Tetanic contraction

Abstract: Twitch potentiation and fatigue in skeletal muscle are two conditions in which force production is affected by the stimulation history. Twitch potentiation is the increase in the twitch active force observed after a tetanic contraction or during and following low-frequency stimulation. There is evidence that the mechanism responsible for potentiation is phosphorylation of the regulatory light chains of myosin, a Ca2+-dependent process. Fatigue is the force decrease observed after a period of repeated muscle stimulation. Fatigue has also been associated with a Ca2+-related mechanism: decreased peak Ca2+ concentration in the myoplasm is observed during fatigue. This decrease is probably due to an inhibition of Ca2+ release from the sarcoplasmic reticulum. Although potentiation and fatigue have opposing effects on force production in skeletal muscle, these two presumed mechanisms can coexist. When peak myoplasmic Ca2+ concentration is depressed, but myosin light chains are relatively phosphorylated, the force response can be attenuated, not different, or enhanced, relative to previous values. In circumstances where there is interaction between potentiation and fatigue, care must be taken in interpreting the contractile responses.

Abstract: The heat produced in a single muscle twitch is made up of two parts, (1) the heat of activation, and (2) the heat of shortening. In the leg muscles of toad or frog at 0 degrees C the heat per cm. of shortening is about 350 g.cm. (expressed in mechanical units) per sq. cm. of muscle cross-section. The heat of activation is usually rather less than the maximum heat of shortening and depends little, if at all, on the length at which the stimulus was applied or on changes of length thereafter: it is equal to the heat which would be produced if shortening were altogether obviated, a condition approximately realized in a muscle brought to a very short length by previous stimulation under a very small load. The heat of shortening occurs at the same time as the shortening. The heat of activation has its maximum rate at the start, very soon after the stimulus, and falls off in rate from then onwards. The heat of maintenance in a tetanic contraction is the summated effect of the heats of activation resulting from successive elements of the stimulus. The effect on the heat production of a sudden arrest of an isotonic contraction is described. Under such conditions the contractile elements of a muscle continue to shorten but at a decreasing rate as the tension rises. The complications due to inequalities of length and contractility in the different fibres of a muscle are discussed.

Abstract: 1. Vibration was applied longitudinally to the fully innervated soleus muscle of the decerebrate cat by attaching its tendon to a vibrator. Vibration at frequencies of 50-500/sec with amplitudes of 10 mu upwards caused the muscle to contract reflexly for as long as the vibration was maintained. The response was recorded myographically by a myograph mounted upon the vibrator, and electromyographically by gross ;belly-tendon' leads. The reflex contraction produced several hundred g wt. of tension and involved too many motor units for their discharges to be separable. The maintained reflex was abolished by making the preparation spinal or by anaesthetizing it with pentobarbitone, but it persisted after removing the cerebellum.2. The minimum latency for the appearance of the reflex response at the beginning of a period of vibration was about 10 msec. The latency of cessation of the response at the end of vibration was similarly short.3. On increasing the amplitude of vibration at any particular frequency in the range 100-300/sec the resulting reflex tension increased to an approximate plateau for amplitudes of vibration of 100-200 mu. Further increase in the amplitude decreased the size of the contraction, though there was no such reduction in records of the ;integrated' electromyogram.4. Such large amplitudes of vibration also reduced the tension, and shortened the duration, of a twitch contraction of the muscle elicited by stimulating its nerve. The strength of a tetanic contraction was much less affected by vibration than was that of the twitch contraction, and the muscle action potential elicited by stimulation of the nerve was unaffected. Thus, large-amplitude vibration influenced the contractile mechanism of the muscle (cf. Buchtal & Kaiser, 1951).5. Increasing the frequency of vibration increased the value of the plateau tension reached on increasing the amplitude. The effect was, however, relatively small and the largest increase seen was 3 g wt. of contractile tension per c/s increase in vibration frequency.6. The primary afferent ending of the muscle spindle is considered to be the receptor whose excitation leads to the reflex response to vibration. The vibration reflex thus appears to be the well-known stretch reflex, elicited by a rather unusual form of stretching. The size of the vibration reflex and its variation with frequency are discussed in relation to the servo theory of muscular contraction.

Abstract: Although the importance of mitochondria in patho-physiology has become increasingly evident, it remains unclear whether these organelles play a role in Ca2+ handling by skeletal muscle. This undefined situation is mainly due to technical limitations in measuring Ca2+ transients reliably during the contraction–relaxation cycle. Using two-photon microscopy and genetically expressed “cameleon” Ca2+ sensors, we developed a robust system that enables the measurement of both cytoplasmic and mitochondrial Ca2+ transients in vivo. We show here for the first time that, in vivo and under highly physiological conditions, mitochondria in mammalian skeletal muscle take up Ca2+ during contraction induced by motor nerve stimulation and rapidly release it during relaxation. The mitochondrial Ca2+ increase is delayed by a few milliseconds compared with the cytosolic Ca2+ rise and occurs both during a single twitch and upon tetanic contraction.