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Biomedical subjects

B Ahmed

Publications and source records attributed to B Ahmed.

At least 55 records · Page 3Linked to original sources
PubMed1988

Factors determining the performance of medical students of the Faculty of Medicine, University of Kuwait.

An investigation was undertaken to determine the factors influencing performance of medical students at the Faculty of Medicine, University of Kuwait. Data were collected for the first batch of 42 candidates who successfully passed the 7-year medical programme. Results showed high correlations between high-school admission grades and subsequent performance during the medical programme. Similar results were also obtained with English proficiency. A partial correlation controlling for English proficiency, however, showed a systematic decline in the magnitude of the correlations between high-school grades and performance during the medical programme. In fact the relationship between high-school GPA and clinical GPA was non-significant. A prominent influence of English proficiency on performance is indicated.

Achievement↗
PubMed1987

On the spatial mismatch of rod and cone mediated inputs to cat retinal ganglion cells.

The determination of threshold sensitivities of cat retinal ganglion cells to 452 nm-blue and 578 nm-yellow narrow-band lights were undertaken prior to the mapping of the rod- and cone-mediated receptive field centres. In some brisk sustained on-centre retinal ganglion cells two distinct and spatially separated regions were noted such that one region was maximally sensitive to 452 nm-blue and the other to 578 nm-yellow lights. The two wavelength-dependent regions of maximal sensitivity are presumably related to asymmetries between rod- and cone-mediated inputs, rather than between inputs mediated by different classes of cones and correspond, respectively, to the loci of the rod- and cone-mediated centres.

Action Potentials↗
PubMed1986

Relative motion sensitivity in cat striate cortex as a function of stimulus direction.

This paper presents the second stage of an investigation into the relative motion sensitivity of complex cells in the striate cortex of lightly anaesthetized adult cats. Relative motion between an oriented bar and a background of random visual texture was generated by moving both stimuli, in-phase or in antiphase, with dissimilar speed. Three configurations were compared: motion of both foreground bar and background texture in preferred directions for either the bar or the background, and also where background motion was other than orthogonal to bar orientation. Providing foreground stimuli elicited substantial responses, sensitivity to relative motion was qualitatively but not quantitatively predictable from discrete responses to foreground or background alone, whatever the angular inclination between their respective directions, suggestive of strongly non-linear interactions between the two. Where the foreground evoked no response, or depressed firing, the pattern of sensitivity to relative motion could not be predicted.

Animals↗
PubMed1986

Relationships between cyclic AMP levels and lipolysis in fat cells after isoproterenol and forskolin stimulation.

Using the flask-incubated fat cell system, alterations in glycerol release (lipolysis) and cAMP accumulation were determined after incubation with isoproterenol or forskolin. These agents caused concentration-dependent increases in both cAMP accumulation and lipolysis. The maximum responses to forskolin for each variable were greater than the corresponding responses to isoproterenol. The maximum responses to isoproterenol for both cAMP accumulation and glycerol release were increased by the presence of either adenosine deaminase or theophylline. Under these conditions, high concentrations of isoproterenol continued to increase cAMP accumulation while having no further effect on lipolysis. These results support the concept that the maximum response to isoproterenol alone was limited by the accumulation of cAMP within the cells. The maximum response to isoproterenol in the presence of either theophylline or adenosine deaminase (and to forskolin) was limited by some step in the lipolytic process distal to cAMP accumulation. The relationships between cAMP levels and lipolysis for isoproterenol and forskolin were found to be different. A 6-fold increase in cAMP levels was sufficient to maximally increase lipolysis with isoproterenol, whereas the maximum lipolytic response to forskolin was associated with a 20-fold increase in cAMP levels. A plot of log cAMP vs. glycerol release resulted in linear relationships for both drugs. The slope of the line for isoproterenol was significantly greater than that for forskolin. At any given concentration of cAMP the corresponding lipolytic response was greater for isoproterenol than for forskolin.

Adenosine Deaminase↗
PubMed1985

Length summation of complex cells in cat striate cortex: a reappraisal of the "special/standard" classification.

A reappraisal of the relationship between length summation and receptive field length assessed by the minimum response field method, was made in complex cells recorded from the lightly anaesthetized feline striate cortex. This relationship has previously been proposed by Gilbert [(1977) J. Physiol., Lond. 268, 391-421] and subsequently applied extensively by others, as a basis for subdividing complex cells into different functional groups: "standard" complex cells length summate, responding preferentially to long contours of appropriate orientation; "special" complex cells respond optimally to short oriented contours). Quantitative comparisons suggest that four-fifths of complex cells can be classified by their length summatory behaviour. Thus length summatory behaviour is a useful adjunct to classification, although one-in-five complex cells cannot be positively assigned to either category and this figure rises to one-in-three following rigid application of the criteria for determining standard complex cells. The need for precise estimates of length summation is emphasized by subjective assessment which leads to greater uncertainty in classification and to several cells being wrongly assigned. The possibility that cells unclassified by length summation constitute a distinct additional class is discussed.

