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Biomedical subjects

D Burke

Publications and source records attributed to D Burke.

At least 127 records · Page 7Linked to original sources
PubMed1997

Mental rehearsal of motor tasks recruits alpha-motoneurones but fails to recruit human fusimotor neurones selectively.

1. As mental rehearsal of movements activates multiple cortical areas associated with movement, we assessed whether this increases fusimotor drive and whether enhanced muscle spindle activity could contribute to the improvement in skill that accompanies mental rehearsal. 2. Microneurographic recordings were made from six muscle spindle afferents innervating extensor muscles in the forearm or tibialis anterior, which were selected because their discharge increased during very weak contractions. Activity was monitored while subjects imagined performing a range of activities including simple and complex movements involving the relevant muscles. 3. No activation of muscle spindle afferents occurred during imagined motor tasks without EMG. When the relevant muscles contracted during mental rehearsal, spindle discharge increased, much as in weak contractions. 4. Mental rehearsal increased background EMG in the involved muscles and also increased H reflex amplitude independently of EMG changes. 5. Although there was no evidence for selective fusimotor activation during imagined movement, skeletomotor activity and reflex excitability increased. Similar changes occur with preparation for movement following a cue. It is likely that mental rehearsal usually involves unintentional performance of the planned motor task.

Adult↗
PubMed1997

Discharge of human muscle spindle afferents innervating ankle dorsiflexors during target isometric contractions.

1. There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the 'acceleration threshold' and the factors contributing to variability of 'acceleration threshold'. 2. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed 'ramp-and-hold' template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required 'ramp' rate and 'hold' force was adjusted between trials (range, 0.5-5% MVCs-1 and 0.5-20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any 'after-effects' on spindle afferent responses in subsequent contractions. 3. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the 'acceleration threshold'. Of twenty-six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01-11.9% MVC). 4. Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. 5. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after-effects are controlled, much of the residual variability in 'acceleration threshold' for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

Acceleration↗
PubMed1997

Unit identification, sampling bias and technical issues in microneurographic recordings from muscle spindle afferents.

Recordings have been made directly from human muscle spindle afferents for some 3 decades, and these have allowed assessment of fusimotor function in human subjects during normal motor acts and in patients with motor disturbances. However, inferences about fusimotor function are indirect, valid only if identification of the axon as of muscle spindle origin is secure and all extrafusal influences on the spindle have been controlled. As is discussed, the identification of spindle afferents and their classification into Group Ia and Group II are more problematic than in the cat, but these problems are probably relatively minor given the insight into normal function that can come from appropriately designed experiments in awake cooperative human subjects.

Action Potentials↗
PubMed1997

Paraesthesiae induced by prolonged high frequency stimulation of human cutaneous afferents.

1. The present study has explored the behaviour of human cutaneous afferents following conduction of prolonged trains of impulses at 200 Hz for 10-20 min, correlating the resultant changes in excitability with the perception of paraesthesiae. 2. Tetanization for 10 min resulted in activity-dependent changes in axonal excitability, with an initial period of hyperexcitability, followed by a long-lasting subexcitability. All subjects experienced paraesthesiae soon after cessation of the tetanic train, and these subsided gradually over 16 min. 3. Longer tetanic trains of 20 min duration resulted in greater changes in axonal excitability, but with paraesthesiae of a similar time course. The post-tetanic increase in excitability was abolished when short tetanic trains were delivered > 30 min before long trains, but all subjects still experienced paraesthesiae. 4. Threshold distributions following tetanic stimulation for both 10 and 20 min established that all axons contributing to the sensory volley underwent a uniform pattern of post-tetanic threshold changes. There was no evidence of a bimodal distribution with some axons hyperpolarized and others depolarized, as occurs with motor axons. However, the excitability changes were graded, with axons of lowest threshold undergoing a proportionately greater increase in excitability than axons of higher threshold. 5. The post-tetanic excitability changes were greater at the site of stimulation than elsewhere along the peripheral nerve. However, DC polarizing currents applied at this site failed to alter the sensation of paraesthesiae in the post-tetanic period. Furthermore, local anaesthetic block of the peripheral nerve proximal to the stimulation site failed to suppress the paraesthesiae. 6. The uniform pattern of post-tetanic threshold changes for cutaneous afferents differs from the bimodal distribution seen with post-ischaemic and post-tetanic motor axons. This difference in behaviour may reflect greater inward rectification and greater expression of a non-inactivating threshold conductance in cutaneous afferents. It is suggested that the ectopic activity responsible for paraesthesiae in the post-tetanic period arises from a more central site than the peripheral nerve.

