CRC critical reviews in bioengineering最新文献

In this chapter, well-known solutions that utilize a Fourier transform method for determining the extracellular, volume-conductor potential distribution surrounding elongated excitable cells of cylindrical geometry are reformulated as a discrete Fourier transform (DFT) problem, which subsequently permits the volume-conductor problem to be viewed as an equivalent linear-filtering problem. This DFT formulation is fast and computationally efficient. In addition, it lends itself to the application of some rather well-known techniques in linear systems theory (e.g., the DFT for convolution and least mean-square (Wiener) filtering for optimal prediction of a signal in random noise). Two specific examples are employed to demonstrate the utility of this discrete Fourier method: (1) the single, isolated, active nerve fiber in an essentially infinite volume conductor and (2) the isolated, active nerve trunk in a similar type of extracellular medium. In each of these, our DFT method is employed to obtain both the classical "forward" and "inverse" potential solutions for each volume conductor problem. In the case where the single, active nerve fiber is the bioelectric source in the volume conductor, simulated action-potential data from an invertebrate giant axon is utilized, and potentials at various points in the extracellular medium are calculated. The calculated potential distributions in axial distance z, at various radial distances r, are consistent with well-known experimental fact. When the active nerve trunk acts as the bioelectric source, the DFT method provides calculated potential distributions that are fairly consistent with experimental data under a variety of experimental conditions. For example, in these experiments, a special, isolated frog spinal cord preparation is used that permits separate or combined stimulation of the motor and sensory nerve fiber components of the attached sciatic nerve trunk. By manipulating the stimulus intensity applied to the motor (ventral) or appropriate sensory (dorsal) roots of the spinal cord, a variety of multiphasic extracellular volume-conductor potentials can be recorded from the sciatic nerve. The excellent agreement of model-generated and experimental data, regardless of the complexity of surface potential waveform, tends to validate the modeling assumptions and offer encouragement that this computationally efficient DFT method may be usefully employed in volume-conductor problems where both the bioelectric source, and the surrounding volume conductor, are of a much more complicated nature.

This article is a review of progress towards a general quantitative theory of photosynthetic productivity or autotrophy in plants. It is not intended to be an exhaustive review, but rather a perspective of the autotrophic puzzle and current approaches to its solution. The review describes attempts to quantitatively describe a generalized plant based on theoretical expressions for its component parts. Particular emphasis has been placed on the source-transport-sink continuum. This continuum can be broken into five subsections: 1. Stomal mechanics and physiology 2. Photosynthesis (within chlorophyllous cells) 3. Mass and energy exchange between the leaf and environment 4. Phloem translocation 5. Sink metabolism models Progress towards the development of physiologically based models in each of the above areas is assessed, relying heavily on the approach and findings of the authors and their colleagues. The problems and possibilities inherent in attempting to couple these components into a generic model of productivity are discussed. Finally, the potential benefits and hazards of genetic engineering of plants are discussed, and weaknesses in the current approach are highlighted.