Animals↗
PubMed1984

Response of cat retinal ganglion cells to motion of visual texture.

Responses of retinal ganglion cells to motion of large fields of visual texture were recorded in the lightly anaesthetised, immobilized cat. Brisk sustained and brisk transient, on- or off-centre, units gave a modulated response to texture motion. The pattern of temporal modulation of the response was dependent upon the particular configuration (sample) of texture crossing the receptive field. The magnitude of the response depended on the size of the receptive field centre. For all units, whether sustained or transient the magnitude of response to a textured field of fixed angular subtense declined as centre-diameter increased from 0.9 deg. For brisk units the response magnitude levelled off for centre sizes smaller than 0.9 deg. Responses to texture were confined spatially to the region of the receptive field, and the overall characteristics of this response were due to interactions between the centre and surround mechanisms of the receptive field. In brisk transient units, no evoked response was evident when texture motion was confined to regions well away from the receptive field of the unit, i.e. no periphery or shift effect could be demonstrated. The results support previous suggestions that the differential sensitivity to texture motion evident in cortical neurones must be due to intra-cortical processing.

Animals↗
PubMed1981

The size and shape of rod and cone centres of cat retinal ganglion cells.

Receptive field centres of cat retinal ganglion cells, as mediated by rod and by cone inputs, were mapped as contours of iso-sensitivity at a mid-mesopic adapting luminance using, respectively, 452 nm-blue and 578 nm-yellow narrow-band lights at an intensity 1 log unit above threshold for the most sensitive locus. Based on the sizes and shapes of mapped or rod and cone centres for 74 ganglion cells, four receptive field centre categories were distinguished. Cone and rod centres were usually elliptical, and in almost 60% of cells the major axis through the receptive field centre was oriented within +/- 20 degrees of horizontal. In 69%, rod and cone centres were non-concentric, 66% rod and cone centres were non-concentric, 66% had larger rod than cone centres - area ratios ranging from 0.6 : 1 to 2.9 : 1, and in only two cases was the rod centre actually smaller than the cone centre.

Animals↗
PubMedDate not supplied

Nirurin: a new prenylated flavanone glycoside from Phyllanthus nirurii.

A new prenylated flavanone glycoside, nirurin, has been isolated from Phyllanthus nirurii. On the basis of chemical and spectral studies, its structure is proposed as 5,6,7,4'-tetrahydroxy-8-(3-methylbut-2-enyl) flavanone-5-O-rutinoside (1). The aglycone, also a new flavanone, has been characterized as 5,6,7,4'-tetrahydroxy-8-(3-methylbut-2-enyl) flavanone, which may be designated as nirurinetin (2).

Acetylation↗
PubMedDate not supplied

Estimates of the net excitatory currents evoked by visual stimulation of identified neurons in cat visual cortex.

The action potential discharge response of single neurons to both visual stimulation and injections of current were obtained during intracellular recordings in cat visual cortex in order to estimate the net excitatory current arriving at the soma during visual stimulation. Of 45 neurons recorded intracellularly, 19 pyramidal neurons and one basket cell were labelled with horseradish peroxidase. The discharge of all neurons adapted to constant current. For 40 neurons, a single exponential provided a good fit to the adapting discharge (r2 = 0.73 +/- 0.03) for all current intensities. Superficial layer neurons were significantly faster adapting [P < 0.001, mean (+/- SEM) time constant of adaptation = 11.5 +/- 1.3 ms; n = 20] than deep layer neurons (mean time constant of adaptation = 51.4 +/- 6.4 ms; n = 10). The percentage adaptation of the spike frequency, %(peak - adapted rate)/peak, was determined from the fitted exponential. Superficial layer neurons adapted significantly more strongly (P < 0.01, mean = 67 +/- 3%) than deep layer neurons (mean = 51 +/- 5%). The mean firing frequency in response to a current step of 320 ms duration had a linear relationship to the amplitude of the injected current (slope 66 spikes/s/nA; origin zero, mean r2 = 0.94; n = 33). This relationship provided a means of estimating the net peak excitatory current generated by visual stimuli. The estimated mean peak somatic current during the passage of a bar across the receptive field was 1.1 nA and the average current for the duration of the visually evoked discharge was 0.64 nA (n = 17). The transfer response of real and model neurons was obtained by differentiating the discharge response to a step input current and was then used to predict the output of the neuron following an arbitrary input. When these transfer responses were convolved with known input signals in model neurons, the predicted output was close to the simulated response of the model neuron to the same input waveforms. The transfer response was calculated for eight real neurons. Estimates of the net excitatory current arriving at the soma during visual stimulation was obtained by deconvolution. The mean peak somatic current for these neurons was 0.62 nA.

Action Potentials↗