PubMed Health⌕ Search

Biomedical subjects

J A Stephens

Publications and source records attributed to J A Stephens.

At least 19 recordsLinked to original sources
PubMed1991

Cross-correlation analysis of multi-unit EMG recordings in man.

This paper discusses the measurement of synchrony between 2 multi-unit EMG recordings. The suitability of a number of previously described indices of synchrony is reviewed. A new index of synchronisation is proposed. This index is independent of the number of units contributing to the correlogram but is dependent upon their firing frequency and upon the bin width of the correlogram. The autospectral density function calculated from each multi-unit record is used to estimate motor unit firing rates. The relationship between the frequency of firing and the chosen index of synchronisation is reciprocal such that if the firing rate is doubled, the index is halved. This may be explained if the proportion of events that is synchronized remains constant and is independent of the firing rate. It is argued that this in turn indicates that the proportion of common and non-common sources driving the neurons also remains unchanged in these experiments. In these circumstances, and to take into account changes in unit firing that may be present in different recordings, it is convenient to normalize an experimentally determined index E/M at a frequency f to some standard frequency fs. The problem of superimposition of spikes that occurs as the contraction strength is increased is discussed.

Adult↗
PubMed1991

Cutaneomuscular reflexes recorded from the first dorsal interosseous muscle of children with cerebral palsy.

Cutaneomuscular reflexes were recorded from the first dorsal interosseous muscles of 21 children with spastic cerebral palsy. The reflex responses in children with cerebral palsy were consistent with dysfunction of the pyramidal system, but differed from those seen in adults with central nervous lesions. This provides further evidence that the effect of a central nervous lesion on the reflex control of movement is dependent on whether it occurs in early or adult life.

Adolescent↗
PubMed1991

Correlation between the discharges of motor units recorded from the same and from different finger muscles in man.

1. Cross-correlation analysis of the discharges of individual motor units recorded from various different finger muscles has been performed during weak, isometric, voluntary contractions in man. 2. The dominant feature in 88% of the cross-correlograms studied was a narrow, central peak, the area of which significantly exceeded that expected for independent processes (P less than 0.001). The highest bin counts in these central peaks were mostly within 5 ms of time zero in the histograms, and the base of these peaks extended between 5 and 31 ms (modal value = 13 ms with 90% of the values lying between 8 and 18 ms). The width and displacement of the central cross-correlogram peaks were similar irrespective of whether the contributory spike trains were recorded from motor units active in the same finger muscle or recorded from motor units in different, co-activated finger muscles. 3. The time course of the central peaks in this study was found to be consistent with the hypothesis that it is generated by the joint occurrence of EPSPs evoked in motoneurones by branches of common stem presynaptic fibres using the theoretical model developed by Kirkwood (Kirkwood & Sears, 1978). The model parameters providing the best fit with our experimental data imply that synaptic contacts on motoneurones made by these common inputs lie on average peripherally in the dendritic tree and generate small (less than 300 microV) EPSPs superimposed on a high level of background synaptic noise. 4. Minima (troughs) were found either side of the central peak in 27% of the cross-correlograms studied, and their appearance was invariably associated with a large central peak. These secondary features could not be modelled with the same operator parameters that describe the central peaks. Their presence was particularly noticed in association with very regular discharges from the output motoneurones. 5. Smaller and broader secondary peaks symmetrically displaced 30-55 ms either side of the large, narrow central peak were observed in 7% of the cross-correlograms studied. We suggest that these secondary features which were found at lags shorter than the interspike interval of the contributory motor unit spike trains reflect the autocorrelation functions of the spike trains of common input fibres. On this basis the observed displacement of these secondary peaks from the primary feature in the cross-correlogram indicate firing rates for common input fibres in the range 18-33 impulses s-1. 6. In a small number of cases (1.4%) the cross-correlogram was flat and indistinguishable from the results of cross-correlating independent spike train data.

Action Potentials↗
PubMed1991

Variation in the degree of synchronization exhibited by motor units lying in different finger muscles in man.

