The mechanical properties of relaxing muscle

Abstract

The response of a muscle to stimulation is classically divided into two phases: contraction and relaxation. The mechanical properties of the muscle in the first phase have been extensively studied (for references, see Jewell & Wilkie, 1958) and much is known about the way in which tension is developed and work is done, but the second phase is much less clearly understood. A. V. Hill (1949b) defined relaxation simply in terms of the external mechanical changes in the muscle, as 'the process by which the muscle returns, after contraction, to its initial length or tension'.Originally our interest in relaxation was aroused by the marked difference between the time courses of isotonic and of isometric twitches. For instance, in Fig. 1, one is confronted by the apparently paradoxical situation that after having lifted a load of 3 g wt. the muscle has completely relaxed at a time when it could have borne tension of 5 g wt. under isometric conditions. Admittedly, with small loads like this, the situation is complicated by an additional factor, for in the isotonic case the muscle has shortened appreciably. However, even with large loads, where the isotonic shortening is much less, mechanical activity has apparently ended at a time when the isometrically contracting muscle is still capable of bearing a considerable tension. METHODSAll the experiments were performed on frogs' sartorii (English Rana temporaria), kept immersed in Ringer's solution (mM: NaCl 115-5; KCI 2-0; CaCl2 1-8; Na phosphate buffer, pH 7 0, 2.0) at 00 C. Isotonic or isometric recordings alone give an incomplete picture of the changes in the mechanical state of the muscle, so we decided to record length and tension changes simultaneously during various different types of contraction. The length changes were recorded in the usual way by connecting the muscle to a light lever, the movements of which were 'sensed' by a photo-electric arrangement (see Jewell & Wilkie, 1958 for further experimental details). In theory the tension in the muscle could have been measured simultaneously and directly in one of three ways: by attaching a suitable transducer to the pelvic muscle clamp; by incorporating a transducer in the connexion to the muscle; or by mounting a transducer at the tip of the lever. The mechano-electronic transducer, RCA 5734, which we normally use to measure tension, cannot conveniently be immersed in Ringer's solution; and to have mounted it at the tip of the lever would have produced an undesirably

The Force-Load-Velocity Relation and the Viscous-Like Force in the Frog Skeletal Muscle

“…(4) might be applicable to such a transient state. Although our experiments were conducted during slowly rising tension, JEWELL and WILKIE (1960) obtained similar results in experiments on the falling phase of tension in a relaxing muscle. JEWELL and WILKIE (1958) also concluded that the change of velocity followed very quickly on the change in force-probably in less than 1 msec.…”

Initial Heat Production in Isometric Frog Muscle at 15°C

“…Stretch produces a pronounced decrease in the rate of relaxation in bull frog sartorius muscles at 15°C (Figs. 4, 6) and in other amphibian muscles (Mittenthal and Carlson, 1971;Jewell and Wilkie, 1960). The difference between the final temperature and the temperature at peak tension (Table I) is the heat evolved during relaxation.…”

“…The stimuli delivered during declining tension after a tetanus and at the peak of twitch tension each would increase the active state in the muscle from about one-half to about one (Jewell and Wilkie, 1960). Hill's analysis (1949 b) in amphibian muscles at 0°C shows that only activation heat is a major heat source shortly after first stimuli.…”

Transient Phases of the Isometric Tetanus in Frog's Striated Muscle

“…Although evaluation of r is more laborious than the methods Hartree and Hill ( 1921) and Abbott ( 1951) used to quantify relaxation, and more laborious than measurement of the half-time of tension fall (Jewell and Wilkie, 1960), r has two merits as an index of relaxation. r may be measured in both twitches and tetani, providing a means for comparing relaxation in these processes.…”

“…During relaxation the fall in active tension is also exponential when P is near zero (Jewell and Wilkie, 1960):…”