Action Potentials↗
PubMed1997

Excitability changes in human cutaneous afferents induced by prolonged repetitive axonal activity.

1. The present study was undertaken to document the excitability changes produced by prolonged high-frequency trains of impulses in cutaneous afferents of six human subjects. 2. Trains of supramaximal stimuli at 200 Hz for 2 min or less produced a prolonged depression in excitability, consistent with activation of the electrogenic Na+-K+ pump. Trains of longer duration resulted in an initial period of hyperexcitability which, with 10 min trains, was associated with the sensation of paraesthesiae in all subjects. This transient hyperexcitability gradually gave way to a long-lasting period of hypoexcitability. 3. The excitability changes were reproducible, and were accompanied by corresponding changes in supernormality, refractoriness, strength-duration time constant and rheobase current, suggesting that the changes in axonal excitability reflected a change in membrane potential. 4. The transient increase in excitability that follows tetanic trains of 10 min had qualitatively similar effects on cutaneous axons as ischaemia or application of a depolarizing current. The post-tetanic changes in the supernormal period of sensory axons were those expected from the changes in excitability, without evidence of a gross distortion in its time course, as has been previously demonstrated in a hyperstimulated human motor axon. 5. It is concluded that the post-tetanic hyperexcitability of human sensory axons is probably driven by increased K+ accumulation in the restricted diffusion space under the myelin sheath, much as in motor axons, the differences in behaviour of sensory and motor axons being explicable by greater inward rectification in sensory axons.

Adult↗
PubMed1997

Weak short-latency spinal projections to the long flexor of the human thumb.

The human thumb is controlled by a muscle, flexor pollicis longus (FPL), that is unique among mammals and contributes to manual dexterity. The present study sought to define whether the spinal reflex circuitry for this muscle differed from that for an adjacent muscle (flexor carpi radialis, FCR). In peri-stimulus time histograms, short-latency, largely monosynaptic excitation produced by median nerve stimulation was significantly less frequent and significantly smaller for FPL motor units than FCR motor units. Thus the motoneurone pools of adjacent muscles differ in their spinal reflex accessibility. The reflex control of FPL may thus be achieved by supraspinal pathways rather than the traditional monosynaptic arc.

Electric Stimulation↗
PubMed1997

The size and number of plaid blobs mediate the misperception of type-II plaid direction.

The misperceived direction of type-II plaids has posed a problem for the intersection of constraints (IOC) model of two-dimensional motion perception. Alais et al. (1994, Vision Research, 34, 1823-1834) examined the perceived direction of type-II plaids and concluded that in addition to the direction signalled by the IOC process, a monocular mechanism signalling the motion of plaid features (blobs) is also involved in plaid perception. It was shown that the prominence of this monocular signal in plaid direction judgements depended on several variables, and the notion of blob "optimality" was introduced. This explained the more veridical direction of "optimal" blob plaids in terms of their more effectively activating the proposed feature-sensitive motion mechanism. One distinction between "optimal" and "non-optimal" blob plaids is their different component spatial frequencies, which necessarily entails a difference in the number and size of the blobs and thus raises potential confounds, since both the nature of the blobs and the components differ, which might affect the postulated blob mechanism and/or the IOC process. In the present paper, by offsetting changes in spatial frequency with changes in aperture size so that blob number is held constant, we examine whether differences in sheer blob number or size can alter perceived type-II plaid direction. The results reveal effects of both blob number and blob size, and their implications for the underlying mechanism are considered. Alternative accounts of the results in terms of the IOC model or revisions of it cannot explain the data. Comparison of monocular and binocular conditions adds further systematic evidence in support of the monocularity of the feature-sensitive motion mechanism.

Analysis of Variance↗
PubMed1997

Lycra garments designed for patients with upper limb spasticity: mechanical effects in normal subjects.

