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Biomedical subjects

D Burke

Publications and source records attributed to D Burke.

At least 253 records · Page 14Linked to original sources
PubMed1988

Interfering cutaneous stimulation and the muscle afferent contribution to cortical potentials.

Cerebral potentials were recorded in response to selective stimulation using microelectrodes of muscle afferents in motor fascicles innervating the intrinsic muscles of the foot or at the motor point of abductor hallucis. The early components of these potentials (P40, N50 and P60) were consistently attenuated by continuous tactile stimulation of related skin areas and by electrical stimulation of digital nerves, timed so that the digital volley reached cortex approximately 5 msec before the muscle afferent volley. The same conditioning cutaneous inputs also attenuated the cerebral potentials evoked by selective stimulation of cutaneous afferents. These findings confirm that there are intermodality and intramodality interactions between low-threshold cutaneous and muscle afferents and between cutaneous afferents, respectively. The findings indicate that 'interference phenomena' (Kakigi and Jones 1986) can occur between different afferent modalities, and within any one modality, and cannot be used to determine the afferent species responsible for the test evoked potential.

Cerebral Cortex↗
PubMed1988

Projection to the cerebral cortex from proximal and distal muscles in the human upper limb.

Motor-point stimulation using insulated microelectrodes has been used to assess the cerebral projection from a number of single muscles in the human upper limb. Previous studies with intrafascicular neural stimulation have revealed a short-latency cortical projection from group I muscle afferents of intrinsic muscles of the hand but this technique is unsuitable to study the projection of afferents from proximal muscles. In 6 subjects, stimuli were delivered to the motor point of the following muscles: anterior deltoid, biceps brachii (short head), flexor carpi radialis, extensor digitorum communis, abductor pollicis brevis and first dorsal interosseous. The mean latency of the early cortical negativity was 15.0 ms for deltoid, 16.4 ms for biceps, 18.8 ms for flexor carpi radialis, 18.8 ms for extensor digitorum communis, 21.9 ms for abductor pollicis brevis and 25.0 ms for first dorsal interosseous. The amplitude of cerebral potentials from individual muscles was smaller than that produced by stimulation of the digital nerves of the index or little finger and did not vary systematically between proximal and distal muscles. This study provides direct evidence for a short-latency cortical projection from proximal muscles of the upper limb. The results are consistent with psychophysical studies which have proposed a role for intramuscular receptors in kinaesthesia at both proximal and distal joints.

Afferent Pathways↗
PubMed1988

Responses to passive movement of receptors in joint, skin and muscle of the human hand.

1. Microneurographic techniques were employed to record unitary activity from afferents associated with digital joints of six conscious human subjects. Of 120 single afferents sampled from the median and ulnar nerves at the wrist, eighteen (15%) were classified as joint afferents; the majority of the sample (72.5%) were of cutaneous origin, and 12.5% were from muscle spindles and tendon organs. 2. Of the eighteen joint afferents six were tonically active in the rest position of the hand. All except two were recruited or accelerated their background discharge during passive joint movement. Three tonically active afferents were responsive to passive movement throughout the physiological range. The majority of the afferents, including the other three tonically active units, responded only towards the limits of joint rotation. 3. As a group, the sample of joint afferents had a limited capacity to signal the direction of joint movement. Nine of the sixteen joint afferents sensitive to movement responded in two axes of angular displacement, and two responded in all three axes. In any one axis of rotation eight afferents were activated in both directions of movement. However, one afferent, associated with the interphalangeal joint of the thumb, responded uni-directionally throughout the physiological range of joint movement and was thereby capable of adequately encoding joint position and movement. 4. Twenty-one of twenty-nine slowly adapting and eleven of eighteen rapidly adapting cutaneous afferents tested were activated by joint movement, but only towards the limits of joint rotation; half of the thirty-two movement-sensitive afferents were bi-directionally responsive. Muscle spindle afferents responded to stresses applied to the joint only if the resulting passive movement stretched the parent muscle. 5. It is concluded that human joint afferents possess a very limited capacity to provide kinaesthetic information, and that this is likely to be of significance only when muscle spindle afferents cannot contribute to kinaesthesia.

Action Potentials↗
PubMed1988

The optimal recording electrode configuration for compound sensory action potentials.

