DISCUSSION

The elastic properties of the cross-bridges determine the principal features of the tension response to stretch (Huxley & Simmons, 1971,; Flitney & Hirst, 1978a). The sliding motion of the filaments deforms the linkages, generating extra force, and ultimately causing them to detach. This event is responsible for the onset of sarcomere yielding and for the abrupt fall in muscle slope stiffness. The results of the experiments described here suggest that a component of the transparency change resulting from stretch is also caused by cross-bridge deformation.

This hypothesis is based, first, on the marked similarity in the form of the optical and tension recordings mentioned earlier. It is especially significant that the transition between phases i and ii invariably coincides with the change in slope stiffness

(Fig 2.11), because this shows that the early part of the transparency change is influenced by an optical process that parallels cross-bridge deformation, and which is terminated at the point where the linkages reach their elastic limit.

The argument is reinforced by the proven quantitative association of a component of the optical signal with tension developed at the point of sarcomere 'yielding': the results show that Al/1^ changes in parallel with AP, when muscle stiffness is varied experimentally, either by altering the extent of actin- myosin filament overlap (Fig 2.9) or by changing the velocity of stretch (Fig 2.10).

The strength of the previous argument depends on the validity - PAGE 2.25 -

FIG 2.11 c

A diagrammatic representation of ; A. Tension transient.

B. length change.

C. Sarcomere movement (increasing length upwards) D Optical transient.

Fig 2.11

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of the method used to analyse the optical signals and there are two aspects of this which require some comment. The first concerns the baseline shift. The origin of this has not been investigated, although it is probable from what was said earlier that changes in the diffracting power of the muscle, as well as changes in its thickness, must make a contribution. Whatever the cause, the steady state measurements of muscle transparency clearly show that the underlying optical processes together combine to make the transmitted light intensity increase with increasing length {Fig 2.3). Moreover it will be seen later that estimates of the amplitude of the offsets, calculated from muscle transparency-length curves like that of Fig 2.3, are in excellent agreement with measurements of actual recordings of dynamic stretches.

The second point concerning the analysis relates to the assumption that the delayed transparency change (phase iii) actually represents a reversal of the process responsible for the initial decrease in muscle transparency (phase i). The argument here has centred on the observation that the two phases are comparable in size for recordings made at short muscle lengths, where any baseline shift is small. At longer lengths, the amplitude of the delayed phase is greater than that of the initial decrease in transparency, because the latter is more strongly attenuated by the baseline shift, which becomes steeper at longer muscle lengths. However, after analysis has been made, the transparency is seen to remain more or less constant through out phase ii and this is so for recordings made at all muscle lengths. It is easy to see that this would not be the case if the delayed transparency change were not equal in amplitude and opposite in sign to phase i. This result is therefore consistent

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with the original assumption, though it does not prove it to be true: it could be that another, quite different process begins at the end of the stretch and that this happens (by chance) to be equal and opposite in sign to the first phase. This does not seem at all likely.

The tension transients generated by the stretch of resting muscles are also thought to be due to the flexural rigidity of myosin projections that form long lasting connections with neighbouring actin filaments (Hill, 1968; Flitney, 1975). The properties of the cross-bridges of active muscle differ from Hill's resting cross-bridges in two important respects: first, in being able to accomodate a larger range of filament sliding before reaching their elastic limit; and secondly, in having a much shorter lifetime. How do the present results compare with those reported by Flitney (1975) for resting muscles?

The mechanical stiffness of a muscle is increased as the result of stimulation. Flitney & Hirst (1978a) showed that the elastic limit of the cross-bridges in active muscle is reached for a relative sliding movement of ca. lOnm. The tension increment resulting from stretch at maximum filament overlap amounts to lO^N.m ^ and so sarcomere stiffness ( A p/ As) is around lO^N.m ^.nm ^ extension. The corresponding value for resting muscle can be calculated from the data in Figs 2A and 7 of Flitney's (1975) paper. His fig 2A shows tension generated by stretching a muscle in Ringer's solution made hypertonic (1.65 x normal osmolarity) by the addition of sucrose. This procedure enhances the stiffness by a factor of ca. 5x compared to its value in isotonic solution. The force increment was lOmN, which would therefore be équivalent to around 2mlSI in isotonic solution.

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2

The cross-sectional area of the muscle was 2mm (Flitney, 3 - 2 personal communication) so that A P is equivalent to 10 N.m The elastic limit was reached for a relative sliding movement of

2 -2 -1