Search PubMed⌕ Search

Biomedical subjects

A Roberts

Publications and source records attributed to A Roberts.

At least 163 records · Page 9Linked to original sources
PubMed1995

The influence of premotor interneuron populations on the frequency of the spinal pattern generator for swimming in Xenopus embryos: a simulation study.

Our aim was to test the hypothesis that the frequency of neuronal rhythm-generating networks is partly controlled by the size of the active premotor interneuron population. We have tested possible mechanisms for frequency changes in a population model of the Xenopus laevis embryo spinal rhythm-generating networks for swimming. After initiation by a brief sensory excitation, the frequency of swimming activity decreases to a steady level determined by the properties of the 24 interneurons and their connections. The initial frequency decrease was dependent on the time-course of initiating sensory synaptic excitation. When some premotor excitatory interneurons were given weaker synaptic connections to reflect the variability in the spinal cord, they could drop out and stop firing during the initial frequency decrease while swimming activity continued. If the synaptic input of such weak excitatory interneurons was graded finely, they could drop out consecutively. This led to further decreases in the level of tonic excitation and in network frequency which depended on the number, type and distribution of excitatory interneurons that stopped firing. Silent weak excitatory interneurons could be recruited by a second sensory excitation and cause an increase in tonic depolarization and frequency which outlasted the sensory input. Such recruitment could occur on both sides after local sensory stimulation to only one region or one side of the model. We conclude that these computer simulations support the hypothesis that premotor interneuron drop-out and recruitment is one mechanism which can control frequency in a locomotor central pattern generator.

Animals↗
PubMed1995

Completion of the La Crosse virus genome sequence and genetic comparisons of the L proteins of the Bunyaviridae.

La Crosse virus is a member of the Bunyavirus genus in the family Bunyaviridae, viruses with trisegmented RNA genomes of mostly negative polarity composed of large (L), medium (M), and small (S) segments. The sequences of the La Crosse/original M and S RNA segments have been previously characterized. Using reverse transcriptase in conjunction with PCR amplification, we have obtained the nucleotide sequence of the L RNA segment, which encodes the viral polymerase in a single large open reading frame. Comparison of the amino acid sequence of the LAC L protein with the sequence of other polymerases from members of the Bunyaviridae, demonstrated the presence of several conserved motifs, some of which are characteristic of many polymerase proteins. A genetic tree comparing the available polymerase proteins of the Bunyaviridae provides insights into the phylogenetic relationships within this large family. Members of the genus Bunyavirus, which are mosquito-borne and infect mammals, have a closer relationship to the plant viruses represented by tomato spotted wilt virus (Tospovirus genus) than to viruses of other genera in the family Bunyaviridae.

Amino Acid Sequence↗
PubMed1995

Evaluation of booster doses of Haemophilus influenzae type b-tetanus toxoid conjugate vaccine in 18-month-old children.

A booster dose of Haemophilus influenzae type b conjugate vaccine in the second year of life is the final step in the recommended series of doses to protect infants from invasive infection. This study assessed the safety and immunogenicity of PRP-T conjugate vaccine booster doses (Act-HIB, Connaught Laboratories Ltd). The participants were 367 healthy children who had taken part in a study of primary immunization with PRP-T. At 18-19 months old, subjects were randomly assigned to receive diphtheria-pertussis-tetanus (DPT) and PRP-T vaccines either mixed in one syringe (n = 183) or separately in opposite limbs (n = 184). Adverse events were monitored for 48 h after immunization. Blood was obtained prior to vaccination in half of the subjects (combined injections group) and following vaccination in all subjects to test for antibodies to each of the antigens administered. Local adverse reactions were infrequent with PRP-T alone and equally frequent at sites of DPT or DPT/PRP-T injection, except for redness > or = 25 mm in diameter which was more frequent after the combined vaccines (25.1 versus 14.1%, p < 0.01). Systemic adverse events did not differ in type or frequency between groups. Before immunization, the geometric mean anti-PRP level in those tested was 0.41 micrograms ml-1; 26.7% had levels below 0.15 micrograms ml-1. Both treatment groups responded strongly to vaccination. In those serially tested, anti-PRP levels rose by over 90-fold, to 38.1 micrograms ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation Studies as Topic↗
PubMed1995

Positive feedback as a general mechanism for sustaining rhythmic and non-rhythmic activity.