1. Cross-correlation analysis was performed on the discharges of motor units recorded from the same or from different finger muscles during weak, voluntary isometric contractions in man. 2. In 88% of cases such cross-correlograms contained a narrow central peak indicating synchronization between the firing of the contributory motor units. Expressed in terms of the synchronization index, b (peak area/total number of reference plus response spikes) the amount of synchronization found in these different recordings ranged from b = 0.0057 to b = 0.1436 (n = 1230). The duration of the synchronization (measured across the base of the peak) ranged from 5 to 31 ms (mode = 13 ms). 3. For each pair of motor units examined the amount of synchronization between their firing varied from minute to minute of a long recording. In around half the cases studied this variation appeared to correspond to variation in the firing rates of the two motor units whereas in the other half of cases tested no relationship was found between the firing rates and the amount of synchronization. 4. Some motor unit pairings consistently showed more synchronization than other motor unit pairings within the same muscle. A frequency histogram of the synchrony measurements from all of the motor unit pairings tested in that muscle showed a unimodal and continuous distribution. 5. Some subjects consistently showed two or three times more motor unit synchronization than others in equivalent recordings. This rank order of motor unit synchronization in different subjects was found to be the same in all muscle pairings tested. 6. A similar distribution in the amount of motor unit synchronization found in different muscle pairings was found in all subjects tested. In the first place the firing of motor units which act on widely separated fingers was less synchronized than the firing of motor units acting on adjacent fingers. Secondly, motor units acting on the lateral fingers (thumb, index) showed less synchronization in their discharges than motor units acting on medial fingers (ring, little). Finally the firing of motor units in the finger flexor muscles were less synchronized than the firing of motor units in either the finger abductor or the finger extensor muscles. 7. The synchronization of motor unit activity in different muscles indicates the presence of a widespread projection pattern for the branches of some last-order input fibres to finger muscle motoneurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1991

Central nervous pathways underlying synchronization of human motor unit firing studied during voluntary contractions.

1. Motor unit firing has been studied during weak voluntary isometric contractions with pairs of needle electrodes in normal human subjects. 2. Pre- and post-stimulus time histograms of the firing time of firing of one event unit before and after the time of firing of another reference (stimulus) unit showed a clear central peak, indicative of synchronization. 3. Synchronization was seen in all the muscles studied. The mean strength of synchronization, expressed as the number of concomitant discharges of the two units as a proportion of the number of stimulus unit discharges, was 0.095 extra event unit spikes/reference unit spike (range 0.042-0.28) for first dorsal interosseous muscle, 0.016 extra event unit spikes per reference unit spike (range 0-0.043) for medial gastrocnemius and 0.056 extra event unit spikes per reference unit spike range 0.016-0.079) for tibialis anterior. 4. The mean duration of synchronization was 11.3 ms (range 5.0-21.0 ms) for first dorsal interosseous, 10.3 ms (range 3.5-21.7 ms) for medial gastrocnemious and 13.5 ms (range 3.0-25.0) for tibialis anterior. 5. Seven patients with radiographically and clinically identified central strokes were studied while they made weak voluntary isometric contractions. The duration of synchronization was significantly prolonged compared to that found in normal subjects. In these stroke patients the mean duration of synchronization on the affected side was longer than that seen in the normal subjects, and in first dorsal interosseous muscle was 35.4 ms (range 12.0-65.0 ms), in medial gastrocnemius was 21.3 ms (range 4.0-43.0 ms) and in tibialis anterior was 28.8 ms (range 14.0-49.0 ms). 6. The mean strength of synchronization of motor unit discharge was found to be greater in the stroke patients than that seen in the normal subjects for first dorsal interosseous muscle (0.161 extra event unit spikes per reference unit spike, range 0.017-0.391) and for medial gastrocnemius (0.030 extra event unit spikes per reference unit spike) but only significantly so when pooled data was compared. There was no difference in the strength of motor unit synchronization in tibialis anterior between stroke patients and normal subjects. 7. Broad duration synchronization among first dorsal interosseous motor units was also found in a patient with a rostral cervical spine lesion (total duration range 43-46 ms; n = 2), but not in a patient with a caudal (thoracic) spinal lesion.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1991

Effect of task on the degree of synchronization of intrinsic hand muscle motor units in man.