There is no uniformity in the published literature from different laboratories on the optimal electrode configuration for recording nerve action potentials, and a number of standard texts omit any reference to the effects that interelectrode distance and electrode orientation can have on the shape, amplitude and latency of nerve action potentials. The sensory action potential from the digital nerves of the index finger was recorded at wrist and elbow using bipolar electrodes with the "active" electrode over the median nerve and the "reference" placed 4 cm laterally or proximally along the nerve using interelectrode distances of 4, 3 and 2 cm. These potentials were compared with that recorded using a remote reference on the ipsilateral shoulder, the assumption being that this configuration eliminated the contribution of the reference electrode to the compound nerve action potential. With different electrode configurations, there were significant differences in the shape of the potential, the latencies to onset and peak and the rising- and falling-phase amplitudes. The shorter the distance between the electrodes the greater the distortions. Overall, the distortions were least with the 4 cm interelectrode separation, particularly for short conduction distances.

Electric Stimulation↗
PubMed1988

Reflex changes in muscle spindle discharge during a voluntary contraction.

1. This study was undertaken to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the foot reflexly affect fusimotor neurons innervating the plantar and dorsiflexors of the ankle during voluntary contractions. 2. Recordings were made from 29 identified muscle spindle afferents innervating triceps surae and the pretibial flexors. Trains of electrical stimuli (5 stimuli, 300 impulses per second) were delivered to the sural nerve at the ankle (intensity: 2-4 times sensory threshold) and to the posterior tibial nerve at the ankle (intensity: 1.5-3 times motor threshold for the small muscles of the foot). The stimuli were delivered while the subject maintained an isometric voluntary contraction of the receptor-bearing muscle, sufficient to accelerate the discharge of each spindle ending. This ensured that the fusimotor neurons directed to the ending were active and influencing the spindle discharge. The effects of these stimuli on muscle spindle discharge were assessed using raster displays, frequencygrams, poststimulus time histograms (PSTHs) and cumulative sums ("CUSUMs") of the PSTHs. Reflex effects onto alpha-motoneurons were determined from poststimulus changes in the averaged rectified electromyogram (EMG). Reflex effects of these stimuli onto single-motor units were assessed in separate experiments using PSTHs and CUSUMs. 3. Electrical stimulation of the sural or posterior tibial nerves at nonnoxious levels had no significant effect on the discharge of the 14 spindle endings in the pretibial flexor muscles. The electrical stimuli also produced no significant change in discharge of 11 of 15 spindle endings in triceps surae. With the remaining four endings in triceps surae, the overall change in discharge appeared to be an increase for two endings (at latencies of 60 and 68 ms) and a decrease for two endings (at latencies of 110 and 150 ms). The difference in the incidence of the responses of spindle endings in tibialis anterior and in triceps surae was significant (P less than 0.05, chi 2 test). 4. For both triceps surae and pretibial flexor muscles the electrical stimuli to sural or posterior tibial nerves had clear effects on the alpha-motoneuron pool, whether assessed using surface EMG or the discharge of single-motor units. Based on EMG recordings using intramuscular wire electrodes, the reflex effects differed for the gastrocnemii and soleus. 5. In this study, reflex changes in the discharge of human spindle endings were more difficult to demonstrate than comparable changes in the discharge of alpha-motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Ankle↗
PubMed1988

Muscle afferent activity and its central projection in man.

Muscle is a complex sensory organ as well as a contractile apparatus, and disease processes that produce muscle weakness and wasting will affect the sensory information transmitted by receptors in muscle. In addition, one of the sensory structures in muscle, the muscle spindle, receives a motor innervation, the gamma-efferent or fusimotor system, with which the brain can alter the feedback that it receives from muscle. Muscle spindle activity forms the afferent limb of spinal reflexes, such as the tendon jerk, and long-loop reflexes that traverse supraspinal reflex pathways. Muscle spindle activity constitutes the major afferent cue for kinaesthetic sensations and contributes to updating the centrally generated programme for movement. This paper reviews briefly some aspects of muscle spindle activity and its fusimotor control as studied in human subjects using microneurography.

Afferent Pathways↗
PubMed1987

Sexual orientation as a continuous variable: a comparison between the sexes.

Sexual orientation was measured as a continuous variable based upon reported percentage of sexual fantasies and percentage of sexual experiences involving members of the same sex. In the present sample (which probably somewhat overrepresented the homosexual end of the continuum) about one-third of both males and females reported at least occasionally fantasizing about sexually interacting with members of the same sex. The survey indicated about one-third of males also reported having had at least one intimate sexual experience with the same sex, whereas only 10% of females did so. Also, virtually all females who sexually fantasized about the same sex only did so occasionally, whereas most of the males who fantasized about the same sex did so much more exclusively. Although these results cannot be considered representative of the distribution of sexual orientation in any natural population, they suggest that there are advantages in measuring sexual orientation as a continuous variable instead of as one with only a few discrete categories. The results also support other studies that have concluded that when deviations from exclusive heterosexuality are present, they are likely to be much more extreme among males than among females.

