Search PubMed⌕ Search

Biomedical subjects

D Burke

Publications and source records attributed to D Burke.

At least 91 records · Page 5Linked to original sources
PubMed1999

Temperature dependence of excitability indices of human cutaneous afferents.

The temperature dependence of different indices of axonal excitability (threshold, latency, refractoriness, supernormality, strength-duration time constant, and rheobase) was studied for cutaneous afferents of 8 healthy human volunteers using threshold tracking. Cooling from approximately 32 - approximately 22 degrees C dramatically increased the threshold for a conditioned potential evoked during the relatively refractory period (average increase 573%) but had little effect on the threshold for unconditioned potentials (increased by 4% with 0.1-ms test stimuli), strength-duration time constant (increased by 18%), or rheobase (decreased by 12%). Cooling increased the latency of the unconditioned test potential by 41%, but this slowing was small compared with the effect of cooling on the latency slowing attributable to refractoriness. This measure of refractoriness was initially 0.17 ms at a conditioning-test interval of 2 ms, and increased with cooling to 1.30 ms at the same interval. With cooling, refractoriness was both greater at any one conditioning-test interval and longer in duration, extending into intervals normally associated with supernormality. It is concluded that, although cooling affects all excitability indices to some extent, the most prominent feature is the increase in refractoriness. By contrast, strength-duration time constant is influenced little by temperature.

Action Potentials↗
PubMed1999

Comparing deficits following excitotoxic and contusion injuries in the thoracic and lumbar spinal cord of the adult rat.

The majority of human spinal cord injuries involve gray matter loss from the cervical or lumbar enlargements. However, the deficits that arise from gray matter damage are largely masked by the severe deficits due to associated white matter damage. We have developed a model to examine gray matter-specific deficits and therapeutic strategies that uses intraspinal injections of the excitotoxin kainic acid into the T9 and L2 regions of the spinal cord. The resulting deficits have been compared to those from standard contusion injuries at the same levels. Injuries were assessed histologically and functional deficits were determined using the Basso, Beattie, and Bresnahan (BBB) 21-point open field locomotor scale and transcranial magnetic motor evoked potentials (tcMMEPs). Kainic acid injections into T9 resulted in substantial gray matter damage; however, BBB scores and tcMMEP response latencies were not different from those of controls. In contrast, kainic acid injections into L2 resulted in paraplegia with BBB scores similar to those following contusion injuries at either T9 or L2, without affecting tcMMEP response latencies. These observations demonstrate that gray matter loss can result in significant functional deficits, including paraplegia, in the absence of a disruption of major descending pathways.

Animals↗
PubMed1999

Reproducibility of tendon jerk reflexes during a voluntary contraction.

OBJECTIVES: The present study explored whether testing tendon jerks during voluntary contraction of the test muscle would improve reproducibility by effectively 'clamping' the excitability of the motoneurone pool at firing threshold. METHODS: Tendon jerk reflexes of soleus, tibialis anterior and vastus lateralis and the soleus H reflex were recorded in 12 healthy subjects at rest and during voluntary contractions of 10-20% of maximum. Recordings were repeated 8-10 times in 5 subjects, in whom reflex symmetry was also determined. RESULTS: Not all tendon jerk reflexes could be recorded at rest, and the variability of latency and amplitude was high. All reflexes could be recorded in each subject during contractions. The latency of tendon jerk reflexes decreased by approximately 2 ms during contractions, but H-reflex latency decreased by only 0.2-0.3 ms. For the tendon jerks, an asymmetry of >3.0 ms at rest and >2.5 ms contracting would be outside 3 SD of the normal mean. In repeat studies, the coefficient of variation of reflex latency was <4% for the tendon jerk. CONCLUSIONS: A voluntary contraction could potentiate the tendon jerk by a number of mechanisms, but the most important is probably enhancement of the excitability of the motoneurone pool. The present techniques should increase the value of tendon reflex testing when assessing possible peripheral nerve, plexus and root disturbances.

Adult↗
PubMed1999

Strength-duration properties and their voltage dependence at different sites along the median nerve.

