult↗
PubMed1999

Corticomotor excitability and perception of effort during sustained exercise in the chronic fatigue syndrome.

OBJECTIVE: We have investigated the possibility of a central basis for the complaints of fatigue and poor exercise tolerance in subjects with chronic fatigue syndrome (CFS). METHODS: Transcranial magnetic stimulation of the motor cortex was used to measure sequential changes in motor evoked potential (MEP) amplitude, post-excitatory silent period (SP) duration and twitch force of the biceps brachii muscle during a 20% maximum isometric elbow flexor contraction maintained to the point of exhaustion. Ten patients with post-infectious CFS and 10 age- and sex-matched control subjects were studied. Results were analysed using non-parametric repeated measures analysis of variance (Friedman's test) and Mann-Whitney U-tests for intra- and inter-group comparisons respectively. RESULTS: Mean endurance time for the CFS group was lower (13.1+/-3.2 min, mean +/- SEM) than controls (18.6+/-2.6 min, P < 0.05) and CFS subjects reported higher ratings of perceived exertion. During the exercise period MEP amplitude and SP duration increased in both groups but to a lesser extent in CFS subjects. Interpolated twitch force amplitude also increased during exercise, being more pronounced in CFS subjects. CONCLUSION: The findings are in keeping with an exercise-related diminution in central motor drive in association with an increased perception of effort in CFS.

Adult↗
PubMed1998

Isometric force-related activity in sensorimotor cortex measured with functional MRI.

Isometric force-related functional magnetic resonance imaging (fMRI) signals from primary sensorimotor cortex were investigated by imaging during a sustained finger flexion task at a number of force levels related to maximum voluntary contraction. With increasing levels of force, there was an increase in the extent along the central sulcus from which a fMRI signal could be detected and an increase in the summed signal across voxels, but these parameters were related in such a way that the signal from each voxel was similar for each level of force. The results suggest that increased neuronal firing and recruitment of corticomotor cells associated with increased voluntary isometric effort are reflected in an expansion of a relatively constant fMRI signal over a greater volume of cortex, rather than an increase in the magnitude of the response in a particular circumscribed region, possibly due to perfusion of an increase in oxygen-enriched blood over a wider region of the cortex.

Adult↗
PubMed1998

Magnetic stimulation mapping of motor cortex: factors contributing to map area.

Transcranial magnetic stimulation (TMS) can evoke an electromyographic response in muscles of the hand with stimulation at a large number of scalp sites widely spaced over the contralateral primary motor area of the brain. To determine the extent to which this is due to current spread from the stimulating coil to a smaller region of excited cortex, excitability curves of motor evoked potential amplitude vs. stimulus intensity were measured at multiple scalp sites. It was found that these curves were of very similar shape, but with different offsets along the stimulus intensity axis. This could be explained on the basis of current flow to a small excitable region of cortex located at some depth in the brain. It is concluded that the surface area of TMS maps is primarily determined by current spread and by the relationship between the position of the coil on the scalp and the depth of the motor output region in the cortex, and does not necessarily provide a true indication of the spatial extent of the motor output region projecting to the target muscle.

Adult↗
PubMed1998

The effects of eccentric exercise on neuromuscular function of the biceps brachii.

This study investigated the effects of a bout of exercise-induced muscle damage on strength and motor skill of the elbow flexor muscles. Eight subjects performed 35 maximal isokinetic eccentric elbow flexions at 90 degrees sec(-1) and maximal voluntary contraction (MVC) force, surface electromyography (EMG), plasma creatine kinase (CK) and tracking error associated with a one-dimensional elbow flexion/extension visuomotor pursuit task were studied at intervals up to 28 days after exercise. Subjects showed a post-exercise decline in MVC (mean = 63 +/- 11% (s.d.) of pre-exercise after 1 day, p<0.02) and were still significantly weaker at 21 days. The delayed-onset plasma CK rise and the absence of any quantitative change in surface EMG suggest that the observed weakness was related to muscle fibre damage. Tracking performance decreased in all subjects with the greatest tracking error occurring 1 day post-exercise (mean = 127% +/- 20% of control value, p<0.02). There was a significant negative correlation between strength and tracking performance following exercise (r2 = 0.724). The results demonstrate that performance in activities requiring fine motor control will be impaired for a number of days following a bout of damaging exercise.

Adult↗
PubMed1998

The corticomotor representation of upper limb muscles in writer's cramp and changes following botulinum toxin injection.

Transcranial magnetic stimulation was used to investigate the properties of the corticomotor pathway and to map the primary motor cortex projection to hand and forearm muscles during a sustained isometric contraction in a group of subjects with writer's cramp of varying duration. Corticomotor threshold, motor evoked potential amplitude and latency, and silent-period duration were normal on both sides in all subjects. The maps of the corticomotor projection were displaced relative to normal in all subjects, and in some cases were distorted in shape, with extensions of the lateral borders and the emergence of almost discrete secondary motor areas. The degree of map distortion and displacement was greatest in subjects with long-standing writer's cramp (> 5 years), and was bilateral in some cases. Injection of botulinum toxin into affected muscles demonstrated that the alterations in map topography were not fixed, and could be temporarily reversed during the period when the clinical effects of the injection were greatest, with the maps returning to their original positions as the effects of the injection wore off. It is concluded from this study that there are slowly evolving reorganizational changes in the primary motor cortex in writer's cramp, and that these changes may be secondary to altered afferent inputs from both clinically affected and unaffected muscles.

Adult↗
PubMed1997

Changes in corticomotor excitation and inhibition during prolonged submaximal muscle contractions.

Changes in motor evoked potential (MEP) amplitude, post-MEP silent period duration, and interpolated twitch torque were measured using transcranial magnetic (TMS) and electrical (TES) stimulation during a 20% maximum voluntary contraction of the elbow flexors sustained to exhaustion. TMS- and TES-induced MEP amplitude increased progressively over the contraction period up until the point of exhaustion. The TMS-induced silent period was prolonged only during the second half of the contraction period, the time course being different from that of the MEP responses, whereas the TES-induced silent period did not change. The findings indicate that corticomotor excitability increases during a sustained submaximal voluntary contraction and that, as fatigue develops, there is a progressive buildup of intracortical inhibition. This may represent a mechanism whereby corticomotor output is maintained at an appropriate level to preserve optimal motor unit firing frequencies during a fatiguing contraction.

Adult↗
PubMed1997

Corticomotor representation of the sternocleidomastoid muscle.

The topography of the cortical motor projection to the sternocleidomastoid (SCM) muscles was investigated in 15 normal subjects using the technique of transcranial magnetic stimulation. Contrary to the long-held view that the representation of the neck muscles in the motor strip is close to that of the face, our findings indicate that the projection to both the ipsilateral and contralateral SCM muscles arises from an area of cortex high up on the cerebral convexity close to the trunk representation and at a comparable level to the sensory representation of the neck in the post-central cortex. The use of shielded intramuscular recordings showed that surface recordings detect activity not only from the SCM muscle but also from the overlying platysma muscle whose cortical representation is lateral to that of the hand muscles. Our findings confirm that the corticomotor projection to the SCM muscle follows both a contralateral monosynaptic pathway, and an ipsilateral pathway, and indicate that the latter pathway may be disynaptic.

Adult↗