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Biomedical subjects

R M Davidson

Publications and source records attributed to R M Davidson.

At least 19 recordsLinked to original sources
PubMed1992

Effect of corticotectal tract lesions on relative motion selectivity in the monkey superior colliculus.

Many cells in the superficial layers of the monkey superior colliculus are sensitive to the relative motion between a small target moving through the classic receptive field and a textured, moving background pattern that fills the visual field beyond the classic receptive field. The cells respond well when motion of the target differs from that of the background, but their responses are suppressed when the target moves in phase with the background. To determine whether this relative motion sensitivity depends on input to the colliculus from visual cortex, we studied colliculus cells in immobilized, anesthetized monkeys after unilateral thermocoagulation, or anesthetic blockade, of the corticotectal tract at the level of the pulvinar. In the colliculus ipsilateral to the corticotectal tract lesions, relative motion sensitivity was significantly reduced when compared either with the colliculus in intact animals or with the colliculus contralateral to the lesion. However, a moving-background stimulus still had a modest suppressive effect compared with a stationary background ("background motion sensitivity"), as is the case for intact animals. Anesthetic blockade of the corticotectal tract had similar effects; relative motion sensitivity, but not background motion sensitivity, was lost following injection of mepivacaine or bupivacaine. Pulvinar cell loss alone, induced by kainic acid injection, had no effect on relative motion sensitivity in the colliculus. The corticotectal tract lesions, but not the anesthetic injections, also had minor effects on flash-evoked responses and spontaneous discharge rates; these effects may reflect a retrograde response of some tectopulvinar cells to injury of their axons by the corticotectal tract lesions. In the colliculus opposite the corticotectal tract lesion, relative motion sensitivity was similar to that in normal animals. However, responses in the presence of a moving background were enhanced, suggesting that removal of cortical input to one colliculus may disinhibit the contralateral colliculus, a phenomenon reminiscent of the Sprague effect in the cat. We conclude that while cortical input to the colliculus may contribute little to the classic receptive field properties of superficial-layer cells, it clearly does contribute to relative motion sensitivity.

Anesthetics, Local↗
PubMed1991

Selectivity for relative motion in the monkey superior colliculus.

1. Cells in the superficial layers of the colliculus were studied in immobilized monkeys anesthetized with nitrous oxide. We examined sensitivity to the relative motion between two stimuli: a small target in a cell's receptive field and a large random-dot background pattern that filled most of the visual field outside the receptive field. 2. Most cells were nonselective for either target direction or speed when the background pattern was stationary but were selective for both direction and speed relative to a moving background. Selectivity for relative motion was independent of the absolute direction and speed of both target and background. When both moved at the same speed in the same direction, the response evoked by the target was strongly suppressed. Changing the background direction relative to the target reduced the suppression; suppression was minimal when the two moved in opposite directions. Selectivity for relative direction was broad: the average tuning width at half-amplitude was 136 degrees. When target and background moved in the same direction, increasing or decreasing background speed relative to the target likewise reduced suppression. Average tuning width for relative speed was 1.4 log units. 3. Selectivity for relative motion was a global phenomenon. Suppression was present even when the background pattern was excluded from a region 10 times the receptive-field diameter. However, suppression gradually diminished with increasing distance between the receptive field and the background pattern. 4. Relative motion selectivity was most common in the deeper part of the superficial layers. Ninety percent of the cells below the middle of the stratum griseum superficiale were selective for relative direction, whereas above this level only 45% of the cells were. 5. Cells in the magnocellular and parvocellular layers of the lateral geniculate nucleus did not show selectivity for relative direction. 6. We suggest that the lower one-half of the superficial grey layer and the stratum opticum together constitute a subdivision of the superior colliculus that is specialized to detect strong discontinuities in relative motion. Descending input by way of the corticotectal tract is probably essential for the detection process. the projections from this tectal motion zone to the pulvinar, and from there to prestriate cortex, may provide a feedback pathway through which motion discontinuities such as occur at dynamic occlusion boundaries can influence local feature detection by cortical neurons.

Animals↗
PubMed1990

Multiple forms of mechanosensitive ion channels in osteoblast-like cells.

Patch-clamp recording techniques were used to examine the direct effects of mechanical stimulation on ion channel activity in human osteoblast-like osteosarcoma cells. Three classes of mechanosensitive ion channels were present and could be distinguished on the basis of conductance, ionic selectivity, and sensitivity to membrane tension. The largest conductance channel (160 pS) was K(+)-selective and showed both a decrease in long closed interval duration and an increase in burst length with increasing membrane tension. For low applied pressures, there was an e-fold increase in the probability of this channel being open (Popen) for every 3.4 cm2 Hg change in pressure. Two additional pressure-dependent channels had smaller conductances, i.e., 60 pS and 20 pS; the 60 pS channel appeared to be non-selective for cations. We propose that one or more of these mechanosensitive channels is involved in the response of bone to mechanical loading.