1. Recordings were made of the firing of pairs of intrinsic hand muscle motor units active under different task conditions in man. The different tasks were defined as isometric contractions producing force in one of three different directions: finger abduction, finger extension, or finger flexion. The degree of motor-unit synchronization associated with each of these task conditions was compared with the use of cross-correlation analysis. 2. The average amount of synchronization between the firing of motor units recorded from within first dorsal interosseous muscle (IDI) was greater during index finger extension than during index finger abduction (n = 8 motor-unit pairings, 3 subjects). In addition, for another sample population of motor units, the average amount of synchronization was greater during index finger abduction than during index finger flexion (n = 11 motor-unit pairings, 4 subjects). 3. In a further series of experiments, one motor unit of each pair was recorded from second dorsal interosseous muscle (2DI), whereas the other motor unit of each pair was recorded from 1DI. The average amount of synchronization for these motor-unit pairings was greater during extension of the index and middle fingers than during abduction of the index and middle fingers (n = 8, 4 subjects). For another sample population of such motor-unit pairings, the average amount of synchronization was found to be greater during abduction of the index and middle fingers than during flexion of the index and middle fingers (n = 11, 4 subjects). 4. In approximately one-third of cases, it was not found possible to maintain the same firing rates from two motor units in 1DI when active under different task conditions. For instance, the "reference" motor unit might consistently fire at a faster rate than the "response" motor unit when active during index finger extension but consistently fire at a slower rate than the response motor unit when active during index finger abduction. Where such motor-unit pairs have been studied in detail, the pattern of task dependence in their synchronization was found to be similar to that described above for motor-unit pairs in which the firing rates remained constant under the different task conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1991

Plasticity of central motor pathways in children with hemiplegic cerebral palsy.

To obtain neurophysiologic evidence for a reorganization of central motor pathways in children who had suffered a cerebral lesion at birth, we performed cross-correlation analyses of multiunit EMG recordings obtained from children with hemiplegic cerebral palsy and marked mirror movements. We found that the motoneuron pools of homologous left and right hand muscles received common synaptic input from abnormally branched presynaptic axons. The results of electromagnetic brain stimulation, cutaneomuscular, and tendon reflex testing suggested that these common inputs are provided by abnormally branched corticospinal tract fibers whose origin is the undamaged motor cortex.

Cerebral Palsy↗
PubMed1991

Diversity of pilin of serologically distinct Bacteroides nodosus.

Pili from 11 distinct serotypes of Bacteroides nodosus were examined for diversity of pilin polypeptide subunits among serotypes and for purity of the pilin preparations. The pilin of all 11 samples was shown to be homogeneous. Mean +/- SD molecular weight of the pilin of 7 serotypes (A198, IV, V, VI, IX, XVII, and XVIII) was 18,500 +/- 100. The pilin of serotypes I, III, and VIII had molecular weight of 17,600, 19,400, and 19,000, respectively. Serotype XV differed greatly from the other 10 serotypes in that 2 distinct polypeptide bands with molecular weight of approximately 7,800 and 6,200 were detected. We suggest that these 2 low molecular weight bands resulted from proteolytic cleavage of the pilin protein.

Bacteroides↗
PubMed1991

Monoclonal antibodies against pili of serologically distinct Bacteroides nodosus.

Several monoclonal antibodies (McAbs) against pili of Bacteroides nodosus were examined to determine their reactivity with 11 different serotypes. One McAb was identified by enzyme-linked immunosorbent assay (ELISA) analysis that bound to nine of the 11 serotypes and another that bound to the remaining two serotypes tested. In addition, some McAbs demonstrated specificity for a single serotype, while others displayed specificities for up to five other serotypes. Comparison of immunoblot analysis with the ELISA revealed that the former method was not as sensitive in that all McAbs positive by the ELISA, were not positive by immunoblot. Possible explanations of these findings are discussed. There appear to be several antigenic determinants on B. nodosus pili and considerable sharing of these determinants between pili types. The 11 serotypes analyzed by the McAbs in this report are representative of all 20 US serotypes as well as the A-set and D-set categories of Australia. Therefore, the two epitopes recognized by two of the McAbs reported herein encompass all of the currently characterized B. nodosus serotypes and may provide a basis for bivalent vaccines efficacious for all types of B. nodosus induced footrot in sheep.

Animals↗
PubMed1990

Synchronization of motor unit activity during voluntary contraction in man.

1. Motor unit synchronization has been studied in human first dorsal interosseous muscle. 2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded. 3. Pre- and post-stimulus histograms of the firing of unit pairs showed a narrow central peak of duration 1.3-9.3 ms (88% of sample in the range 1-6 ms; mode 3.0 ms), together with a variable amount of synchronization of somewhat longer duration. 4. For the duration of the whole synchronization peak (85% sample in range 5-15 ms; mode between 6.1 and 8.0 ms (31% of sample], units fired between 8 and 485% times more often than would have been expected had the units been firing independently of one another. Amplitudes of the peak of the recorded histograms expressed as a proportion of control ranged from 1.8 to 10.9 (mean 3.9; bin width 160 microseconds). 5. The strength of synchronization between the firing of motor unit pairs was inversely related to differences in recruitment threshold. The largest amount of synchronization was observed for pairs of units in which both had recruitment thresholds less than 0.5 N or greater than 1.0 N. Less synchronization was found between pairs of units in which one had a recruitment threshold less than 0.05 N and the other a threshold greater than 1.0 N. 6. The time course of synchronization was well matched by the predictions of a theoretical model based on the hypothesis that underlying the observed synchronization is the joint arrival of EPSPs from branched last-order input fibres.

