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1. Introduction | 3. Design Solution | |
2.1 Muscle structure and contraction.
2.2 Monochromatic light diffraction experiments.
2.3 X-ray diffraction experiments.
2.4 The need for a positive feedback circuit.
Muscles comprise bundles of fibres a few centimetres in length and 20-100 µm in diameter, surrounded by connective tissue (See figure 1). Every fibre is filled with approximately a thousand
rod-like structures called myofibrils, each about 1 µm in diameter. Each myofibril is divided up into cylindrical sections by thin partitions called Z-lines or Z-discs. These also cross from
myofibril to myofibril throughout the fibre, dividing it into repeating units called sarcomeres.
Thin filaments of actin emerge from the Z-line, making up the I-band, while thick myosin filaments make up the A-band. The actin elements of the I-band extend into the A-band, leaving a less dense region in the centre, where there is no overlap. This is the H-zone. This variation in density is the cause of diffraction of a monochromatic light beam, allowing changes in sarcomere length to be observed. The cross section (figure 1) shows how the filaments from the I- and A-bands interleave in a hexagonal
structure.
On stretching or shortening of the muscle fibre, the A-bands remain at constant length, while the I-bands expand and contract. This was shown by Dr. H. Huxley, and leads to the sliding filament theory of muscle contraction: contraction occurs by the interdigitation of the I-band (actin) and A-band (myosin) filaments. The process by which the filaments pull themselves past each other to interleave in this way is complex, and is still not fully understood; it is the subject of considerable investigation in this field.
Figure 2 shows a typical experimental set-up that is used for obtaining diffraction patterns from plaice fin muscle. The muscle is mounted inside a chamber and immersed in a cool solution known as ringer's solution; this closely matches conditions in the fish plasma, which helps to preserve the muscle for a long time. Monochromatic light from a laser passes through the muscle, and the resulting diffracted beams are observed using an array of photodiodes. The central part of the array is usually masked to avoid overloading due to the high intensity of the central, undiffracted beam.
The array is scanned by a computer, to produce a chart similar to that shown in figure 4. The first order peaks can be easily seen. Statistical analysis can be performed, using the computer, to filter the noise and obtain two best-fit Gaussian curves to fit peaks. From this, the sarcomere length can be calculated. The tension in the muscle can also be measured, using a force transducer connected to the wire holding the muscle in place inside the muscle chamber. The muscle is stimulated using a number of
short (2 mS) voltage pulses occurring every 20 mS, for a period of say 200 mS. During thc resulting contraction, the sarcomere length and tension in the muscle change and are recorded by the computer
for later analysis. Even when the muscle is fixed at both ends, the sarcomere length is still found to change by up to 8%. In addition, the intensity of one of the diffracted beams can be up to 7 times that of the other.
In a similar way to that decribed above, X-rays may be used instead of visible light, to obtain higher resolution diffraction patterns, due to their shorter wavelength. The differences are that the machine is a lot larger: a typical distance of the screen or photographic plate to observe the pattern, from the
muscle, is six metres. The images which are produced are highly complex and must undergo mathematical transform analysis using a computer, before any useful information can be extracted. A typical pattern is shown in figure 3. Again, the observations are performed during muscle stimulations, so that the changing muscle structure during contraction can be accurately observed. In this way Biophysicists hope to gain a greater understanding of the processes behind muscle contraction.
Biophysicists would like to be able to see changes in muscle structure during contractions at constant tension, or constant sarcomere length (Isometric contractions) Unfortunately merely clamping the muscle at its two ends is no guarantee that the intervening tissue will remain at a constant length: the length
seems able to re-distribute itself so to speak, parts of it contracting and parts lengthening. In fact light diffraction studies have shown that the sarcomere length changes by 7-8% during some contractions (previous section). Therefore some means of keeping sarcomere length constant is required.
One way of doing this is to shine a laser beam through the muscle in addition to the X-rays; the passage of this beam will not affect the characteristics of the muscle, nor will it alter the X-ray diffraction pattern. The position of the two first order diffraction peaks could be measured in some way and used to
provide a positive feedback signal, in such a way that a decrease in sarcomere length would cause a servo-motor to lengthen the muscle as a whole, while an increase would have the opposite effect. Thus while the whole muscle's length is not kept constant, the sarcomere length at the position of X-ray and optical measurement is. This was the subject of this project.
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1. Introduction | 3. Design Solution | |