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Biomedical subjects

M Colding-Jørgensen

Publications and source records attributed to M Colding-Jørgensen.

8 recordsLinked to original sources
PubMed1995

Analysis of interaction between TGF and the myogenic response in renal blood flow autoregulation.

The present study investigates the interaction between the tubuloglomerular feedback (TGF) response and the myogenic mechanism by use of a mathematical model. The two control mechanisms are implemented in a spatially distributed model of the rat renal juxtamedullary afferent arteriole. The model of the afferent arteriole is based on in vivo measurements of the stress-strain relation in muscle strips. Analysis of experimental data shows that the myogenic response can be modeled by a linear relation between the transmural pressure and the level of activation of the vascular smooth muscle cells. The contribution of TGF to smooth muscle activity is assumed to be a linear function of the glomerular capillary pressure. The results show that the myogenic response plays an important role in renal blood flow autoregulation. Without a myogenic response, mechanisms such as TGF that are localized in the distal segments of the microvasculature would not be able to achieve autoregulation because of passive, pressure-mediated effects in the upstream vascular segments. In addition, it is shown that a strong myogenic response may lead to both propagation and enhancement of vascular effects mediated through mechanisms located in the distal part of the afferent arteriole. An ascending myogenic response could enhance the regulatory efficiency of the TGF mechanism by increasing the open-loop gain of the system. However, such a synergistic interaction will only be observed when the two mechanisms operate on more or less separate segments of the afferent arteriole. In the case where they operate on common segments of the arteriole, the outcome of the interaction may well be antagonistic.

Animals↗
PubMed1993

[Is the new chaos- and fractal research of importance to medical science?].

Living systems are characterized by a complexity of structure and function which goes well beyond the descriptive capacity of classical physical and mathematical methods. During the latest decade, the development of non-linear dynamics has introduced a series of new concepts and tools to describe even extremely complex systems. This has led to a better understanding of the processes through which complex structures can arise from very simple interactions. The present and a following paper will give a brief introduction to some concepts and methods of this new science.

Biological Evolution↗
PubMed1993

A model of the mechanics of smooth muscle reservoirs applied to the intestinal bladder.

A model of the mechanics of smooth muscle reservoirs was derived from the stress-strain relation in smooth muscle strips. The model was applied to the intestinal bladder reservoir but may also be applied to natural reservoirs such as the bladder. The relation between reservoir volume and pressure was calculated at different values of smooth muscle tone and was found to accord with clinical results. The muscle tone of the bowel when measuring off the bowel to construct the reservoir was found to have a profound influence on its ultimate volume. Stiffening of a part of the reservoir wall as a result of edema or fibrosis will cause a substantial decrease in functional capacity, both directly as a result of a reduced expansion of the diseased part and indirectly by enhancing the stretching of the adjacent normal wall. The pressure rise produced by a slow peristaltic contraction was calculated to depend both upon the strength of contraction and on the degree of reservoir filling: Almost no pressure rise will occur in the near empty reservoir, while the highest pressure rise was calculated to occur at a volume of about 300 ml. If only a part of the reservoir contracts, more filling is needed before the pressure increases and the maximum pressure will be reached at a substantially higher volume. Localized contractions will stretch the resting reservoir wall and so activate stretch receptors, which may cause a desire to void, resulting in reduced functional capacity.

Animals↗
PubMed1993

Mechanical characteristics of tubular and detubularised bowel for bladder substitution: theory, urodynamics and clinical results.

A theoretical analysis of the mechanics of tubular and detubularised bowel bladders is presented. The results are compared with clinical findings following bladder substitution with the tubular ileocaecal and the detubularised urethral Kock reservoir. The theoretical results are consistent with clinical observations showing that detubularisation increases reservoir capacity substantially, delays the onset and reduces the amplitude of the pressure rise produced by contractions. These findings account for the markedly improved nocturnal continence (80 versus 17% at 2 years), the longer voiding intervals (4 versus 2.5 h at 1 year) and the predisposition to urinary retention (25 versus 0% at 1 year) with detubularised bladder substitution. A simple equation is derived to calculate the capacities of both near-spherical and cylindrical reservoirs. The capacity of the U pouch is calculated to be less than that of the W-shaped, S-shaped and Kock reservoirs. Altering the shape of a reservoir from spherical to ellipsoid is calculated to have only a slight effect on its mechanical characteristics. Consequently the essence of detubularisation is to create a reservoir with high capacity, while shape is of secondary importance.

Adult↗
PubMed1992

Minimal model for Ca(2+)-dependent oscillations in excitable cells.

A minimal model for calcium controlled oscillations is presented. The model considers only an exchange of potassium and calcium ions over the plasma membrane. Calcium ions leak into the cell through a potential dependent channel and is extruded by a pump. Potassium leaks out through a calcium dependent, but voltage independent, channel. The cytosolic calcium concentration is buffered, so a fixed fraction is free. Inactivation, membrane capacity, and time delays for the conductance changes are not included, so the time dependence is solely introduced through the temporal changes of the intracellular Ca(2+)-concentration. With continuous parameter changes the model can switch between five states: (1) a non-excitable, stable state; (2) single-spike excitability; (3) slow, spontaneous oscillations; (4) reverse-spike excitability; and (5) another non-excitable, stable state. One of the key parameters for this switching behavior is the rate constant for the calcium pump.

Action Potentials↗
PubMed1983

A model for the firing pattern of a paced nerve cell.

When a nerve cell is paced by another cell or by external stimuli, its firing pattern is generally not regular. In a given stimulus and frequency interval the cell responds only on a fraction of the stimuli, and this fraction is not necessarily a nice simple fraction as 1/2, 1/3, etc. but may be any fraction less than one, why the firing pattern becomes accordingly irregular. The paper demonstrates the rules for these irregularities and supplies a mathematical tool to analyse the firing pattern. In the analysis only a cell with the so-called impulse dependent type of adaptation is considered, but similar analyses may be done with other cell types. The analysis is restricted to a case where a resting cell is stimulated by repetitively applied stimuli of constant strength and repetition frequency. Cases with more than one input, spontaneously firing cells, and transient phenomena are not included in the analysis.

Action Potentials↗