Our aim is to reassess our proposal that various states of motor output may be sustained by positive feedback generated within the premotor neural circuitry. The evidence for this proposal came from the Xenopus embryo which when touched can swim for many seconds even after all movement has been prevented by a neuromuscular blocking agent. Experiments showed that even the spinal cord could sustain its own swimming activity for a few seconds after stimulation. We proposed that this was the result of the glutamatergic excitatory spinal interneurons synapsing with each other. Because this excitation is of long duration compared to the swimming cycle period it can sum from cycle to cycle to sustain swimming by a form of positive feedback. We have tested the plausibility of these ideas by making realistic computer simulations of the spinal networks and have shown that positive feedback can sustain stable swimming activity. Pharmacological evidence recently suggested that acetylcholine contributes to the excitation underlying swimming in spinal embryos so we investigated the central synapses made by motoneurons. Recordings from pairs of synergistic motoneurons then showed: a) cholinergic chemical synapses from more rostral motoneurons activate nicotinic receptors and produce excitation; and b) local intrasegmental electrical synapses also lead to mutual excitation. The presence of central motoneuron synapses suggested that they could contribute to excitation during swimming. We therefore used local drug applications to see if spinal neurons received cholinergic or electrical excitation during fictive swimming. The results show that motoneurons received both types of excitation while interneurons received only cholinergic excitation. This evidence suggests that when motoneurons are active during swimming they contribute positive feedback excitation not only to themselves but also to the premotor interneurons of the spinal rhythm generating network. This excitation would sum with that from 'glutamatergic' excitatory interneurons. We conclude that in addition to our original proposal of feedback between excitatory interneurons, there are other forms of positive feedback during swimming in the Xenopus embryo spinal cord. Motoneurons feed excitation back to each other. They may also contribute cholinergic excitation to premotor interneurons which could sum with the excitation from 'glutamatergic' interneurons and help to sustain swimming. If they do this, motoneurons may be a component part of the central pattern generator for swimming. Since central motoneuron synapses are a feature of most vertebrate groups, these results suggest a reevaluation of such synapses in these groups also.

Animals↗
PubMed1995

Cross-cultural comparison of US and Nigerian adolescents' HIV-related knowledge, attitudes, and risk behaviour: implications for risk reduction interventions.

Attitudes towards AIDS prevention, AIDS-related knowledge, and sexual behaviour of Nigerian adolescents and their African-American counterparts in two geographic regions of the United States were compared to assess cross-cultural similarities and differences between the adolescents. This study was conducted as the first in a programmatic sequence of activities leading toward the adaptation of a risk reduction intervention that was developed in the US for implementation with Nigerian youths. Subjects (n = 511) completed self-report measures of AIDS-related knowledge, attitudes towards condoms, attitudes towards AIDS prevention, sexual behaviour over the past two months, self- and response efficacy, and perceived vulnerability. Between-group comparisons revealed that US adolescents were more knowledgeable, held more favourable attitudes towards prevention and toward condoms, reported more sex partners, engaged in higher frequencies of unprotected vaginal intercourse, and became sexually active at later age than Nigerian adolescents. The findings are discussed with respect to their implications for translating risk reduction interventions developed in the US for African-American adolescents for later cross-cultural implementation with adolescents in developing countries where such risk reduction efforts are urgently needed.

Adolescent↗
PubMed1995

A multistate program to educate physicians: successful, economical, but endangered.

The Southern Regional Education Board (SREB) was formed in 1948 to facilitate multistate contracts for education exchange; at present, 15 states participate in this program. West Virginia is one of four states in the SREB that offer training in osteopathic medicine, and the West Virginia School of Osteopathic Medicine (WVSOM) is the only professional school in West Virginia that is a member of the SREB program. The authors describe (1) WVSOM's mission to provide primary care physicians to meet the medical needs of Appalachia, and (2) the success that an WVSOM-SREB partnership has had in training primary care physicians for participating states at a fraction of the effort and cost that would otherwise have been necessary. Concern is expressed for the program's future, and suggestions offered for the establishment of similar multistate programs to foster the production of physicians in a variety of specialties throughout the nation, since the WVSOM has clearly been successful in producing primary care physicians and in motivating graduates to return to their home states. For example, through the WVSOM-SREB osteopathic medicine program, students from states that share parts of Appalachia (Georgia, Mississippi, Alabama, and Maryland) and Florida have received their education at WVSOM. From 1978 until 1994, WVSOM graduated 148 SREB physicians; 122 of these are practicing physicians, 23 are in internships or residencies, and three have died. Of the 122, 53% have returned to practice in their home states; 85% of these graduates and 84% of all the SREB graduates who have returned to their home states or to other Appalachian states are practicing in primary care fields.