OBJECTIVES: There is some evidence that the ease with which ectopic activity can be induced varies systematically along the course of a nerve and is greater at more proximal sites. Recent studies have implicated a non-inactivating threshold conductance, possibly due to persistent Na+ channels, in ectopic activity associated with ischaemia and hyperventilation. This conductance is largely responsible for the voltage dependence of strength-duration time constant (tauSD), and changes in it can explain the time constant changes that occur during hyperventilation and ischaemia. METHODS: To determine whether the strength-duration properties of motor axons of the median nerve vary along the course of the nerve, tauSD and rheobase were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC polarizing current to change axonal excitability. RESULTS: tauSD was similar at the 3 stimulating sites but increased less at the axilla with depolarizing current. CONCLUSIONS: These data indicate that the greater tendency for ectopic activity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to tauSD and underlie its dependence on axonal excitability. The findings provide further support for the view that the precise relationship of the stimulating electrodes to the nerve has little effect on tauSD, at least when it is measured in the forearm.

Action Potentials↗
PubMed1999

Value of homonymous and heteronymous monosynaptic reflexes in the diagnosis and follow-up of cervical spinal injuries.

Needle electromyographic examination constitutes the only neurophysiological test used routinely to assess the integrity of proximal nerve pathways. A heteronymous reflex, with a latency consistent with a monosynaptic pathway, can be evoked in the contracting biceps brachii muscle by stimulation of the median nerve at the elbow. This reflex response provides a complementary technique to the biceps and supinator jerks for accessing the C(5)/C(6)motoneuron pool because its afferent limb involves the median nerve and the C(5)/C(6)posterior roots. Three cases are described to illustrate the clinical value of this heteronymous monosynaptic reflex of biceps brachii and of conventional H reflexes in the diagnosis and follow-up of lesions involving the C(5)/C(6)segments. Reflex function can be assessed for most spinal segments innervating the upper and lower limbs, thus allowing both objective confirmation of the clinical findings and documentation of changes over time. Copyright 1999 Harcourt Publishers Ltd.

Journal Article↗
PubMed1999

Determinants of fusion of dichoptically presented orthogonal gratings.

Liu, Tyler, and Schor (1992 Vision Research 32 1471-1479) reported the surprising finding that dichoptically presented orthogonal sine-wave gratings do not always produce binocular rivalry. Gratings of high spatial frequency, and especially of low contrast, fuse to produce a stable percept of a dichoptic plaid. Using a somewhat different perceptual task, we replicated those findings and extended them. The probability of a plaid percept is higher for square-wave gratings than for sine-wave gratings, and higher still for rectangular-wave gratings with high duty cycles (with very thin light or dark bars). Experiments were conducted to test whether this duty-cycle effect was due to changes in overall luminance, or in the size of the regions of luminance congruity (which may reduce the probability of rivalry), but no such effects could account for the results. The presence of locally conflicting contour information in the two eyes was shown to be an important determinant of rivalry onset, but, since removing such regions did not eliminate rivalry, other factors also have a role to play. The spatial frequency composition of the gratings is one such factor which is consistent with all of the findings we report.

Contrast Sensitivity↗
PubMed1999

Comparison of 0.25% S(-)-bupivacaine with 0.25% RS-bupivacaine for epidural analgesia in labour.

We have compared the efficacy of 0.25% S(-)-bupivacaine with 0.25% RS-bupivacaine in providing epidural analgesia for labour in a randomized, multicentre, double-blind study. Analgesia was initiated with 10 ml of the study solution and maintained with 10-ml top-ups. We studied 137 women and treatments were found to be equivalent for onset, duration and quality of block. Median onset of pain relief was 12 min for both drugs and median duration was 49 (range 3-129) min and 51 (7-157) min for S(-)-bupivacaine and RS bupivacaine, respectively. The estimated treatment difference for duration of pain relief was -4 (90% CI -13, 6) min. Thirty patients failed to achieve pain relief after the first injection (20 patients after S(-)-bupivacaine and 10 after RS-bupivacaine; P = 0.039). However, median duration of pain relief from the first top-up was 82 (range 3-164) min for S(-)-bupivacaine and 76 (22-221) min for RS-bupivacaine. There were no significant differences in the quality of analgesia, as assessed by the investigators. There were no significant differences in the extent of sensory block, percentage of patients with motor block or incidence of adverse events.