Adult↗
PubMed1987

Hyperexcitability of cutaneous afferents during the supernormal period. Relevance to paraesthesiae.

The excitability of cutaneous afferents in the median nerve of 20 normal human subjects was investigated using a supramaximal conditioning stimulus and a submaximal test stimulus that produced an afferent volley of 20 to 40% of maximum. The intensity of the test stimuli was 1.1 to 1.25 times threshold for the first recruited axons (defined as a compound action potential of approximately 0.5 microV). The amplitude of the test volley did not reach the control amplitude until conditioning-test intervals of 3.4 ms. This was followed by a period of increased axonal excitability which reached a peak at intervals of 5 to 8 ms and declined gradually into a late phase of subnormality at 15 to 20 ms. The amplitude of the submaximal test potential was potentiated by, on average, 47% (range 9-146%) during the supernormal period. There were no significant changes in latency. The time course and the degree of potentiation were reproducible in the same subject on separate occasions. When the limb was cooled, the latency of the onset of supernormality was increased and the degree of supernormality was decreased and ultimately abolished. There was an inverse relationship between the degree of potentiation and the size of the test volley. High-frequency repetitive stimulation of the digital nerves for 10 min modified axonal excitability and produced spontaneous paraesthesiae that lasted for 20 min after the stimulation. Following this prolonged stimulus train, single submaximal stimuli evoked larger cutaneous afferent potentials than before the train, and the extent of the potentiation during the supernormal period was increased. It is concluded that prolonged repetitive stimulation of cutaneous afferents has two effects, one on axonal threshold, the second on the supernormal period. It is suggested that, in this experimental model, paraesthesiae result from ectopic impulses generated when axonal excitability is highest, namely at the peak of the supernormal period.

Adult↗
PubMed1987

In-parallel and in-series behavior of human muscle spindle endings.

1. The responses of 62 putative muscle spindle afferents innervating the pretibial flexor muscles of normal human subjects were studied during graded twitch contractions of the receptor-bearing muscle to search for possible in-series coupling between spindle endings and motor units. 2. The majority of afferents (n = 57) responded unequivocally in an in-parallel manner to the twitch contractions, regardless of contraction strength. There were two patterns of in-parallel response: afferents without background activity discharged during the relaxation phase of the twitch; afferents with a background discharge were transiently silenced during the contraction phase and resumed their discharge on the relaxation phase. 3. Evidence of in-series coupling was found for five afferents during submaximal twitch contractions, to which each afferent responded in a mixed "biphasic" manner, with increases in discharge during both the contraction and relaxation phases of the twitch. Background discharge, response to stretch, and response during isometric voluntary contractions suggested that four of the afferents innervated primary spindle endings and the fifth a secondary spindle ending. 4. It is argued that the five atypical spindle endings responded in an ambiguous manner during twitch contractions of the receptor-bearing muscle because there was an in-series mechanical coupling between motor units and the spindle. The incidence of demonstrable in-series responses has serious implications for the mechanisms of spindle activation during normal motor behavior, but has only minor implications for the use of the twitch test to identify muscle spindle endings.

Action Potentials↗
PubMed1986

Coupling between human muscle spindle endings and motor units assessed using spike-triggered averaging.

The electromyographic activity (EMG) generated by voluntary contraction of a muscle was averaged using the potentials from 18 identified muscle spindle afferents as a trigger. In post-spike averages of 1000-10,000 sweeps, no evidence of reflex excitation of the homonymous motoneurone pool was detected. In pre-spike averages there was no evidence of a motor-unit EMG potential that was closely correlated to the trigger spike. A single spindle afferent has only a weak reflex effect on an active motoneurone pool and must be part of a synchronized volley to affect motoneurone discharge significantly. No evidence was found for spindle activation via beta-motoneurones in weak voluntary contractions.

Animals↗
PubMed1986

The refractory period of the visual evoked potential produced by pattern reversal in multiple sclerosis.