Education, Medical↗
PubMed1995

Transverse limb deficiency, facial clefting and hypoxic renal damage: an association with treatment of maternal hypertension?

Transverse limb defects are reported in a fetus and an infant born to mothers on treatment for hypertension. One pregnancy resulted in an intrauterine death at 20 weeks, and in addition to the limb defects, there was bilateral cleft lip and palate and renal hypoxic damage. It is proposed that the drugs caused maternal hypotension which led to reduced uteroplacental blood flow, fetal hypotension and hypoxia and that the anomalies seen in the two babies are a consequence of these events.

Abnormalities, Drug-Induced↗
PubMed1995

Effect of bran particle size on gastric emptying and small bowel transit in humans: a scintigraphic study.

Bran is an effective treatment for constipation but its use is often limited by heartburn and bloating. This study examined the effect of fine and coarse bran (15 g) on the gastric emptying and small bowel transit of a 325 kcal rice test meal. Twelve healthy volunteers underwent a three way cross over study, ingesting the technetium-99m labelled rice meal with or without 15 g of indium-111m labelled fine or coarse bran, in random order. Serial scintigraphic images were obtained to define gastric emptying and colonic arrival of label. Compared with control values (99 (9) minutes) (mean (SEM)), the time to 50% gastric emptying was significantly delayed by coarse but not fine bran, being 121 (6) and 104 (9) minutes respectively, p < 0.05, n = 12. Fundal emptying was unchanged but both brans seemed to increase the proportion of isotope in the antrum at 90 minutes. Small bowel transit was slightly faster with both bran types but in this study the difference was not significant. Both the bran and rice labels moved down the gut without significant separation. Fine bran causes less disturbance of gastric physiology than coarse bran.

Adult↗
PubMed1995

Properties of networks controlling locomotion and significance of voltage dependency of NMDA channels: stimulation study of rhythm generation sustained by positive feedback.

1. We have built a realistic 24-neuron model based on data from the spinal pattern generator for swimming in Xenopus embryos with the use of the SWIM programs. The neurons have dendrite, soma, and axon compartments with voltage-gated Na+ and K+ channels. Dendritic synapses were modeled as modulated ionic conductances with currents that have different reversal levels. One of these conductances was voltage dependent to model N-methyl-D-aspartate ("NMDA") synapses in the presence of Mg2+. 2. In this model, rhythm generation is initiated by a brief excitation, depends on rebound from reciprocal inhibition, and is sustained by long-duration "NMDA-dependent" feedback excitation. 3. Without NMDA voltage dependency, rhythmic activity is stable over a wide range of synaptic conductances. Its frequency decreases with more inhibition and increases with more excitation. The introduction of normally distributed variation in soma size or excitatory synaptic conductance extends the lower stable frequency range. Without such variation the frequency of the 24-neuron model is the same as a 4-neuron model provided that the synaptic conductances for each neuron are the same. 4. The effect of introducing NMDA voltage dependency on rebound after negative current injections or synaptic inhibition was investigated in single depolarized model neurons. With NMDA voltage dependency, hyperpolarizations and rebound spike responses were increased. 5. Network activity with NMDA voltage dependency was similar to that without it, but inhibitory postsynaptic potentials (IPSPs) and spikes were larger, and frequencies were lower and more sensitive to changes in excitatory and inhibitory conductance. 6. We conclude that in the model, mutual reexcitation among excitatory spinal interneurons can sustain rhythm generation by positive feedback and that NMDA voltage dependency can enhance postinhibitory rebound, stabilize swimming activity and extend its lower frequency range, and steepen the dependency of frequency on synaptic drive.

Animals↗
PubMed1995

Cholinergic and electrical motoneuron-to-motoneuron synapses contribute to on-cycle excitation during swimming in Xenopus embryos.