OBJECTIVE: To assess the stretch of pronator muscles produced by a specifically designed upper-limb Lycra garment that could have a better acceptability than rigid splints in treating upper-limb spasticity. DESIGN: Double-blind comparison among three garments. They were designed to produce a supinating, a pronating, and no torsional force, and were individually manufactured and tested in 10 healthy volunteers. MAIN OUTCOME MEASURE: Angular position and passive rotational stiffness of the forearm were measured with and without each of the garments immediately after the garment was fitted and every hour for 6 hours. RESULTS: When put on by a trained person, the supinator garment supinated the forearm in all subjects (mean, 17 degrees; p < .01; range, 5 degrees to 44 degrees) while the pronator garment pronated the forearm in 8 of 10 subjects (mean, 5 degrees; p < .01). These effects gradually decayed over 6 hours, as garment position was not readjusted. Passive rotational stiffness of the forearm increased by about 30% with each type of garment. The garments designed to produce no torsional force exerted no intrinsic rotational effect. CONCLUSION: Individually made Lycra garments can produce continuous stretch of muscles for several hours and may be useful in the treatment of spasticity. The garments, however, must be put on by a trained person and their position adjusted when necessary.

Adult↗
PubMed1997

Development of the St. George Hospital Memory Disorders Clinic Occupational Therapy Assessment Scale.

The St. George Hospital Memory Disorders Clinic Occupational Therapy Assessment Scale (OTAS) is a performance-based assessment of activities of daily living (ADL) administered in the home environment to monitor function among patients with early cognitive decline. Of the 30 items investigated, 19 items that measured "higher" functional domains were found to be useful in discriminating between subjects. Psychometrically, these 19 items had high internal consistency (coefficient alpha = .89) and high interrater reliability (kappa coefficient range: .71-1.0), and correlated well with other measures of higher function (Lawton's instrumental activities of daily living scale: r = .75; Nottingham extended ADL scale: r = .67). However, unlike these latter measures, the OTAS also correlated well with the Mini-Mental State Examination (r = .73), indicating that OTAS is a valid instrument that should be sensitive to functional problems among patients in the early stages of cognitive impairment.

Activities of Daily Living↗
PubMed1997

Folie à deux: three cases in the elderly.

Folie à deux is a rare, interesting disorder involving delusional beliefs shared by two persons. We report three cases of elderly couples referred to our service during an 18-month period with this disorder, and discuss the salient clinical features and the possible risk factors for folie à deux in the elderly.

Aged↗
PubMed1997

Hepatic arterial cannulation for regional chemotherapy is safe in patients with a liver metastasis volume of less than 1 litre.

Hepatic arterial cannulation for regional chemotherapy was achieved in 121/134 (90.2%) of colorectal liver metastasis patients. The perioperative mortality (5.8%) was significantly greater (P = 0.004) in patients with > 1000 ml (5/22, 23%) compared with < 1000 ml (2/83, 2.4%) metastasis volume. Colorectal liver metastasis patients should be considered for regional chemotherapy before metastases become extensive.

Antineoplastic Agents↗
PubMed1997

Excitability changes in human sensory and motor axons during hyperventilation and ischaemia.

This study was undertaken to compare the excitability changes of sensory and motor axons during hyperventilation and ischaemia, and to determine why ectopic impulse activity develops more readily during hyperventilation, and in sensory fibres. During hyperventilation for 20 min, all six subjects reported paraesthesiae in the hand and face, and four out of the six developed muscle twitching and cramps, associated with significant decreases of 20-30% in the threshold current required to produce sensory and motor potentials of constant size. During ischaemia four out of the six subjects reported paraesthesiae, but none reported muscle twitching. There were significant decreases of 15-20% in threshold for sensory and motor fibres. Ischaemia produced a marked decrease in supernormality, an increase in refractoriness and an increase in latency of the test compound sensory or motor potential, changes that were not seen with hyperventilation. The decrease in threshold during these manoeuvres was associated with a significant increase in strength--duration time constant (tau SD), indicating a relatively greater decrease in rheobase current. Using the technique of latent addition, we found that the changes in tau SD were consistent with a recently proposed model in which non-inactivating, voltage-dependent 'threshold channels' (presumably persistent Na+ channels) are active at resting potential. The failure of hyperventilation to alter conduction velocity, refractoriness or supernormality appreciably indicates that, unlike ischaemic depolarization, hyperventilation does not increase inactivation of conventional Na+ channels or activation of K+ channels, and this implies that the hyperventilation-induced increase in excitability is not the result of conventional depolarization, as seems to occur during ischaemia. These results suggest that hyperventilation has a rather selective action on the threshold channels, and they help to explain its greater effectiveness compared with ischaemia in provoking ectopic discharges. The greater expression of threshold channels in sensory than in motor fibres can explain why hyperventilation induces paraesthesiae before fasciculation and why only paraesthesiae occur during ischaemia.