Bone Neoplasms↗
PubMed1990

Metastatic calcification in a patient with malignant parathyroid carcinoma. Correlation of clinical, surgical, radiographic, and scintigraphic findings.

A patient whose bone scan showed features typical of a "superscan," having extensive pulmonary, cardiac, and renal calcifications is described. Metastatic parathyroid carcinoma with renal insufficiency and phosphate retention are cited as the cause. Early diagnosis and surgical extirpation offer the best chance for cure or palliation. Pitfalls of plain film radiography in identifying metastases and pathologic soft tissue calcifications in the setting of severe hyperparathyroidism are discussed, and the advantages of functional radionuclide imaging assessments are emphasized.

Bone Neoplasms↗
PubMed1989

Indium-111 WBC scan in acute toxic centrilobular hepatic necrosis.

In this case of prolonged fever and abnormal liver functions, dual tracer scintigraphy with In-111 WBCs and Tc-99m SC led to a biopsy-proven diagnosis of severe acute toxic hepatitis (hepatocellular necrosis). Correlation of the Tc-99m SC scan findings with those previously reported for "pseudotumors" of the liver is discussed. A "pseudonormal" scan pattern is described for the In-111 WBC scintigraphy. Discordance between In-111 WBC and Tc-99m SC scintigraphy in this clinical setting should raise the possibility of hepatic necrosis as a diagnostic alternative to hepatic abscess.

Adult↗
PubMed1987

Schoolboy Rugby injuries, 1969-1986.

For 18 years (1969-1986) a casualty station has been operating during all Saturday interschool Rugby matches at a private school. Since its inception, both the diagnosis and the early management have been documented for every injury that presented to this station. Over this period, 1444 boys have been seen, of whom 116 sustained injuries that were classified as being severe; of these, only two injuries--a skull fracture and a fracture--dislocation of the cervical spine--were regarded as clinically serious. Over all, the injury rate was 176/10 000 player-hours, or 1.56/100 player-games; the rate of severe injury was 14/10 000 player-hours, or 0.12/100 player-games. Injury rates were highest in older boys and in those who were in the most advanced grades of play. Fullbacks experienced the most injuries; otherwise, little variation occurred in injury rates by player position. No consistent trend in injury rates was observed over the period of the study.

Adolescent↗
PubMed1987

Sensory and motor integration during mandibular function.

The subject of pain is intimately related to that of mandibular function. It is now clear that certain types of temporomandibular disorders, particularly myofascial pain dysfunction, result, in part, from rhythmic muscle activity produced by parafunctional oral habits such as diurnal or nocturnal bruxism. Furthermore, in addition to phasic hyperactivity, evidence also suggests that masticatory muscles of patients with MPD are tonically hyperactive. The pain associated with such hyperactive musculature prompts many patients to seek professional help. This article provides an updated historical review of one important aspect of mandibular function and gives insight into the general "state of the art."

Animals↗
PubMed1986

Global visual processing in the monkey superior colliculus.

Neurons were recorded in the superficial layers of the superior colliculus in anesthetized monkeys. As classically described, cells were non-selective for target direction and speed when the target moved through an empty visual field. However, these same cells were sensitive to target direction and speed relative to a textured moving background. The target's response was suppressed when its direction and speed were similar to that of the background, irrespective of the absolute direction of background movement.

Animals↗
PubMed1979

Cross-sectional echocardiography. I. Analysis of mathematic models for quantifying mass of the left ventricle in dogs.

Cross-sectional echocardiography was used to quantify left ventricular mass noninvasively in 21 dogs. Short- and long-axis cross-sectional images of the left ventricle were reproducibly traced at endocardial and epicardial borders during stop-motion video-tape replay. We used area, length and diameter measurements to calculate left ventricular mass by seven mathematic models, including the standard formulas used with M-mode echocardiography and cineangiography. Calculated mass was compared with excised weight of the left ventricle by regression and percent error analyses. Formulas using short-axis areas and long-axis length resulted in higher correlation coefficients (0.94--0.95) and lower mean errors (6--7%) than for standard formulas. Since short-axis areas account for regional left ventricular irregularities, noninvasive quantification of left ventricular mass by cross-sectional echocardiography in dogs is most accurate with formulas using short-axis areas.

