The role of non-uniform sarcomere behaviour during relaxation of striated muscle.

Abstract

It has previously been shown that there are marked non-uniformities in the pattern of sarcomere length changes during relaxation of a muscle fibre. This problem has now been studied in more detail during both twitch and tetanus in frog single muscle fibres and some of these results are presented to elucidate the mechanical consequences of the non-uniform sarcomere movements. The sarcomere behaviour during an isometric twitch is essentially the same as that observed during a tetanus in a given fibre: the sarcomere pattern is stable during the rising phase, the peak and the initial portion of the relaxation period. The pattern becomes non-uniform at a point (approx. 73% of peak force) which coincides with the beginning of a rapid fall in tension. At this point some segments (altogether c. 70% of the fibre length) start to shorten at the expense of others which are overextended. Maximum shortening, and elongation, is attained as the isometric tension approaches zero (2–5% of peak force). The majority of the sarcomeres in the fibre thus produce mechanical work (against a decaying load) during the isometric relaxation phase. The onset on non-uniformity varies with the time at which peak twitch force is attained. Thus, if a fibre is released to redevelop tension at different times during a twitch, the onset of non-uniformity is found to shift correspondingly to coincide with the beginning of the rapid tension fall in each case. The results suggest that the mechanical activity decays at different rates along the length of the fibre. The cross-bridges (reduced in number) in the weaker segment(s) will be stretched and when the stretch exceeds approximately 18 nm/half sarcomere the bridges start to slide. It appears that the yielding somehow further reduces the activity in this segment leading to a steeper decline in tension. It is possible to induce the non-uniform behaviour (and the rapid relaxation phase) at an earlier time by applying a stretch to the fibre so that the weak segments start to yield. In this way the rapid relaxation phase may be initiated already at the peak of the isometric twitch. The results suggest that the non-uniform behaviour provides a mechanism by which the isometric relaxation is speeded up. Evidence is presented to show that a similar mechanism may be effective also during relaxation of cardiac muscle.