1. We have previously shown that Xenopus spinal motoneurons make both chemical and electrical synapses with neighboring motoneurons. Because motoneurons are active during swimming, these synapses would be expected to contribute excitation to their neighbors. The significance of central motoneuron to motoneuron synapses was therefore investigated by analyzing the composition of the fast on-cycle excitation underlying spiking activity during fictive swimming in spinal motoneurons. To accomplish this we developed a method for very local application of drugs around a caudal recorded neuron while still being able to evoke and record essentially unaltered fictive swimming rostrally. 2. Intracellular recordings were made from spinal motoneurons during fictive swimming. Bicuculline (40 microM) and strychnine (2 microM) were used continuously to block inhibitory potentials locally around the motoneurons. The amplitude and duration of the fast excitation underlying spiking activity was measured before and during local applications of excitatory antagonists. 3. The nicotinic antagonists d-tubocurarine (10 microM) and dihydro-beta-erythroidine (10 microM) reduced the amplitude of this excitation by approximately 20%. Nicotinic antagonists also reduced the duration of this fast on-cycle excitation. The kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) reduced the amplitude (by approximately 30%) but not the duration of the on-cycle excitation. In the presence of 100 microM Cd2+, which blocks all chemically mediated transmission, a considerable amount (50%) of on-cycle excitation remained. 4. These results suggest that 20% of the on-cycle excitation comes from activation of nicotinic receptors by naturally released acetylcholine (ACh), presumably from other motoneurons.(ABSTRACT TRUNCATED AT 250 WORDS)

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

Cholinergic contribution to excitation in a spinal locomotor central pattern generator in Xenopus embryos.

1. We have investigated whether in Xenopus embryos, spinal interneurons of the central pattern generator (CPG) receive cholinergic or electrical excitatory input during swimming. The functions of cholinergic excitation during swimming were also investigated. 2. Intracellular recordings were made from rhythmically active presumed premotor interneurons in the dorsal third of the spinal cord. After locally blocking inhibitory potentials with 2 microM strychnine and 40 microM bicuculline, the reliability of spike firing and the amplitude of fast, on-cycle, excitatory postsynaptic potentials (EPSPs) underlying the single on-cycle spikes were measured during fictive swimming. 3. The nicotinic antagonists d-tubocurarine and dihydro-beta-erythroidine (DH beta E, both 10 microM) reversibly reduced the reliability of the spike firing during swimming and reduced the amplitude of the on-cycle EPSP by 16%. DH beta E also reduced the EPSP amplitude in spinalized embryos by 22%. These results indicate that interneurons receive rhythmic cholinergic excitation from a source within the spinal cord. 4. Combined applications of nicotinic and excitatory amino acid (EAA) antagonists or cadmium (Cd2+, 100-200 microM) resulted in complete block of the fast EPSP, suggesting that interneurons do not receive electrical excitation. 5. The nicotinic antagonists mecamylamine and d-tubocurarine (both 5 microM) reduced the duration of episodes of fictive swimming recorded from the ventral roots, in spinal embryos. When applied in the middle of a long episode, d-tubocurarine decreased the swimming frequency, ruling out an effect on the initiation pathway. The cholinesterase inhibitor eserine (10 microM) increased the duration of swimming episodes.(ABSTRACT TRUNCATED AT 250 WORDS)

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

A noninvasive method to measure splanchnic glucose uptake after oral glucose administration.

We have developed a noninvasive method to estimate splanchnic glucose uptake (SGU) in humans (oral glucose clamp technique [OG-CLAMP]), which combines a hyperinsulinemic clamp with an oral glucose load (oral glucose tolerance test). We validated this method in 12 nondiabetic subjects using hepatic vein catheterization (HVC) during an oral glucose tolerance test. During HVC, splanchnic blood flow increased from 1,395 +/- 64 to 1,935 +/- 109 ml/min, returning to basal after 180 min and accounted for 45 +/- 7% of SGU in lean and 19 +/- 5% in obese subjects (P < 0.05). SGU estimated during the OG-CLAMP was 22 +/- 2% of the glucose load, and this was significantly correlated (r = 0.90, P < 0.0001) with SGU (35 +/- 4%) and with first pass SGU (24 +/- 3%; r = 0.83, P < 0.001) measured during HVC. SGU was higher in obese than in lean subjects during OG-CLAMP (27 +/- 1% vs 18 +/- 3%, P < 0.01) and HVC (44 +/- 4% vs 26 +/- 5%, P < 0.05). In conclusion, SGU during the OG-CLAMP is well correlated to SGU measured during HVC. An increase in splanchnic blood flow is a major contributor to SGU in lean subjects. SGU is increased in obese subjects as measured by both methods.