Action Potentials↗
PubMed1990

Maturation of the cutaneomuscular reflex recorded from the first dorsal interosseous muscle in man.

1. Cutaneous reflexes have been recorded from the first dorsal interosseous muscle following electrical stimulation of the digital nerves of the index finger in man. Recordings have been obtained from 127 children, aged from 3 to 18 years while each performed an isometric abduction of the index finger and whole hand grips. 2. In the adult, reflexes recorded while subjects performed an isometric abduction of the index finger consisted of three readily identifiable components within the first 100 ms. These were an initial short spinal latency increase in EMG (E1), followed by a decrease (I1), followed by a prominent longer-latency increase (E2) which was normally larger in size than the shorter-latency increase. 3. The configuration of the reflex changed during development. When recorded during an abduction, the E1 component was present in all children and showed a small but progressive reduction in size with age. With increasing age, both the I1 and E2 components were present in an increasing proportion of children and increased in size until the adult configuration was reached at 14-15 years. 4. Comparison of reflexes recorded during sustained whole hand grips with reflexes recorded during an isometric index finger abduction revealed differences in all three components. When recorded during those grips used in the present study the E1 component was generally significantly larger while the I1 component was generally significantly smaller than when recorded during index finger abduction. At all ages the E2 component was significantly smaller when recorded during a grip. 5. The configuration of the reflex during an isometric index finger abduction was compared with the ability of the child to perform rapid finger movements. Those children who had no E2 component tended not to perform well in the tests of rapid finger movement. However, there was no simple relationship between the size of the E2 component and ability to produce rapid finger movements.

Adolescent↗
PubMed1990

Mirror movements studied in a patient with Klippel-Feil syndrome.

1. Electromyographic (EMG) recordings have been made from upper limb muscles in a patient with well-defined congenital mirror movements occurring in association with Klippel-Feil syndrome and the results compared to those obtained in normal control subjects. 2. In the patient, liminal percutaneous electrical or magnetic brain stimulation applied over either hemisphere elicited bilateral and symmetrical short-latency muscle responses in relaxed intrinsic hand muscles. In the normal subjects unilateral brain stimulation only elicited contralateral muscle responses. 3. F response and H reflex studies for the patient's ulnar-supplied intrinsic hand muscles were normal. No crossed responses were recorded in the homologous muscles of the contralateral hand. 4. Scalp-recorded somatosensory-evoked responses following ulnar or median nerve stimulation were of normal latency and distribution in the patient. 5. In the patient, cross-correlation analysis of on-going single and multiunit needle EMGs recorded between muscles of left and right hands revealed a central peak in the cross-correlogram. No cross-correlogram peaks were found between left- and right-hand muscles in normal subjects. The magnitude and time course of the central peaks in the cross-correlograms constructed between the firing of motor units on opposite sides of the body in the patient were similar to those found in cross-correlograms constructed between the firing of motor units from muscles on the same side of the body in the patient and in normal subjects. 6. The magnitude of cross-correlogram peaks detected within a muscle and those detected between left and right homologous muscles showed a gradient in which the largest peaks were found in the intrinsic hand and forearm extensor muscles. The smallest peaks were observed in the forearm flexor muscles. No peaks were detected between left and right biceps brachii muscles. In intrinsic hand muscles, the size of the cross-correlogram peak detected between the EMGs of homologous muscle pairs was greater than that found for non-homologous muscle pairs. 7. Cutaneous reflex responses were recorded from first dorsal interosseous muscle following unilateral electrical stimulation of the digital nerves of the index finger. In the patient, this produced an early excitatory (E1) response on the stimulated side. Later excitatory (E2 and E3) responses, of approximately equal size and latency, were distributed bilaterally. In the normal subjects, reflex responses were confined to the stimulated side.(ABSTRACT TRUNCATED AT 400 WORDS)

Arm↗
PubMed1989

Task-dependent changes in cutaneous reflexes recorded from various muscles controlling finger movement in man.