Refractoriness of the visual evoked potential to pattern-reversal stimulation has been studied using a light-emitting diode stimulator, the effect on a test P100 of a conditioning pattern reversal being determined for conditioning-test intervals of 20 ms, 50 ms and 100 ms. In 20 normal subjects, the latency of the test P100 was increased by a mean of 9.8% (SD 5.0%) with the 20-ms interval, 5.8% (SD 6.6%) with the 50-ms interval, and 2.8% (SD 4.7%) with the 100-ms interval. Patients with clinically definite multiple sclerosis were divided into 3 groups based on the latency of P100 in the conventional VEP: within 2 SD of the normal mean (7 patients), plus 3-6 SD (7 patients), and greater than 10 SD (7 patients), representing normal, mildly abnormal and grossly abnormal latencies, respectively. Only 4 patients had "abnormal" results in the conditioning-test paradigm, 2 of whom were in the "grossly abnormal" group and 1 in each of the other two groups. The prolongation in latency tended to be greater the longer the control P100. The conditioning-test paradigm did not reveal evidence of a significantly prolonged refractory period of demyelinated axons in the optic nerve, and did not assist in the identification of changes "typical of demyelination" in patients with normal or mildly abnormal control P100s. It is concluded that testing the refractoriness of the VEP may not enhance the diagnostic yield of the VEP in multiple sclerosis.

Brain↗
PubMed1986

Reflex influences on muscle spindle activity in relaxed human leg muscles.

The study was designed to determine whether low-threshold cutaneous and muscle afferents from the foot reflexly activate gamma-motoneurons innervating relaxed muscles of the leg. In 15 experiments multiunit recordings were made from 21 nerve fascicles innervating triceps surae or tibialis anterior. In a further nine experiments the activity of 19 identified single muscle spindle afferents was recorded, 13 from triceps surae, 5 from tibialis anterior, and 1 from extensor digitorum longus. Trains of electrical stimuli (5 stimuli, 300 Hz) were delivered to the sural nerve at the ankle (intensity, twice sensory threshold) and the posterior tibial nerve at the ankle (intensity, 1.1 times motor threshold for the small muscles of the foot). In addition, a tap on the appropriate tendon at varying times after the stimuli was used to assess the dynamic responsiveness of the afferents under study. The conditioning electrical stimuli did not change the discharge of single spindle afferents. Recordings of rectified and averaged multiunit activity also revealed no change in the overall level of background neural activity following the electrical stimuli. The afferent responses to tendon taps did not differ significantly whether or not they were preceded by stimulation of the sural or posterior tibial nerves. These results suggest that low-threshold afferents from the foot do not produce significant activation of fusimotor neurons in relaxed leg muscles, at least as judged by their ability to alter the discharge of muscle spindle afferents. As there may be no effective background activity in fusimotor neurons innervating relaxed human muscles, it is possible that these inputs from the foot could influence the fusimotor system during voluntary contractions when the fusimotor neurons have been brought to firing threshold. In one subject trains of stimuli were delivered to the posterior tibial nerve at painful levels (30 times motor threshold). They produced an acceleration of the discharge of a spindle in soleus at a latency of approximately 125 ms, in advance of detectable activity in skeletomotor neurons and before an increase in muscle length was noted. It presumably resulted from activation of gamma-motoneurons innervating soleus by small myelinated afferents (A-delta range).

Electric Stimulation↗
PubMed1985

Effect of training on voluntary activation of human fusimotor neurons.

The study was designed to determine if human subjects could develop a strategy that would allow them to activate muscle spindle afferents selectively, without contraction or stretch of the receptor-bearing muscle. Recordings were made from 19 identified muscle spindle afferents using insulated tungsten microelectrodes inserted into motor fascicles innervating ankle dorsiflexors, ankle plantar-flexors, and intrinsic muscles of the hand. The discharge of nine of the spindle endings accelerated in voluntary contractions at low levels of effort (less than 10% of maximum force). The remaining 10 endings had relatively high thresholds for activation in voluntary contractions. Despite periods of relative freedom to move and prolonged feedback of the spindle discharge and relevant electromyographic signals, subjects did not develop a strategy with which they could activate any of the afferents selectively. The findings suggest that fusimotor neurons in awake human subjects cannot be activated voluntarily without also activating low-threshold alpha-motoneurons. This is in contrast to reports of selective activity in muscle spindle afferents in freely moving cats. There are two possible explanations: 1) the motor tasks studied in man and cat are not equivalent, or 2) there is a species difference in the control and excitability of fusimotor neurons.

Action Potentials↗