Studies on the latent period of muscular contraction. Method. General properties of latency relaxation

Abstract

Research on the niechanical latelit period of contraction -the short interval of time between the instant of application of the stimulus and the onset of tension develowent -has given rise to one of the more controversial subjects in the field of muscle physiology. A major point of contention, in fact, has been the question of the very existence of mechanical latency. It is not necessary to review the general literature of the subject here for detailed summaries will be found in the work of Peo (1888), Rauh ( '22)' Fulton ( '26)' and Lindhard ( '31). Two points of special interest will be discussed, however. The first is the undue credence (e.g. Ritchie, '33) given to the relatively recent research of Roos ( '31, '32) whose experiments seemed to give incontrovertible support to the long-held view that no latent period will be registered if the inertia of the recording system is reduced to zero. The attention paid to ROOS' claim is hardly justified, for as Snyder ('36) states: "ROOS publishes very few of his observations, and bases his argument not on a statistical datum but apparently upon a few readings, that he considers crucial, and upon theoretical grounds. " The second point is the general lack of attention suffered by the investigation of Rauh ('22), which demonstrated for the first time the existence of a minute relaxation of frog skeletal muscle just prior to the beginning of tension rise.2 This finding is obviously of such great significance for the whole problem of mechanical latency that it Will be discussed in some detail.Rauh, having in mind the criticism that the latent period had no real existence in muscular response and that its presence in meclianograms Z A n earlier observation of a precontractilr relaxation seems to br present in the work of Ye0 (1888). It is not clear, however, whether this relaxation was similar to that observed by Rauh, or a stretching effect at the point of recording due t o the contraction of other parts of the muscle. A t any rate, Yeo states that "the elongation of the muscle commences after the ordinary latency is over," and this indicates that, whatever the nature of his actual records, the author did not think he was dealing with the type of relaxation later demonstrated by Rauh.

Proceedings of the Physiological Society

“…Initial relaxation was always present, as found by Rauh (1922) and Sandow (1941), the tension falling by about 20 dynes which is about 1 part in 1000 ofthe tension later developed. The tension changed over and became positive between 11 and 20 msec.…”

A large birefringence signal preceding contraction in single twitch fibres of the frog.

“…Latency relaxation and the early optical signal As has been well described (Rauh, 1922;Sandow, 1944Sandow, , 1966Abbott & Ritchie, 1951), the earliest mechanical response detectable during a muscle twitch is actually a slight drop in tension (fibre lengthening) before active shortening. At 200 C this early mechanical response, called 'latency relaxation', begins about 2 ms after stimulation (Sandow, 1944(Sandow, , 1966Mulieri, 1972).…”

“…At 200 C this early mechanical response, called 'latency relaxation', begins about 2 ms after stimulation (Sandow, 1944(Sandow, , 1966Mulieri, 1972). Although latency relaxation is usually measured as a mechanical change at the fibre end, with the longitudinal stimulation used in these experiments its underlying basis probably involves a local sarcomere change propagating along the fibre length with the speed of the action potential (Sandow, 1966).…”

Tension due to interaction between the sliding filaments in resting striated muscle. the effect of stimulation

“…The compliance factor alone could not be the cause of the variation of the latency relaxation with length, because although a change of compliance would alter the rate of fall of tension, it would equally affect the rate of rise of the positive tension, and the time at which the tension curve re-crossed the base line ought to remain unchanged. In fact, the duration of the period in which the tension is below the resting value increases when the muscle is stretched (Sandow, 1944;Abbott & Ritchie, 1951). However, it could be supposed that in an isotonic muscle all three of the above factors together combine to give the observed critical dependence on length.…”