Adolescent↗
PubMed1999

Muscle spindle activity in the affected upper limb after a unilateral stroke.

Weakness, loss of dexterity and exaggerated reflex responses to proprioceptive and cutaneous stimuli are typical features of hemiparetic stroke. Since the extent to which altered fusimotor drive contributes to these deficits has not been established, this study was designed to assess fusimotor function in stroke patients by comparing three aspects of muscle spindle afferent behaviour (background discharge rate, responses to reflex inputs and responses to voluntary contractions) in 11 subjects affected by recent cerebrovascular lesions, with those in 18 healthy volunteers. The mean background discharge rates of muscle spindle afferents in the radial nerve when subjects attempted to relax the recorded limb completely were 6.6 +/- 5.3 Hz (n = 26) in patients and 6.4 +/- 6.1 Hz (n = 76) in control subjects. The variability of discharge rate of active afferents was also similar (0.12 +/- 0.07 and 0.09 +/- 0. 10, respectively). Reflex activation of fusimotor neurons was assessed using trains of electrical stimuli to the superficial radial nerve or to the palm of the hand, and using natural skin stimuli. Neither type of cutaneous stimulation affected muscle spindle afferent discharge in the absence of an EMG response. During deliberate voluntary contractions muscle spindle discharge rates were enhanced similarly in both the control and patient groups, indicating that volitional drives could access fusimotor neurons in the patients. Qualitatively, spindle behaviour was similar in patients and control subjects. These findings suggest that fusimotor function is not disturbed any more or less than skeletomotor function in hemiparetic patients and it is concluded that fusimotor dysfunction probably contributes little to their deficit.

Acoustic Stimulation↗
PubMed1999

Conduction block in carpal tunnel syndrome.

Wrist extension was performed in six healthy subjects to establish, first, whether it would be sufficient to produce conduction block and, secondly, whether the excitability changes associated with this manoeuvre are similar to those produced by focal nerve compression. During maintained wrist extension to 90 degrees, all subjects developed conduction block in cutaneous afferents distal to the wrist, with a marked reduction in amplitude of the maximal potential by >50%. This was associated with changes in axonal excitability at the wrist: a prolongation in latency, a decrease in supernormality and an increase in refractoriness. These changes indicate axonal depolarization. Similar studies were then performed in seven patients with carpal tunnel syndrome. The patients developed conduction block, again with evidence of axonal depolarization prior to block. Mild paraesthesiae were reported by all subjects (normals and patients) during wrist extension, and more intense paraesthesiae were reported following the release of wrist extension. In separate experiments, conduction block was produced by ischaemic compression, but its development could not be altered by hyperpolarizing currents. It is concluded that wrist extension produces a 'depolarization' block in both normal subjects and patients with carpal tunnel syndrome, much as occurs with ischaemic compression, but that this block cannot be altered merely by compensating for the axonal depolarization. It is argued that conduction slowing need not always be attributed to disturbed myelination, and that ischaemic compression may be sufficient to explain some of the intermittent symptoms and electrodiagnostic findings in patients with carpal tunnel syndrome, particularly when it is of mild or moderate severity.

Action Potentials↗
PubMed1999

Validation of the Ottawa ankle rules in children.

OBJECTIVE: To assess whether the Ottawa ankle rules can be used to accurately predict which children with ankle and midfoot injuries need radiography. METHODS: Prospective study with historical control group of all children aged 1-15 years presenting to Sheffield Children's Hospital accident and emergency department with blunt ankle and/or midfoot injuries during two five month periods before and after implementation of the Ottawa ankle rules. RESULTS: In the study group 432 out of 761 (56.76%) patients received radiography compared with 500 out of 782 (63.93%) in the control group. This was a statistically significant reduction in radiography rate of 7.2% (95% confidence interval 2.3% to 12.1%, p <0.01). The sensitivity of the Ottawa ankle rules was 98.3% and the specificity 46.9%. There was no increase in the number of missed fractures (one in each group). CONCLUSION: The Ottawa ankle rules can be applied in children to determine the need for radiography in ankle and midfoot injuries. Their implementation leads to a reduction in the radiography rate without leading to an increase in the number of missed fractures.

Adolescent↗