Animals↗
PubMed1976

Significance of S-T segment elevations in acute myocardial ischemia. Evaluation with intracoronary electrode technique.

A method is described for measuring intracoronary S-T segment elevations in the closed chest, a technique that appears to provide more reliable measurements of myocardial ischemia. Electrodes were inserted through intracoronary balloon catheters that were placed within a coronary artery and its adjoining vein both proximal and at several points distal to a coronary occlusion. Intracoronary arterial and adjacent venous electrocardiograms produced equivalent tracings. The intracoronary S-T segment elevations after coronary occlusion resembled those recorded from the epicardial surface but were free of artifacts noted in open chest studies. Study of progressive alterations of the intracoronary S-T segment after proximal occlusion of the left anterior descending coronary artery in 18 closed chest dogs revealed a peak segment elevation of 3.2 +/- 0.6 mv within 5 minutes, followed within 2 to 3 hours by spontaneous reduction by more than 40% of the S-T elevation over the occluded zone. In 44% of these animals, the S-T elevation decreased spontaneously to less than 1 mv, and in 22% it decreased to the preocclusion control level within 2 hours of occlusion. This spontaneous decrease in S-T elevation was frequently followed by a secondary increase and then S-T segment fluctuations. Reperfusion of the left anterior descending coronary artery after 30 to 60 minutes of occlusion generally led to a prompt reduction in S-T elevation. In some cases S-T elevations persisted up to 14 hours of occlusion, were reduced after reperfusion and exhibited a renewed pronounced increase after subsequent reocclusion of the left anterior descending coronary artery. During the 1st hour after occlusion, the early S-T segment elevation followed by spontaneous reduction reduction generally corresponded temporally with the derangements in myocardial lactate extraction and potassium loss. However, after 1 hour of occlusion no clear-cut correlation could be established between S-T fluctuations and changes in hemodynamic or myocardial metabolic measurements. We conclude that the new closed chest intracoronary electrocardiographic S-T technique might be of use for monitoring the early ischemic myocardial derangements and to assess benefits or drawbacks of treatment in both the experimental animal and man. Correspondence of S-T segment elevation with lactate and potassium alterations in the coronary-occluded region in the 1st hour after occlusion indicates that S-T segment elevation might represent an index of early myocardial ischemia. The spontaneous S-T changes that follow coronary occlusion must be taken into consideration when investigators utilize S-T segment modification as a sign of effectiveness of treatment.

Acute Disease↗
PubMed1976

Regional and global myocardial effects of intravenous and sublingual nitroglycerin treatment after experimental acute coronary occlusion.

The consequences of sublingual and intravenous nitroglycerin treatment after acute coronary occlusion were studied in 18 closed chest dogs. Intravenous (0.1 mg/min) or sublingual (0.4 mg/15 min) nitroglycerin therapy was instituted 1 hour after occlusion and the effects were observed over a period of 2 hours. Hemodynamics and global and regional cardiac function were measured in both the coronary occluded and nonoccluded segments of the left ventricle before and during coronary occlusion, and after administration of nitroglycerin. A similar nine dog control series was used to establish the significance of the measured effects of nitroglycerin. Intravenous nitroglycerin therapy after 1 hour of occlusion resulted in a marked increase in heart rate (37 +/- 12 [mean +/- standard error of the mean] percent), reduction of systolic blood pressure (9 +/- 3%), decrease in left ventricular end-diastolic and end-systolic volumes (32 +/- 5% and 34 +/- 5%), increase in coronary sinus flow (64 +/- 24%) and decrease in left ventricular stroke work (29 +/- 8%). Sublingually administered nitroglycerin produced similar trends but much less pronounced effects. However, intravenous or sublingual administration of nitroglycerin provided no improvement or caused further deterioration in ischemic region lactate extraction and potassium loss. The left ventricular ejection fraction, which was severly depressed after 1 hour of occlusion, changed minimally after administration of nitroglycerin, and there was no evidence of any correction of regional left ventricular akinesia or dyskinesia. Whereas mean systemic vascular resistance changed minimally as a result of nitroglycerin therapy, it increased 19 +/- 8% during a corresponding period of an untreated coronary occlusion series suggesting that nitroglycerin prevented an anticipated increase. Postocclusion S-T segment elevation in the electrocardiogram persisted after treatment. Our data corroborated that nitroglycerin reduced left ventricular volumes and increased coronary sinus flow; however, these improvements were accompanied by persisting metabolic and mechanical derangements in the ischemic region.

Animals↗