1. Cutaneous reflex responses have been recorded from muscles involved in the control of finger movement following electrical stimulation of the digital nerves of the fingers in man. 2. Recordings have been made while subjects performed various manual tasks. 3. Reflexes recorded while subjects performed a relatively isolated finger movement consisted of an initial short-latency increase in muscle electrical activity, followed by a decrease, followed by a prominent longer-latency increase. The long-latency excitatory component was smaller or absent during those grips used in the present study. 4. The short-latency excitatory (E1) and inhibitory (I1) components of the cutaneomuscular reflex response are mediated via spinal pathways. The second longer-latency excitatory component (E2) is of supraspinal origin, requiring the integrity of the dorsal columns, sensorimotor cortex and corticospinal tract (Jenner & Stephens, 1982). The results of the present study suggest that one or more of these supraspinal pathways is more active when a finger is used in a relatively isolated manner than when the same finger participates in any of the gripping manoeuvres used in the present experiments.

Adolescent↗
PubMed1989

Task-dependent changes in the size of response to magnetic brain stimulation in human first dorsal interosseous muscle.

1. Electromyographic responses have been recorded from human first dorsal interosseous muscle (FDI) in response to magnetic and transcutaneous electrical stimulation of the brain. 2. Following magnetic but not electrical stimulation of the brain, the recorded EMG response was larger when FDI was active during voluntary isometric index finger abduction than during a power grip. 3. In the same experiment, cutaneous reflex responses have been recorded from FDI following electrical stimulation of the digital nerves. The long-latency excitatory component at about 60 ms (E2) was larger when recorded during voluntary finger abduction than during a power grip. This difference in size of E2 with task bore no simple relationship to the difference in size with task of the motor response to magnetic brain stimulation. 4. The results are discussed in relation to the presumed site of action of magnetic and electrical brain stimulation. It is concluded that the results may best be interpreted by assuming a higher level of cortical activity during a voluntary index finger abduction than during a grip and that this could in part explain the task-dependent changes in the long-latency response to cutaneous stimulation.

Adult↗
PubMed1985

Maturation of cutaneous reflex responses recorded in the lower limb in man.

Cutaneous reflex responses from tibialis anterior muscle following electrical stimulation were recorded for 39 normal children between the ages of one and 16 years. In the first year of life stimulation produces a monophasic excitatory reflex response at short latency; in the second year it produces long- as well as short-latency increases in activity. Maturation of reflex function continues during the school years, with progressive emphasis on long- rather than short-latency components. These results are discussed in relation to mechanisms underlying the acquisition of motor skills during childhood.

Adolescent↗
PubMed1985

Cutaneous reflex responses recorded in children with various neurological disorders.

Cutaneous reflex responses were recorded from tibialis anterior or first dorsal interosseous muscles of children with hemiplegia, spinal-cord compression, necrotizing sacroid granulomatosis, acute encephalomyelitis, myalgic encephalomyelitis, and a group of children attending the Learning Difficulties Clinic. Abnormalities of response are reported and are compared with the different reports in the literature of abnormal reflex EMG responses recorded by various methods. It is concluded that cutaneo-muscular reflex testing may have a part to play in the diagnosis of difficult paediatric problems.

Adolescent↗
PubMed1984

Immune-recognition of Schistosoma mansoni primary sporocysts may require specific receptors on Biomphalaria glabrata hemocytes.

Cellular interactions, leading to cell-mediated cytotoxicity when Biomphalaria glabrata hemocytes encapsulate Schistosoma mansoni sporocysts, have been investigated. Rabbit antibodies (IgG), when bound to antigens on sporocyst surfaces, prevent the normal cytoadherence (CA) of hemocytes from both susceptible and resistant host snails. Since interference with CA occurs with even fixed sporocysts, the effect is not due to IgG stimulated modulation of the parasite surface. Using two antisera with some overlapping specificities, and quantitative immunofluorescent antibody technique (QIFAT), we determined the concentrations of IgG needed to place equivalent amounts of IgG on the sporocysts. At these concentrations, CA was affected differentially, implying that interference was due to the specific antigens bound, and not due simply to the presence of IgG. Also with QIFAT we determined how much F(ab')2 and IgG from anti-sporocyst serum were needed to block an equivalent amount of antigenic determinants from access by whole FITC labelled IgG. Sporocysts whose surface antigens were equally blocked were equally unadherent for hemocytes, supporting the notion that the nature of obscured antigens, and neither the Fc portion nor the larger size of intact IgG protein, was responsible for the effect on CA. These surprising results imply a role for specific antigen binding sites on snail hemocytes.

Animals↗