Action Potentials↗
PubMed1997

Improving care for minority children with asthma: professional education in public health clinics.

OBJECTIVE: Recent studies have shown that lack of continuing primary care for asthma is associated with increased levels of morbidity in low-income minority children. Although effective preventive therapy is available, many African-American and Latino children receive episodic treatment for asthma that does not follow current guidelines for care. To see if access, continuity, and quality of care could be improved in pediatric clinics serving low-income children in New York City, we trained staff in New York City Bureau of Child Health clinics to provide continuing, preventive care for asthma. METHODS: We evaluated the impact of the intervention over a 2-year period in a controlled study of 22 clinics. Training for intervention clinic staff was based on National Asthma Education and Prevention Program guidelines for the diagnosis and management of asthma, and included screening to identify new cases and health education to improve family management. The intervention included strong administrative support by the Bureau of Child Health to promote staff behavior change. We hypothesized that after the intervention, clinics that received the intervention would, compared with control clinics, have increased numbers of children with asthma receiving continuing care in the clinics and increased staff use of new pharmacologic and educational treatment methods. RESULTS: In both the first and second follow-up years, the intervention clinics had greater positive changes than control clinics on measures of access, continuity, and quality of care. For second year follow-up data these include: for access, greater rate of new asthma patients (40/1000 vs 16/1000; P < .01); for continuity, greater percentage of asthma patients returning for treatment 2 years in a row (42% vs 12%; P < .001) and greater annual frequency of scheduled visits for asthma per patient (1.85 vs .88; P < .001); and for quality, greater percentage of patients receiving inhaled beta agonists (52% vs 15%; P < .001) and inhaled antiinflammatory drugs (25% vs 2%; P < .001), and greater percentages of parents who reported receiving patient education on 12 topics from Bureau of Child Health physicians (71% vs 58%; P < .01) and nurses (61% vs 44%; P < .05). CONCLUSION: We conclude that the intervention substantially increased the Bureau of Child Health staff's ability to identify children with asthma, involve them in continuing care, and provide them with state-of-the-art care for asthma.

Black or African American↗
PubMed1997

In vivo skeletal responses to porous-surfaced implants subjected to small induced motions.

Cylindrical porous-coated implants were placed in the distal femoral metaphyses of twenty dogs and were subjected to zero, twenty, forty, or 150 micrometers of oscillatory motion for eight hours each day for six weeks with use of a specially designed loading apparatus. The in vivo skeletal responses to the different magnitudes of relative motion were evaluated. Histological analysis demonstrated growth of bone into the porous coatings of all of the implants, including those that had been subjected to 150 micrometers of motion. However, the ingrown bone was in continuity with the surrounding bone only in the groups of implants that had not been subjected to motion or that had been subjected to twenty micrometers of motion; in contrast, the implants that had been subjected to forty micrometers of motion were surrounded in part by trabecular bone but also in part by fibrocartilage and fibrous tissue, and those that had been subjected to 150 micrometers of motion were surrounded by dense fibrous tissue. Trabecular microfractures were identified around three of the five implants that had been subjected to forty micrometers of motion and around four of the five that had been subjected to 150 micrometers of motion, suggesting that the ingrown bone had failed at the interface because of the large movements. The architecture of the surrounding trabecular bone also was altered by the micromotion of the implant. The implants that had stable ingrowth of bone were surrounded by a zone of trabecular atrophy, whereas those that had unstable ingrowth of bone were surrounded by a zone of trabecular hypertrophy. The trabeculae surrounding the fibrocartilage or fibrous tissue that had formed around the implants that had been subjected to forty or 150 micrometers of motion had been organized into a shell of dense bone tangential to the implant (that is, a neocortex outside the non-osseous tissue).

Animals↗