Adult↗
PubMed1995

Functional morphology of the vanes of the flight feathers of the pigeon Columba livia

The functional morphology of feather vanes was examined by combining morphological examination with mechanical tests. A geometrical model was derived which related the in-plane behaviour of the vane to the orientation of the barbs and barbules. This predicted that the small branching angles of both barbs and proximal barbules should result in a vane which is easier to move distally than proximally. These predictions were verified by mechanical tests on primary and secondary feathers of the pigeon Columba livia. A further prediction, that the inclination of the barbs' cross section should make the vanes more resistant to forces from below than those from above, was also confirmed by mechanical tests. Differences in the mechanical behaviour of feathers are related to differences in their morphology and function. The vanes of outer primaries are more resistant to out-of-plane forces than those of the inner primaries and secondaries, particularly towards their tip, a property which will help them withstand the larger aerodynamic forces to which they may be subjected in flight. The outermost primary vane also showed the least asymmetry to out-of-plane forces as a result of the more vertical orientation of its barbs. This may help it to act as a reversible aerofoil during take-off.

Journal Article↗
PubMed1995

Controversies in vascular imaging and intervention: a panel discussion.

PURPOSE: To express for educational purposes, different and often opposing views on controversial topics of current interest in vascular radiology. And to explore whether or not consensus can be reached on any of these controversial issues. METHOD: Panel discussion among experts in the field. With the help of a moderator, controversial topics were introduced for discussion. For each topic the moderator underscored the principal issue and asked relevant questions. All panelists were asked to express their views on all issues. The following issues were discussed: Atherectomy vs balloon angioplasty; Balloon angioplasty vs stents; Digital vs analog Imaging; TIPS as an emergency procedure; Regional thrombolysis; MRA vs conventional angiography; Laser angioplasty; Carbon dioxide angiography; Lymphangiography vs CT; The training of surgeons in angiography. The panelists were urged to avoid extensive references to the literature but rather to express their own personal opinions based on experience and practice. For each topic an attempt was made to arrive at consensus. RESULTS: Ten issues were presented for debate and discussion. Despite divergent opinions it was possible and relatively painless to reach consensus on seven controversies. On the remaining issues the panelists agreed to disagree. However even when there was disagreement certain trends became apparent. Local considerations and resource availability accounted for varied approaches to the solution of certain controversies. CONCLUSIONS: A panel discussion among experts may be a useful way to address controversies for educational purposes. Despite divergence of opinions consensus may be reached, or in absence of consensus general trends may become apparent.

Angioplasty, Balloon↗
PubMed1994

Nicotinic and muscarinic ACh receptors in rhythmically active spinal neurones in the Xenopus laevis embryo.

1. Intracellular recordings were made from presumed motoneurones in the Xenopus embryo spinal cord, and their response to cholinergic agents was investigated. Nicotine and 1,1-dimethyl-4-phenylpiperazinium (DMPP; both 1-10 microM) strongly depolarized, and muscarine and oxotremorine (2-20 microM) weakly hyperpolarized, these neurones. Tetrodotoxin (1 microM), which blocks action potentials in Xenopus neurones, did not affect either of these responses. 2. The extrapolated reversal potential of the nicotinic depolarization was -12.1 +/- 8.3 mV (mean +/- S.E.M.) suggesting the opening of a mixed conductance. The nicotinic response was antagonized by dihydro-beta-erythroidine, d-tubocurarine and mecamylamine (10-20 microM) but not by alpha-bungarotoxin (10 microM). 3. The muscarinic response was not reversed when recorded with electrodes filled with potassium chloride but was antagonized by atropine (0.1 microM). 4. Acetylcholine (ACh, 10 microM) caused a strong depolarization of the neurones which was blocked by d-tubocurarine and dihydro-beta-erythroidine, suggesting that its effects are mediated predominantly by nicotinic ACh receptors. 5. ACh and nicotinic agonists applied to the spinal cord produced a barrage of IPSPs that were blocked by TTX and strychnine.

Acetylcholine↗
PubMed1994

Impact characteristics of two types of police baton.

A comparison study of the American style ASP extendible police baton with the conventional UK police wooden truncheon was performed. It was found that the ASP baton generated approximately twice the contact pressure than the conventional wooden truncheon. A discussion on the medical/legal aspects of the results are given.

Biomechanical Phenomena↗